(first posted 1/24/2015) If I haven’t learned something new each day here, it’s been a waste. A couple of days ago, at the V5 diesel post, I got into a spirited debate about diesel versus gas engines. The issue was that every naturally-aspirated (non-turbo) car diesel engine I’ve ever encountered has a lower torque output than a comparable gas engine. I and some other commenters found stats on numerous car and truck engines, all with the same result, like these two compared above.
Others insisted that diesel engines intrinsically make more torque due to their higher compression and because diesel fuel has 12-15% more hydrocarbons per volume than gasoline, and other aspects too. Well, that all sounds good in theory, and I almost had the right rebuttal in hand, but was missing one key detail. I little research found the answer, and it explains perfectly why gas engines make more torque.
The short version: It’s all about the air-to-fuel ratio.
In the combustion reaction, oxygen reacts with the fuel, and the point where exactly all oxygen is consumed and all fuel burned is defined as the stoichiometric point. Gasoline and diesel fuel have essentially the same stoichiometric air-fuel-ratio by mass; 14.7:1 for gas, and 14.5:1 for diesel.
A gasoline engine can and does typically run at or near stoichiometry; modern electronically-controlled engines can control the fuel mixture very accurately. Obviously, to start a cold engine, a richer mixture is needed, and certain conditions may require some deviation. Running at perfect stoichiometry tends to create a very hot exhaust; in the old days, carbs jets were often set to run high-performance or truck engines rich, because it provided a cooling effect, but with some loss of efficiency.
Gasoline is highly volatile, and vaporizes easily, typically outside of the combustion chamber (except for direct injection gas engines). Thus in order to burn properly, gas and air must be premixed at or near stoichiometry, generally speaking. A gas engine will not run (or properly) in excessive lean condition.
The typical gas engine has a throttle plate to control the amount of pre-mixed air-fuel intake. This creates pumping losses, which makes the gas engine even less efficient at lower engine speed/load than at higher ones, when pumping losses are reduced through greater throttle opening. (Note: all these conditions may not apply to very certain modern gas engines with variable valve timing, direct injection, etc. We’re speaking of traditional gas and diesel engines).
With full throttle at its torque peak (the point where each combustion event creates the greatest force), the gas engine is essentially limited by how much air-fuel mixture it can “inhale”, which depends on a variety of factors such a valve size, porting, etc. The point being, the better it “breathes”, the more power/torque it can generate, since the amount of fuel will proportionately increase with the amount of air “inhaled”.
Diesel fuel, essentially a light oil, is drastically less volatile, which is why it can’t be mixed with air via vaporization. The heat of the compressed air in the combustion chamber begins to evaporate the droplets of diesel fuel after it is injected, and they begin to burn, causing more evaporation and additional burning. Because of not being atomized, diesel burns much more slowly, as the droplets break up, which explains why no diesel engines rev much more than 4,000 rpm, even the smallest ones. Even Audi’s LeMans racing cars max out at 5,000 rpm. It’s essentially impossible for a diesel to rev any higher.
That slow burn rate explains why even though the stoichiometric ratio for diesel fuel is almost the same as gasoline (14.5:1), in reality, diesel engine always run much leaner; usually drastically so, because if a diesel runs at or near stoichiometric ratio, it simply cannot burn all the fuel and emits a large quantity of soot (black smoke). Unlike in an oil furnace, there’s just not enough time during the combustion cycle for stoichiometric combustion.
Therefore, diesel air-fuel-ratios are typically from 100:1 or more (at idle) and roughly 40:1 to 30:1 at normal operation. Even the Audi LeMans racer runs no less than 19:1. Running anything greater than that results in prodigious plumes of black smoke, which “coalers” readily accomplish by tampering with their electronic injection controls. It may be approaching the stoichiometric ideal, but in reality, it’s just massive amounts of unburned diesel.
The upshot is this: for a given volume of intake air, a diesel engine simply can’t burn nearly as much fuel as a gasoline engine can; it has to run very lean to avoid smoking massively. And this explains why a diesel inherently makes less torque and power than a comparable gas engine.
And it explains why naturally-aspirated diesel engines have a lower BMEP (Brake Mean Effective Pressure), the maximum working pressure from combustion that determines torque. The typical BMEP range for naturally-aspirated gas engines is from 8.5 to 10.5 bar; for NA diesel it is 7 to 9. There’s a bit of overlap, given the wide range of designs covered.
Yes, a diesel engine is intrinsically more efficient, meaning it generates more power and less waste heat from a given volume of fuel than a gasoline engine. But it simply can’t burn as much fuel as a gas engine; the difference is big enough that the gas engine for a given displacement invariably makes more power. Important note: we’re talking about naturally-aspirated engines in both cases.
Diesel engines thrive with boost (forced induction); in fact they operate more efficiently with boost, as the heat from the exhaust can be partially converted to more energy. Diesel engines can be boosted to very high pressures, because there’s never a risk of pre-detonation, as in a gas engine. The more air can be forced in, the more fuel can be burned, offsetting the intrinsically lower power output of a diesel. Of course, gas engines can and increasingly are turbocharged too, and comparing turbo diesel and gas engines in terms of power output is essentially futile, as almost any amount of boost can be dialed in, although a diesel will take higher pressures more readily.
The key word is “comparable”, because ultimately, it’s impossible to compare two engines (diesel and gas) perfectly, although there are numerous engine families that have gas and diesel variants. I have come up with a few obvious ones, like the original VW 1.6L diesel/gas engines:
VW’s 1.6L Golf/Rabbit engines (1980s):
Diesel: 74 lb.ft.
Gas: 92 lb.ft.
Another typical comparison is the Mercedes W123:
240D: 97 ft.lb. @2400rpm
230 (four): 125 lb.ft. @2400 rpm
Similar comparisons to similar-sized gas-diesel comparisons for engines from Audi, Peugeot, Oldsmobile, etc, all yield the same results: the gas engines generate considerably more torque.
These are two V8 engines as used in light/medium truck use, the Navistar/Ford diesel 7.3 V8 and the Ford 460 gas V8. Not only does the 460 make considerably more torque at its peak (400 lb.ft. @ 2200 rpm), it also makes more torque at the diesel engine’s torque peak rpm (1650 rpm).
I decide to look at some big truck engines too. From the 1930s into the 1960s, Hall-Scott’s big gas engines were legendary for their torque and power output, and were favored by trucker in the mountainous West as well as fire departments everywhere, who kept their Hall Scotts running well into the 70s. Hall Scott will get a detailed history here, but let’s compare just a couple of their engines to similar-sized Cummins diesels.
The legendary H-S 400 was not exactly what one might expect in a low-rpm truck engine: it had an overhead cam hemi head, which promoted good breathing. Hardly any car engines in the US had a head design like that. And the results were accordingly impressive.
Here’s a dyno chart of a 400 engine from 1943. The 400 had a 5.75″ bore and a massive 7″ stroke, which yielded 1090 cubic inches from six cylinders. Peak output was 950 lb.ft. @1300 rpm, and 295 hp @2000 rpm.
I don’t have a dyno chart, but the same basic engine in the early 60s (then called the 6182) was yielding 1020 lb.ft. @ 1300-1400 rpm, and 370 hp @ 2300 rpm.
Cummins was the most progressive and successful diesel engine builder in the US, starting out in a crowded field back in the 1930s. Although founder Clessie Cummins understood the advantages of forced induction on diesel engines early on, and used it for his numerous Indy 500 diesel racers, forced induction was not common on US diesel truck engines until well into the 1960s, although superchargers were available on some models in the 50s.
The final evolution of the long line of Cummins natural-aspirated engines big six truck/marine diesels was the NH 250, which was built from the 60s all the way until 1989, as used in the military M939. It had a 5.5″ bore and 6″ stroke, displacing 855 cubic inches. Max torque is 685 lb.ft. @1500 rpm, and max hp is 250 @2100 rpm.
To compare the two, we need to increase the Cummins outputs by 28%, since the Hall Scott is bigger in displacement by that amount. The adjusted results are 877 lb.ft. and 320 hp; both well below the H-S gas engine.
I ran the same comparison between two smaller Cummins and Hall-Scott engines, and the result was essentially the same.
The Hall-Scott engines both made about 1 lb.ft. of torque per cubic inch; the Cummins made about .8 lb.ft. per cubic inch. In fact, most reasonably healthy gas engines make roughly 1 lb.ft. per cubic inch, as a general rule of thumb. Naturally aspirated diesels seemingly never do, to the best of my knowledge.
Needless to say, once Cummins’ turbo diesels became commonplace, the Hall-Scott engine quickly disappeared, given its prodigious thirst.
But some trucker’s thirst for power was so great that they installed the V12 version of the big Hall Scott; 2269 cubic inches, and up to 900 hp. Sorry; I don’t have ready specs for the torque, but it must have been over 2000 lb.ft. These were the monster trucks of their day.
In a way, this subject is mostly moot, as almost all modern diesel engines are turbo-charged, with the exception of some smaller agricultural and industrial engines. Forced induction solved the inherent issue of a diesel’s inability to burn an equal amount of fuel per air volume as a gas engine, and increased its output and efficiency further. The most efficient engines in the world, the giant 100,000+ hp oil-burners in cargo ships, achieve up to 55% efficiency.
And gas engines have come a long way too; in fact automotive gas engines have improved their efficiency in the past few decades more than diesels. Whereas the difference in efficiency between automotive gas and diesels was once 35-40%, today that number is down to some 20-30%. As super-high compression gas engines like the Mazda Sky-Active lead the way, in the future the difference between the two will only decrease. But until someone can make diesel fuel burn faster, they will always be intrinsically less powerful, not factoring the forced induction. The laws of physics are hard to bend.
A cogent explanation of a complicated subject. Well done.
Aside from the near-universality of turbocharging, I think the other factor in the efficiency of modern turbodiesel engines is the adoption of “common rail” injection at extremely high pressure, which as I understand it promotes more rapid atomization to compensate for diesel fuel’s inherent limitations in this area.
It might to some extent. But even the most modern diesels still can’t seem to get any closer to stoichiometric air-fuel ratio than about 19:1, which means that the same fundamental issue is still very much there.
It certainly improves efficiency some, as well as make them quieter, which is the best benefit of all, from my POV.
Gosh, do I get any credit for this article? I didn’t help write it or do any research but it seems to me a more detailed and better researched version of what I said in a response to you the other day.
Here’s one more kernel of knowledge from my foggy old brain that you can turn into whole new thread:
With the recent advent of direct injection in gasoline engines, the upper limit for compression will probably be bumped higher and higher little by little. Ford’s new ecoboost engines are probably the future of gasoline engines. With direct injection, combustion engineers should be able to solve(or partially solve) the problem of detonation by delaying the fuel injection until the spark plug is sparking. In fact, they might even be able to turn off the spark plugs entirely once the engine is heated up good and hot and just rely on the heat of compression for ignition just like a diesel engine does.
The air and fuel has to mix well before ignition in gasoline engines. The direct injection is done as the air is being compressed, and the evaporation of the fuel cools the air, allowing for a higher compression ratio. If you delay fuel injection until top dead center, then the liquid gasoline would start burning like diesel fuel, although faster, but not being well mixed the process would limit performance I think.
Yes you do; sorry it’s belated. You and I were both very close; I kept saying it was because diesel burned slower, but didn’t quite connect the dots that it also meant there was intrinsically less fuel that could be burned, due to the diesel requiring to be run with excess air. And you were very close too, but not quite all the way there either, in terms of the specific issue.
The Diesotto engine is essentially what you’re talking about. it’s been in development for some time.
Good!
Now that I got some credit, I’m going to criticize your article. 😀
I’ll start out by saying that I do not believe you have proven your hypothesis,,,that being that gasoline engines intrinsically generate greater torque than diesel. What you have done, I think, is to inform us of the disadvantage of excess air in diesel engines.
Those are some very persuasive graphs. I have some problems with them. I think they are dishonest, a little bit. As I am sure you are well aware, it is not fair to compare engines if one has a turbo and one does not. I noticed you are not guilty of this.
It is ALSO not fair to compare engines of unequal displacement!
AND it is UNFAIR to compare horsepower and torque values THAT ARE NOT AT EQUAL RPMs!
AND it is a little dishonest to present the two graphs with the vertical scales not at the exact heights!
Finally, I notice that you are guilty of all these things which would help your side of the argument and none of the things that would help the other side of the argument. This seems really suspicious to me.
Here’s what you do to be completely fair. You re-do those graphs with the vertical scales at the exact same heights. then you use CORRECTED data and instead report ft-lbs PER CUBIC INCH, not ft-lbs total for two engines that are not the same displacement. Then you label, NOT THE MAX TORQUE, but the torque output at 1000 RPM for BOTH engines.
Anything less than this is essentially lying to people.
PS, I already ran the numbers and it appears the torque numbers still come out in your favor, you lucky bastard.
One more criticism for now…you only did one engine comparison. That is hardly enough evidence to back up your hypothesis.
John, Frankly, your tone is more than a bit off-putting. I have not been “essentially lying to people”. I presented the facts as they were readily available to me. Those charts aren’t my creation; I found them on the web. I post two to four articles a day here; how much time do you think I have in a day? 60 hours? Seriously; you’re telling me I should redo everything I’ve shown here? Meanwhile I have knowledgeable folks leaving comments that this post is the best explanation they’ve ever seen on the subject. I’m sorry you feel lied to.
If you’d like to take a crack at doing it to your standards, please do so and send it in. It’s easy to criticize; step up and do it better.
It’s essentially impossible to find engines of the same size, although I did give several examples. And it’s not that hard for folks to read the charts and interpret them. And I did adjust the numbers for the Cummins and Hall-Scott for the difference in displacement.
I did point out that the Ford 460 made more torque at 1650 rpm, which is the Navistar 7.3’s torque peak.
But your demand that I show torque for both sets of engines at 1000 rpm is completely irrelevant. The whole article and point of it is that gas engines make more torque; meaning the maximum amount. If a diesel engine happens to make its torque at a lower rpm or a gas one at a higher rpm, that easily compensated with gearing; that’s why we have transmissions, right? The issue is maximum torque, not at 1000 rpm.
I don’t need to show any more evidence, because I’m utterly convinced that the diesel’s inability to burn roughly equal amounts of fuel as a gas engine is a fundamental reality, and will essentially apply to all diesel engines, except for possibly some very specialized, low-speed engines for which there are no gas engines to compare to anyway. And it would seem pretty much everyone except you is too.
If you’re not convinced, show me some examples that contradict mine.
Gee, so much for giving you partial credit; Thanks for…nothing. 🙁
OH jeez, I’m not trying to start a fight. I’m just pointing out constructive criticism and I was trying for a little humor in the process . If they aren’t your creation you have no reason to be offended. Actually even if they are yours I don’t see why you would be offended. They are a good find they just aren’t perfectly suited for this debate. Back in grad school if I had used charts like that I would’ve been made to do it over and got teased in the process.
Look, all you gotta do is read both charts at a given RPM, and divide the number by the displacement. Then if you think those numbers are not easy to understand you can normalize them by multiplying by an imaginary displacement…say 450 cu in. That way you are doing a sensible comparison without the appearance of trying to deceive. I did not actually think you were trying to deceive and that should’ve been obvious when I stated that I checked the numbers and they were still in your favor.
Now, a less-than-perceptive person will just look at the charts and see that one torque curve is much higher than the other. The way the charts are constructed, one might think they were intended to conceal truth from the less-than-perceptive people…like a sales brochure is designed to do.
I realize you put way more effort into it than I have. And unfortunately I do not have the time to be much more than an armchair quarterback here, so that’s not likely to change.
BTW, the point of the article appears to be that the reason a gasoline engine makes more torque is because it uses less excess air(or negative excess air), which means it gets more fuel per combustion event, which more than makes up for the lower compression ratio. If that is the case then you need to compare torque outputs at the same RPMs to support the hypothesis. If you choose to use a gasoline torque output at a higher RPM what you are proving is that the gasoline engine produced more torque because it has the ability to generate peak torque further out in the RPM range than a diesel and that is a completely different thing. Now you have introduced other variables into the system like cams, valves, exhaust, etc, which are affecting torque output and are not being accounted for.
@John-
In terms of theory, I think Paul Niedermeyer has proven that diesels can’t generate the same amount of torque without turbocharging. But I think that one point must be understood, modern diesels are not using the diesel cycle which requires the fuel to be burned isobarically. Diesels actually burn the fuel more rapidly than that making the thermodynamics more like the Otto cycle. (google these terms for an explanation).
So the practical question is whether a diesel, burning half the fuel as a gas engine but having compressed the air into half the space, can get about the same peak pressure and therefore about the same torque. A number of examples of production engines have been presented, some by Paul Niedermeyer, and, below, some by me, all of which support the hypothesis.
“So the practical question is whether a diesel, burning half the fuel as a gas engine but having compressed the air into half the space, can get about the same peak pressure and therefore about the same torque”
Logical and agree. Very well stated and I wish I had stated it this clearly myself. This absolutely IS the hypothesis. I would make only small changes to be just right.
Hypothesis: Does a diesel engine, burning less fuel than a gas engine, and having compressed the air into less space, achieve less measured torque at the crankshaft because of these two differences?
“I think Paul Niedermeyer has proven that diesels can’t generate the same amount of torque without turbocharging.”
Illogical and disagree…for the reasons I already stated in previous comments.
“A number of examples of production engines have been presented, some by Paul Niedermeyer, and, below, some by me, all of which support the hypothesis.”
Supporting a hypothesis and proving a hypothesis are not one in the same. I have not looked at examples presented by you.
OK, I suppose “proven” is overstating what we have done. But we have a plausible explanation as to why a naturally aspirated diesel probably will have less torque than a comparable gas engine. Most modern diesels are turbocharged, but I have found some industrial engines made by Perkins that are naturally aspirated in smaller 4 cylinder (or less) sizes.
> With direct injection, combustion engineers should be able to solve(or partially solve) the problem of detonation by delaying the fuel injection until the spark plug is sparking.
I think that would be bad. To my knowledge, DI gas engines still inject the fuel while the intake valve is open (mostly). They can’t wait to inject until the piston is near TDC on compression stroke. Not only will it not be well-mixed (vaporized), but because gasoline ignites so quickly, I think you’d get a hot spot that would damage the nozzle end of the injector.
> In fact, they might even be able to turn off the spark plugs entirely once the engine is heated up good and hot and just rely on the heat of compression for ignition just like a diesel engine does.
Paul mentioned a “DiesOtto” engine. I never heard that terminology until now. I just looked it up and it’s basically the same thing I was going to suggest, which is the Homogeneous Charge Compression Ignition (HCCI) engine. GM was working on HCCI before and supposedly had an engineering prototype running in a vehicle. Mercedes has their DiesOtto engine prototype, and one of the Japanese companies was working on one too. Hydrogen and electric is getting all the press these days, but hopefully auto manufacturer R&D groups are still working towards bringing these to market.
“I think you’d get a hot spot that would damage the nozzle end of the injector.”
I don’t see it. The nozzle squirts fuel only, not fuel plus oxidizer.
Rudolph Diesel intended his engine for operation on vegetable oils,notably ground nut oil from German colonies in Africa.Ackroyd -Stuart was the first to use solid injection rather than air blast.The ability of ci engines to run on any old crap[ as my fitter mate put it]was at the heart of their adoption,they are no longer in favour with legislators in Europe thats for sure .
You might want to tweak your headline. At the moment, it states the opposite of your argument.
The TDI Audi racing cars make the point well. When they first ‘competed,’ the ACO allowed them a 5.5 liter twin-turbo V12. “Aren’t diesels fast!” exclaimed the rubes. Sure, but the last time a 5.5 liter turbocharged 12 cylinder gasoline powered car was allowed to road race was 35 years earlier, and even then it made twice the power of the ‘fast’ diesel.
Good idea! It’s amazing how one can overlook such obvious mistakes in one’s writing; the mind sees what it wants to say, not what’s actually written.
Nicely said. The air/fuel ratio reality of a diesel engine sure explains its superior fuel efficiency (in most cases).
The air fuel ratio has nothing to do with their efficiency. It does have to do with their lower power output per displacement, at least comparing NA diesel to NA gas.
Paul explained the reasons for the fuel efficiency differences.
1. Gasoline engines have pumping losses due to the need for the throttle.
2. Diesel fuel contains more energy per gallon than gas.
3. Diesel engines have higher compression ratios.
Now modern gas engines have some answers to those issues which is part of the reason the gap has closed, and diesels are finally subject to stringent emissions standards which is also causing the gap to close.
#1 is changing in engines that inherently operate on some form of the Atkinson cycle or have variable intake valve timing. By closing the intake valve late the air being forced back out of the cylinder is being forced into the next cylinder in the firing order. Turbo charging is also reducing the pumping losses. If you size the turbo so that there is essentially no vacuum or boost created you eliminate that pumping loss caused by the throttle. Some diesel engines now have a throttle, it is still not used to control power or rpm but it is used for EGR operation and/or DPF regeneration.
#3 gas engine compression ratios are climbing thanks to direct injection and more sophisticated engine controls that can adjust timing in response to very sensitive and often multiple knock sensors.
I’m not an engineer, but I understand your points. Please clear this up for me. Doesn’t straight ethanol (theoretical) require an air/fuel ratio of under 10:1? Therefore, it takes more ethanol to provide the same power than gasoline due to its lower energy content per gallon.
Now, since ethanol has a higher resistance to detonation, some of that inefficiency can be regained by raising the compression ratio. Flex fuel vehicles don’t have that luxury, so their mileage on E85 suffers.
If Diesel fuel has more energy per gallon, enough that it can run 19:1 AFRs, how can that not affect its efficiency to some extent (granted, in combination with the above factors that you mentioned)? I’m just a backyard mechanic, so I’m simply curious here.
It’s not the higher BTU content of diesel that affects its AFR. It’s the way diesel burns.
One has to think of diesel combustion in different way than most other (volatile) fuels. With gas and such, the whole quantity of the intake is an essentially homogenous mixture; a gaseous vapor, because the gasoline has vaporized. In order for a gaseous mixture to ignite and combust properly, the AFR has to be within a certain ratio. Too rich, and it wont ignite. You know about the trick of throwing a lit match into a can of gasoline. And if it’s too dilute (thin) it won’t ignite either.
In a diesel engine, it’s very different. It sucks in straight air, and then even the smallest amount of liquid diesel injected will ignite, because of the heat of the compressed air. But it never becomes a homogenous mixture/vapor. Whatever minute amount of diesel is injected, it essentially burns by vaporizing in minute amounts and burns, regardless of the unburned air all around it.
At idle, the AFR is 100:1 or even greater. It doesn’t matter, because there’s no homogenous vapor that ignites; just the microscopic particles if diesel fuel itself, using up a very small quantity of the available air around it, of course. But 99% of the air is exhausted again, unaffected by the combustion.
The AFR in a diesel at idle can be as high as is needed to create just enough energy to keep the engine idling.
As a larger quantity of diesel is injected, the engine will speed up and create more energy. But it always combusts in the same way: the liquid diesel droplets breaking up and igniting due to the hot air, not because the whole load of diesel has vaporized before combustion, as in a gas engine.
That is the key essential difference, which is why it’s called the Diesel Cycle. If one could vaporize (theoretically) diesel and then inject it, it wouldn’t be a true diesel anymore. The diesel engine is so efficient, because it doesn’t need the whole intake air to be a homogenous vapor. Which is the greatest difference/advantage of a diesel. And which s why it doesn’t need a throttle. It doesn’t care how much air is sucked in, just how much diesel is injected.
Stratified charge gas and some direct injection gas engines have tried to take advantage of this, by creating just a pocket of fuel-air mixture in the combustion chamber to ignite at low load conditions, in order to run more efficiently. But the results have been less than stellar, to date.
Actually oily fuels(heating oil, kerosene, diesel, jet fuel, etc) do vaporize when the temp is high enough. They may not fully vaporize until they are burning, but they do vaporize.
Got it…thanks!
Yes straight ethanol does require a richer air fuel ratio but again that doesn’t have anything to do with the fuel economy when comparing two different fuels. The difference is again in the lower energy content per gallon. What does make a difference is that due to that lower A/F ratio you can stuff more fuel in a cylinder and make more power. Getting air into and the resulting exhaust out of an engine is what limits how much power you can make. So use a fuel that requires less air and the power can go up even with a fuel that contains less energy. That is why many drag racers have switched to E85 because they can make more power with a given displacement vs gasoline and why methanol with an even lower energy content has been used in drag racing for years. The bonus is that E85 and methanol is way cheaper than a similar octane gasoline.
The people who don’t understand E85 often state the reason that it gets worse MPG is the richer A/F ratio it needs, but again it is the lower energy content that causes the loss of MPG.
I think that the main reason that people think the A/F ratio impacts efficiency is because with a given fuel the A/F ratio does in fact impact efficiency. Run too rich and fuel goes out the exhaust without being burned completely and thus creating no or less energy. But when comparing different fuels the A/F ratio of them isn’t a factor in its efficiency.
Since E85/Ethanol is a much higher octane it can be used in an engine with higher compression ratios and if you build an engine with that in mind it can approach or in some cases exceed the efficiency of a gasoline engine of the same displacement. The fact that we have FFVs instead of dedicated E85 engines of course means that MPG suffers when run on E85.
Diesel and gas are ignited in different ways. Gasoline can only be ignited by a spark in a relatively tight A/F range. Diesel on the other hand is exposed to a temperature where it self ignites. You need that extra air to produce the cylinder pressure to cause the in cylinder temp to rise to the autoignition temp of diesel. Otherwise the extra air in the cylinder is just along for the ride.
To look at it another way a particular vehicle requires a certain amount of energy to push it down the road at a given speed which is based on the vehicle. Use a fuel that contains more energy per gallon and you’ll need fewer gallons, use a fuel that contains less energy per gallon and you’ll need more gallons.
Of course the other factors mentioned play a part in the higher efficiency of the diesel engine.
The alcohol molecule contains oxygen. I’ll let you ponder that on your own.
There extra air is not “along for the ride” it expands with the heat of combustion and does work.
Consider water injection in hotrods and drag racers.
Thanks Eric,
In all my time tinkering, I’ve never messed with Diesels; therefore, I’ve never thought much about them. It’s only been over the last few months that I’ve been studying them to any extent, making up for lost time, I suppose!
Yes ethanol contains oxygen which in one of the reasons it needs a richer A/F ratio, because it brings some of the needed oxygen along with it. What really maters, when discussing the energy content per gallon, is how many hydrogen atoms it brings to the party.
I think you mean carbon atoms. carbon bonds contain far more energy than hydrogen bonds and a hydrocarbon chain contains far more carbons than hydrogens.
Combustion is simply the breaking of H-bonds and C-bonds in a molecule. This is done by giving the C and the H something else to bond to. That something else is oxygen. The richer fuel air mixture as car buffs call it is really a confusing way to talk about it when talking about different kinds of fuel. That terminology really only makes sense when discussing one kind of fuel.
Alcohol is a smaller molecule which itself contains oxygen, and that is partially why it requires less air to combust a given quantity of fuel.
Here is the EXACT reason why alcohol is favored by drag racers. A cylinder only holds a finite amount of air-fuel volume. The fuel is where the energy comes from, not the air. If you switch to a fuel that requires less air, then you can shove more fuel into the cylinder and get more power per combustion event.
Top fuel drag racers have switched to a fuel that has such a low air requirement and they run such a high compression ratio, that they are approaching “hydraulic lock” at top dead center on the compression stroke. This means that the combustion chamber is nearly full of liquid and has very little gaseous volume.
H2 burns quite nicely and since it doesn’t have any other atoms the only product is H2O, none of that pesky C to make CO or CO2. The H is what it is all about. The C is just about giving it something to bond to that results in the fact it can be transported easily in a liquid form.
“The H is what it is all about. The C is just about giving it something to bond to that results in the fact it can be transported easily in a liquid form.”
That is pure nonsense, Eric! The carbon is where all the energy is. A double carbon-carbon bond releases at least 50% more energy than a carbon-hydrogen bond…not to mention in gasoline the average hydrocarbon chain has twice as many carbons as hydrogens.
Look at the energy density of H2 per mass. Way higher than any Hydrocarbon.
“Look at the energy density of H2 per mass. Way higher than any Hydrocarbon.”
Who cares? That’s not relevant. H2 gas is not a component in gasoline or diesel.
H2 may not be a component of gasoline or diesel but it is becoming a motor vehicle fuel, albeit in limited use at this point. Now it does have a lot of drawbacks compared to liquid fuel and the gaseous fuels currently in use but its high energy density is one of the reasons that it is appealing and gaining traction. The other is the lack of those pesky carbon atoms which means the only vehicle emission is water.
H2 is a pipe dream.
Natural gas(methane) is starting to take off, but it will go nowhere in the long run. The fuel tanks are too cumbersome. Every natgas fuel tank is manufactured with an expiration date stamped onto it. They have a short lifespan and they are expensive to replace. Hydrogen is the same problem but orders of magnitude greater and 16 times as complicated. I’ve often wondered why no one has ever tried/proposed butane. it does not require such an unwieldy container and it should be fairly simple and economical to manufacture from natgas or crude once economies of scale kick in. Butane is a very short hydrocarbon if you didn’t know, and as such has a high percent of hydrogen.
The thing is that 2 octane molecules (sort of like gasoline) will burn to 16 carbon dioxides and 18 waters (molecules). I think that the H2O bond releases more energy than the CO2 bond (but could not find a number from wiki, “burning carbon” comes up with a fusion process).
I found a webpage that (assuming I understand it) says that the CO2 will release 1598 kJ/mole and the H2O will release 920 kJ/mole. If I am not mistaken, a mole of something contains a certain number of atoms (or molecules), like 6,02x10to23 maybe.
So the carbon in the gasoline makes more energy than the hydrogen.
CNG has more than just taken off. It has been used for many years and you’ll find the vast majority of new garbage trucks and city buses use it. A huge portion of the UPS fleet uses it too. It is currently the cheapest fuel to use which is why it is being widely adopted by those that consume a lot of fuel. Are there drawbacks, certainly, but every fuel has its drawbacks.
“Are there drawbacks, certainly, but every fuel has its drawbacks”
and the main drawback right now is the container you put it in. It is expensive and doesn’t last. The only people who do it are those who get a tax credit or those who are spending other people’s money(government entities who spend our tax dollars) or those who are getting some kind of PR kick out of it. Nobody else with enough brains to do an honest cost-benefit analysis would ever convert their car to run on natgas/methane.
yet you think H2 is a good idea?? Ha!
UPS and Refuse haulers use CNG because until the recent crash of oil prices it was much cheaper than diesel or gas. Yes there are a few governments that run the refuse hauling in their jurisdiction, but the vast majority is now done by private companies. UPS of course is a private enterprise. City buses are another story because they are usually operated by some gov’t entity but again the previously much lower overall cost is why they chose it. Not only was there lower fuel cost per mile the clean burning nature of CNG can potentially mean fewer oil changes and longer engine life.
If only the cost of diesel were less than that of gasoline. I remember when both were less than $1.00.
I remember when premium (100 octane) was 40 cents per gallon.
Jason, life is full of “if only” propositions – if only property prices were what they once were, I would have an 1800 square foot house on 20 acres for around $50,000 and I could save $0.13 per month for not sending off a mortgage payment. If only. 🙂
Diesel is a great fuel in some applications, but the market right now is simply not working in its favor in a number of ways. I just paid $1.82 per gallon for gasoline; diesel across the street was around $2.92. At this point in time, any advantage to diesel will take eons to pay for itself. The market, like politics and phases of the moon, is cyclical; give it a few months or years and it will look completely different.
That’s too bad. I’d buy a diesel powered vehicle I were in the market for a new car, or even a used car.
John, fleet operators love CNG and LPG, and it exists largely for them. Engines on these fuels basically have ZERO wear. I have seen SBC’s go one million km on LPG, and no, no government was subsidising us.
I am in the fleet business, and I still stand by this maxim: for less than five tons, gas (or LPG), over five tons, diesel.
It is also, in my opinion, not worth buying a used diesel unless you have a complete history.
That sounds wonderful for CNG but it doesn’t change the fact that practically ZERO private owners are willing to do the conversion. I’ve already explained why.
“My dad says butane is a bastard gas.”
Bobby Hill, King of the Hill
Diesels have a higher compression ratio.
This is why they are more efficent and produce more Torque.
I guess you didn’t read the article? Here’s the short version: For a given amount of fuel, diesels do make more torque/power; but gas engines can burn more fuel, thus make more power than a comparable-sized diesel engine..
He read the title, which agreed with his comment.
Very interesting and well done !
And now a comparison between a gasoline and diesel engine with the same displacement and both with a turbo….just kidding, just kidding….
The fact is that putting diesel engines in cars (from A-segment to F-segment) is a European-affair. Thanks to turbo chargers, intercoolers, direct injection and certainly common rail injection the performance of diesels has become fully on a par with gasoline engines. Engines like Audi’s 3.0 liter TDI BiTurbo and BMW’s 3.0 liter TriTurbo-diesel are downright brutes. Power-wise, torque-wise and performance-wise. Go to YouTube and search for BMW M550d for example.
The Fiat Group (the company that introduced both direct injection and later on common rail injection on cars), PSA, Renault, VAG, Mercedes and BMW are the undisputed experts when it comes to diesels for cars.
In big trucks Volvo and Scania (both from Sweden) are north of 700 hp with their 16 liter turbo diesels. A straight six in the Volvo and a V8 in the Scania.
One of them diesel brutes. Performance is adequate, to say the least.
700hp of Volvo cannot stay with 600hp of Cummins Freightliner on the same hill both trucks empty and Ive driven both of them, The T610 KW I drove had the 615hp Cummins in it torque was limited to 1950 ftlbs because thats all an Ultrashift Eaton Fuller AMT can cope with reliably and with 8 drive tyres the truck lacks enough traction to use it all, road surface and available traction are limiting factors in how much useable power high hp trucks can make use of, our roads are being shredded by trucks wheel spinning while grossing 50+ tonnes and when it rains it just gets worse..
I’m seeing in the Hall – Scott dyno chart that they used 73 octane gasoline. Would using modern high octane gas would yield different results or the fact that modern gas is unleaded would make a difference?
Absolutely, with higher compression ratio. Which explains why the later version of that engine makes 1020 lb.ft. I meant to point that out, that the gasoline back then was significantly poorer quality. Whereas diesel is diesel, except of course for the modern low-sulfur diesel.
Regarding the fuel efficiency. I remember my dad and three other guys went to Lyon (France) more than 35 years ago to visit an exhibition. Two of those guys were brothers and my dad’s employers. One of them had a shiny and luxurious BMW E23 7-series and the other had a rather muddy corn-yellow and trailer-towing Mercedes W123 240D.
Shall we take that nice and fast Bimmer to drive to France ?? Hell no ! Way too expensive to drive ! We’ll take the Benz !
Were there transmissions that could actually handle the power of the V12 Hall Scott? Or did the power mad back in those days just accept that transmissions were going to be “wear items” if they felt the need for 2000+ cubic inches of power?
Transmission? With 2000 lb.ft of torque? Why bother? 🙂
ha
Take it one step further and remove the drive shaft and even the crank shaft. LIke an old fashioned steam locomotive or the Hildebrand & Wolfmuller motorcycle(which was internal combustion).
Transmission of course you need one,2000lbft +- is not uncommon in trucks now with modern turbodiesels weight it up to 60 tonnes and you still need low box to get it moving, I do this every day.
One thing that my Cadillac history book comments on is when Cadillac went to the V16, they did not have to beef up the transmissions because the peak torque was the same as the V8s. So only one cylinder fires at an instant and that torque was probably less than the V8s.
Interestingly, I had heard of this truck before and there is some info on it from a poster on the American Truck Historical Society forum. Sounds like it was de-stroked,had a lower compression ratio, and made about 450 hp and 1300 ft lbs of torque. Apparently it got about 4 mpg and had to be driven with “some discretion”. The article is from 1951 and states that it was a factory installation, considered experimental. It would be another 25 years before drivelines caught up with that kind of power.
Gasoline truck engines were a dying breed by the time I started wrenching on trucks for a living, but there were a few big inch Internationals and Fords around. Fuel consumption and not lack of power was what did them in.
Great article, I’m looking forward to the story on Hall-Scott!
Interesting. It seemed a bit excessive to have such a powerful engine in a truck.
Very interesting article and succinctly written. I like the look of the Hall-Scott engine ohc layout.
Another example of petrol/diesel versions of the same engine is the Nissan Patrol 4.2 litre 6-cylinder, which had 168hp @ 4200/244lb-ft @ 2800 as a petrol and 114hp @ 4000/198lb-ft @ 2000 as a diesel. Bore and stroke are the same for both.
An interesting element is the pumping loss of the throttle plate. I talked to a guy who put a supercharger on his Falcon V8 (390hp 5.4 dohc Boss motor) and claimed his highway fuel economy had improved because the very slight boost pressure, presumably 1-3 psi range because it only had 7 psi peak, was reducing the pumping loss of the nearly-shut throttle.
Johannes- what a can of worms that could be! Not many engines would have the same boost level to make a valid comparison. With direct injection petrol engines I think the playing field has changed again and while diesels may make more torque with higher boost levels they still won’t get there on power.
letg- they could no doubt make more power but would likely need hardened valve seats assuming the head was cast iron.
Modern diesel car engines are geared to run at peak torque even my old Citroen cant use peak hp in virtually any gear above 2nd legally as our speed limit is too low and it barely has the traction to use whats available anyhow at peak torque. Peak hp is at an instant license losing 150kmh in 4th.
Paul,
A well done piece! Your explanation as to why the naturally aspirated diesel has combustion limitations was the best I have read to date.
C.D.G.
I read the comments in the other article with a great deal of interest. As you approached the subject, you and your detractors seemed sometimes like the blind men describing the elephant and it is a very big elephant. I think this does much to settle the argument and I look forward to reading all the comments.
Very good job Paul. I have approached this subject in my reading in the past and understand it better now by far.
Excellent article. I have a question, maybe slightly off-topic. To what do you attribute the greater longevity of Diesel engines? Does it have to do with the fact that diesel fuel is a light oil, whereas gasoline basically acts as a solvent and washes lubricant from cylinder walls? Or is it just the lower RPM that diesels run at, or maybe the heavier components needed to deal with the higher compression ratios?
I don’t have a lot of diesel experience, although I happen to be driving one this morning.
That’s a very complex question. It used to be a given (that diesels last longer), but I’m not so sure anymore. Back then, gas engines often ran with excess richness on cold starts, and that would wash fuel into the oil. And diesels just needed to be stronger to handle to extra forces from the high compression.
But modern gas engines have overcome some/all of those deficiencies. Manufacturing processes and materials have improved. Lubricants and oiling systems have improved. Engines are incredibly clean inside. It’s not uncommon to hear of gas engines running 500-600k miles without ever being opened up.
I really can’t support the statement that modern automotive diesel engines intrinsically are longer-lasting than comparable gas engines. There are so many factor that go into that, and lets face it, few people run engines that long.
In Europe, diesel taxis are the norm, and many probably last 500k miles. But in the US, gas taxis are known to go thta long too; the Toyota Previas taxis in Eugene all have 500-600k miles, and these are mostly on original engines.
It’s an argument that was based on facts in the past, but things change. I can’t find any support for it anymore.
Of course, industrial/truck engines last longer, but there’s no real gas engine equivalents. Although in the past, the Hall-Scott engine would run over a million miles without a major overhaul. It really depends more on how the engines are built than any intrinsic difference based on what fuel they burn.
I would have to say that diesels supposedly last longer because most of them, until rather recently, have been used in commercial fleets, where they got regular maintenance. And the advancements in oil technology certainly has not hurt either. Fact is, some people can break a crowbar in a sandbox; it’s all in how you treat/take care of your equipment.
I only get things second hand now, but according to my BMW mechanic nephew, diesels require a lot of maintenance when they get older, and any repair costs a fortune. There’s a lot of high tech in diesels, now, from common rail injection systems to emission controls.
As a younger man, I was a huge diesel fan. However, the complexity, higher up-front cost, more expensive fuel and maintenance issues mean I am no longer interesting in buying one.
I was in a Scania 420 the other night its previous life was as a milk tanker and racked up 1.2 million kms, its now a bulk tipper enjoying a second lease of life it ran great for such kms and will probably do many harvest seasons before being scrapped.
Fuel injection is a significant reason that modern gas engines last longer than they did in the past. With a carb on warm up liquid fuel does make it to the cylinder and to the oil where it washes the oil off the cylinder wall and decreases the lubricity of the motor oil.
The strength of the components used in diesel engines does help their longevity but gas engines designed for Medium, Heavy duty and stationary applications have traditionally outlasted their lighter duty counterparts used in cars and light trucks. My favorite is the International SV family of engines that would regularly make it to 500K or more back in the day despite using a carb, less precise tolerances and the lower quality oils available.
As Paul mentioned there are a number of light duty gas engines nowadays that can last and last. See the “million mile van” who’s original 4.6 didn’t give up the ghost until just under 1,300,000 miles. It was not babied either with oil change intervals were typically between 10K and 20K with one case where it went 55K w/o an oil change.
http://www.millionmilevan.com/
That of course is an extreme example but in livery service they are known to go 500K or more.
Diesels also have become more “computorized” and technically advanced in the past 15 years. You just have to pay more attention to the way you use and treat it, to proper maintenance and to good quality diesel fuel. For example, if it was very cold in the old days you added gasoline to the diesel, up to 20%. Do that now you can say goodbye to the (expensive) injectors immediately.
A few years ago I read a pdf-file from some official Belgian institute about diesel engines in cars.
It was recommended that you’d better not buy a diesel when you drove less than 10,000 km a year or when you only drove short distances in and around town. Otherwise premature failures may occur to the battery, the glow plugs, the DPF, the injectors, the EGR valve and the turbo charger. Modern diesel engines have to reach full operating temperature as often as possible. And just let it rev and spin freely once it’s warmed up. Just step on it, high speeds for a prolonged time are no problem at all.
Simply said: they’re made for the long haul, not for getting your groceries a few blocks from your house.
Anybody here who drives more than 30,000 to 35,000 km a year has a diesel. That’s why so many executive D-, E- and F-segment cars have 6 cylinder diesels with a displacement around 3.0 liter. If you can’t offer one you’re out.
Exactly for that reason high-mileage cars (say 400,000 km and up) here almost always have a diesel under the hood.
I’ve got a turbo diesel with an intercooler and common rail injection myself. Just routine maintenance so far, with 265,000 km on the odometer. But I use it as a diesel should be used.
diesel pickups have become a huge fad here in the US in the last 10 years. In my opinion, mostly stupid people are buying them now. The people who can actually make use of the additional expense of a diesel engine and make it pay for itself are a small minority and they already had diesels 20 years ago. The increase market share of diesels after that point is, in my opinion, going to people who are not smart enough to do an honest cost-benefit analysis and purchase a vehicle accordingly. They are buying based on emotion only.
The problem with “fads” is that they come and then they go. Car companies try diesel engine to see if customers will buy them, but even when enough customers buy them to justify continuing production, companies discontinue production. Why the hell is that? I find it difficult to believe that cost is a reason. When customers buy your product, you make money. if enough customers buy your product, you make enough money to continue. So you should continue production. As long as there’s a demand, you should continue to produce.
The basic problem was that emission standards were imposed that technology to meet them did not exist. So light duty diesels went away.
You’ve posted similar sentiments before, Jason. They are wishful thinking, and those of us with tastes too far out of the mainstream are invariably in for a hard time.
Companies do not have infinite resources. Presuming they behave logically, a company will try to use its resources most efficiently. If there is a product being manufactured at one of their plants, but they can convert it to produce another product that generates higher profit, then they will do that. This applies whether a company makes car parts, microchips or shoes.
Expanding on what SOITWW mentioned, there are also added considerations such as changing safety standards, fuel efficiency and emissions requirements. If the light duty diesels, for example, are successful enough that they’re turning a modest profit, but new emissions legislation mandate an expensive R&D effort to clean them up for future production, management may decide that there’s not enough return on the investment to continue offering them.
Similarly with the Jeep GW from the other day. Chrysler was building an outdated vehicle, likely with high assembly cost. It still used their former competitors engine, which they probably weren’t too happy about, and still carbureted in the face of ever stricter emissions standards. It would need massive retooling to make the GW class-competitive again. Not worth the effort… repurpose the plant capacity to build something else.
After WW2 there was a massive and consequent switch to diesels in Europe in anything that was “commercial”.
Trucks (all sizes), vans (all sizes), buses, fork lifts, generators, farm and construction equipment and boats. Air cooled and water cooled. When it comes to cars Peugeot and Mercedes were the post-war diesel pioneers. By now they are well-mannered and powerful enough to be in Jaguars, Porsches and Maseratis.
Light trucks (like flatbed trucks with dual rear tires) typically have a four cylinder turbo diesel, displacement around 3.0 liter. They’re not even available with gasoline engines.
In America it is different. Here there is a substantial price differential when upgrading a specific make/model to the diesel engine. The diesel engine is an option you must pay extra for. Also, in America, diesel fuel costs more per volume than gasoline. It is about 50% more money for diesel fuel where I am. Also, diesel mechanics get paid more than regular mechanics and diesel engine work is much more expensive. It is very difficult to justify purchasing a diesel pickup truck instead of the gasoline version of the same make/model.
@John: European turbodiesels typically cost more than petrol engines, although because different engines are increasingly associated with distinct trim/equipment packages, making a direct comparison is sometimes difficult. However, in some markets, diesel fuel is significantly cheaper than gasoline and there are sometimes other tax breaks based on lower CO2 emissions. (This is also market-dependent: The U.K.’s CO2-based company car tax penalizes diesels compared to gasoline engines in the same emissions tier.)
European emissions standards have also had separate scales for gasoline and diesel engines. (I haven’t looked at the details of the Euro VI standards, but I assume that’s still true.) So, while European emissions standards for spark-fired engines are about as stringent as the U.S.’s or Japan’s, diesels have gotten certain allowances, which isn’t true in the States.
The Oldsmobile diesel cost $1000 extra. One would have expected it to last 100,000 miles (before we knew about problems). It was rated around 24 and the gas engine was about 17 combined. So the gas engine would take about 5900 gallons of fuel to the diesels 4200 gallons. About 1700 or so gallons difference @$1/gallon. Well diesel was cheaper than gasoline then.
“It was rated around 24 and the gas engine was about 17 combined. So the gas engine would take about 5900 gallons of fuel to the diesels 4200 gallons.”
Hmmm, 5900? Duration of ownership?
Expected lifetime of the engine (which one would have expected to be at least 100,000 miles until the problems started to show up). Probably most people might have expected it to last like a Mercedes diesel, 500,000 miles.
Among the pick up crowd a three quarter ton quad cab long bed 4wheel drive turbo diesel is viewed like a Porsche Carrera. They are status symbols. A lot of the young guys I work with have them. Like you need over ten thousand pounds of towing ability to drag your ass to work. I saw a beautiful 2015 Ford F150 quad cab 4 wheel drive, eco boost fully optioned out xlt truck with the list price of over 57 thousand dollars! Whatever-a Nissan GTR costs lots more.
“…over ten thousand pounds of towing ability…”
Even better, a 6~7 liter turbo diesel should be enough for over thirty thousand pounds of towing ability. That kind of displacement is Herculean for a pickup-truck.
Johannes my C5 has almost 350,000kms on it now it runs as well as when I bought it, consumes around half a litre of oil between 20,000km oil and filter changes which is the recommended interval it warms up very quickly to normal operating temp and sees the motorway on all 3 of my commutes 6 days a week it may run forever or untill it eats something more expensive than its value.
I am going slightly off topic here. Hopefully it provides food for thought.
Diesel fuel does not vaporize well and therefore the method of injection is rather important in getting the most out of the fuel. The commonly used method of injection is comparable to dropping a pellet of fuel into the combustion chamber which will result in a sudden combustion with a peak of the high temperature. Thus a lot of energy is going into the exhaust.
If it were possible to feed the fuel over longer time (picture feeding a spaghetti) into the combustion chamber it would result in a flat(ter) temperature curve of the combustion and much of the loss of energy could be avoided.
A second problem is the use of piezo elements to produce the high pressures needed for direct injection. These elements are self destructing. However, there is another method of accomplishing the high pressures for direct injection without the inherent disadvantages of the piezo elements.
An injector that promises to solve both problems exists: http://greatplainsdieseltechnologies.com/Content/Explore-The-Technology.aspx
Feeding the fuel over a longer time (what Rudolph envisioned) means that the last fuel in burns when the piston is near the end of the expansion cycle so the heat is wasted in the exhaust. Burning more of the fuel at the top of the cylinder maximises the efficiency. The fuel injector should spray the fuel in small droplets not a blob.
Nice, nice job Paul! And maybe I should mention that because diesels “process” fuel at a lower rate, and do run cooler, there have been known instances of marine engines freeze cracking in sub-zero temps…while they are running! I will take a gas engine over a N/A diesel any day. I will also mention that the first gen T1N Sprinter van I drove for work for a brief while, with it’s 2.7 5cyl 4 valve turbodiesel, had a redline of 5500 rpm according to the service manual. The trucks have no tach. And as a sidenote, if you use the incorrect taillight bulb in these, it will make the transmission shift funny. I kid you not. German engineering at its finest…
A good explanation. Rudolph Diesel’s idea of how the diesel engine cycle should work was that fuel would be injected while the piston is moving downward and that the fuel burning would maintain a constant pressure in the cylinder. Gasoline engines supposedly have an explosion with the piston at the top of the cylinder creating a burst of pressure which then pushes the piston down. This creates a peak torque in the gas engine, whereas the diesel would have steady torque.
But production diesel engines inject the fuel more quickly than Rudolph envisioned, so there really is a burst of pressure as the diesel fuel burns, but as Paul Niedermeyer points out, diesel fuel burns slower than gasoline, limiting the peak pressure.
Light duty gasoline engines (such as found in cars) produce more than 1 lb-ft of torque per cubic inch of displacement (depending on tuning and depending on net or gross). Light duty diesels produce less. Heavy duty industrial gasoline (or natural gas) engines are a different matter. Ford has a V10 of 415 CID that will run continuously at 3000 RPMs making 160 hp or 283 lb-ft @2000 RPMs. There is 280 lb-ft at 3000 RPMs @160 hp. This is not a truck engine though, and is designed to run continuously at the rated power output. Comparable diesel engines now are all turbocharged so comparison is not really possible. However, there is a Ford truck engine, V10, that runs on gasoline and produces 457 lb-ft. This engine is the same as the industrial engine above.
Industrial engines that run at continuous speed for long periods of time are typically de-rated, since they don’t need the burst of maximum torque for acceleration, so putting them into the comparison doesn’t really work either.
I have found that Perkins makes industrial diesels that are not turbocharged. This link
http://www.perkins.com/cda/files/334144/7/404D-22%20Industrial%20PN1819.pdf
for a 4 cylinder has less than 1 lb-ft per cubic inch. The point here is that an industrial diesel does not seem to produce much more torque than the light duty car diesels without turbo charging.
It does make sense to me that all things being equal (not sure what that would mean exactly), a gasoline engine should produce more torque than the diesel simply because the fuel burning is slower in the diesel producing a lower peak pressure in the cylinder, as Paul Niedermeyer points out. The amount of diesel fuel that can burn is limited by the amount of air in the cylinder, as pointed out by Paul Niedermeyer.
Ford has a small 4 cylinder industrial gas engine comparable to the perkins above (link)
http://www.mgbryan.com/shop_image/product/Ford-DSG-423.pdf
This engine has 130 ft-lb continuous (156 peak) from 140 CID. This is 0.93 lb-ft per cubic inch vs the diesels 0.78 lb-ft per cubic inch. These are both modern engines with most of todays technology.
And generally, stationary engines are run on either CNG or LPG.
And I have been aware of Hall-Scott engines pretty much most of my 48yrs on this planet; funny thing is, for the last 12 or so, I have been good friends with a man named Scott Hall. I think of these engines almost every time I visit him.
It’s unforgivable that while diesel engines are used in heavy duty trucks and medium duty trucks got diesel engines, light trucks like the Ford F150, the Chevy/GMC C/K1500 and the Dodge Ram D150 (until recently), were never available with diesel engines. It’s even more so that you can’t get a van, an SUV, or a car with a diesel engine, even as an option. Is diesel for everyone? No, not really. The clattering sound of a diesel engine may be too much for some people to tolerate. The smell that comes out of the tailpipe may be too much. And the cost of filling the tank of a diesel vehicle may turn some people away. Some people, like those who try to control emissions, seem to think they can control the emissions that come out of a diesel powered car or truck. I believe that’s unrealistic. I don’t believe it’s possible to completely eliminate the pollution that comes out of the tailpipe of a car or truck.
The reason that light duty “trucks” don’t get diesel power is simply cost: the diesel is easily going to be 15% of the price of the vehicle, and this is a price sensitive segment. With gasoline hovering at historic low prices, I don’t see much demand for a small diesel truck in the USA. We can be very sure that the car companies have examined it, too.
Modern diesels do not clatter, and are no more noisy than a direct injection gasoline engine. There is zero soot coming out of tailpipes, and no diesel smell. Again, the real issue to adoption in North America is cost. At $2.00 a gallon, it isn’t going to happen any time soon, and in my experience the guys screaming for a car maker to make something very often don’t go to the dealership with a cheque book.
I was in Beijing last march, to change planes coming back from Manila. The pollution was so bad there was smog INSIDE the terminal building. Visibility was no more than 100 metres. So while we will never completely make diesel and gasoline clean, we have come a long, long way. A modern gasoline only emits a tiny fraction of the pollution of an engine with no emission controls. Do we really want our children breathing this crap in day in and day out? The last time I was in Nanjing, I was hacking and choking withing a couple of days. There was coal dust on everything and my face was black after going out for an hour. Cities smell like it did here forty years ago: a mix of soot, smog and unburned fuel.
I do not want to return to that. The basic problem is gasoline is a horrible fuel. Give my CNG or LPG any day.
I want to breathe clean air as much as anyone, but I believe these emissions standards are unrealistic at best.
Why are they unrealistic? Both gasoline and diesel engines have never been more efficient or clean, nor have cars ever been cheaper. The real issue is we are using inherently dirty fuels. CNG or LPG really are they way to go, and I am speaking from experience. However, we are doing pretty well with gasoline now, less so with diesel.
How can we breathe clean air and not have emission standards? They are mutually exclusive. Want to breathe clean air? Well, then we have to regulate the pollution that goes into said air.
I’m sorry, but what is CNG?
Compressed Natural Gas.
if they were unrealistic, they’d be unachievable. but they’ve been met. so they’re realistic.
Ram seems to be selling, or at least were selling a lot of the EcoDiesel equipped 1500’s. We will see how the current price of gas vs diesel impacts that in future months. Of course it will be hard to say how much of the drop will be because of fuel prices and how much will be because the people who wanted one already bought it. The reality is currently it would be cheaper to fuel a Hemi which costs less to purchase and has greater capability.
There were diesel engines available in “1/2 ton” pickups in the 60’s and 70’s. IH and Dodge did it in the 60’s while GM did it in the 70’s.
You can get a diesel engine in a van, in fact that is the only way you can get a Sprinter in the US. The Ford Transit and Ram Promaster are both available with diesel engines and of course you could get a diesel engine in the Econoline and GM vans in the past.
No you will never completely eliminate emissions from an ICE powered vehicle but we have come a long long way. For the longest time diesel engines got a pass on emissions are were allowed to create much more than their gas powered counterparts. That has come to an end which I am glad to see. Why should one class of engines be allowed to create more emissions than another class of engine in the same application?
Well said, Eric. There is a lot of CNG conversion of diesels going on here now. All the garbage trucks are running it, and the buses are being switched over, too. When I was living in Seoul in the lat 1990’s, all the diesel buses were converted to CNG, the pollution problem was much abated.
The offerings of diesel in the 60’s and 70’s was very rare, I have only ever seen one, a Dodge D100.
Are the diesel engines being converted or is a CNG engine replacing the diesel?
In the new vehicles they are being built as CNG but there are conversions for diesel engines. The common method of conversion is to keep the diesel as the ignition source and use the CNG for the bulk of the power production. Those converted engines idle on diesel only and continue to inject diesel at that rate under load. As more power is requested they start introducing the CNG and the diesel acts as the “spark” to start combustion.
The concept isn’t all that new though as there have been people playing with propane injection to increase the power output of diesels for many years. They inject the propane into the intake tract and in then consumes the rest of the available oxygen in the cylinder.
The Westport/Cummins 8.3L CNG engines use spark-ignition.
No doubt, the 1/2 ton diesels were vary rare in the 60’s though less so in the 70’s after GM started installing the Olds 350 in pickups.
Is bio-diesel a factor of any importance in North America ? Like refrigerator trucks that run on the animal fats from the abattoir. Or the McDonald’s delivery trucks that run on recycled cooking oil.
(Photo courtesy cleantechies.com)
Bio diesel is huge in the US, though not pure bio-diesel. At many stations the only thing you can buy is B5 or diesel with 5% bio-diesel content. The new Ford built 6.7l diesel engines in their pickups are B20 ready.
Oops meant to post a picture.
During my brief stint (20 months) as a fleet manager, we had 200 or so dump trucks that ran biodiesel during the warm months, approximately April through the end of October. They ran regular diesel the rest of the time.
The biodiesel was generally more expensive at that time (2010 – 2011) and it was mixed to a B20. Every spring, when converting back to biodiesel, fuel filters would plug up continually and it would happen again during the transition to straight diesel. If a piece of equipment, say a loader or motor grader, had biodiesel in the tank and it wasn’t used much, the fuel would gel horribly. There were also occasional problems with gelling in the bulk storage tanks in both summer and winter.
It should also be noted these trucks could take B5 but were fed B20.
From my experience, biodiesel is great in theory but less so in actuality. That said, this is just my experience and not indicative of everyone.
GM offered diesels in their light duty pickups starting in 1978. Dodge offered a Mitsubishi-supplied diesel in its half-tons from 1978-1979.
So I’ve read somewhere. The problem with the Mitsubishi diesel was that it was too small, and delivered way too little horsepower and torque for the vehicle to do any kind of hard work. I don’t know that much about General Motors’ diesel engine.
GM put the Olds V8 in their pickup trucks.
briefly. later the (much better) 6.2 could be had in the 1500-series trucks.
I should have stated that GM joined the party in the 70’s because both Dodge and IH revisted the pickup diesel in the 70’s after dropping them due to poor sales. Of course IH offered it in a “1 ton” pickup but it was not a full size rather it was a midsize, the Terra. They were also the first to offer a turbo diesel in a pickup.
This has been a facinating discussion and Paul you did a good job presenting the subject clearly. While great progress has been made in automotive performance and emission reductions the increased weight of the current fleet has probably reduced some of the benefits. Let’s face it, most cars today are really fast compared to cars of the 60s. V6 Camcordias are quicker than the average muscle car was while providing significant higher mileage. Threre’s a lot of “wasted performance”. So much so that the engines have to electronically speed limited to around 110 mph. Wouldn’t it be better to design the car with maybe 75-80 max. cruising with a top speed of maybe 100mph? The displacement could be reduced and the power curve optimised for that operating envelope. Wouldn’t that return even better mileage? This could be seen even better with motorcycles which have extremely (even excessively )high levels of performance. Now I really sound like an old man,but Honda has brought out a new line of twins that feature increased fuel economy and tractability. I rode motorcycles for over thirty five years so it’s not like I wasn’t an enthusiast. But I wonder, isn’t a 100 mpg. bike possible?
Horsepower requirements are dictated by acceleration, payload, and hill climbing abilities, not by top speed. This became the reality when they started deigning bodies for minimal air friction.
A modern engine really doesn’t give up much efficiency by running at low load levels. Which explains why cars are so fast, yet efficiency continues to increase. The modern downsized turbocharged engines even more so: they run very efficiently at low load, but can generate lots of power on demand. In this day and age, there’s little need to sacrifice a decent level of performance for good efficiency.
Recently, I watched on YouTube a series of videos about climbing a steep highway in Colorado. The did it in big trucks and were impressed in their power to reach the 14,000 foot summit.
Then, sneeringly, they got a Fiesta 1.6. Haha, they thought, it won’t be able to make 65 mph all the way up!
It did, with no problem.
So, they loaded the little car with four big people and set out again. Haha, they thought, it won’t be able to make it.
It still did, no problem.
The upshot is, we drive cars that are much more powerful than we need. I doubt I use 20% of the power my car makes on a daily basis.
I remember a hill climbing race a few years ago. 2nd and 3rd place were taken by a hummer and I believe and all wheel drive dodge charger or something similar. Both were extensively modified and owned by filthy rich guys. were’ talking hundreds of thousands of dollars invested and somewhere around a thousand horsepower each.
Guess what won first place?
A 2WD model T ford with 1930s vintage aftermarket upgrades that included OHV conversion and a different transmission. It might’ve had an antique supercharger on it too, I can’t remember.
Weight makes a huge difference when going up hill.
This remindes me of an old, little known racing factoid. In hill climb races of short distance, it took a very long time for cars to reach the point where they could beat a mule up a steep hill. I wish I could quote the year in which a car was finally able to beat a mule but my memory is not that good.
“Diesel engines thrive with boost (forced induction); in fact they operate more efficiently with boost, as the heat from the exhaust can be partially converted to more energy. Diesel engines can be boosted to very high pressures, because there’s never a risk of pre-detonation, as in a gas engine. The more air can be forced in, the more fuel can be burned, offsetting the intrinsically lower power output of a diesel.”
this is key. Nobody blinks when the peak boost of a 6.7 Powerstroke is ~21-22 psi (at a really high airflow) but when people found out Ford was pushing the 2.7 Ecoboost to pressures that high some people had a heart attack.
for no good reason, I might add. gas engines (especially DI) can tolerate that just fine. It’s all about avoiding preignition/detonation. Those are what kill forced-induction gas engines, not a lack of mechanical strength.
You need both the proper strategy to avoid detonation and the mechanical strength to have a highly boosted gas engine have a long life. Strap a high pressure turbo on a gas engine not designed to handle that level of boost and it can go boom even if the proper strategy is employed to prevent detonation. However designing a gas engine with the needed mechanical strength is a simple engineering exercise.
“designing a gas engine with the needed mechanical strength is a simple engineering exercise.”
Yep. Basically they design it for diesel-like strength if they want it to stand up long term.
Great article. Anyone who lived in Los Angeles (city or county) before 1980 would remember well the magnificent Hall-Scott gasoline engines, which powered much of the fire apparatus used in Southern California during that time. They had a very distinctive sound and were prone to loud backfires when downshifted, sometimes blowing blue tinted fireballs out of the exhaust pipe. Nothing quite like a classic Crown Firecoach (built in Los Angeles) or Seagrave powered by a 1091 Hall-Scott.
I am not going to get into a diesel vs. gasoline debate, each has their place and that’s all you can say about the matter. However, a very interesting comparison between gasoline and diesel power, most all other things being equal, can be found comparing GMC’s old 478 V-6 gasoline engine with the Toro-Flow DH-478 diesel. GMC’s truck V-6’s were designed to be offered in gasoline and diesel versions, and shared a common bore and stroke. The DH478 was naturally aspirated and direct-injection. These figures are from 1967:
Gasoline 478 V-6: 192 h.p. @ 3200 r.p.m., 371 ft. lbs. @ 1400 r.p.m. (net).
Toro-Flow DH478: 155 h.p. @ 3200 r.p.m., 298 ft. lbs. @ 2000 r.p.m. (net).
Since the Toro-Flow adopted the gasoline engine’s oversquare design (5.12 x 3.86) it was quite high-revving for a diesel. Unfortunately, that was one of the reasons for the early Toro-Flow’s less-than-perfect reputation for durability. In any event, it is reasonably clear the gasoline engine will out-power a similar sized naturally aspirated diesel. Of course, the diesel will deliver much greater fuel economy and will better benefit from turbocharging.
“I am not going to get into a diesel vs. gasoline debate”
Oh really? You sure do talk a lot for someone who isn’t getting into it.
No, I just wanted to point out an example of a gasoline and a diesel engine that come about as close as possible in specifications.
““Pedant.” Merriam-Webster.com. Merriam-Webster, n.d. Web. 25 Jan. 2015.”
Hmm, lets see…what kind of name-calling could I stoop to, to describe someone who likes to quote dictionary definitions?? Nah, I don’t want to stoop, my back is too old and stiff.
Since you have your dictionary handy, you can look this up for me, would you please:
ad hominem
That’s directed at me. I deleted my comment, because I realized I’d stepped over the line. I apologize.
In the future, I’ll attempt to use an adjective instead of a noun so John doesn’t think I’m attacking him personally. I’ll simply attack his tone: pedantic. I won’t apologize for that.
“In the future, I’ll attempt to use an adjective instead of a noun so John doesn’t think I’m attacking him personally. I’ll simply attack his tone: pedantic. I won’t apologize for that.”
In the future, I will read your adjectives as nouns since I now know what you really mean and now realize that you choose the adjective form merely to dance on the line. You’re apology is clearly disingenuous.
John, wow, you’re a regular Dale Carnegie! Um, do you think you could please leave some friends and influence for the rest of us?
Daniel, you just made me laugh out loud, not for the first time might I add. Thanks for brightening my day. (And you missed an empty-net goal by not commenting on John calling Aaron an “apology …”.)
Eh, I wasn’t in the mood for ballistic barrelfishing that day.
Diesel makes only 70% of the gasoline engines horsepower with the same displacement. That’s a known fact. Diesel engines have at most 95-99% higher values of NOx in the exhaust than a comparable gasengine, and who want’s a diesel in a car? The noise of the engine alone is enough to stay away. The only reason that diesel is/was so popular in Europe is because of the fuel economy, but the SMOG-problems created by dieselengines in cars is huge in the bigger citys like Paris.
Diesel is for big rigs and tractors.
I have given some thought to the comments following the first post of this article, mainly those by John. I am not going to claim the following discussion “proves” anything. This link to a calculator may be useful. (click on the word link)
Let us consider an engine with a cylinder displacement of 40 cubic inches (to make computations simple). For a gasoline engine with a compression ratio of 10:1, the cumbustion chamber is 44 cubic inches (engine displacement is the volume swept out by the piston). A diesel engine with a compression ratio of 20:1 has a combustion chamber of 42 cubic inches.
The calculator above allows us to compute the ideal efficiency of each engine. The gas engine is just over 60% and the diesel is 67%. The parameters for the diesel assume some time is required for the fuel to burn. The amount of gasoline ingested (or fuel injected direct or not) can be represented by dividing 44 by 14.7 = about 3 units of gasoline. We have 44 cubic inches of air. In the diesel there is 42 cubic inches of air and lets assume we can burn diesel at 21:1, which is about 50% more air than stoichiometric. This means we have 2 units of diesel fuel which is injected around the point where the piston is at top dead center. Details here are not important as this is an ideal engine remember.
Lets assume that the diesel fuel has 20% more energy than the gasoline. So there are 3 units of energy in the gasoline engine and we can get 60% of that to the flywheel, so the gas engine is rated at 1.8 units of energy (horsepower). The diesel has 2.4 units of energy and we get 67% out, so the diesel is rated at 1.6 units of energy. This does not say anything about torque just yet.
Horsepower = engine speed (RPMs) times TORQUE (lb-ft) divided by 5252
or HP = RPM x TORQUE / 5252
For the gas engine – 1.8 hp = RPM x TORQUE (gas) / 5252
for the diesel — 1.6 hp = RPM x TORQUE (diesel) / 5252
Divide the gas by the diesel
1.8/1.6 = TORQUE(gas) / TORQUE(diesel) = 1.125
Basically this says that the gas engines torque is 12.5% more than the diesel. Most diesels run on a 25-30:1 ratio making the difference in an ideal engine even greater. Real engines are not ideal. However, I think Paul Niedermeyer title is true for most comparable engines.
Interesting. I’m really not qualified to comment on your calculations scientifically, although there is a perceived logic to them. FWIW, diesel has only 12-15% more BTUs than gasoline.
I was trying to give the diesel as much help as possible, knowing that it was going to lose. There are some missing details, My calculation is only for a single power stroke, horsepower is an energy rate. The assumption though is that the combustion chambers fill to atmospheric pressure on each intake stroke at all engine speeds. Real engines don’t work that well. For a diesel running at a 30:1 ratio of air to fuel, the gas engine is at least 50% more torque.
You were certainly on the right track recognizing that the naturally aspirated diesel is limited by the amount of fuel that it can burn effectively.
Wild West – you clearly know your stuff. Thanks for sharing.
Can I talk you into taking a stab at why adding a turbo changes the equation so significantly?
Because it allows the diesel to burn much more fuel! As long as there is excess air, an ever increasing amount of diesel can be burned. And unlike in a gasoline turbo engine engine, pre-detonation from too much compression is a non issue, as diesels simple don’t pre-detonate (generally speaking) due to the slow burning nature of the fuel. One can just keep increasing boost and fuel, to crazy levels.
Understood. But there must be limiting factors beyond the physical strength of the components. Otherwise we would see more turbo diesels with tiny displacements and huge torque ratings.
Reduce mechanical compression ratio and you can have as much boost as you want in a gasoline engine. Upgrade to direct injection(like a diesel) in your gasoline engine and you don’t even have to reduce the compression ratio.
This is good, but…
There are some problems here.
The first and obvious one is your 44 units of air for gasoline vs the 42 units of air for diesel. You are not comparing equal displacement engines. You are comparing a 42 cubic inch engine to a 44 cubic inch engine. When the piston moves up and down and draws in the air, it draws in 40 cubic inches of air….both engines do so. That is why they are termed “40 cubic inch displacement”. Think of the volume above the piston at TDC as dead space. It does not have anything to do with the volume of air drawn into the cylinder. For the purpose of your calcs, that air can be considered “spent” air after the first combustion event…meaning it is essentially exhaust gasses and does not contribute to combustion.
The second problem, I believe, I have not yet verified this, is that you estimate the gasoline efficiency too high. My memory says it is about 40%. I will come back to this later when I can research it. I think your diesel efficiency is also high. I recommend using 55% for diesel until we can verify more accurate numbers.
The next problem is the way you estimate the amount of fuel. The air fuel ratios you cite are POUNDS OF AIR to POUNDS OF FUEL. Yet in your calculations you are calculating cubic inches of air to pounds of fuel.
This may be a moot point though, as long as you remain proportional. I will come back to this later after I think about it more.
The 17:1 compression ratio you use for diesel I believe is the typical compression ratio for turbocharged engines. I think a non-turbocharged engine would have a higher compression ratio. Maybe 18-24 to 1, I think. This will also take more checking as I do not trust my memory.
Also, the fuel air mixture in a diesel engine is variable, unlike a gasoline engine. A diesel engine is richest at full power and leanest at no load while idling. I do not know the full power air fuel mixture in a diesel engine. Your method of estimating this may or may not be realistic. I have no way to know without researching since you did not show how you arrived at this number or from where you get 50% above stoichiometric for full power air fuel ratio. If you simply multiplied the gasoline air:fuel ratio by 1.5 I can just about guarantee you this is incorrect.
You need to google engine displacement. What I did is exactly right. You also obviously misunderstand what is meant by an Ideal engine. Obviously you know nothing about thermodynamics.
I should make some clarifications here perhaps. First of all displacement is the volume that the piston sweeps out. Whether or not one should consider the top of the combustion chamber in computing the ideal gas or diesel cycle is debatable. It does not make much difference. What I would like to do next is to compute the ideal cycles for a 3 liter engine for both the gasoline and diesel cycles. There are Mercedes engines of this size in both gasoline and non-turbo diesels (M103, M104 and OM603.912).
Second the amound of fuel depends on the mass of air, which is proportional to the volume. The stoichiometric ratio is based on the mass, so it determines the mass of fuel in the gasoline and diesel engines, not the volume of fuel. What this means for a 3 liter engine depends on the following information:
at standard pressure (atomospheric sea level – 760 mm Hg) and temperature (freezing point of water) a mole of gas takes up 22.4 liters of space.
A mole of air weighs 28.96 grams
gasoline contains 42.4 megaJoules/kg of energy (see wiki)
Diesel contains 43.1 mega-Joules/kg of energy (see wiki) <-link
So ideally a 3 liter engine would have 3/22.4 x 28.96 = 3.88 grams of air. With a 14.7:1 ratio, the mass of gasoline would be .264 grams. The diesel would probably get about half that.
The energy for one cycle in the gas engine is 11,193.6 Joules.
For the diesel (29:1 ratio) is 5689.2 Joules.
To be continued…
The next step is to run the “engine” at some speed. To make calculations simple, lets start off assuming the engine runs at 600 RPMs, which is equivalent to 10 revolutions per second. We have 4 cycle engines, so that is 5 power strokes per second and 5 exhaust strokes, etc.
So the energy per second in gasoline engine is 55,968 Joules. To clarify what did with the diesel, I assumed half the weight of diesel fuel compared to gasoline. So, for the diesel the energy is 28,446 Joules per second. Now a Joule per second is 1 watt. One thousand watts is a kilo-watt, a common unit used in your electric bill. One horsepower is about 745.7 watts or 0.7457 kilowatts.
My next step when I continue will be to look at the Mercedes engines and compute the ideal power output and compare with the real output.
I tried to add some links but they are being see as spam.
If you want to check my work I can’t give you a link – too bad.
Heat lost to the coolant appears to be about 30% of the energy of the fuel, possibly more for a diesel. What this means is that the power for a gas engine is about 40%.
The Mercedes engine (M103) is rated at 138 kW at 5700 RPMs with torque of 260 Nm at 4400 RPMs or 188 HP and 191 lb-ft. Compression is 9.2:1
The Mercedes diesel (OM603.912) is rated at 84 kW, 113 HP @ 4600 RPM with torque 191 Nm @2800. Compression is 22:1
So for the gas engine (M103) the efficiency is about 59%. So at 5700 RPMs the energy in the fuel is 5700/600 x 56 kW = 532 kW. We need 70% of that and the Otto cycle is about 59% = 220 kW. This would be gross power, not net. At the peak torque rating we get 4400/600 x56 = 170 kW. This is 369 Nm (gross). This is not far off. Remember that the 500 CID Cadillac engine had 550 lb-ft gross, 400 net.
Calculations for the diesel give us 99 kW assuming a 22:1 compression and 2 for the fuel cut off. Efficiency is 66% of 70%. Torque is 205 Nm.
I am done at this point.
Yeah…
Forget about all the unit conversions. They are not that important at this point. First you need to do some more fact collecting. What you need to focus on is justifying your WAG (wild-ass-guess):
“The diesel would probably get about half that.”
All your computations that come after, and are based on, that WAG are useless until you can demonstrate your WAG is somewhere in the correct solar system.
Look, I am not trying to tell you your hypothesis is incorrect. At this point I am only telling you, you haven’t come close to providing evidence to support it. There are too many flaws and holes and fishy numbers in your method.
“Obviously you know nothing about thermodynamics.”
I suppose I can let that go and consider it a hasty outburst in a moment of frustration and bruised ego.
This paper (link here) states that a diesel engine runs on less than 25:1. The gas engine run t 14.7:1, so by using half as much I am making the diesel 29.4:1, which as Paul Niedermeyer pointed out in the article, is in the right ball park. I do have a masters degree in physics, so I am competent in thermodynamics.
“I do have a masters degree in physics”
Good, then I am not wasting my breath.
“This paper (link here) states that a diesel engine runs on less than 25:1. ”
Excellent. This paper provides you with some justification to use 25:1. Don’t use 29:1 because you cannot justify doing so.
Here is the money quote from your link:
“The lowest average A/F ratio is often found at peak torque conditions. To avoid excessive smoke formation, A/F ratio at peak torque is usually maintained above 25:1”
The key word here is USUALLY. That means NOT ALWAYS. So even using 25:1 could end up being an unfair approximation in gasoline’s favor. If you wanted to be cautious, you would use 22.5:1 for your calcs until more info is obtained. Not 29:1. Using 29:1 is unconservative, unjustifiable, and intellectually dishonest.
I have given you all the information that you need to do those calculations your self. The number that we should use is the actual number that Mercedes used in their OM603.912 engine, but we don’t know the number. However, once I get to the point of comparing the ideal engine (adjusting for cooling loss, which is non existent in an ideal engined), we might make a guess at the fuel ratio.
Note that a 21:1 ratio would give you about 40% more fuel and 25:1 would give you 17.5% more fuel.
It is really interesting to read this article again six years on. The whole VW dieselgate thing has come and gone and there are not many diesel passenger cars available.
My own experience of diesel vs gasoline torque is with my all time favourite car, the Golf I. As Paul points out, the diesel has significantly less torque than the gas engine and at higher rpm, too.
1.5 gas motor 79.7 lb⋅ft at 2,500 rpm
1.5 diesel 59.0 lb⋅ft at 3,000
1.6 diesel 72.3 lb⋅ft at 2,300
1.6 turbo diesel 95.9 lb⋅ft at 2,600
My experience was the post 1980 1.6 diesel was the best of the lot. It appears VW did some clever engineering to get more torque at a lower RPM.
From the early 1960s until the late 1970s most US built farm tractors offered a choice of diesel or gasoline engines with similar horsepower ratings. The gasoline engines were invariably smaller in displacement. As tractors grew larger the gas engines died out. Greater durability for the diesels was important, as well as easier starting. Early on tractor diesels required glow plugs, decompression pedals, engines that started on gasoline then switched to diesel, and pony motors (large John Deere two cylinder tractors). But the biggest reason farmers moved away from larger gasoline engines was their extreme thirst under load. These large gas engines burned insane amounts of fuel and after the early 1970s the diesels simply paid for themselves. A half century later tractors from that era with diesels still sell for substantially more than the gas versions.
Another reason the diesel tractors from the 50-60-70-80 are selling so well these days are their reliability. The latest emissions are so complicated that the average farmer can no longer repair his equipment. Service call and maybe the tech can figure out what’s gone wrong now. I agree with the idea of cleaning up the emissions on diesels but the product put out was just slapped together and hope it works. If batteries and hydrogen fuel cells get worked out diesel is going to be in trouble. Trucking and farming have one thing in common, what works the best at the least cost.
One comparison that’s very close are the Oldsmobile 350 engines, both gas and diesel. As I own and drive both versions, I can say that the gas engine has way more power and torque, and the diesel is way more fuel efficient. Unrelated to the discussion, is that the diesel starts and runs perfect every time, because it is “fuel injected”. The carburetor is primitive and prone to trouble especially with the ethanol mix of today’s fuels.