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.