Inspired by this thread, why does diesel return a higher mpg than regular fuel?
You need a little less of it to make equal amounts of power…
Diesel engines create a lot of ‘low-end torque’ which means it can pull your car/truck along at very low RPMs (and engines breath better and are more efficient when the RPM band in which the operate is narrow)…
They require less support (no spark plugs and ignition system to drain power), as the diesel simply explodes when compressed…
Most of the reason is simply that diesel contains more energy per volume. (about 10% more according to this.
They also run at a more even temperature and have smaller losses in the aspiration part of the cycle. (I believe)
Gasoline engines maintain a constant ratio of air to fuel and adjust the amount of fuel/air mixture to provide more or less power. Since the cylinder volume is constant, this means that the air/fel mixture is compressed less at lighter loads and may not burn completely.
Diesel engines always compress a large amount of air, at higher pressures than in a gasoline engine, and adjust the amount of fuel burned in this air. The fuel is completely burned, regardless of how much power is produced.
From what I’ve heard, diesel trucks tend to get much higher mileage when lightly loaded at highway speeds. Under high loads, their mileage is still better than gas vehicles, but not by as big of margin.
Cornflakes,
You might find this article to be of interest: ;j
Another result of this is that diesels use a lot less fuel at idle. I used to drive an old diesel Rabbit, and it got very similar mileage in city and highway driving, where nearly all gas cars get significantly poorer mileage in the stop and go of city traffic.
Slower RPM engines tend to burn their fuel more completely than faster revving engines, since the fuel has longer to burn before it’s exhausted out of the cylinder.
That is contradictory to what you usually see in diesels, which is a lot more black smoke (which is unburned fuel) compared to gasoline. Anyone know why?
The unburned and partially burned fuels in the exhaust of a gas engine are carbon monoxide and short chain hydrocarbons, plus a fairly small amount of solid carbon. The carbon monoxide and short chain hydrocarbons are colorless gasses at typical exhaust temperatures. You can’t see them, no matter how much of them are coming out of the exhaust pipe. (In most modern cars, though, the catalytic converter burns them, so they never actually make it out of the exhaust pipe. They still represent invisible waste from the engine).
In a diesel engine, the unburned fuels consist of carbon monoxide, short chain hydrocarbons, long chain hydrocarbons, and a larger amount of solid carbon. There’s also an amount of “ash”, stuff that’s completely oxidized but is in solid form at exhaust temperatures. There’s just more stuff that you can see in diesel exhaust.
This is an outdated observation. If you eliminate vehicles older than 10 years or so, you will see little to no visible exhaust smoke/soot except when not warmed up, using very poor quality fuel, in very poor tune or heavily modified. Modern, computer controlled, direct injected diesels mostly burn very cleanly. In Europe with Ultra-Low Sulphur diesel (which will be available here next year), you can’t even smell “diesel” in the exhaust when you’re standing behind the running car.
A gas engine MUST operate at stoichiometric fuel/air ratio(14.7:1 I believe). Diesels, on the other hand, can operate at nearly any fuel/air ratio. On older, non-computer controlled diesels, for maximum power under heavy loads they would inject more fuel than could be burned with the available air charge, therefore lots of smoke.
If you tried that with a gasser, it would just stall.
Another factor resulting in high-MPG from diesels is the high compression ratio. Higher compression engines are inherently more efficient, but in gas engines the compression ratio is limited by the tendency of gasoline to predetonate. New direct-injection gas engines eliminate this problem.
Right (14.7:1, air to fuel by weight, sounds right to me too.)
Most gasoline (and alcohol, nitromethane, etc., etc.) engines and diesel engines use the Otto cycle design. The difference is that the temperatures just before combustion (point 3 in the graph) are below flash point for a gas engine and above flash point for a diesel.
In a gasoline engine, the air and fuel enters the cylinder at the same time. The fuel is vaporized and mixed with the air during the compression stroke, and the mixture burns at the end of the compression stroke. The air and fuel are mixed, and the mixture has to be more-or-less stochiometric.
In a diesel engine, each fuel injector acts sort of like a blowtorch; oil is injected at top dead center on compression and rapidly burns in the hot air, expanding as it burns. Just as a propane torch will burn in any size of room, the air to oil ratio doesn’t have to be stochiometric inside the cylinder. There just has to be enough air to burn the oil.
Actually, AF dragsters (aka Gassers) were usually jetted rich. The carburetors were set up to provide more fuel to cool the cylinder and to compensate for any fuel that failed to atomize (Hey, who called me Clifton Cleban!?)
Also with a high compression engine, more energy is recovered by the piston during the power stroke since there is a a larger volume for the expanding combustion gas to expand into.
Diesel engines also run cooler, more kinetic energy than thermal energy is liberated in the combustion chamber…but for reasons I don’t know why, exactly. :rolleyes:
I’m not sure, but this doesn’t sound right to me. Timing on gasoline engines tends to remain constant above 2000-3000 RPM or so (assuming that the manifold pressure remains constant.) Most of the time, I have heard suggestions for “all in” timing of 35 to 40 degrees before top dead center at 2500RPM or so (meaning that the timing should reach this full advance at the recommended RPM.) This would imply that, for higher RPMs, the fuel burns at a rate proportional to engine speed.
Verrrry Innteresting… :dubious:
That’s not true. Gasoline engines can be operated over a fairly wide range of mixture ratios. It was common practice in the past for gasoline powered airliners and military aircraft to operate at extremely lean fuel/air ratios. Even today, gasoline powered aircraft run extremely rich for takeoff, and slightly rich or very lean for cruise. Aircraft would be falling out of the sky if pilots had to maintain a stochiometric ratio to keep their gasoline engine running.
My source for this is a History Channel program on Honda. When emission control laws started to go into effect, Honda had to do a lot of work on their engines, because Honda engines tend to run at higher RPMs than most engines. Honda’s can run at around 6K all day long with no problems, whereas many other cars redline before this.
I’ll mention, in passing, that the engine Tucker originally planned to install in his cars was going to run in the low hundreds RPMs. I don’t know if Tucker was thinking about emissions, but I do know that he was thinking about improving fuel economy when he concieved of the design.
“A fairly wide range of mixture ratios” for a gasoline engine . According to the chart on this page , A gas engine will run at A/F ratios of about 8:1 to 20:1. Diesels operate at far wider range of A/F ratios, and are not prone to self-destructive detonation at very lean A/F ratios.
I’ll amend my statement to say "a gas engine must operate near stoich in order to produce reasonable power(in other words, keep running).