Why are diesels more efficient? 9

[ornerynorsk]

I'm not into engines that much, but could someone tell me why diesels SEEM to be more efficient than gasoline engines? Is it the fuel volatility, heat content, compression, or a combination of all factors? Thanks!

 

[SomptingGuy]

It's mainly because of the (much) higher compression ratio coupled with no throttling losses.

 

[TheBlacksmith]

The last two. Diesel is generally considered less volatile, but it has more BTUs per gallon or pound mass than gasoline and the higher compression begats a longer expansion stroke that extracts more energy.

Blacksmith

 

[dgallup]

#1 No throttling losses
#2 Higher expansion ratio
#3 Higher BTU/Gallon fuel

There are a lot of other factors depending on what type of engine you are talking about: Automotive, class 8 truck, stationary, etc. Lower engine speeds, turbocharging, intercooling, etc.

 

[ornerynorsk]

Thanks everyone!

 

[globi5]

Some additional factors that are generally the case:

#4 higher BMEP = lower rpm = less frictional losses
#5 higher stroke/bore ratio = lower volume/surface ratio = less heat-loss

 

[globi5]

Oops, I meant to say lower surface/volume ratio

 

[Turbinator]

Also, the compression ignition permits higher a/f ratios to be used.

 

[crysta1c1ear]

Not all fuel present before TDC advantage?

In a gasoline engine, all the fuel is present when the spark ignites, before TDC. In a diesel, just a small pilot quantity of fuel can be ignited initially, and further fuel injected later.

So is there an effect where compression losses are not so bad and less fuel burns prior to TDC. I don't know, I'm asking?

===================================================

Stratified rather than homogenous advantage?

The Volkswagen fuel stratified injection engines can run with the fuel injected into the middle of the cylinder, giving a near homogenous charge in the middle of the cylinder, but an insulating layer of air around the cylinder walls thus reducing heat loss from gas to cylinder wall contact. That is the stratification as I understand it: burn stuff in the middle and not against the walls.

I would assume that diesels can profit from the advantages of fuel stratification over a completely homogenous charge, and that most gasoline engine (not being direct injection) cannot.

That's another question! Have I got that right?

==========

I agree with the other reasons listed above, but I guess two of them deserve numbering.

#6 tubocharging is the norm - c/o dgallup
#7 lean burn - c/o turbinator

 

[drwebb]

The Carnot cycle is of course more efficient the higher the operating temperature. Does this have any bearing on diesel vs. gasoline thermodynamics?

 

[hydrae]

In thermal efficiency the OTTO cycle is more efficient due to higher heat input temperatures (close to 4000) but the losses of throttlling, heat transfer, incomplete combustion and friction reduce the efficiency below that of the diesel cycle.
In normal vehicle operation the full output of the engine is rarely used, so the throttling mechanism of the diesel cycle of limiting the fuel provides an overall gain in operating cost.
Hydrae

 

[globi5]

^ I don't see why the heat input temperatures with gasoline should be higher than with diesel.
Both fuels have about the same heat energy per mass (not volume). So if you burn the same mass of fuel at TDC the diesel engine should end up with higher temperatures due to the higher initial air temperature at TDC (because of the higher compression ratio).
Unless the amount of diesel fuel burnt is significantly less than the amount of gasoline burnt in a equally sized combustion chamber, it should be expected that diesel engines generally have higher peak combustion temperatures. Actually, since most diesel engines are turbocharged they have a higher air density within the combustion chamber anyway and might even inject more fuel than equally sized gasoline engine despite the leaner fuel/air ratio. And it that case more fuel + more air + higher initial temperature = higher peak temperature (well, very simplified).

Back to the question whether the carnot cycle is more effient at higher temperatures temperatures: Yes I believe this has bearing on diesel vs. gasoline thermodynamics. After all higher peak pressures go along with higher peak temperatures. (p*V=n*R*T)

 

[ornerynorsk]

Thanks everyone, I appreciate all of the input.

 

[crysta1c1ear]

Sort of on-topic ....

In thermal efficiency the OTTO cycle is more efficient due to higher heat input temperatures (close to 4000) but the losses of throttlling, heat transfer, incomplete combustion and friction reduce the efficiency below that of the diesel cycle.

In thermal efficiency the OTTO cycle is more efficient .....
That's what we get from people studying thermodynamics and defining an idealised Otto cycle as constant volume combustion, and an idealised Diesel cycle as being constant pressure combustion. But constant volume combustion would mean you can burn all the fuel instantaneously at top dead centre. The combustion is of course slower than that in practice. And the constant pressure idea from (thermodynamisicst's) idealised Diesel cycle doesn't seem too accurate to me either.

Then, the themodynamics guys go on to deduce that in thermal efficiency the OTTO cycle is more efficient, but we should qualify that, and say for the same compression ratio. So the real way to increase efficiency of petrol/gasoline engines ought to be to increase the compression ratio and to prevent the fuel autoigniting by not injecting it into the cylinder until near TDC. Efficiency could then be further increased by running lean. If the fuel air mixture were near stoichiometric in the centre of the cylinder and the excess air were around the outside of the cylinder insulating it from heat transfer to the cylinder walls, and there to pick up and burn any leftovers hydrocarbons from the combustion in the centre, then the engine should be even be more efficient. I believe that is pretty much the VW fuel stratified injection that has been winning at Le Mans.

If we look at the differences between what we might call Diesel and petrol engines, or might call spark ignition and compression ignition engines, we can list quite a few.

Fuel used
Throttle
Compression ratio
fuel delivery method
fuel ignition method
fuel equivalence ratio

I think we can see many of the differences disappearing as people look for Diesel engine efficiency without using Diesel fuel. For example BMW used IVT (intake valve throttling) to eliminate the need to suck air past a throttle. Fuel ignition method? Does it really matter? The main body of fuel in a so called compression ignition engine is ignited by injecting it into an already lit pilot flame, so compression ignition is a misleading description. It is almost irrelevant how the pilot flame was lit: maybe with the aid of a glow plug.

 

[crysta1c1ear]

Sort of off-topic ...

I don't see why the heat input temperatures with gasoline should be higher than with diesel.

Neither do I. A factor limiting temperatures is NOx formation. There is a trade off between efficiency (want higher temperatures) and NOx emissions (want lower temperatures). Since the trade off doesn't really depend on the fuel: I'd expect both types of engines to be designed to run at similar temperatures.

I agree that higher temperatures before ignition result from higher compression ratios and also result from turbocharging. Burning more air and fuel however shouldn't cause higher temperatures: a bit like two stones don't drop faster than one. Burning twice as much air and fuel should just create twice as much hot gas, but at the same temperature. The higher temperatures before ignition would cause higher temperatures after combustion - all things being equal. But as I mentioned before something may get adjusted in the engine design (eg air fuel ratio) to keep the temperature at a desirable level.

From the work you do to heat the air in the cylinder before combustion, at best you are going to get a high percentage of it back later, so I do not see that as contributing to diesels being more efficient.

Adiabatic flame temperatures (AFT) are calculated by distributing the heat of combustion among the exhaust gases. globi5 said: "both fuels have about the same heat energy per mass."
I imagine he was thinking that if you burn the same mass of fuel at TDC you should have similar temperatures. Diesel temperaturs might be lower in practice if the engines are typically burning less fuel. But I suspect that the AFT for diesel might be higher due to an ability to burn more fuel. (If the same fuel were to require less air due to a difference in soichiometric air fuel ratios - say diesel 14.55 petrol 14.7 - then the same amount of air would be able to burn more diesel fuel than petrol and thus produce more heat and a higher temperature.) I may check some information on AFTs for diesel and gasoline when I get a chance.

 

[globi5]

"Burning twice as much air and fuel should just create twice as much hot gas, but at the same temperature."

I think you make a valid point.
If you have an equally sized combustion chamber and burn double the air and fuel you'd end up with double the heat energy in that volume, which will be experienced in double the pressure and not double the temperature.
However since you have higher initial temperatures at TDC due to the higher compression ratio, the combined heat energy from the compression and burning the fuel air mixture the temperature should be higher. The fact that Diesel engines run a leaner fuel air mixture will reduce that temperature, as you mentioned as well, but is this temperature reduction high enough to keep the temperatures at the same level or even lower? Maybe at low load conditions but probably not at high load conditions where the fuel air ratio is richer.
You also mention NOx emissions as an indicator for peak temperatures. Aren't diesel engines known to produce more NOx than gasoline engines despite the fact that they run a very lean mixture (cooler) most of the time?

Regarding higher temperature and higher efficiency: The same engine with a higher compression ratio is more efficient according to the ideal Otto-cycle. Higher compression ratio also leads to higher temperatures at TDC. So it is fair to say that a more efficient engine usually deals with higher peak temperatures as well.

 

[globi5]

The 'Swiss EPA' claims that Diesel engines on average produce 8 times more NOx than gasoline engines:

http://www.umwelt-schweiz.ch/buwal/de/fachgebiete/fg_luft/themen/dieseltreibstoff/

(Sorry couldn't find an English link.)

 

[turbocohen]

Modern Diesels use boost as a means of providing Nox control via maintaining enough excess air to prevent air fuel ratios that lead to nox generation. The area ratios suport more mass flow at part load than in the past which does not improve efficience but does reduce nox and some particulates.

 

[crysta1c1ear]

I just tried to quantify some of the savings for somebody and took a formula for Otto cycle efficiency from here

http://sitemaker.umich.edu/mhross/files/fueleff_physicsautossanders.pdf

10 to 1 compression ratio : Otto cycle efficiency is 60%.
30 to 1 compression ratio : Otto cycle efficiency is 74%.

So there is around a 23% fuel economy improvement (calculate 74/60) available just by injecting fuel in the engine around top dead centre, instead of beathing it in and forcing a low compression ratio.

 

[globi5]

Here's another one (It has more to do with direct injection though):
In an engine without direct injection part of the fuel can leave through the exhaust without even getting burnt due to valve overlap. (Probably not a significant factor - if at all.)