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Why are diesels more efficient?
9

Why are diesels more efficient?

Why are diesels more efficient?

(OP)
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!

RE: Why are diesels more efficient?

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

RE: Why are diesels more efficient?

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

RE: Why are diesels more efficient?

#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.

RE: Why are diesels more efficient?

(OP)
Thanks everyone!

RE: Why are diesels more efficient?

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

RE: Why are diesels more efficient?

Oops, I meant to say lower surface/volume ratio

RE: Why are diesels more efficient?

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

RE: Why are diesels more efficient?

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

RE: Why are diesels more efficient?

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

RE: Why are diesels more efficient?

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

RE: Why are diesels more efficient?

^ 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)

RE: Why are diesels more efficient?

(OP)
Thanks everyone, I appreciate all of the input.

RE: Why are diesels more efficient?

Sort of on-topic ....

Quote:

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.

RE: Why are diesels more efficient?

Sort of off-topic ...

Quote:

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.

RE: Why are diesels more efficient?

"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.

RE: Why are diesels more efficient?

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.

RE: Why are diesels more efficient?

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.

RE: Why are diesels more efficient?

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.)

RE: Why are diesels more efficient?

Globi5
A clarification
The fuels are not the same even though they both have close to the same heat value per pound
Diesel fuel is an oil, it burns slow, it is selected for compression ignition engines because is burns slow,  this allows so the cylinders can be lighter that they would if a fast burning fuel was used,  since it burns slow, it works better in lower rpm and long stroke engines.  
Because the fuel is an oil the fuel system is easier to make and not wear out the moving parts at high pressures required.
If you put gasoline in a diesel engine it will operate, produce more power, torque, efficiency, and at higher rpm but the engine will not last (and maybe not even that first power stroke) if the engine did not break, its performance would be closer to the Otto cycle because the heat addition would be nearer to constant volume not constant pressure...
Hydrae

RE: Why are diesels more efficient?

hydrae, actually I remember a Nissan 2.8 l diesel engine (1988) that stopped running because someone tanked gasoline. (I'm not claiming that it wouldn't work per se though.)

Back to the temperature question: Since diesel engines produce more NOx is this proof that they generally have higher peak temperatures (regardless how fast diesel burns)? (I don't know - I'm asking)

RE: Why are diesels more efficient?

Diesels have much heavier components for the size and speed range than a spark ignition engine.

I think that alone indicates higher loads which means higher pressures as the speed is lower.

Regards

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RE: Why are diesels more efficient?

you are all jumping around the answer,

diesel engines can perform a significant lean burning environment. i.e. the cylinder fills with air and at idle or at any rev range lower than full throttle, only a relatively small quantity of fuel is in the cyclinder compared to the air quantity. The final air temp is much lower than if you choked the air like in a petrol engine, thus less heat energy is wasted in transfer to engine components, petrol engines are restricted to compression ratio about 8:1 to 12:1. latest diesels are direct injection which gives better response, and higher power to weight ratio, getting them closer to p/w ratio of petrol engine.

diesels because they must handle higher compression ratios and direct injected fuel = heavy engines in general which is why most are turbo charged.

NOx generation comes from the lean burn lots of air in cylinder = lots of nitrogen = lots of NOx when you put your foot down. diesel operate where your average catalitic converter for a petrol engine wont work, some diferent technology needs to be used. hence they produce a truck load of it as most diesels on road now are untreated but in future they will be restricted just like petrol

RE: Why are diesels more efficient?

The nox production from either gasoline fueled SI engines or direct injection diesel fueled CI engines is the result of oxygen that has not reacted with the hydrogen in the fuel. Both style engines develope combustion chamber temps. that exceed the threshold temp. needed for the formation of nox. SI engines have a fairly homogenous air fuel mix so most of the oxygen gets consumed as part of the combustion process leaving little available for bonding with nitrogen compared to the traditional CI diesel fueled engine. Homogenous charge compression ignition engines (HCCI) emit very low nox #'s when fueled to consume all the oxygen available and the combustion process is almost instant.-------------Phil

RE: Why are diesels more efficient?

The temperature componet of NOX formation has a peak value less than 4000° F.  g if you get the temperature hotter than a certain value the amount of NOX is formed starts to drop again.  but NOX has many parameters that govern its formation besides temperature, such as the availability of oxygen, the absense of NOX destroying chemicals (such as urea)
Hydrae

RE: Why are diesels more efficient?

Phil, Atchooley problematic nox starts forming quite easily due to inadequate homogeniety even in todays engines.

Time, O2 availability, thermal and chemical stratification, surface temps, diluent (egr) and pressure all influence Nox generation.  Same is true for dissociation and selective reduction.

A decade & a half + ago some work was done with true peripheral ignition and injection along with low quench chambers and piston using a center squish chamber (no edge squish, fuel diected to center as opposed to radially (can explain later).  The problem was the cost with the articulated piston and material life.  In short, the work was performed to minimize nox via short burn time for lean methane mixtures on a 7.3L Si diesel based block.

The missing link at the time was a suitable cng injector.  The lessons learned might work with gasoline of other fuels but that opinjion was not shared by the project sponsor..

tmi for the day, ciao

RE: Why are diesels more efficient?

The simple answer to the original question lies in the theoretical models:
A diesel cycle engine has heat addition at constant pressure,  while the Otto cycle (S.I.) engine has heat addition at constant volume.
The former cycle has a significantly larger area under the curve in a P-V diagram, if memory serves.
This is also of course why temperatures are higher in the S.I. engine.

RE: Why are diesels more efficient?

Chemistry is definitely not my area of expertise, however; some further thoughts:-

In the conventional diesel engine, although the global air/fuel ratio is lean, the local air/fuel ratio on the periphery of the fuel plume in the region of the flame, is close to stoichiometry and that is true whether you are at light load low idle or at full power rated speed.  For acceptable fuel consumption (sfc), the full load injection duration must be kept under about 30 crank degrees, but the heat release can extend much longer than that.

Now, the flame is virtually stationary since it is anchored by the fuel plume, even in swirl supported combustion systems.  So you have a near constant heat source with a flame that is in excess of the ~1900°K thermal NOx formation temperature, and this is surrounded by plenty of extra air that is wafting over and feeding the flame.  My assumption had always been that the NO and NO2 was formed from thermal breakdown of the nitrogen in the air rather than the oxygen others have mentioned, and since there is plenty of that in a diesel engine even at full load (since the fuel plume is not able to reach all the air in the chamber), we get NOx.  This is still true but less so, even when we use a diluent such as EGR.

In the homogeneous stoichiometric SI engine though, the fuel burns in a flame front that radiates out from the spark plug.  In this scenario and assuming good homogeneity, the flame may well be over the thermal NOx threshold as it is in the diesel engine but it fully consumes the air behind the flame front and progresses into the heated air in front of it perhaps limiting its opportunity to form NOx?  There is also a time constant in the formation of NOx.

In the case of the true HCCI engine, the air/fuel ratio is both homogeneous and way lean of the flammable range so combustion is a near simultaneous auto-ignition event distributed all over the chamber, with no flame front at all.  As such, the AFT remains below the thermal NOx threshold.

PJGD

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