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!

 

[hydrae]

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

 

[globi5]

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)

 

[patprimmer]

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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[HydroScope]

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

 

[SMOKEY44211]

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

 

[hydrae]

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

 

[turbocohen]

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

 

[Rob45]

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.

 

[PJGD]

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