diesel vs gas

[Toddr8541]

has anyone done a comparison or know what the differences in cylinder press and cylinder temps on a diesel engine vs SI engine of comparable size?

 

[MiketheEngineer]

MANY books and articles discuss this - try Google

 

[Lou Scannon]

Your criteria are too vague. You need to narrow down the application, and how much of an apples-to-apples comparison you are looking for.

 

[patprimmer]

This sounds very much to me like an exam question. Are you a student?

Regards
Pat
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[TheBlacksmith]

Geez, I wish somebody had though of this before (you got your homework assignment).

 

[Toddr8541]

I am a engineer. I am not looking for a detailed report on the differences with temp and pressure graphs. All I am asking for, is if someone has experience with both types of engines to say engines of comperable size and power rating. ie: "Diesel cylinder pressures are about 10 to 15% higher and temps are about 20-30% higher."

I am familiar with gas engines but never took the time to investigate diesel engines. This question was more for curiosity than anything else. I don't know enough about diesel engines to know if there are any comporable sized engines( same size and power rating).

Theoritically for the same size engine, with same bore and stroke temps and pressure should be the same. However, diesels use the compression to ignite the feul. So does the diesels have slightly pressures for the same engine or are they significantly higher? Also if the diesel is smaller to make the same power of a larger gas engine, is it the increased pressure from compression ignition that allows the bore to be smaller.

Are the same pressures seen in the cylinder because the diesel feul reguires more compression pressure to ignite it and the gas engine just simply does the same by using a spark.

Again this was meant for general discussion. Not a specific project im working on and don't need the pressure and temp graphs.

 

[LostHippie]

small SI engines usually can get to about 10:1 or 11:1 compression ratios on conventional gasoline before you start having problems with autoignition.

small CI engines running on No. 2 Fuel oil (standard diesel fuel) run anywhere from 15:1 up to 25:1. Diesel engines typically see higher cylinder pressures (not sure about temperatures) which is why diesel engines, in general, are more robust in their design.

 

[patprimmer]

If they have higher pressures, they must also have higher temperatures as temperature and pressure are directly related as expressed in the combined gas laws.

A hint will be the rpm they make their power at. If two engines are the same dimensions and make the same power, but one does it at substantially lower rpm, the lower rpm engine will also be the high cylinder pressure engine.



Regards
Pat
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[MiketheEngineer]

Diesel was FIRST!!

 

[1gibson]

Didn't Chevy (briefly) make a 350 diesel? That would probably be a pretty direct comparison.

 

[patprimmer]

I think the 350 Chevvie was a pretty good SI engine for the day and age, but a pretty crappy diesel, mainly because it was not robust enough to withstand the pressures generated in a diesel.

Regards
Pat
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[TheBlacksmith]

Actually, it was Oldsmobile. The Olds 350 was a very stout small block (basically the same casting as the 455), but obviously not enough. Another shortcoming was the use of a fuel injection pump that was fuel lubricated. A typical failure was water in the fuel damaging the pump, followed by the pump sticking and over injecting one or more cylinders, blowing the head gasket. The Olds 350 ran between 9-1 to 10.5-1 in it's heyday and the diesel if I remember was 18-1. I cannot think of any other direct comparisons, but typically a diesel makes more torque at a lower rpm and therefore less peak horsepower, so pat's rule would dictate a higher compression. In fact, I've don't remember seeing a SI engine with more than 12-1 (Chevy L-88)or a CI engine with less than 16-1 (my current project, a Pielstick PA6B).

 

[GregLocock]

I worked on two SI to CI projects. Neither was a success. Main problem is that you need completely different cooling strategies.

Cheers

Greg Locock


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[JSteve2]

Some pretty typical numbers - a commercial diesel engine will run (at some loaded value) around twice the compression, and per unit volume maybe 4 times the moles of cylinder gas, and maybe 2.5 to 3 times the mass of fuel. The peak combustion pressures and temperatures are both much higher than a gasoline engine. Not as high as those numbers work out to in the raw math, because if you allowed that to happen you would break something, melt something, or produce prohibitive amounts of NOx. Therefore, among other things, you retard the fuel timing, introduce inert gases, and intercool the compressed air - largely to reduce the peak temperature. Around loaded values, diesels will exhibit a tendency for a flattened peak combustion temperature that is near the point where the NOx creation takes off.

 

[Lou Scannon]

My experience developing SI lean burn turbocharged natural gas engines from base diesel engines is that the peak pressures and temperatures are not that different, for the same power rating. The diesels have slightly higher compression ratio, and the gas engines run a little richer. The material limits are the same, typically, although the diesels do get pushed to higher ratings with commensurately upgraded materials; this is no longer apples-to-apples.

 

[izzmus]

Greg, that is interesting, VW didn't seem to change much. What problems did you encounter?

 

[GregLocock]

Ecessive cooling of the cylinder head, leading to excessive smoking and poor fuel consumption. This was 30 years ago, admittedly.

5 years later they did manage to do it on a different block by working closely with a diesel engine manufacturer.

Cheers

Greg Locock


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[BrianPetersen]

Generic question, generic answer. For a certain automotive turbodiesel engine that I have some familiarity with, I understand that the engine is designed for a peak cylinder pressure of 185 bar. This is some three times higher than a typical peak cylinder pressure for a normally-aspirated gasoline engine. Obviously there is a lot of give or take depending on the specific engine design that one picks for comparison, on either side of the comparison. In other words, "your mileage may vary".

Peak temperature is probably in the same range. The diesel has higher compression ratio and has boost pressure without perfect intercooling, so the temperature at the end of compression stroke is certainly higher. But the diesel runs leaner (less fuel in the charge means more dilution) and it's designed so that the moment of ignition - not the same as the moment of fuel injection! - is near or slightly after top-dead-center, as opposed to being quite a bit before for the gasoline engine. This is for both noise control and for emissions (and also, likely to prevent cylinder pressure from going through the roof).

For emissions reasons, newer diesels often run much more dilution with significant exhaust-gas recirculation even near full load, to try to reduce the peak temperature in the interest of reducing NOx.

 

[dicer]

If they are being motored and of course depending on the compression ratio, most normally the CI is higher.
Some SI's can have the same or higher CR as a CI.
At unloaded idle the CI runs cooler due to the nonhomogeneous fuel air mixture.
The type of fuel makes a difference as well.

 

[BigClive]

CI mostly runs cooler due to its greater 18:1 (or thereabouts) expansion ratio.