Questions about diesel engines

[curmudgeon]

Also some other food for thought.
All the forces to move the piston comes from expanding air which comes from heat which is generated by the burning of the fuel/air mixture, is this correct?

If so we want it to be as hot as possible without knock. But then when it gets too hot we add in extra fuel to cool it down. So why don't we save that extra fuel and just add in less air and fuel to start with so we get the temperature we want without wasting extra fuel?

Because the air is the working fluid as well as the source of oxygen. So obviously a small mass of air will produce less work than a larger mass at exactly the same temperature.

[GTSBoy]

but how can that difference of fuel make a difference to the temps?
Can the latent heat (Not sure if thats the correct term) of just that bit of fuel actually absorb that much heat to cool the chamber down?

Excess liquid fuel will heat up to vapourisation temperature then will vapourise and then the vapour will heat up to flame temperature. That is sensible heat (specific heat capacity of the liquid) followed by latent heat (of vapourisation of the liquid) followed by sensible heat again (of the gaseous fuel) all soaking up heat from the combustion heat release. The latent heat is actually a very significant fraction out of the three of them, but it is the three of them together that results in the reduced combustion temperature.

Also does this mean, if you are running lean (>14.7) wouldnt the exhaust temps be lower? I thought in practice exhaust temps are higher when you go above 14.7 (maybe a misconception of mine), but obviously they should be highest at 14.7.
EDIT: I think curmudgeon answered it, when you run lean you have burning gasses passing through the exhaust now because of the later burning time? Wouldnt that mean you have burning gasses going back into the intake with an EGR setup? I guess that would cause the intake temps to be higher increasing the chance of detonation.

Yeah, as you go above 14.7:1 you will get colder combustion temperatures again. EGTs may continue to climb for some space above 14.7 because of the slow burn effect.

I wouldn't worry about burning gases being returned to the intake through EGT. They will likely burn out and/or quench out before they get that far, unless perhaps it is an old crude system like the Holden 6 system where the manifolds are bolted together!! Anyway, you'd have to run the wrong mixtures first.

PS: EGTs are measured through thermocouples on the exhaust, how are CHTs measured? (is it the heat in the cooling system?)

You have to specially instrument the combustion chamber. Put in a special spark plug with a thermocouple tip, or put a separate thermocouple through the chamber wall. You only do this on development engines.

[GTSBoy]

I'd imagine that it would be well past the point where the exhaust has turned to a black, opaque cloud.

Yuh, I've been to the truck racing too.

[pauly24]

Awesome, thanks GTSboy and curmudgeon for the responses.
I think I'm out of questions for once in my life! well... for now. :p

Can I have my certificate in engine theory 101 now? haha