Gas verse Diesel exhaust temps?

[Skydiver]

This is a question that has been on my mind since I became a member of TDR.

I have learned that too much fuel on a diesel causes high egt's. But why is it that?Is it we are running a stock diesel so lean that it does not have the fuel to create high egt's.

What I was comparing this to (and I know it is not apples to apples)is when a gas engine runs too little fuel,it gets hot enough to burn out the exhaust valves.

Remember on the NASCAR restrictor plate racing if the engine does get enough fuel it burns a piston.

I know that a diesel is more efficient than a gasser but,is a diesel running so lean (in stock fueling) that it doesn't have the fuel to create much heat?

>More diesel=heat+soot

>More gas=cooler+soot

 

[RustyJC]

A diesel is running on the lean side of a theoretically perfect air/fuel ratio (stoichiometric mixture) - it runs with excess air. Therefore, up to a point, if you give it more fuel, the air is there to burn it, and EGT's will rise.



A typical gasser runs on the rich side of a stoichiometric mixture. There's no excess air to burn added fuel, so if you pour more fuel to it, the excess fuel just cools the combustion process. (Note - exceptions to this exist. The direct injection lean burn gasoline engine is an example. )



Rusty

 

[Joe G.]

Skydiver,



First of all the terms lean and rich have no meaning for a diesel. There is no throttle butterfly valve so the intake air is never restricted. A gasser has to have a mixture close to 14:1 or it won't run. When our engines are idleing the mixture is over 100:1. At WOT it's still more than 20:1. For all practical purposes, during decelleration the fuel is cut off for a diesel. Not so for a gasser, 14:1 for all conditions. If you inject more fuel than a diesel can burn cleanly you get black smoke regardless of the air/fuel mixture. If you lug it that runs up the EGT. This can happen at high RPM if there is not enuff air to burn the fuel and the engine cannot excellerate any more. A diesel also burns fuel for more degrees of crankshaft rotation instead of very quickly like a gasser. What all this means is that comparing a diesel to a gasser just does not work. I'm doing this from memory so if anyone sees something wrong, please jump in and correct me.