AFR vs EGT ( SI vs CI ) 3

[Ibrahim M]

Hello, can someone explain me why :
More fuel results in lower EGT in a SI engine
More fuel results in higher EGT in a CI engine ?


I belive, that SI engines, past some point too rich will cause the unburned fuel to evaporate and carry heat away thus lowering combustion temps and EGTs correct ?


Now with diesel, i believe rich mixtures causes about the same combustion temps,  and combustion temp reduces as oxygen gets rare.  But higher EGT  due to more energy.

But  can injection duration play a role here ?
Theoretically Richer = longer injection duration, and  burn closer to Exh opening, but normally  SOi  would advance to prevent that ??

And how come too rich diesel  will melt pistons ?

Thanks

 

[Compositepro]

It is not the ignition method that causes the difference. Gasoline engines operate at near stoichiometric fuel/air ratios, which results in the highest exhaust gas temperature. Diesel engines do not have a throttle valve in the inlet air flow. They always operate at full air flow and inject the amount of fuel required for the power output. Thus they almost always operate at very lean fuel/air ratios.

 

[BrianPetersen]

SI engine traditionally operates rich of stoichiometric when the engine is under load. In this situation, moving leaner moves the mixture towards stoichiometric, which raises EGT, and moving richer moves it away from chemically correct and increases dilution (total mass of air + fuel relative to total chemical energy released), which lowers EGT.

CI engine traditionally operates lean of stoichiometric all the time. In this situation, moving leaner takes energy away and increases dilution which reduces EGT, richer adds more fuel which adds more energy and moves it towards stoichiometric, which raises EGT.

If you can find a way for spark ignition to reliably ignite a very lean mixture, going lean of stoichiometric with a SI engine will also reduce EGT.

If you grossly overfuel a CI engine so that the overall cylinder is rich of stoichiometric then that will add dilution and reduce EGT. You will not be able to see across the exhaust plume ...

So what is actually happening is that moving closer to stoichiometric raises EGT and moving away from it reduces EGT, regardless of in which direction (leaner or richer) you are doing that, but practical limitations generally dictate that SI will normally operate on the rich side and CI will normally operate on the lean side.

* * * The above is an oversimplification in the interest of making the concept understandable * * *

There are some secondary confounding factors ... the speed of combustion and the completeness of combustion varies with lambda, which means the actual peak EGT is probably going to be slightly on one side or the other of stoichiometric rather than being right at lambda = 1.000.

Engines subject to stringent emission control tend to operate at lambda = 1.0 (or at least, spend a considerable amount of time near that) to keep their catalysts happy. SI maintains lambda 1.0 at part load by throttling (or valve-timing trickery, or to some extent EGR). CI maintains lambda where the catalysts want it to be via some combination of boost pressure regulation and EGR (exhaust gas recirculation). CI engines under given loading use varying lambda setpoints (achieved by playing with EGR and boost pressure) depending on whether the DPF and SCR catalysts are happy (slightly lean operation is OK), or the DPF needs to be regenerated (operate at lambda close to 1.0 for a while to get it good and hot, and then switch to lean operation while the accumulated soot in the DPF burns itself off).

 

[enginesrus]

SI engine, more fuel and less O2 means more incomplete combustion. If there was no O2 there would be no combustion thus very cold EGT. This is the simple answer.

 

[Lou Scannon]

Simple and less accurate than the excellent answer by BrianPetersen.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz