air fuel ratio
air fuel ratio
(OP)
Hi guys,
Can anyone explain or guide me to appropriate literature, why a diesel engine exhaust temperature is high with a rich air to fuel ratio and a gas engine exhaust temperature is low?
Can anyone explain or guide me to appropriate literature, why a diesel engine exhaust temperature is high with a rich air to fuel ratio and a gas engine exhaust temperature is low?





RE: air fuel ratio
Let's tackle the diesel first because it's easier to follow.
Understand that the diesel engine is always lean of stoichiometric (we are assuming proper calibration here, and not a dense-black-smoker). The only question is by how much.
When the engine is idling or at light load, there is a huge amount of excess air for not much fuel. So the heat of combustion (the portion that isn't used for operating the pistons!) is diluted over a great amount of mass (the excess air). At maximum load, there is not much excess air, so there is more heat relative to the amount of air, so it's hotter.
Now ... Spark ignition engine.
These are, for the most part, always stoichiometric or nearly so. With conventional engines, if they are not stoichimetric, they are rich of stoichiometric. And with that ... the situation is simply the upside-down version of what was just described. Too rich, combustion becomes incomplete, less energy is released, but there is more mass flow due to the extra fuel, so there is less heat spread over more mass ... it's cooler (but dirty).
If you can coax a spark-ignition engine to operate on the lean side, the situation is just as it is with the diesel and for the same reasons.
Normally with spark-ignition engines, the exhaust gas temperature is at a peak very close to stoichiometric (because that's where the "heat release per amount of total mass flow" is greatest) and it drops off on either side.
In reality the peak EGT is not exactly at stoichiometric because of various combustion-speed-related matters, but it's pretty close.
When people are tuning race engines (who are not scientists or engineers), their word "lean" should usually really be interpreted as "less rich" ... very rarely will a racing spark-ignition engine ever actually operate lean of stoichiometric. The way I've seen this stated in understandable terms is that if your goal is maximum power output, the amount of air will be constrained, and it has to run a little on the rich side to make sure you burn all the air. If your goal is maximum fuel economy, it has to run a little on the lean side to make sure you burn all the fuel.
RE: air fuel ratio
RE: air fuel ratio
RE: air fuel ratio
Another fact affects the exhaust temperature, the engine compression ratio. Diesel engines has higher compression ratio (14:1 to 23:1) meanwhile gasoline/gas engines compression ratio are lower than 11,5:1.
RE: air fuel ratio
RE: air fuel ratio
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RE: air fuel ratio
RE: air fuel ratio
Assume initial conditions at the completion of heat release, and solve for T at the end of expansion.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: air fuel ratio
RE: air fuel ratio
Keep in mind that there are combustion-speed and fuel/air mixing-rate considerations which mean the peak temperature (of what? Exhaust? Pistons? Peak cycle temperature?) in real operating conditions will be slightly on either side of stoichiometric.
RE: air fuel ratio
http://www.gaceflyingclub.com/member%20download/Le...
I believe the models referred to have fixed ignition timing around 20-25 degrees BTDC. Changing ignition timing can have a significant affect on EGT.
RE: air fuel ratio
Diesels normally run lean of stoich' so richer mix means moving towards stoich => increasing exh temp.
SI normallly run close to stoich' so richer mix is moving away from stoich => decreasing exh temp.
je suis charlie
RE: air fuel ratio
It is common in the homebuilt aviation community to run engines at cruise in lean-of-peak EGT. They do this by staying rich until they are in a steady flight configuration, then pulling the fuel back behind the AFR where detonation is likely. They can keep power high enough for cruise and stay there as long as they do not need much additional power. This is very efficient at saving fuel on long trips and it keep engine temps down. But in these very light engines, it doesn't take much detonation to destroy the engine, so the practice is done carefully and judiciously.
RE: air fuel ratio
RE: air fuel ratio
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