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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?

RE: air fuel ratio

2
It's not that simple.

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

By the way, if you grossly over-fuel a diesel engine, the EGT relationship will eventually do the same as for a spark-ignition engine. Tractor-pullers are making sure they are burning all the air. You won't be able to see past the smoke cloud ...

RE: air fuel ratio

(OP)
This clarify a lot thank you.

RE: air fuel ratio

greetings,
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

How compression ratio affects exhaust temperature?

RE: air fuel ratio

I once ran a "low idle" test just for laughs on a 1000 cu in V8 turbo diesel where we just cranked up the fueling to ridiculous levels while keeping the engine speed at 600 PRM. If I recall we were able to get about 100 hp as we cranked up the fueling. At some point the power started dropping off then the smoke started to fall instead of rise it was so cold and dense.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: air fuel ratio

Temperature is directly proportional to the pressure increase. I operate same engines series with slightly different compression ratios and exhaust temperature are different as well.

RE: air fuel ratio

Quote (Deividas)

How compression ratio affects exhaust temperature?
PV=nRT
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

BrianPetersen - so, peak temperature should be close to stoichiometric a/f ratio, and when going to lean or rich side temperature should drop in both, ci and si engines? :)

RE: air fuel ratio

That will be approximately true.

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

Deividas summed it up.

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

Tmoose, et al
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

TXS, super lean operation with aircraft recip engines is nothing new, it was done in the war days and in the 50's with large multi engine commercial planes. Like over rich, over lean also cools because of the excess of air. Similar to CI engines.

RE: air fuel ratio

Diesel fuel is also more energy dense and are quality governed as opposed to quantity governed like in SI engines. Hyundai made a CI gasoline engine, maybe there are some EGT numbers out on that one for more comparison

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

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