petrol vs diesel EGT'S?

[bj70cruzer]

So I have been thinking for a while as to why a petrol motor can see egts as high as 900degrees whereas the limit for a diesel in 650 degrees?

anything above 650 in a diesel is said to have chances of melting pistons etc but why dosnt the same apply for a petrol motor.

Does the compression ratios have any effect on this? (e.g greater cylinder pressures in an diesel?)

Thanks!

 

[The Phoenician]

are you sure it's only 650 for diesels? I thought it needed to be below 1200 degrees before it melts anything. Mine runs in the 900 to 1000 range, sometimes much higher depending on incline, load, ambient temperature and acceleration. Most Cummins Diesels I know run in 800-900 on highways.

EDIT: are you talking Fahrenheit or Celsius? If you're talking Celsius then yes it roughly comes out to 1200 and are right

 

[inkpot]

A supercharged petrol can idle at 700-800 and hit as high as 1300-1400 on a long pull. Kinda depends on where the temp sensor is mounted also. John

 

[Wheelingnoob]

I had seen up to 1600f on my gas engine in a performance car. Never had an issue with melted pistons.

I'm sure there is a reason for running diesels cooler I just do not know why.

 

[Tapage]

for some reason diesel ( At leats Toy pistons ) on our old engines seems to be weak going over 1200°F I've been there plenty of times for few secs .. and never had an issue, but I'm not trying to push my luck that far ..

To speak about are the 2JZGTE or 7MGTE pistons material that different from 2H or 1HD-T piston material ?

 

[Wheelingnoob]

for some reason diesel ( At leats Toy pistons ) on our old engines seems to be weak going over 1200°F I've been there plenty of times for few secs .. and never had an issue, but I'm not trying to push my luck that far ..

To speak about are the 2JZGTE or 7MGTE pistons material that different from 2H or 1HD-T piston material ?

Well the 1200F seems to be a universal Diesel EGT limit from Toyota to Cummins.

On my 7M-GTE and 1JZ-GTE I had stock pistons same materials as the Toyota Diesel pistons. In the 2JZ-GTE I had stock pistons and cracked the ring lands when the ECU fed it 40deg timing at 18psi boost (bad). The Engine was rebuilt with Forged pistons and H beam rods after that as well as a new $3000 ECU, no issues since. Think the current owner has well over 60k kms on it now running 23psi daily. That engine had EGT's high enough to turn the tubular manifold white hot and the turbine housing bright orange/white on the dyno when tuning.

 

[BEE-JAY42]

Your EGT is only the temp of the exhaust gas in the manifold, not the temp of your combustion chamber/piston, that's the trap.

Because diesel burns slower and the engines turn over slower (generally there are always red herrings though) combustion is more complete, so by the time the exhaust hits your manifold most of the fuel is burnt and that heat has been absorbed into the combustion chamber.

A petrol engine of the other hand is turning over a lot faster especially when it is making a lot of power, the cycle becomes so quick that they fuel is still burning as it is being pushed out of the combustion chamber into the manifold, a larger part of combustion is taking place in the manifold so still burning fuel makes your EGT higher.

So basically a diesel has a lower limit because more of the combustion heat is absorbed into the combustion chamber due to more complete combustion as a result of the air/fuel mixture spending longer in the combustion chamber, the really high EGT's seen in petrol engine are not indicative of the combustion chamber temps because you are still burning fuel as your exhaust leaves the combustion chamber. If you ran a petrol engine at 2200RPM with an exhaust temp of 900*C it would not last long.

Hope this helps.