beep-beep

Ok, let say I know physics quite well. Still I do not understand, why lean mixture overheats engine.
I mean less fuel produces less heat and it dicipated in the same amount of the air.

Evilclown

Less fuel means the spark is burning mostly air, which will create more heat in the engine than if the right amount of fuel were getting to the combustion chamber. I understand your logic, but this isn't a normal fire; the spark is just as powerful if there's nothing there to burn as it would be at optimal mixture. So, in this case, more fuel means the combustion chambers are kept at a constant temperature, and not detonating or overheating.

fusilier

Sparks don't make that much heat. It has nothing to do with the sparkplugs or the arc.

Generally when the fuel is vaporized to a mist it readily absorbs heat from the air going into the engine which in the case of turbos and superchargers is very hot even after an intercooler. So the fuel cools the air-fuel mixture. Lean mixtures cause spontaneous detonation because the air fuel mixture is too hot with alot of oxygen readily on hand. Detonations are uncontrolled explosions of gas at 18,000 F as opposed to controlled burning of fuel that occure ahead of the flame front. It occurs after the combustion process has started and is usually located in the area last to burn (the last hot cylinder as new lean air-fuel is injected). The explosions release more heat and also much more pressure which means even more heat. Temperatures and Pressures from Detonations are about 10 times higher than regular combustion, thus increasing engine temperature even though your introducing less total energy into the engine. Keep in mind that the majority of the energy in a controlled fuel burn in lost as heat out the exhaust pipe. Whereas a detonation is a full release of energy inside the engine compartment in places where thats not supposed to occur.

And to answer you next question, no engines can be designed to operate from detonation of fuel as a more effecient method of extracting energy from gasoline. The working stresses and impact loads are too great.

Hope this is useful,
James Bufkin, P.E.
Austin TX
2001 A8L
1983 UrQ
1998 A6

Evilclown

I have a hard time explaining things sometimes.

beep-beep

Thanks, it makes sense for me.

Julien Cordon Caron

Dear all , i need help here , i am having the following problem : Temperature gauge goes up and down from 90 to 120 , completely randomly sometimes it doesn't happen and sometimes it does, no difference if i drive , or stand at a traffic light , or drive smooth or hit the road hard. My car is an aud s6 2007 model 5.2 V10 engine , radiators have been checked , flushed , and reconditioned perfectly, thermostat is brand new , temperature sensor is brand new , expansion tank is new too. Than problem occurred after i had a blown gasket due to a malfunction of my thermostat couple of month ago. Gasket has been replaced at that time but after 600 hundred km without any issue , i am facing this constant problem of the temperature gauge going crazy and for whatever reason it comes from 90 steady up to 120 in about 30 to 40 seconds and then it comes back down while im still driving,or if i stop the engine and then all the cycle again when i restart it, up and down up and down , sometimes more than 120 and sometimes less. The engine is not over hitting like a blown gasket or something i ve seen before , coolant is not boiling ,cooling system pipes are not hard and can be pressed bare hands. The system has been already bleed properly and filled back with a special vacuum to avoid any air bubble in the system. Cylinder head have been checked by pressure test and have shown a perfect health condition , no cracks no damages . Cylinders have been checked the same way and are also in perfect condition , there is no air lick and no crossover in the system from the cylinder to the cooling. As well please note that this problem has happen just after i had a severe overheating above 120 degrees due to the thermostat that was not working and which caused my fans to not properly cool my engine at a certain point and consequently overheated the full engine ( coolant boiling) After than incident we were not aware that the gasket was blown and we just changed the thermostat. At that point the car was doing that up and down with the temperature gauge , so we replaced it , ..result was nothing changed it was still doing it again, so we thought of a problem such as air in the cooling system, so we did the bleeding and again nothing changed. Result : same problem , so we opened the engine and found the blown gasket , we thought that it was the cause and we replaced it . Result after 600 km , the **** happened again . Temp Gauge is going up and down again . Do you guys have any clue of what the fk is happening to my car?


After reading your thread i decided to replace the injectors and see if it solves the problem and im quiet sure it will , as we completely dismantled the engine since, timing has been done at perfection, all new tensioners, everything checked so if anyone could just let me know if he think im going in the good direction by replacing all 10 injectors by brand new ones.

To be said my engine was shaking with the old injectors , we tried some other injectors from another s6 and my engine stopped shaking so now im kind of persuaded that with the new ones it will be allright but im still scared of the over heatting and i just wanted you to tell me if you guys think im going to finaly end this overhiting crap i was getting.

i would say yes but what do you guys think ?

Best regards .