sohcpwr

sorry, im still with JD on this one.

baldur

Your call, but your opinion isn't worth a damn.

Tom-Guy

Neither is your knowledge of thermodynamics.

baldur

You're the one who were trying to argue that heat buildup wasn't a dominant factor in causing destructive detonation. And that engines have less heat buildup at peak power than they have at peak torque.
It's obvious you're not the one in the position to insult the thermodynamics knowledge of others.

USS

Show me one post where JD said that heat buildup wasn't a factor in detonation. His argument was that more heat occures at peak torque than higher rpm, which is correct. What do you think horsepower output is a factor of? Why do you think the desiel guys care so much about EGTs? It's because they spin at a lower rpm and make a ton of torque. Since every thermodynamic equation that I've ever seen involves time, this proves you wrong.

baldur

Diesels don't have knock.

sohcpwr

your a ------- idiot. Your wrong, you know your wrong, but your trying to prove yourself right. But since you cant get it through your head..... right from the physics book

work done by a gas (combustion) = - integral from Vi to Vf of PdV where P is the pressure, d is the distance the gas is compressed, and V is the volume of the compressed gas.

now, the power of detonation P = W/delta t (change in time)

that is all

baldur

Oh brother. Let me summarize the facts for you who obviously don't understand and therefore resort to insults.

- Peak cylinder pressure is the highest at peak torque.
- Peak torque usually also happens within the window where the motor has peak thermal efficiency.
- Thermal efficiency is dictated by compression and the time available to complete combustion becomes a factor at some point. Friction also plays a part in brake mean efficiency.
- Detonation is caused by the pressure/heat of the mixture reaching autoignition points while there's still something left to burn.

At low revs the cylinder pressures are higher and there is lots of time for the combustion to complete. Combustion speed is roughly the same so the mixture will have to be ignited much later than at high revs to bring the peak pressure point in the right place of the power stroke.

At high revs (peak power and above) the cylinder pressures are in theory lower, but consider this. The engine is doing a whole lot more work, and thermal efficiency has gone down a little bit.
Engines run on events, each event nets a certain amount of joules. There's a lot more events happening every second, a lot more fuel and air being burned and a lot more heat to be dissipated.
What's also interesting to note is that with a turbo engine exhaust back pressures are higher than at peak torque and overall volumetric efficiency is lower so there's more residual exhaust gas in the cylinder in the inlet stroke, furthermore increasing charge air temperatures in the cylinder.
Hotter parts are also more vulnerable when detonation does happen because the heat reduces their tensile strength.

Lets look at the thermodynamics again. There's double the amount of heat generated in the engine at maximum revs than at lower revs. Simply the number of cycles per second is a lot higher. Heat transfer area and material stays the same. Heat transfer increases because the differential temperature between the combustion chamber and the outside is higher. In other words, the combustion chamber has to run hotter to dissipate the extra heat.
The heat doesn't have the courtesy of just leaving out the exhaust without touching anything else in the engine, like Joseph Davis likes to think. Yes, the residual heat from each cycle spends less time inside the cylinder at high revs, but the duty cycle at which there is combustion heat inside the cylinder just goes up as the revs go up.

I've run engines under a steady load, full throttle at both peak torque rpm and at peak power rpm. I'm comfortable sustaining peak torque for longer periods than peak power. It takes a manly cooling system (and oncoming wind) to keep the engine happy at peak power, much more than it takes at peak torque.

Tom-Guy

*cough* pulse converter *cough*

You're going to have to point out where I said that. IIRC, I said the *majority* of the exhaust mass was unable to come in contact with the chamber to shed it's heat, I never said there wasn't thermal transfer taking place. The catch to high speed engine operation is that *there isn't as much thermal transfer taking place for the power level as thermal transfer @ naturally occuring tq peak would suggest*

You mean *aluminum* parts. Never seen a cast iron piston?

You might want to contact Bruce Plecan, he has some interesting factoids about fuel requirements and the nature of dynamic combustion across the powerband and at varying loads. I posted some of the early stuff up in one of Ron Chinoy's threads about ignition systems, and has since morphed into something strange and wonderful.

sohcpwr

This could probably go on for days, i give up.... let him believe what he wants