Why are cooling systems necessary ?

[hypermecanix]

Hi,

I'm a student and right now we're working on designing a cylinder head for a truck engine. The problem is mainly to reduce the stress.
Stress in the material is produced by the pression in the combustion chamber, and by the thermal dilatation.
And the thermal stress is the worst. The higher the temperature gradient is, the higher the stress is. So here is my question : by setting cooling channels everywhere we make the gradient bigger and bigger (the low part of the cylinder head is at 600K, and the cooling fluid is 370K just at 15mm from the bottom), and thus the stress becomes unacceptable.
Why must we cool engines ?

[manchild]

Because engine parts expand under heat and if not cooled engine would get stuck, blow the gasket and/or bend the cylinder head and damage the internal parts etc.

[DaveKillens]

The process of combustion generates heat. If that heat was not removed from the critical head area, it would eventually reach the same temperature as the combustion process, and suffer catastrophic mechanical failure.

[hypermecanix]

Yes but there is no part that can burn. What's the true mechanical reason ? Why would something break ?

The cylinder head bending is increasing with cooling. That's what makes me ask the question.

[manchild]

You're wrong about burning. Everytning burns oh high temperature - pistons, valves... look at the pics






Sudden cooling like puting cold water in hot engine or boiled engine can bend cyl head but when the thermostat works fine including the rest of cooling system the head won't bend.

[hypermecanix]

Wow now i understand. The material just doesn't like heat.
Does someone know the pressure in the combustion chamber ? (For a standard car engine).

Thanks Manchild

[manchild]

np

Pressure depends on various factors like whether the engine has compressor or not, it also varies from car to car depending on compression ratio. Diesel engines create much greater pressure because their compressing ratio is usually double or at least for 1/2 greater than compression ratio on 4 stroke Otto engines.

You can search for some tech data of truck engines similar to ones from your task and use that data for calculations.

When engine cooling system fails than the friction between the moving parts increases which usually ends up in engine stalling, blocking, self-welding or burnouts which make holes in pistons or valves. Raise of temperature due to cooling system failure causes additional raise of temp due to high friction and lubrication system becomes useless.

[hypermecanix]

Thank you for the advice.
The pressure in my case is 25Mpa, and it creates a lot of stress.

[DaveKillens]

The temperature reached by combustion can easily exceed 2,000 F. An aluminum piston would melt. So would an aluminum head. Even iron would lose a lot of it's structural strength. As described in a previous post, oil would break down quickly, to eventually burn off.
That heat is carried off by the exhaust gasses, and the cooling system. It used to be common to see air cooled engines, but when they start to reach high power levels, it isn't sufficient. For instance, even the venerable Porsche flat 6 now has liquid cooled heads.

[manchild]

For instance, even the venerable Porsche flat 6 now has liquid cooled heads.

They sound like s**t compared to old air cooled engines

[DaveKillens]

Well, it's hard to make a flat six sound sexy. But it sure hauls good.

[Spencifer_Murphy]

I always wondered how on earth they managed to get an AIR COOLED flat six to cool properly.

Especially when its mounted in the back. I mean in the font...lots of nice cold air, in the back...no such luxuary!

[manchild]

Well, the big fan does the job. Air cooled had that great metalic sound while water cooled is a sissy.

[salilp]

Just one more thing, if u dont cool the engine, wont it affect the combustion process?

[MrT]

I have seen a manufactures engine data sheet (dyno results), peak pressure is approximatly 73 bar for a 1.5 4 cycle engine......... Although some times flash pressures of 90 bar are encountered......

73 Bar = 73 * 10^5 Pa

Mr T