Heat Loss in Engine

[Shiv]

Hi everyone,

Is the heat transfer down the piston and cylinder head significant during the combustion process? I have read articles saying that it is, but isnt the ignition/combustion process so fast that the time for heat transfer by conduction is too short to occur ?

Thanks

Topic

[Mike]

In round figures, an internal combustion engine converts only about 25% of the available heat energy into work, with the remaining 75% of the heat being sent to the atmosphere, mostly by way of the exhaust and cooling systems. Suppose you have an engine which produces 250 horse power. What’s really happening is that you’re burning fuel at the rate of 1000 hp in order to harvest 250 hp.

If an engine retains more of the heat released by the fuel, the more work it can do for the same amount of fuel burned. There are limits, however. The first thermal limit (in gasoline engines) is the auto-ignition temperature of the fuel, where detonation occurs. The second limit is the temperature at which the lubricating oil stops lubricating and breaks down. And the third limit is the ability of the engine’s materials to withstand the heat load.

[Shiv]

Thanks for the reply. I suppose heat lost to the coolant happens when heat is transfered to the cylinder head, cylinder walls and down the piston?

[James]

The piston and cylinder head absorb a lot of the heat left over that didn’t go out the exhaust. Heat into the cylinder head is transferred to the coolant system mostly and heat into the piston is transferred to the cylinder wall via the piston rings. Engine oil carries away some of that heat and the rest is absorbed by the coolant system. The combustion process is fast but it’s happening a lot so you can almost consider the heat transfer to be a steady state process for a given RPM/load.

[Daniel Weith]

First apologies my intentions are not to hijack the thread, but after reading I have a question regarding the heat transfer from the cylinders.

As more modern engines are cast aluminum blocks versus cast iron would the percentage of heat to work still hold at 75:25?

Since aluminum is a better conductor of heat does the same ratio still apply, or are they able to burn a bit higher to increase the ratio to 70:30 and get more HP?

[Shiv]

Isnt the Expansion and compression process so fast that the opportunities for significant heat transfer is minimal? I suppose over time, heat will be lost.

Daniel – I am not sure about the thermal part. But I suppose the lightweight aluminum helps with the reciprocating mechanism.

[Author]

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