Is it better to have warm or cool air for an external combustion engine?

Submitted by Past-Loquat-4184 t3_10eux6v in askscience

I've heard that for internal combustion engines, cold air makes the engine more efficient because it has a higher oxygen density. From my understanding, this is because more oxygen allows more fuel to be burned per unit volume, providing more power with each stroke.

For an external combustion engine, is there any reason you would prefer cold air to hot air? As in, would air temperature affect the energy output of burning a fuel? Assuming 100% heat transfer from the fire to the boiler, can you make these engines more efficient by controlling the air temperature?

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BobbyP27 t1_j4uluok wrote

For external combustion, you want the air as hot as possible. The hot gas temperature after combustion depends on the air temperature and the heat released from the fuel. For the same fuel, hotter inlet air means hotter hot gases, or if you have a fixed target hot gas temperature, less fuel is needed.

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Deerescrewed t1_j4urxa4 wrote

Replying to [deleted] (#1,394,244)

Cold air is a killer of an external combustion system. You want to have the charge air pre heated already so you aren’t using the energy of the fuel to get the air up to combustion temperature and then heat the water.

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kilotesla t1_j4x8ruw wrote

Replying to BobbyP27 (#1,394,366)

This is specifically for the air supplying the combustion process. There's also a "cold side" of the heat engine, for example a steam condenser or a heat sink on a Stirling engine. If this is cooled by air, that air should be as cold as possible. (It might instead be cooled by water, in which case you want the water as cold as possible.)

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kilotesla t1_j4xb3jg wrote

The answer for internal combustion engines is a little more nuanced. The simplest answer is that for a race car engine, where you want the maximum power output with the throttle wide open, you get that with cold air because the cold air has higher density (at the same pressure), and so you can get more air into the cylinder and can burn more fuel. And the efficiency goes up because the fixed losses are a smaller percentage of the overall power.

For regular internal combustion used for transportation, you rarely have the throttle all the way open and are often running with the throttle way back from wide open, creating substantial throttling losses. Lots of work on improved engine efficiency is finding ways to mitigate that problem. Variable valve timing is one example. But another is actually to deliberately heat the intake air. The lower air density will reduce the amount of mass flow of air into the cylinder, enabling scaling back the power towards the lower level that's actually needed with the throttle open more, or ideally even wide open.

So warm air can result in better efficiency given a specific engine size and a power output requirement well below its capability, because it effectively degrades the engine power without degrading its efficiency. However, if you are just operating the engine at its maximum efficiency point, with a wide open throttle, perhaps because you are using it for something other than driving a car on public roads, you'll get better efficiency with cold air.

A little more detail and links to papers with a lot more detail are on this Wikipedia page:

https://en.wikipedia.org/wiki/Warm_air_intake

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