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Name: Leon
Status: other
Grade: other
Location: TX
Country: USA
Date: N/A 

Does pre-heated air used during combustion improve gas mileage? if so, why it does it?

Hi Leon -

I think it might be bad for horsepower (opposite of using an intercooler) but a little bit good for mileage. Provided the engine is not igniting early or detonating, warmer air might help the mix combust more completely or uniformly, producing less soot or other under-burned gases which must then be burned off in the catalytic converter. In particular, gasoline can use some warmth to help more of it evaporate before the spark demands prompt and complete combustion. Also, if you are driving for mileage you are trying to use a fairly small fraction of your engine's peak horsepower, and warm (expanded) air might help each cylinder get a smaller charge without going to the most extreme vacuum or mix-leanness the engine can handle.

Of course I have no idea if the advantage is large enough to measure, and I am not even sure I am right.

Jim Swenson

If it is used in a modern computerized engine, it can increase mileage, but not because it has that affect it has on the combustion process. The computers used in modern engines sense inlet air temperature, and if the air coming in is very warm, the computer will lean out the mixture, thus using less gas.

Obviously, if the air coming in is too warm, and the engine runs far too lean as a result, engine damage can result. How warm is too warm is a function of the individual vehicle.

David Brandt

Hi Leon,

Heating the air being "inhaled" by an engine, will not improve fuel mileage, and may actually result in worse mileage in many cases. Heated air is less dense than cold air, and requires less fuel to be mixed with it for proper combustion.

With most fuel injected engines, the mass of the air being "inhaled" is measured, and the correct amount of fuel needed is injected. The volume of air an engine "inhales" is proportional to the engine's displacement, and its throttle opening, which means that if neither of these change, the volume of air "inhaled" is the same whether cold or hot. The problem is that a given volume of heated air has less mass. Less "inhaled" air mass requires less fuel, and produces less power. To get back your lost power, you have to open the throttle more (i.e. step on the gas pedal), and this results in increased fuel consumption.

If the engine has a carburettor instead of fuel injection, things get worse. Carburettors meter fuel to the engine based only on the air volume being "inhaled", not air mass. If the air is preheated, a carburettor will not correct for the reduce air mass in heated air, and the result will be an excessively "rich" fuel-air mixture (too much fuel for the amount of air mass being "inhaled"). The result will be not only less power, but significantly worse fuel mileage.

As a general rule, best engine performance results when an engine is "fed" cool air, and many car designers go to some trouble to have the engine draw its air from a cooler location under the hood.


Bob Wilson

No. The objective of combustion is to increase air pressure and volume by changing its temperature. If you start with hot air, you get less of it into the combustion cylinder because its pressure and volume have already been increased, and you have to heat it to a higher temperature to get the same temperature difference.

Tim Mooney

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