Relative Humidity via Intake

[NSXLNT]

Does the relative humidity of air drawn into an engine affect performance in any way? Does
the car's ECU compensate? Does dense, cool, dry air combust more efficiently than dense, cool moist air? Just curious,Thanks.

 

[Lud]

The only thing in the air that produces power is oxygen. The more oxygen contained in a fixed volume of air, the more power you get.

Temperature, humidity, altitude, barometric pressure all affect HP because they all affect the amount of oxygen in a fixed volume of air.

Higher temperature means the air is less dense. Less dense means less oxygen in a fixed volume.

Humidity is just water vapor in the air. More water vapor means less of everything else, including oxygen. It is actually better to use dewpoint instead of relative humidity for the calculation since dewpoint does not change as radically through the day as relative humidity typically does.

Increased altitude means less dense air. Less dense air means less oxygen in a fixed volume.

Lower barometric pressure by definition means less dense air. We all know what that means!

As an example, at 85 deg F, 30.14 in-Hg barometer reading, 58 deg F dewpoint and 5000 ft altitude, the engine only produces about 81.1% of the rated horsepower.

There is a very dense paper on the subject; SAE J1349 Revision JUN90.

[This message has been edited by Lud (edited 28 August 2001).]

 

[NSXLNT]

Lud, thanks for your answers, here and to my other question. Everyone's answers have helped. I did know that the Oxygen in the air is the only thing that burns with the fuel. I would assume that moist air would be harder for an engine to utilize, not only because of less room for Oxygen, but because water vapor is harder for the engine to compress through the cylinders. I wonder if it's possible to provide some sort of de-humidifier on the intake side to dry out incoming air, while at the same time cooling it off. Is that what an intercooler on a supercharger/turbo would do?

 

[Lud]

It would take far more energy to dehumidify or cool ambient air at the consumption rate of the engine than you would gain by doing so.

Intercoolers dissipate the (considerable) additional heat generated by compressing the air in a forced induction system. Since they use ambient air as their exchange medium (either directly in an air-air setup or indirectly in an air-water setup), they wouldn't help at all on a naturally aspirated engine.

[This message has been edited by Lud (edited 30 August 2001).]