Relative Humidity effect on HP

ginsu · Post 26 Mar 2006, 01:36

I am wondering what effect Relative Humidity has on engine power.

I found this quote on a Thermodynamics site:

Quote:

The performance of an internal combustion engine depends on the amount of air or, more precisely, the amount of oxygen, which can be burned in one engine cycle. This amount can be expressed as a function of the density of air, which is depending on atmospherical conditions. The density specifies the mass of air, which is contained in a certain volume. Because the atmosphere is a compressible gas, pressure and temperature have a strong influence on its density. Additionally, the amount of oxygen is reduced, when the humidity is increased.

Would this confirm that a high RH would put the engine down on power? Despite the hot temp. of Malaysia, does the high RH cause the engine to ingest less oxygen, thus not burning as hot, therefore loosing power?

If so, is there a rough index of how much? Say 10hp, for every 10% increase in the RH. Of course, temp would have to be held fixed.

DaveKillens · Post 26 Mar 2006, 02:01

Relative humidity definitely does have an affect on the power levels of an angine. But it's not a huge variation it definitely is there.
http://www.findarticles.com/p/articles/ ... i_n9240601

Sodder · Post 26 Mar 2006, 03:18

Theres a backwoods, country saying at the race tracks around here....Its hard to make fire when theres water in the air.

It makes sense to me... The dryer the air is the more "pop" an engineer would be able to get from each combustion.

DaveKillens · Post 26 Mar 2006, 04:58

Well, the more oxygen you get, the better. But moisture in the air displaces it, so that you get less air, and thus less oxygen per intake. But moisture has a cooling effect, so that it's beneficial too. One trick for superchargers is to spray water inside the intake, to cool it before it enters the combustion chamber. So having water inside the intake isn't that much a bad thing, it just displaces that precious oxygen, one of the essential elements in producing power.

ginsu · Post 26 Mar 2006, 05:45

That's a good point, but it seems it only applies to Super/TurboCharging. I have only seen reference to water injection systems with forced induction.

It seems to me that relative humidity works a bit differently, especially since it is not injected and cannot transfer heat from the air. Besides correction factors for power output (with associated equations), I have found very little information on this subject of RH vs Power.

DaveKillens · Post 26 Mar 2006, 07:34

The ambient air that the engine inhales has three properties that we have to deal with. All three relate directly to the important factor that you want as much air (and thus oxygen) per intake stroke. There is air pressure, temperature, and humidity. If the air pressure is low, then you have less air per cubic meter, less oxygen. More humidity, water is displacing the air, less oxygen. Higher air temperature makes the air less dense, and of course, less oxygen.

wowf1 · Post 26 Mar 2006, 16:14

I think it's true to say that intake mixture temperatures won't be affected by humidity. The air & water in the atmosphere will be at similar temperatures I would assume, so the water in the atmosphere would not exhibit any cooling effects.

However, once the mixture enters the combustion chamber and combustion occurs, it's clear that the water will not burn. And if we say that the temperature of the water vapour as it enters the engine is eg. 35deg C, then the water vapour will take some heat out of the combustion process. Could this decrease the possibility of detonation (knock)? I wonder what other effects water has during the combustion process.

rob

ginsu · Post 27 Mar 2006, 01:16

I'm wondering about that too. It seems if it does reduce combustion temperatures then you can run a higher compression ratio in humid weather, especially to gain back any lost power from lower volume of oxygen in the air. I wonder if any F1 teams have experimented with this.

Also, you could effectively increase the boost pressure on a turbocharged car if the weather were very humid. I wonder if there are any forced-induction road cars that take advantage of this? It may be very marginal. I guess I'm really wondering if it's a marginal effect or not.

wowf1 · Post 27 Mar 2006, 12:08

I'm not sure you would increase the compression ratio. I suspect the components in an engine are engineered to withstand the stress at a certain maximum compression ratio, and not really above that ideal ratio.

I would expect that you could advance the ignition timing though, without the combustion turning to detonation. I guess if you could have an ECU with a humidity sensor attached, then you could run a more complex engine map that took humidiy into account.