The effect temperature/humidity has on power on turbo cars

[Thumper]

Ok I was driving the other night and it got cool at night, a good 15 degree drop, and of course my car picked up some power. Then as the night went on the temp stayed the same but humidity went up by 30% and power went down. So my question is there a formula for calculating how the temperature AND humidity can effect your power? It would be interesting to see how big of a factor humidity is. My guess is it will not be near the difference the temperature can cause but still significant.

 

Also is the only reason the humidity hurts power is because instead of getting 100% air you are getting water molecues mixed in so less amount of air in the same volume? Or is there other reasons why humidity effects power? Thanks

[TheNeedForZ]

humidity or water vapor is a flame front retardant too. It slows down the speed of combustion.

 

The other scenario I can think of is on carbureted engines or engines with electronic management system that is not equipped with oxygen sensor. In those systems, fuel is metered according to vacuum and airflow only. When there is water content in the air, the fuel setting becomes rich because the metering isn't adjusted according to the oxygen content in the intake charge.

[zguy36]

Think of it as free water injection and turn up the boost.

[z_cars_rule]

scary thing i have found is when the road it wet, or if its raining, i get a light spray of water on my FMIC, which ups the power a bit, and my tyres are wet, so less traction, evil combo.

[cygnusx1]

Humid air is less dense than dry air. Fuel injection systems and carburetors will compensate for less dense air and cut back fuel to maintain the correct mixture density.

 

Nothing beats a cold dry day in winter with a turbo Z....but like you said, traction gets worse.

[TheNeedForZ]

Humid air is less dense than dry air. Fuel injection systems and carburetors will compensate for less dense air and cut back fuel to maintain the correct mixture density.

 

Hmmm I am not sure about that. Imagine a scenario in which the air blowing thru the carb is 100% gaseous water. When the gaseous water flow thru a carb, will the carb compensate and provide zero amount of fuel? It won't; a certain amount of fuel will still be metered in response to the gas flow thru the venturi. The venturi only "feels" air flow, but does not distinguish the content of it. This is why carbs get "out of tune" so often, the fact is altitude and weather is "out of tune" more often. A EFI setup WITHOUT oxygen sensor suffers the same.

[veritech-z]

How is humid air less dense than dry air? You now have a liquid suspended between the molecules, filling up the spaces between them. Shouldn't that actually make the air MORE dense?

[mario_82_ZXT]

That's what they'd expect you to think

 

from here:

http://www.usatoday.com/weather/wdensity.htm

 

The water molecules, which replace nitrogen or oxygen, have a molecular weight of 18. (One oxygen atom with atomic weight of 16, and two hudrogen atoms each with atomic weight of 1). This is lighter than both nitrogen and oxygen. In other words, replacing nitrogen and oxygen with water vapor decreases the weight of the air in the cubic foot; that is, it's density decreases.

Wait a minute, you might say, "I know water's heavier than air." True, liquid water is heavier, or more dense, than air. But, the water that makes the air humid isn't liquid. It's water vapor, which is a gas that is lighter than nitrogen or oxygen. (Related: Understanding water in the atmosphere).

Compared to the differences made by temperature and air pressure, humidity has a small effect on the air's density. But, humid air is lighter than dry air at the same temperature and pressure.

 

Mario

[veritech-z]

I suppose in retrospect, that that is the reason the pressure suddenly drops when there is a storm coming? It still seems counter intuitive, but since that came from USA Today and not Wikipedia, it has a bit more credibility for me...However, is the water vapor "replacing" the N2 and O2, or merely filling in the blanks between those molecules which would otherwise be empty space? If it were replacing the air, you should be unable to breath in 100% humidity...

[cygnusx1]

Yah it is counterintuitive....but I learned it in engineering school a long time ago. It's all about molecular weight. Water has a lower molecular weight than air. Thus humid air is less dense. Lessdense=lessfuel=lesspower.

 

Humid AND Hot conditions both create less dense air. Cold and dry is ideal.

[Tony D]

Yes, that is the key: "MolWeight"

When designing flows and capacities in stationary multi-stage centrifugal compressors, humidity is a critical facto that has to be accounted for when determining flow delivered to the customer, as well as determining what size the condensate removal piping will have to be.

After the first stage of compression, all compressed air (short of refrigerating the air for desiccation) is 100% RH, and that is one of the reasons the first stage is usually disporportionately larger than the relative sizes between second and third stage, or subsequent stages of the machine.

 

Which leads into the next subject: Freon intercoolers....

 

if you have 100% RH at 9-10 degrees F above ambient from your A/A interfooler, just imagine the density boost you would get by desiccating the air in a freon intercooler and dropping the inlet temperature down to 35F.

 

For a short run drag car (er, or Bonneville Car) a battery powered electric compressor would make a big difference in power. And even with a tap off the vehicle's A/C system, the net gain in power would more than offset the drag of an intermittent compressor operating drag...

 

But I digress...

 

Water displaces air that could be there. And the hotter the air, the more water it can hold. The amount of water that air hold jumps dramatically from 85F to 104F, it's a factor of what can be held at a lower temperature, and for all intents and purposes, if the air is above 120F the amount of water it can entrain as water vapor is jumped by another factor!

 

In many cases, with multi-stage stationary compressors that are air cooled (interstage cooler), design will dictate savings by deleting condensate removal on the first stage of cooling, simply by desinging the second stage inlet to operate at <120F, and spinning the second stage slightly faster to make up for the less dense air.

[TheNeedForZ]

Tony D,

 

Now that's some good stuff you don't hear everyday In fact just the other day I was wondering "What if AC is used to cool the intercooler...? Has any one tried that before?" A couple days later I got a book(by David Vizard) about nitrous and I realised that whomever wants more power from denser intake charge is probably just going to run liquid wet shot nitrous....chemical supercharging/cooling and there is no limit to the amount of power nos can make except the strength of the block. The AC cooled intercooler is still interesting because unlike nitrous, it's never spent or going to blow up

[cygnusx1]

Hmm. So if I run my A/C backwards, my evaporator under the dash will get hot and the condenser out front near the intercooler will get cold and cool off the intake charge....off to find a Mobius Strip belt to run my AC compressor backwards..... Just Kidding of course. LOL.

[(goldfish)]

I liked the idea of using the a/c to cool a water-air IC.

 

But back to the program, the density of the air can side track you. What is important is how many oxygen molecules are in every liter of air your engine breathes. That water vapor is taking up some of that room, i would think.