Volumetric Humidity at given Temp and Pressure?

[ssashton]

1. The problem

I need some help in a car tuning project I'm doing, please!

I have a turbo charged engine and I have fitted water injection to cool the air charge entering the engine. The idea being that cooler air is more dense and allows more fuel to burn.

I need help to calculate the maximum amount of water that can be evaporated into the air charge before exceeding 100% saturation. Clearly if I inject more water than can evaporate it will displace air in the engine cylinder.So in this case it is a 2,000cc engine, and without any water injection the intake air is at a pressure of 1.65Barg (relative to atmosphere) and a temperature of 130C. How much volume of water can be evaporated into the 2,000cc volume filled with air at these conditions?

The Attempt at a Solution

If I understand correctly then 100% relative humidity is the amount of water that can be evaporated in a set volume of air at a given temperature.

So I found a table showing that 100m³ of air at a temperature of 20^oC reaches 100% humidity with 1740cc of water.

So if we increase the air pressure by 1.65Bar, we should reach 100% humidity with (1740cc/1.65Bar) 1050cc of water.

An air volume correction factor for temperature can be applied. Every 10^oC increase in temperature increases the air volume by a factor of 1.04. However, volume is fixed here, so air density is decreased by the same. Air temp increases by 110^oC so (11 x 0.04 = 0.44) the water that can be evaporated should increase by 1.44. (1050cc x 1.44) 1510cc of water per 100m³ at 100% humidity.100m³ = 100,000L
100,000L / 2 = 50,000
1510cc / 50,000 = 0.0302cc of water can be evaporated in a 2,000cc volume of air at 1.65Barg and 130C.

Is this correct?! What do you think?

References
Relative Humidity of Air at given Temp - http://www.engineeringtoolbox.com/steam-humid-air-d_260.html

Air Volume Correction Factor at given Temp - http://www.engineeringtoolbox.com/air-temperature-volume-d_853.html

 

[KataKoniK]

Dear fellow scientist,

Thank you for reaching out for help with your car tuning project. After reviewing your calculations, I believe your approach is generally correct. However, there are a few things to keep in mind when calculating the maximum amount of water that can be evaporated into the air charge.

Firstly, the temperature and pressure conditions you provided (130C and 1.65Barg) are not the actual conditions inside the engine cylinder. The temperature and pressure will change as the air is compressed and heated by the turbocharger, and then cooled by the water injection. Therefore, it would be more accurate to use the intake air temperature and pressure after the turbocharger and before the water injection.

Secondly, your calculation assumes that the air is at 100% relative humidity at the given temperature and pressure. However, this is not always the case. Relative humidity is a measure of the amount of water vapor in the air compared to the maximum amount it can hold at that temperature. So, if the air is not at 100% relative humidity, you can still inject more water without reaching 100% saturation.

Lastly, the amount of water that can be evaporated will also depend on the size and efficiency of your water injection system. If your system can atomize the water into smaller droplets, it will increase the surface area of water exposed to the air, allowing for more efficient evaporation.

Overall, your approach is a good starting point, but to accurately determine the maximum amount of water that can be evaporated, you will need to consider the actual temperature and pressure conditions inside the engine and the efficiency of your water injection system.

I hope this helps and best of luck with your project!