turboholic wrote:Yes there is alot of debate over there on running water pre turbo. It's hard to argue with the results the guys are getting and they show pictures of their compressor wheels before and after and there is no damage. They have been running these setups for years.
They say it allows for better atomization.
Anyways. Interesting ideas.
Yeah, there really isn't any debate on whether or not pre-turbo injection is effective or not. However, it has to be implemented correctly to maximize effectiveness, as well as to prevent damage to the compressor wheel.
In order to produce the proper droplet size, a small jet, preferably with a narrow-angled dispersion pattern and high pressure (100+ PSI) is required to get the water as finely misted as possible, somewhere on the order of 40 microns or lower. The objective is not to introduce the majority of the water/meth injection at this point...only a small fraction of the total flow rate required needs to be introduced pre-turbo. The jet needs to be positioned close to the compressor inlet, where the fog can be directly aimed at the compressor wheel nut...by introducing the fog here, a higher proportion is consumed at a point on the compressor wheel with lower relative rotational speed. Also, care has to be made to not point the jet to where some of the fog could possibly make contact with the tubing wall and allow the fog to condense into droplets, which would then contact the compressor blade tips, where relative rotational speed is the highest.
Turbos normally operate under adiabatic compression, as a gas is compressed, it gets hot. When employing forced induction, especially in high boost scenarios, we need to get rid of the heated charge air to avoid detonation...in addition, it takes work for the turbine shaft to perform the heating, so we are losing efficiency.
Introduce the correct amount of water injection and compression begins to approach isothermal...we change the pressure-temperature profile inside the compressor wheel itself. The water droplets absorb the heat produce by the compression of the air and we can achieve increased mass flow at the same pressure ratio. The speed of sound of the air in the wheel would also be affected, resulting in a change in the mach number at the wheel tips...most likely altering the surge and the choke limits. As a result the process is more efficient and we can enjoy either:
- additional boost at an equal amount of exhaust flow
or
- less exhaust flow to produce an equal amount of boost
Water by itself has a very high latent heat of vaporisation, with a poor saturation partial pressure...only so much can evaporate before the air is at 100% relative humidity. Any additional water droplets in the air will not provide a cooling effect until compressed in the cylinder. But Dalton's law of partial pressures says that if you add a second fluid, ie. methanol, even though the air is saturated with water vapor, the methanol is unaware that water content is maxed and the methanol evaporates into the air as well.
Pre-turbo injection changes the operating point to the left on the compressor map. Because of this, you typically employ pre-turbo injection when we know we are starting to max out a turbo...we don't want to use it while on the left side of the map and operate under compressor surge. Also, we are only introducing a small amount of the total injection flow rate pre-turbo and only under higher boost scenarios, so condensation in the intercooler should be of little concern.
Bottom line is that with a proper mix ratio, you can cool the air to slightly below ambient temp AND increase the mass flow through the compressor essentially making it act like a bigger turbo than without pre-turbo injection. A win-win situation.