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[Guest Bob L]

Making a few reasonable simplifying assumptions, the temperature at the output of the compressor is a multiplier (which depends on pressure ratio and efficiency) times the temperature (in absolute terms) at the input of the compressor.

 

The latent heat of vaporization decreases the temperature of the intake charge by a rather fixed amount, assuming constant specific heat capacities (a good assumption). For a 12.5:1 AF ratio, the evaporation of the fuel causes a 25 degree K drop in the mixture temperature. If the compressor efficiency is 70% and the pressure ratio is 1.5, the output of the compressor is about 10 degrees © cooler if you add the fuel before the compressor.

 

There are at 3 additional benefits [1] for adding the fuel in front of the compressor:

 

1. the compressor is further from the surge limit

2. there is a more homogeneous mixture at the entry to the cylinders

3. the compressor work is reduced

 

The main issue with this approach is that it's harder to tune. A secondary issue is with the blow-off valve (BOV). For safety and emissions reasons, you can't vent a fuel-air mixture into the atmosphere, it needs to go back to the compressor input. This raises the intake charge temperature.

 

[1] Internal Combustion Engines, Richard Stone, 2nd ed.

[Guest gtmattz]

Wow.. Great explanation.. It seems a little safer to me now. I take it that at the low levels of boost that it is designed to run at, the blowoff problem wouldnt be much of an issue. I can see how tuning would be a fun job. I still like the idea of a twin turbo/intercooler/EFI system (thinking 2 of the turbos off the 280zxt) which is what I will do if i decide to go turbo.