Toyota Supra twin turbo and water injection.

Greg Hermann bearbvd at sni.net
Mon Apr 5 05:07:03 GMT 1999


>Ok, so we know that H2O injection is good because is suppresses
>detonation allowing one to run higher effective compression ratios, and
>that it increases VE through charge cooling.  The question I have is if
>it increases power though increased cylinder pressures...

IF you can get the water atomized and into the cylinder and the intake
valve closed , without first evaporating the water (which would lose power
by displacing large amounts of oxygen)---------------------------------

1. It increases the net power by greatly reducing the work the engine must
give back to the compression stroke. (Makes the compression process more
nearly isothermal than adiabatic.)

2. It reduces the peak temperature in the chamber considerably, thus
increasing power and efficiency  by reducing the amount of heat rejected to
the water jackets substantially.

3. It reduces net power and cycle efficiency to a significant degree
because of the lowering of peak cycle temperature. (Even though cylinder
pressures are higher.)

The NET result if this is all done RIGHT can be a net gain, in both power
output AND bsfc, of about 8%. More if you take advantage of the ability to
run a higher effective compression ratio on a given fuel. Together with a
reduction of 350 to 400 degrees in EGT, and the improvement in reliability
which that implies.

Doing it right includes NOT using any more water than what will produce gas
at TDC which is saturated with respect to the partial pressure of water
vapor in it at whatever specific gas volume and temperature you end up with
at TDC!!! Any more water than this will just detract from power, bsfc, and
cycle efficiency by overcooling things.

With a turbo(s) you will need a somewhat higher pressure ratio across the
turbine (more backpressure) to get the same boost, because of the lower EGT
going into the turbine(s).

You will also need a smaller trim turbine, tighter A/R ratio housing
because, even though the exhaust gas MASS flow is a bit more, the lower
temp means it is significantly denser. But not proportionally smaller,
Because the lower EGT also means that Mach 1 for the exhaust gas is lower,
and a properly sized turbine nozzle usually is running at about Mach .7 or
.8 exhaust gas velocity. (Mach 1 for any given gas varies only with its
absolute temperature, not with its density!)

Have calculated all of this in quite a bit of detail, got the tee shirt,
eaten the Excedrin! Now, wanna build an engine to prove it!! Suggestions
for an ecu that can run FOUR injectors PER cylinder now  being accepted!

Regards, Greg
>





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