Originally Posted by 18psi
I've not seen this done before.
However, here's my take on it: you want to prevent det/knock/heat BEFORE it takes place, not after. Just like with knock, you want to prevent it from happening, not react to it.
So if your EGT's shoot up, you're already hot and have to cool the engine off. I'd want to prevent it from getting hot in the 1st place.
From the FAA studies on aircraft, knock develops quickly, but it still takes hundreds of revolutions for it to go from warmer than typical to all out knock.
My thinking was more that the EGT rises in response to boost, more so with more boost. If that were all there were to it, boost linked injection would be as good as is possible. Making up some numbers to make it easier to explain:
- Lets say 950 F is the max EGT with boost disabled, on a hot day.
- Let's say 1125 F is detonation.
- Let's say typically the engine can go to 10 psi of boost with EGT at 1050 without water injection.
- Let's say the EGT that indicates the onset of pre-ignition is 1075 F measured at an EGT probe just at the turbine flange (This is the only number that didn't come out of my ***, it came out of Corky's
See where I'm going with this? We could set such a system to start injecting water at 1025 F, ramp up to maximum injection at say 1050 F, and turn on an indicator light while doing max injection above that. Since it is reading EGT and has an indicator light, we could even make it blink in anger at 1075 F if we wanted. Of course, the EGT wouldn't keep rising with boost, since boost would cause a rise that makes the system inject water which lower the EGT. The feedback could dramatically lower the amount of water consumed, but yet increase it at the critical moments that might normally blow the engine on a decent tune under bad conditions.
Alternately, the control could be set to a lower value to extract more HP, or higher to be a backup fail safe.
The effect of such a system would be to inject water in response to rising likelihood of detonation, rather than in response to rising boost, so it would inject less water at higher boost under cooler conditions and more water at lower pressure under hot conditions.
TL-DR: I'm basically saying it is the cyl temps we want to control. Generally, when boost rises, so do temps, but the starting point and rate of rise can differ depending on other factors. I'm saying the EGT is directly proportionate to cyl temps, which are directly proportionate to knock probability for a particular engine. So boost linked injection is like open loop, and EGT linked injection is like closed loop. With a smart program, closed loop is always better.