To get back on topic I thought I'd made a post that represents my oppinions on WI tuning ...
General Tuning, and Making the Switch to Water Injection
Mixture for a Fixed Injection Rate:
If stuck wtih a fixed relative injection rate, I would go with a 70/30 methanol/water mixture at a rate of 50% of your fuel flow on an intercooled engine. I'd have it setup to switch on 2 psi lower than you first start to see knock at. This would put the overal water injection rate at 15% of your fuel flow, and methanol at 35% of your fuel flow. Other people may say much less is advisable. This is just my oppinion.
I like that number because it keeps the water level down relatively low, yet the alcohol level high. Water and alcohol will cool the charge, and alcohol will raise the blended octane of your fuel right when you need it most.
On 91 AKI octane, the blended octane with 30% methanol would be 98 AKI octane (RON + MON)/2. That means you have the benefits of near race fuel + water injection when your WI is active.
If your engine doesn't knock at all at any PSI on this mixture, try reducing to a 40% injection rate, then 30% and so on so that you only use just barely more than you need.
Ideal Mixture and Setup:
A
truly ideal WI setup would have a variable Meth/Water ratio. However, given the unnecessary complication of such a setup, I say that a 70meth/30water setup is a good place to start. The reason I chose this mixture is primarily that it allows brief trips into the severely lean section without knock.
My ideal controller would use zero WI whenever possible, and upon detection of light knocking begin adding WI while holding ignition timing constant if possible. (It'd need to pull timing for one or two rotations as WI cannot always be added quickly enough.) Then as the knock persisted continue to add more WI until a certain limit, at which point it would begin to lower boost pressure and slowly pull ignition timing until the knock abated.
That is essentially how my stock Saab's ECU works in the first place, with the exception that it first pulls timing, then adds fuel (interernal coolant), then slowly lowers boost as necessary. I think given enough programming skill, most ECU's could be altered to do the same. Instead of adding to the fuel curve, they just add WI from supplimentary injectors on the intake runners.
My Oppinion on Certain Encountered Problems:
Again I'm far from an expert, but these are just my thoughts to add to the discussion ...
Hesitation:
I think that when WI seems to be causing a loss in power this may be due to the over-quenching of the spark. Most cars use quench pads to create a faster burn. With WI this may be "blowing out" the flame Kernel.
I think one possible solution is to advance the timing several degrees. Remember that the quench pads only work near TDC, so the further you are from TDC the easier it should be to ignite the mixture. If timed properly the flame kernel should develop before quench takes place, then as the flame kernel is quenched the flame wave begins to propogate quickly. If you're experiencing hesitation, you're probably nowhere near the knock threshold, so advancing the timing a little to compensate for quench and a very low Flame Development Angle should be ok.
Another, slightly more obvious, solution is to lean the mixture. If you are running just straight water see how close you can get it to 14.7:1. As shown by the previous graphs, when running lots of water (IE enough to make th engine hesitate) 15:1 should be just as resistant to knock (or nearly) as 12:1.
If none of that works, either change the mixture to a mostly alcohol mixture with just a little water, or reduce the quantity of the mixture until there is no hesitation. (I think that should be a last resort, as clearly even a 50% mixture can work if setup right ... just perhaps not on all cars.)
Some Thermodyamics to Consider:
Since most engines are NOT like the engine in those graphs I posted, I thought I'd make list of things that can make
your engine different. (Aside from the name of your engine's manufacturer.)
1. Higher compression. The NACA engine was 7:1 compression. More compression means more heat, which means more cooling is needed. It also means it's harder to ignite the mixture, especially if you're injecting lots of water. The further away from TDC the charge is ignited the easy it will be to ignite, but too far advanced and cyllinder pressure's skyrocket.
2. Large Bore X Short Stroke. The NACA engine had a long stroke, and thus a relatively small bore. This means it's easier for the piston to dissipate heat as there is less distance for that heat to go to make it to the coolant through the cyllinder walls. Large bore engines may need more internal cooling.
3. Single Spark Plug. The NACA engine was a twin plug engine. But, because the plugs were both on one side of the combustion chamber, the burn rate was still somewhat sluggish. A DOHC or Hemi chamber should burn just as quickly, but with a single plug igniting a heavily water laden mixture is harder.
4. Computerized Spark Control. Some cars, like my newer Saab, have a pre-set spark voltage which is designed to be JUST enough to ignite the mixture at various boost levels. A high water content can bog down the car as it's not expecting to require that much ignition power at that boost level.
5. Quench Pads. As discussed earlier, these can blow out your spark if you have enough water. Quenching is greatest near TDC, so sometimes a very advanced ignition can compensate to some degree. High compression engines usually have a great deal more quenching, so high compression turbocharged engines may need a disproportionately large spark energy, or high ignition advance ... or a little less water. ::lol: (Easiest solution last!)
Anyway ... those are my thoughts for now. Any correlation to the real world is purely coincidental.
ops:
Adrian~