Re: Methanol corrosion
Water injection properly setup, slightly re-writes the rule book. Almost all internal combustion engines are knock limited for power. If you put a high enough octane fuel in them, and make the appropriate changes to take advantage of that octane you can substantially increase power output.
In typical street applications you only get small gains with WI because most people do not optimize the tune and engine build for the higher effective octane. In boosted applicatons you can use all that octane by bumping up the boost.
WI does several things many people are not aware of besides increase effective octane.
It reduces negative work performed on the compression stroke because the compression is more isothermal than adiabatic, ie the charge does not heat up as much under compression due to the high latent heat of evaporation of the water and alcohol injected. This returns some "free power" that other wise would have been wasted.
It lowers combustion temperatures and raises the effective octane of the fuel air charge. This gives two confilicting results, (1) -- your peak cylinder pressures drop a bit, but (2) the engine has less thermal load so it can now be pushed to levels of performance that would have melted valves or triggered runaway pre-ignition before.
It also has a little appreciated side benefit that following combustion there are more combustion products produced, this means cylinder pressure does not fall as quickly as the piston descends on the power stroke. Your effective power stroke is longer, so again another small incremental power increase.
It also slightly slows combustion time, so it moves the peak pressure time to later in the power stroke, depending on the engine this may be good for mechanical effeciency or require more ignition advance to get peak cylinder pressure to occur at the proper time.
The above is where most people end their tuning. If you use WI with a rich fuel mixture it will absolutely cost you power. To make power with WI you need to run fuel mixtures that would turn your pistons into molten slugs without the WI. Ignition timing, compression ratios and manifold pressures also need to be pushed into areas that most people would not even consider based on normal tuning experience.
For example when NACA was investigating WI for use in ADI systems (Anti- Detonation Injection) on WWII military piston engine aircraft here are some of the numbers they came up with.
Tests were run on a 6.65:1 compression ratio dual spark plug 4 valve engine.
The convention when these tests were run was to use water injection scaled to the fuel used. IE a 0.5:1 ADI rate ment you were injection .5 lb of water for each pound of fuel the engine was using. As a result water usage actually drops as you lean out the mixture. The also prefered to specify mixture in terms of fuel air ratio rather than the common air fuel ratio used in the automotive performance community.
Report E5E18
The following tests were run at a fuel air ratio of 0.08, inlet air pressure 50 in-hg absolute (about 10psi boost).
Peak imep psi
internal coolant water only
gasoline + 0 internal coolant/fuel ratio = 229 psi spark timing of 31.5 deg
gasoline +.2 internal coolant/fuel ratio = 241 at spark timing of 31.8 deg
gasoline +.4 internal coolant/fuel ratio = 250 at spark timing of 42.5 deg
Simply adding timing and increasing internal coolant rate was worth about 9% increase in imep. In effect you are creating a synthetic compression ratio by over advancing the spark.
Higher ratios were tried but did not return any additional power.
With a 50/50 mix of water and ethanol alcohol they got the following.
Gasoline + 0 internal coolant/fuel ratio = 229 psi at spark timing of 29.5
gasoline + .2 internal coolant/fuel ratio = 236 psi at 30 deg
gasoline + .4 internal coolant/fuel ratio = 233 psi at 31.5 deg
gasoline + .6 internal coolant/fuel ratio = 235 psi at 34 deg
gasoline + .8 internal coolant/fuel ratio = 237 psi at 37.6 deg
As you can see above the alcohol mix needs less timing to get to peak imep because of its faster burn speed, but in this case produces slightly less output.
Note that these tests were all run with the typical fuel air mixture you might see on any street engine, of 12.5:1 AFR, 0.08 Fuel air ratio.
Now lets look at what happens in a WI application if you lean the fuel air mixture out.
Data from NACA report 812
On gasoline only peak in imep was achieved at 2 different fuel air ratios.
lean max imep = 285 psi @ 0.05 FAR ( 20:1 AFR)
rich max imep = 290 psi @ 0.082 FAR ( 12.2:1 AFR)
with 100% water mix at coolant to fuel ratio of .5:1 we get:
lean max imep = 340 psi @ 0.06 FAR (16.66:1 FAR)
rich max imep = 360-365 psi @ 0.105 FAR ( 9.52:1 FAR)
on 70%/30% methanol water mix at coolant to fuel ratio of .5:1 we get:
lean max imep = 580 psi @ 0.044 FAR ( 22.7:1 FAR)
rich max imep = approx 460 psi from 0.075 - 0.09 FAR ( 13.3 - 11.11)
The engines running with internal coolants could run much higher manifold pressures to achieve these numbers. Knock limited manifold pressure for the 70/30 water alcohol mix went up to 150 in-hg (75psi), on pure water they got to the same manifold pressures at slightly leaner mixtures.
The maximium relative power ratio that they achieved in this test on 100% water internal coolant, was at a coolant to fuel ratio of 0.6:1 = 1.52, on 70/30 methanol water at a coolant to fuel injection ratio of 0.08:1 they got to 1.80 relative power ratio.
Most tuners would not even think of attempting these tuning numbers so this is, for the most part unexplored territitory outside the turbocharged imports, unlimited air racing, and tractor pull environments.
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