Quote:
Originally Posted by Daox
I don't think you'd ever want nearly that much water injected into the engine. In fact, I don't think you want to inject much at all. If the point is to create steam from the water so the expansion of the steam aids in powering the engine, you want to inject already hot water (easy enough, just use coolant to heat). But, you don't want to inject a ton because the phase change takes quite a bit of heat energy to complete. You might end up just cooling down the combustion, not turning much water to steam, and thus loosing power and decreasing efficiency.
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I agree, and with what you said earlier. Water vapor isn't combustible and so acts as a buffer in the chamber, which slows the burn speed, or flame propagation as some call it. This is generally bad, especially during times of loading when power is needed. The system would have to be programmable to coincide water delivery with the proper conditions.
Liquid water would necessarily need to be introduced into the intake manifold in the presence of low pressure to ensure that it is vaporized. You would need a pressure switch to cut delivery at a certain pressure level in the manifold so water would not pool there. A sink at the bottom of the intake manifold would also be a good idea to protect the engine.
The main advantage of water injection is that it allows you to take advantage of much higher than normal cylinder pressures. You see it used in boosted applications, but you could also use it with high static compression ratios as an anti-detonate. Using water injection in combination with a high static compression, and low octane fuel is a winning combination for FE. This has been proven by the work of David Vizard in the 1970's, (book: "Performance with Economy").
As was stated, injecting liquid water into the chamber will absorb a significant amount of combustion heat, which will reduce power as well as slow the flame travel. Ideally you'd want vaporization to take place by the front side of the intake manifold (throttle area), in order to ensure good distribution, and to take advantage of increased mass in the intake ports/runners.
I have done very little with water/steam injection myself.
I have noted differences in power, FE, and detonation/knock (or misfire) threshold with changes in relative humidity.
As for FE, the higher humidity is bad at times when you need the power, but can be good when little power is needed (driving condition dependent).
A car running stoich (closed-loop), but with the maximum possible spark advance can see a gain in FE due to increased humidity. That's not to say that you couldn't get the same FE gain or more by improving the tune without the humidity.
I've also noted that it's common to see a FE improvement while doing test loops in the rain. I'm pretty positive that the gain comes from reduced rolling resistance rather than the increased water in the air. The rain usually cools the air, and actually reduces the relative humidity in a lot of cases.
If you make a water system and test it thoroughly I'd love to see the results.