How accurate are these statements about Water injection?

[jt2ma71]

I have a book that was given by a good friend. I'm on the chapter about water injection and was wondering how accurate this is, especially the last couple sentences...

"When we inject water it can provide a number of benefits, depending on where our turbocharged engine has a deficiency. So far we have only mentioned that water helps when we are low on fuel octane. The manner in which it helps here is that during combustion the water is converted to steam. That conversion pulls heat out of the combustion process, slowing the rate of pressure rise in the combustion chamber. Additionally, these steam particles get in between fuel and oxygen molecules which again slows combustion and pressure rise rate. While this is going on the piston continues moving to TDC squeezing the gases tighter and tighter, but with less heat in the chamber and a slower pressure rise things don?t get out of hand to induce detonation. Then as the piston slowly begins to descend a controlled burn continues, but the combustion flame now has to duck around steam molecules plus an increasing number of exhaust gas molecules to get to fuel and oxygen molecules. As such, peak cylinder pressure will be reached at about 15 deg. after TDC. Any later than 18-20 degrees we lose power; earlier than 12-14 degrees we could precipitate detonation."

--Ron

[hotrod]

That's a reasonably accurate description. It's aimed at an unsophisticated audience ( i.e. little scientific background) so he is using some simplifications and easily understood explanations for a very complex process.

Yes water injection increases the effective octane.
Yes one of the ways it accomplishes that is by cooling the peak combustion temperatures.

No it does not significantly modify the fast burn portion of the combustion process, but its effects on initial cylinder temps and pressures, tends to slightly modify the early stage of combustion, and slows things down a bit. The end result is a slight delay in reaching peak cylinder pressure.

The ideal point for peak cylinder pressures varies some from engine to engine, but he is correct that excess early pressure rise before the piston is past about 11 deg ATDC is usually a contributing factor to detonation damage, as the piston is not able to quickly accelerate and relieve high pressures and capture the energy as useful work. If the peak pressure comes too soon the mechanical arrangement of the connecting rod and the crank shaft is very nearly in line, and most of the pressure is transferred directly to high loading in the crank and con rod bearings, and side thrust on the piston which causes high friction.

Short answer the book is accurate enough for a general understanding of the processes, with out being confusing. There is lots of highly technical info out there like the paper at :

http://www.waterinjection.info/

and the papers published by NACA and others that go into very great detail about how WI works.

Larry

[jt2ma71]

Thanks, so how do we make sure the later statements don't happen. Water injection, at least my aquamist is not crank timing based. Is it then basically just injecting the right amount of water. Thanks for the link.