water into efi system before injectors.

[Bernd Felsche]

Greg Hermann writes:

>>so, without getting  into details, a normally aspirated high compression
>>motor could use this to prevent knock by lowering combustion temperatures.
>>Now could this be knock sensor triggered? would the H2O anti knock rob
>>more.or less. power than retarding the spark? (rough numbers).

>Water injection won't rob any power if you keep the water quantity from
>going too high.

[It's only a small factor, but...]

The humidity of the intake air is highly variable. Anywhere from
3g/kg (water to air) to 40g/kg i.e. (from 200:1 to 25:1 AWR).
Measuring the humidity isn't done so easily, nor rapidly.

One could saturate the intake air with water vapour bringing the RH
(relative humidity) to near 100%; that way you know how much water
vapour is present and how much more you want inject as liquid for
in-chamber cooling. A pre-filter "misting-chamber" using an
air-shrouded, or air-venturi injector could be used - excess water
would condense and drain off before getting past the filter.

The gasoline FAQ indicates a 4:1 relationship (approximately)
between water _vapour_ and octane number requirement (ONR).
i.e. 4g/kg water vapour results in a reduction of 1 point in ONR
That makes a large difference in practice on knock-sensitive
engines.

>From that, it would frequently be sufficient to just use the
misting-chamber approach - especially for NA engines. The extra cost
of high-pressure water injection could be avoided.

But if money is no object, do both. :-)

As to determining how much water can be injected, one could consult
steam tables, etc and determine from the amount of energy, the
maximum amount of water which can be evaporated during compression.
You get a small margin of "safety" due to the heat stored in the
walls of the combustion chamber from the previous cycle. Don't rely
on it too much or the thermal cycling could become a problem.

There are apparently a large number of SAE papers on the subject of
water injection. I've read none, so don't ask me which one is "best"! :-)

-- 
Real Name: Bernd Felsche
    Email: nospam.bernie at perth.DIALix.com.au
     http://www.perth.dialix.com.au/~bernie - Private HP
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