Really long water injection post, $0.04 worth

Edward C. Hernandez ehernan3 at ford.com
Tue Sep 3 16:34:55 GMT 1996


>...an article involving water injection to reduce detonation.  My >question is: What happened to this wonderful idea?  I saw an ad toward >the back of the magazine in which Edelbrock that was selling their >Vari-Injection

I've been running my Vari-Jection unit for 6 years without problems.

> If you forget to keep the tank filled and you don't have sophisticated >controls to either tell you of this or to shut the advance curve down >to stock when there is no injection taking place, you will cause >yourself a problem....

I just include this in my list of fluid levels to check, like how much
gas do I have...

>Octane rating has *nothing* to do with how fast the fuel burns.  An
>octane rating is a measure of an engine's resistance to detonation...

No, octane rating measures a FUEL's resistance to detonation.

>Flame front speed is very similar for all gasolines regardless of >octane rating.

That depends on what you call similar. Different octane fuels do, in
fact, burn at different rates. How important that is depnds on how well
calibrated the seat of your pants is or how important that last 0.01 in
the 1/4 mile is.

>Water does not slow the combustion process.  Its primary function is to
>reduce the intake charge and combustion chamber temperatures, which >help the engine avoid detonation.

No, it's primary function is to asborb some of the heat of combustion,
reducing peak in-cyulinder temperatures, which improves the knock limit.
Driving a phase change of water(ie, liquid to steam) takes a lot of
energy. That kind of energy isn't available in the intake tracts of a
normally aspirated engine. There's more in a turbo. I think I agree
about the flame speed not being affected.

>To my knowledge, knocking is not a fast burn. The burning
>process takes some time (for the flame front to travel accross the
>cylinder). As this burning is happening the pressure in the cylinder
>rises dramatically. The pressure is also rising (BTDC) due to the
>piston compressing the mixture. If these two pressure rises combine
>(due to over-advancing the timing), the resulting pressure = internal
>energy of the gases may exceeded the activation energy of the
>combustion reaction. This energy will be present at all points in the
>unburnt gas, so all molecules of unburnt gas are likely to react, and
>when the do the pressure rises very suddenly (around TDC) with shock
>damage resulting to your precious lump o' steel.

Excellent and accurate explanation! But...

>Knock resistance is a measure of the activation
>energy required to combust (react) the mixture. Slowing the
>burn rate can be achieved a number of ways, but its net effect is
>that the peak unburnt gas energy density is lower because the
>actual peak will occur when the piston is somewhat past TDC.
>Knocking is not burning, so knock resistance cannot be a measure of
>burn rate.

I disagree. The best analogy I can think of is an old fashioned bomb
fuse burning its way to a big black bomb. Think of the bomb as the knock
limit. If it goes off, you've ignited the end gas as explained above: it
explodes al at once. The fuse represents the parameters that control the
knock limit(pressure, temperature and TIME). Your goal is to burn the
mixture completely before the fuse reaches the bomb. Anything you do to
increase pressure or temperature shortens the fuse, and the converse is
true. Time is defintely a variable, which means that the knock limit is
related to burn rate. Therefore, another approach is to outrun the fuse,
which fast burn combustion chambers can do. Anyone notice how high the
compression ratios of modern 4V engines (and even some 2V) are getting?
 

Ed Hernandez, still catching up...
Ford Motor Company
ehernan3 at ed8719.pto.ford.com



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