water injection questions

[Darrell Norquay]

At 09:38 AM 7/22/96 -0400, you wrote:

>>With Methanol, you have a more pronounced effect
>> (The equivalent Octane rating for Methanol is 105).
>> 
>> <snip>

The amount of methanol you would be injecting is really very small compared
to the amount of gasoline, (you generally use a gallon or so of mixture per
tankfull of gas) so the effect of methanol's 105 octane rating is minimal.
The main principle behind water injection is that the steam produced when
the gasoline burns in the cylinder slows down combustion, thus giving a
pseudo octane increase.

>I've always been told and under the impression that the octane ratings on
>gasoline (not sure how it compares with Methanol) were not ratings as to
how >slow the fuel burned, but a rating of its resistance to pre-mature burn
-- i.e. >ignition from combustion chamber heat while being compressed...
But here, you >are saying the octane determines the rate it burns ...  If
that were the case, >wouldn't increasing the octane w/ignition advance give
exactly the same results >as lowering the octane w/ignition retard? -- I
guess the point is to have a >repeatible mixture that is consistant (i.e.
not having pre-mature ignition on >some cycles and delayed ignition on
others)...

>Donald Whisnant

Octane is derived mainly from experiments done on a special engine (motor
octane) along with a chemical method (research octane).  The two are
averaged to get "pump octane".  While octane rating is not stictly a measure
of combustion speed, it is closely tied to it.  It all has to do with peak
cylinder pressures. The peak cylinder pressure is reached sooner with low
octane fuels, so with even a small amount of ignition advance, the peak
pressure may be reached before the cylinder reaches TDC, causing knock.
With a higher octane fuel, the mixture burns slower, so you can ignite it
sooner and still reach peak cylinder pressure just after TDC, which results
in an increase in power.  If you run less advance, the peak cylinder
pressure occurs too late in the cycle, wasting power and pushing unburned
fuel out the exhaust, because combustion has not completed before the
exhaust valve opens.



regards
dn
dnorquay at awinc.com