Re: Octane Boosters - Methanol & MBTE

["Fred Munro" <munrof@isys.ca>]

-----Original Message-----
From: Robert Houk - WorkGroup Server Firmware <Robert.Houk@East.Sun.COM>
To: quattro@coimbra.ans.net <quattro@coimbra.ans.net>
Date: Thursday, July 09, 1998 11:17 AM
Subject: Re: Octane Boosters - Methanol & MBTE


>   > Another concept I remember reading about is water injection -- cools
>   > down the air/fuel mix . . .
>
>   It's more complex than that - water heated to steam through the same
>   temperature range that petrol/air goes through in combustion expands
>   about three times as much.  The military use water injection quite a
>   lot, but then they can afford to trash engines frequently.
>
>Yeah, I kinda remembered that too.
>
>The big question I have is: other than the immediate lowering of the
>air/charge temp before ignition, does adding water/vapor/steam affect
>the detonation characteristics (i.e., is it in effect an "octane
>booster")?
>
> -RDH

    From what I recall, the higher octane fuel actually burns more slowly
than low octane fuel. "Knock" occurs when the flame front propagates too
quickly and impacts the piston before it is capable of moving with the
expanding gas (or the fuel pre-ignites, but that's a different problem). The
octane boosters decrease the propagation speed of the flame front by making
the fuel burn more slowly. Water would have the same effect - the energy
absorbed by turning the water into steam would slow down the burn. I seem to
recall that the steam generation results in a increase in power, albeit a
small one.
    I researched the water injection issue in the late '70's when I built a
system for my Rover 2000TC, but I'm afraid a lot of the info has since
vanished in the mists of time. It did work well on the Rover engine which
was designed for leaded high test British petrol and not the low test
unleaded crap we were getting in Canada at the time. The water eliminated
detonation and smoothed out the engine nicely under load. It also cleaned
the carbon from the piston tops and plugs. I do recall that you had to be
careful how much water was added - overdoing could result in a blown head
gasket. It doesn't take much water to do the job - I was metering it into
the SU intakes through VW carb jets with a windshield washer pump.
    I haven't thought about water injection for years. I just happen to have
an Edelbrock water injection kit kicking around somewhere. Hmmm.... - might
be a nice fit with a boost increase........

Fred Munro
'91 200q  255k km