Date: Wed, 24 May 2006 08:56:01 -0400
From: Fred W <Malt_Houndnospamm-me-not*yahoo.com>
Subject: Re: Regular gas in a 9.5?


Bob wrote: > On Tue, 16 May 2006 22:19:45 -0400, Fred W > <Malt_Houndnospamm-me-not*yahoo.com> wrote: > > >>But if the reason the ignition is that the engine is pinging due to >>early combustion of the fast burning, low octane fuel, doesn't it make >>sense that the fuel will still be fully burned during the power stroke? > > > I'm certainly not an engineer - but my crude understanding is that we > advance the timing to start the burn earlier in the stroke and gain > more power, but that it requires higher octane in order to do this. > > If I am not misunderstanding your statement, it seems like you are > inferring that we advance the timing to compensate for higher octane > gas and that therefore we get the same power from lower octane and > retarded timing. If that was true, there would be no reason to have > higher octane gas - we'd just retard the timing on all cars. > > What you say makes a great deal of sense. I think the idea is that we want the majority of combustion to be occurring during the optimum part of the power stroke so that the angle of the crankshaft is as close to perpendicular to the cylinder as possible for best energy transfer from reciprocal to rotational. Obviously, it is not a binary situation. There is a ramp up and ramp down of the combustion and the expansion of gasses. To achieve this optimum angle at higher engine rpms requires igniting the mixture before the piston reaches TDC (advance). But with low octane fuel the mixture burns too rapidly and starts to expand before the piston reaches the top, causing pinging (preignition). The ECU retarding the ignition can reduce the preigintion, but that will not necesarily put the maximum combustion at the optimum crank angle. And then there is the whole valve timing thing... The other thing that does happen with low octane fuel is spontaneous combustion if/when the combustion chamber pressure and temperature are high. Higher compression engines make more power than lower compression ones (all other things being equal) and so the need for higher octane fuel. Boosting the intake pressure (supercharging or turbo charging) is the functional equivelent of increased compression (and increased displacement) and so requires higher octane fuels. -- -Fred W

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