Date: Tue, 18 May 1999 15:16:47 -0400
Reply-To: David Beierl <synergx@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <synergx@IBM.NET>
Subject: Re: How do anti-ping additives work?
In-Reply-To: <3741A71D.B21E9787@cp.net>
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Big mouth steps in where angels fear to tread <g>.
At 10:45 5/18/99 -0700, Dave Bayer wrote:
>I know we used to have someone who worked for one of the
>big gas manufactures on the list... Maybe someone can help
>me with this...
Not me. I just play an expert on TV.
>I am trying to figure out if anti-ping additives merely slow
>the burn rate of the fuel-air mixture or actually just increase
>the charges' resistance to detonation (and if it does the latter,
>how does it accomplish this).
Increase resistance to detonation. People talk about the charge exploding
in an IC engine, but that's just shorthand. The burn actually takes
(guessing here) a couple of milliseconds or more; whereas detonation *is*
an explosion, with burn times in the microsecond range. This really puts
the two conditions in different neighborhoods.
>The reason I am asking is that I am trying to figure out why
>some engines with kncock sensors require "premium" gas. I am
>also trying to figure out if there is any truth to the
>statement that running regular gas in say an Audi engine would
>retard the timing and therefore reduce the power the engine
>puts out (the arguement was that since the timing was delayed,
>the enigne would lost power - which does not make sense to me
>because the lower octane gas will complete its burn quicker
>than the premium stuff anyway thus offseting the delayed
>spark).... Of course, that is my "logically" view of a system
>I know little about.
An engine turning at 6000 rpm will make 100 revs/second, or 10
milliseconds/rev, or 5 milliseconds/stroke. Each 10 degrees of ignition
advance or retard will alter the timing by just over a quarter of a
millisecond. I don't know offhand why retarding the spark will reduce
detonation, although I know it does; so I'll leave that alone. But
retarding the spark will certainly make the engine lose power, b/c there is
a certain amount of energy in a charge of fuel. Any energy not extracted
as mechanical energy is wasted as heat. If the optimum advance is say 25
degrees, retarding it will allow more of the energy from the charge to
escape into the exhaust, where it is wasted. In extreme cases this can
melt the exhaust manifold. So you're saving the piston tops at the expense
of the exhaust system and of economy, since all the wasted heat is really
money out of your pocket (but maybe not enough to offset the cost of the
premium gas, I don't know).
david
David Beierl - dbeierl@ibm.net