Re: Plug Knock

Whoever <nobody@xxxxxxxxxxxx> writes:

> On Fri, 26 Aug 2005, Dennis wrote:
>
> > I remember reading in the Ford Motor Co magazine several years ago (last or
> > 2nd the last issue before they abandoned it) that the use of higher octane
> > gas was actually detrimental to a modern car engine. They remarked that the
> > knock sensor actually needed to sense a slight knock (which they stated was
> > not-harmful) and the use of high octane prevented the sensor from detecting
> > a knock, sending a signal to the computer that the engine was running too
> > rich. (Sorry for the non-technical terms as I'm writing this from memory and
> > don't know exactly what control the anti-knock sensor has over the PCM.)
> > Their summary was to use ONLY the gas octane recommend in the owners manual.

I'd love to read what was actually said, and to find out when it was
said. I'm having a really hard time following why somebody doing an
engine management computer would look at something other than the O2
sensor to adjust the mixture.

> I won't totally disagree with you, but rather make some
> comments. Knock can be caused by over-advanced timing and by other
> factors that can increase temperature: such as a lean mixture or
> over-retarded timing. Also, to remark on anther poster's question:
> knock takes place when there is spontaneous combustion. Hot spots can
> help this to happen -- and hot spots can be created by isolated pieces
> of carbon (by isoloated, I mean a spur of carbon which may not conduct
> heat into the block efficiently).
>
> In the days before catalytic converters became mandatory (later in
> Europe), engine designers were working on the "lean burn
> engine". However, this never yielded the necessary emissions
> performance and so the fuel/air mixtures in modern engines are largely
> determined by the chemistry of the cat.

Not quite. "Lean burn" was leaner than earlier carbs, but still not
especially lean (I put an O2 sensor on my '78 Newport, which has both
lean-burn and a catalytic convertor. There are conditions under which
it runs leaner than the ideal mixture, but not often). The mix in a
modern car is set as close as the computer can get to the ideal
stoichiometric ratio, which is what produces the least emissions.
It's tru that this gives the cat the least amount of emissions to
clean up (the curves are interesting to see: as the mixture gets
leaner, the HC output goes down nice and smoothly. When you get just
a smidge lean, the NOx suddenly skyrockets).

> Thus, I doubt that knock sensors were ever used to control
> mixtures. Their first use was, I think, to control turbo boost and
> later, to control timing.

My recollection is that it all happened pretty close to
simultaneously -- and roughly a decade after Volvo started using O2
sensors.
--
Joseph J. Pfeiffer, Jr., Ph.D. Phone -- (505) 646-1605
Department of Computer Science FAX -- (505) 646-1002
New Mexico State University http://www.cs.nmsu.edu/~pfeiffer
skype: jjpfeifferjr
.

Relevant Pages

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