[Mgs] Engine maths...and spare time

[Richard Lindsay]

   Max, as Paul says, finding the sweet spot is the key and it's a moving
target! Take a look at the attached scan from Campbell's book on sports car
engines. Notice please the pressure curve and how it doesn't really rise
much at all until well after the spark alights the charge on fire - perhaps
only doubling cylinder pressure by TDC. So although lighting off the charge
before TDC would appear to reduce power, the negative effect is minimal
when compared to placing the six-fold pressure increase where it can do the
most work.
   Paul, *et al. *have astutely pointed out the counter-productive and even
damaging effects of pre-ignition and 'pinking', or 'pinging' as we say here
in the States. Both of these phenomena are potentially damaging to the
engine. And their occurence is a whole other topic - if somewhat allied to
timing. Here's just a teaser and stated at the limits of my knowledge.
   Pre-ignition is as the word describes, when the charge is ignited before
ignition by the spark. That can happen in high compression petrol engines
by dieseling. That is, ignition by the heat of compressing the charge
alone, or by some 'hot spot' within the combustion space. E.g. the plug's
tip or a glowing carbon buildup. Pinking is believed to be caused by the
'end gas' flash-burning all at once rather than progressing smoothly. It's
rumored to have gained its name from the sound it makes. Again, I am
paraphrasing words from Campbell, not claiming intimate knowledge.
   Suffice to say, so many poorly understood things are happening in a
running engine, it's no wonder that most parameters are set, or at least
fine tuned, by testing rather than modeling. Repeating: "Practice always
works in theory, but theory doesn't always work in practice."

   Thank you everyone for your questions and answers. Fascinating topic.

Rick

On Sun, Apr 5, 2020, 5:19 AM PaulHunt73 via Mgs <mgs at autox.team.net> wrote:

> That's exactly what does happen, because ignition is anything from 10
> degrees upwards BTDC.  It's a trade-off between how much power you lose by
> too much of the fuel burning while the piston is still coming up - over
> advanced, against how much you lose with fuel still burning far into the
> expansion stroke - retarded.  There is a sweet-spot, but that continually
> changes with rpm and  throttle opening, which is why we have centrifugal
> and vacuum advance.  Centrifugal increases the advance as the rpm
> increases and the likelihood of detonation decreases, vacuum advance backs
> off the timing as the throttle is opened and more mixture goes into the
> cylinder giving higher cylinder pressures, which increases the likelihood
> of detonation.
>
> PaulH.
>
> ----- Original Message -----
>
> Well, it seems to me that if you advance the spark to a point on the
> compression stroke where the piston is still coming up, the expanding
> ignited mixture would be fighting the piston, and it would actually reduce
> power. I’m not sure if that is what you are talking about, though.
>
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