Maximum advance

[Bernd Felsche]

Eric Fahlgren tapped away at the keyboard with:

> Eric Fahlgren wrote:
> 
> > actually doing useful work (or so it seems to me).  The Q term
> > is interesting in and of itself, so I think I'll write a little
> > plotter that produces a bunch of curves for various rod/stroke
> > ratios (and maybe with bore offsets, too).

> See http://members.home.net/pfahlgren/cars/Qplot.jpg for some
> curves with the bore centerline directly over the crankshaft
> centerline.  I'll do some later with offset bores.

> Note that based purely on geometry terms, you should have the
> longest rods possible.  This is, of course, completely too
> simplistic in a real motor, where the P curve varies shape
> grossly depending on this geometry and ignition timing et cetera,
> but it is almost certainly the case that small L (short rods)
> should be avoided.

That's not necessarily true. Note on your graph that the short rod
lengths will impart a "harder bang" if peak pressure occurs early in
the cycle. In an extreme stratified charge, there's very little fuel
left to burn after initial burn so there would be little additional
pressure after the first 15 degrees at moderate engine speeds so the
additional mechanical advantage of the short rod near TDC may be
more valuable. Even in a homogeneous mix, the show is basically over
at about 75 degrees in terms of pressure build-up. The long-rod
assists in maximising the torque gained from residual pressure, at
the expense of losing maybe 10% of torque near TDC.

Without seeing the integral of the product of pressure and
mechanical advantage (i.e. gross torque), it's impossible to judge.

The long-rod engine will seem smoother, the short-rod one coarse.

> Now if I can only come up with a thermodynamic model for combustion,
> as Bernd suggests, then I can model timing and torque variations.

-- 
Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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