Max Advance

[Bernd Felsche]

Eric Fahlgren tapped away at the keyboard with:
> Roy wrote:
> 
> > Basically the time a cylinder needs for complete
> > combustion of the fuel is constant, you want max
> > pressure developed on the piston (BMEP) just after TDC
> > and therefore as the engine speed increases you need
> > to start combustion earlier so as this can be
> > achieved.  This is a very simple explanation of why
> > you need to use ignition advance.
> 
> Well, it's not quite so simple.  What you really want to do is
> maximize the integral of the PQ curve (see my mail from 10 minutes
> ago), however you can.  After running a couple of thought simulations
> using the old meat computer, I think this means that you _probably_
> want the pressure in the cylinder at maximum when the rod-to-crank
> angle is about 90d for engines with big L (rod/stroke ratio), but
> probably a little earlier in the stroke as L gets smaller.
> 
> (You can tell by all the "probablys" and "I thinks" that I'm
> sure about this. :)

I'm sure about that! :-)

Long rod ratios wouldn't be as sensitive to timing to obtain optimum
torque as the angles are closer to "right" for more of the time.

This is given by the tangential crank force

	Ft = Fg sin(a + b)/cos(b)
  Fg being the gas force

cos(b) is larger for longer rod lengths (the angle of inclination is
smaller) so that factor remains closer to 1.

But because the torque is a result of the integral of the product of
two factors over the stroke, the relationship is quite complex.
Putting the pressure peak well before those 90d allows the product
of the reducing pressure and the Q factor, later in the combustion
process, to be more effective. Another side-effect is that the
earlier you ignite (within reason), the higher your pressure peak.

Near the 90d position, piston velocity is also near maximum, so the
pressure is levelling off at higher engine speeds as the piston
speed approaches and maybe exceeds that of the flame front.

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