"Sweet Spot"

[Robert Harris]

Force applied is a function of ( (area x pressure )  x lever arm )

Picture a 1000 cc cylinder with a 100 cc chamber.  At top dead center this is
a 10 to 1 ratio.   

Moving the piston downwards a small amount will increase the volume of the
chamber from 100 cc to 200 cc - effectively halving the pressure.   If the
pressure was 1000 psi at TDC - it is now 500 psi - with no other changes.

Moving the piston down another 100 cc increases the volume to 300 cc or 1/3
what it was at TDC and a corresponding decrease in pressure.

The reason for the sweet spot being so near TDC is quite simply the result of
the pressure falling far more rapidly ( result of volume increasing and work
extraction ) than the lever arm is increasing.

Increasing the dwell time near the sweet spot results in disportionate
increases in power again because of the pressure area lever arm relationship.
Longer rods do this nicely.

Piston pin / offset bores increase power by effectively lengthening the lever
arm near TDC with minimal change toward BDC - thus increasing the power.

Any emphasis any - pressure applied before piston top dead center -  ( but not
necessarily crankshaft - think pin/bore offset ) is negative work - work
applied to OPPOSE the rotation motion of the engine.  With the pressure area
lever arm combination working to extract the greatest energy near TDC it does
not take much to see how a few extra degrees of advance can send the power
into the toilet - not to mention detonation etc.

Raising the speed of combustion - "fast burn"  not only reduces the amount of
negative work produced, but also delivers more pressure earlier in the cycle -
closer to the sweet spot, and thusly increasing power.

With regard to the "broad" insensitivity to MBT / MBP advance, we need only
visit data about combustion pressure.  Tests reveal that even with excellent
mixture control, other environmental controls, exacting spark etc, there will
be minor variances from cycle to cycle of pressure and when peak pressure
occurs.  This establishes sort of a small band where the peak pressure occurs.

Small changes to advance simply move the center of the band.  Larger changes
are easier to determine than small. Knowing this, getting the fuel and
ignition and EXHAUST consistent narrow this band of pressure variances and
allow more bettor control of peak pressure occupance and thus more effective
power.

Fuel that is consumed after the "sweet spot" is passed continue to increase
the pressure over motored pressure, but the increase is less effective at
translating into crankshaft power / torque than if it had been consumed
earlier.

>Date: Fri, 18 Jun 1999 10:38:50 -0700
>From: Eric Aos <EOA at spartek.com>
>Subject: RE: "Sweet Spot"
>
>> If you set up a series of tests such that the only thing that 
>> varied was the
>> spark advance,  and you moved it such that the pressure peak 
>> varied from 10 to
>> 20 degrees atdc
>
>Just imagine for a moment though if you could change that to 73 degrees
>(for a 350 Chevy 3.48 stroke, and 5.7 rod) this is where leverage is
>maximum (90 deg between rod, and crank throw). For a 228% increase in
>leverage...