Combustion Pressure Sensor - revisited

[Robert Harris]

The current interest in continuous combustion pressure sensing is driven IMHO
by two conflicting needs - to improve milage for CAFE and to reduce emissions
further.

As far as I understand it, there is a combustion "sweet spot" about 15 ATDC (
varies from 10 to 20 ) where depending on bore, stroke, pin offset, combustion
chamber design, fuel etc, placing the maximum pressure at will result in
maximum torque.   This is probably empirically determined by the manufacturer
after extensive dyno testing.

Combustion speed varies by load, temperature, rpm, mixture and other factors.
The purpose of all spark advance schemes is to quite simply - place the
maximum combustion pressure as close to this sweet spot as possible to
maximize both power and efficiency.  This is done by starting combustion early
"advance" such that as the pressure peaks some time later, its at the optimum
point.

All spark advance schemes work to compensate for change and bring the advance
to the point where peak pressure is over the sweet spot - some better than
others.  Vacuum advance adds a lot of advance to compensate for the radically
reduced part throttle flame speed.  Centrifugal advance compensates for the
reduced time available for combustion as rpm increases.  Static advance sets
up for idle baseline.  Boost retard takes advance out as pressure goes up
because higher pressure mixtures burn faster.  And the beat goes on.

Advancing to knock and retarding works well only when the knock advance is
near the optimum point for peak pressure.  At any point less than knock limit,
it will result in excess spark advance leading to higher than needed
combustion temperature, pressure and emissions.  Knock detection is however
critical to prevent engine destruction - and that is a MAJOR good thing.

All of these methods are still approximations - albeit getting much better
with the aid of electronic engine management.  Close but no emissions cigar.

If on the other hand, you control spark advance by directly measuring peak
combustion pressure, you are always tuned for maximum power ( torque ) from a
given amount of fuel.  This matching of pressure to sweet spot results in the
minimum pressure possible to maintain the required output.  This reduces the
heat produced, fuel consumed and minimizes the total emissions.

Think of hit like pushing a swing.  Increasing the effort at the wrong place
results in less movement than properly timing when to push.

Additional analysis of  the combustion pressure can reveal impending knock
prior to occurrence by the presence of certain characteristics of the pressure
rise.  If these sharply rising peaks are present, then knock is imminent.

By watching pressure in the lean region, it can also be used to run close to
lean limits.  This is done by monitoring pressure turndown and if the peak
begins to decrease, slightly richen the mixture.

Directly measuring the combustion pressure reduces the utility and criticality
of other measure measurements as it directly reads the desired result instead
of divining the entrails.

Certain year imports are rumored to have direct cylinder pressure sensors.
Wouldn't it be sweet to instead of compensating for to optimize changes on the
fly.  Conceive of each cylinder having optimum timing independent of all
others, each cylinders power exactly matching the others etc.  It will happen
as the combustion pressure sensor ( Trionic or otherwise ) moves mainstream.