AFM measurement/backpressure (was Turbo speed sensor)

[Bernd Felsche]

Mike writes:
>At 08:15 PM 7/03/2000 +0100, you wrote:
>>What I tryed to say was that the AFM cant give info for the retard of
>>ignition advance that the backpressure is indicating.

>OK - I understand now, the backpressure affects ignition timing but the
>AFM still manages to report actual air flow - over time as it were.

>I suppose in those conditions a simple pressure sensor off the exhaust
>before the turbo would be OK.

>ie Short (cooled) piece of pipe to carefully mounted sensor.

Most pressure sensors will handle only about 180C. Cooling the pipe
reduces the pressure as far as I know; you need to know the amount
of cooling (temperature sensors?) to determine the actual
backpressure.

That's a bit more number crunching than you might want to do. Even
then, the pressure you determine will be the pressure at some time
in the past - that might be as long as 10 milliseconds ago. You
almost certainly will not see the pulsations resulting from the
opening and closing of valves.

>The speed wouldn't be much of an issue for this reason. Once its installed
>and calibrated then the conditions which lead to a immediate ignition
>retard can be 'pre-emptive' ie. a differentiator off the pressure sensor
>could initiate an interrupt for immediate retard by a set number of
>degrees - or even better, the differentiator sets the interrupt pulse
>width depending on rate of 'upward' change of pressure in exh before turbo.

The condition which is being avoided is that of knock. A knock
sensor is apparently out of the question to aid in learning the
engine's response under a range of conditions. It is theoretically
possible that a single knock event can destroy a fragile engine.
However, you don't need to run the engine to severe knock
conditions, and once you've learnt the response, you can use that to
avoid the knock condition by a safe, but minimal margin.

An alternative may be to build pressure sensors into the head of the
engine; those devices must exist (presumably you could buy them if
you had the money). Another option may be to monitor the speed of
deflection of the crankshaft - the knock pressure will cause the
angular velocity to exhibit a characteristic "jitter". You won't be
able to measure that easily at the flywheel end - the inertia will
"squelch" the jitter, but a measurement at the other end could be
worthwhile if there are no inertial loads there.

A PLL-type logic could be used to detect the jitter from a
"crank-angle" sensor if the resolution were high enough.

Of course, that's an empirical approach...

The exhaust backpressure determines the internal exhaust gas
recirculation and hence the remaining displacement to be filled with
fresh mixture. That's the air-fuel ratio - remembering that the two
differ by about 400C (if not more) in temperature, making for a more
volatile mixture (hence the need to retard with more EGR).

At high speed, we can consider the two gas mixtures to be
stratified; they won't change significantly in volume, nor will they
mix to a significant degree.

We can measure "exactly" how much fresh mixture enters the cylinder
by measuring the flow through the inlet tract, but only if the
exhaust valve has already closed. What we need for that is a
high-speed differential pressure sensor and two pressure taps ahead
of the inlet valve. The inlet temperature is fairly constant and we
should already know what this is for calculating the amount of fuel
to inject...

If there's an exhaust valve overlap with inlet, and we don't know
the exhaust backpressure, then we must measure if the overlap causes
exhaust to be drawn back into the cylinder, or if a significant
portion of the fresh mixture is being "scavenged" into the exhaust.
Enter your trusty O2 sensor. You need it to react fast enough to
detect a rise in O2 after the exhaust valve to indicate scavenging,
or to remain "level" indicating only exhaust gases downstream of the
exhaust valve (obtaining such a sensor's a problem for higher engine
speeds).

	Even knowing the mean exhaust backpressure wouldn't be
	sufficient on it's own to determine the degree of EGR. Flow
	dynamics of the exhaust valve closing will often result in a
	disproprotionately-high exhaust flow for the closing valve
	as the columns of exhaust gas "sucks" harder through the
	diminishing opening.

Any exhaust gases actually drawn back will add to the base EGR of
the engine and will be "cancelled out" by a reduced fresh mixture
intake. We can still calculate the AFR based on our knowledge of
volumetric efficiency of the engine.

Prior testing with the engine will indicate appropriate ignition
timing to avoid knock at a particular AFR for a specific load/boost.

-- 
Real Name: Bernd Felsche
    Email: nospam.bernie at perth.DIALix.com.au
     http://www.perth.dialix.com.au/~bernie - Private HP
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