AFM measurement/backpressure (was Turbo speed sensor)

[Bernd Felsche]

Mike writes:
>At 12:01 AM 9/03/2000 +0800, you wrote:
[pressure tapping via tube]

>> 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.

>Doubt it - flow is effectively zero and the sensor need only be about
>300mm from the exhaust and behind the heat shield. I'm not thinking of
>using any forced cooling normal convective is fine. Once its in steady
>state the pressure at the sensor will follow the point at which the pipe
>enters the exh manfiold (I would exspect) quite quickly, actually 200mm
>behind heat shield would make it OK (torch into garage at 1am, trip over
>bin I left there - ouch),

So you're treating the cooler air as a static column - fair enough.

>>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.

>Not interested in pulsations (at moment) but the average pressure over a
>exhaust cycle (though any pulsations could be interesting, though I'm
>not sure of the b/w of the sensor, but IIRC its better then 1mS), I'd be

1 millisecond is typical on the MXP-type sensors according to the
spec sheets - for a step of 10% to 90%.

>plumbing the pipe into a point just before the turbo flange and the length
>of pipe to the sensor may very well dampen any fluctuations well major
>peaks anyway.

>>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

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

>mmmm <groan> seems a bit complex when it should be fairly predicatable for
>a given engine when the bulk of knock inducing parameters are known
>in advance, unless its a test engine with frequent alterations,

Other conditions vary; fuel quality and humidity are variables that
are typically not measured.

>>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...

>mmm But surely the AFM - even a slow one will point to the overall
>flow, cycle to cycle variations are interesting but I can't see
>how its information value is of immediate use given the engines
>unlikely to have instantaneous speed changes - hence we can use
>the s/w to pre-empt the knock condition.

The AFM will indicate an average of how much air has been drawn in
by the turbo-charger and pumped into the pressure-vessel commonly
known as the intake manifold which the turbo tries to maintain at a
nominal pressure.

The actual amount drawn/pumped into cylinder will depend on the
amount of EGR, the boost pressure, valve timing, etc, etc. . As the
backpressure can result in some (more) EGR during intake if the
exhaust valve is still open, there is a reduced volume in the
combustion chamber to be filled by the fresh mixture.

>Don't get me wrong, its an interesting proposition but, I seem to
>have got myself fixated on the pattern matching pre-emptive value
>in AFM, speed, advance, exh pressure etc. Especially as I hope to
>push my own setup a bit higher and not having a lot of engine
>instrumentation has me at a loss - exhast backpressure sounds like
>a useful qty to know - at least for experience - that and EGT.

EGT is significant; the energy in the recirculated exhaust gases 
will increase the possibility of knock as the total energy of the
mixture is higher than with all fresh mixture.

>>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).

>mmm OK got it, but why measure it when its known from the engine
>layout that its going to happen at combination of AFM, speed, etc

Just in case you encounter a load combination in practice which you
haven't already learnt about.

It's an _adaptive_ system. The complexity of implementing an
adaptive control is frequently unattractive due to the dealing with
a (comparatively-)large number of parameters and sensors.

If you can acquire sufficient information from the dynamics by other
means, then use those to adapt to the changing conditions. i.e. if
you can determine the amount of EGR from engine speed, backpressure,
EGT, boost, mainfold temperature and airflow, then that would be great.

>>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.

>Yes, does sound like a potential issue thats helpful, I need to think
>a little more how far I go on my own setup.

With hind-sight, working out the EGR (and its energy) is probably a
better way of determining the adjustment to avoid knock.

>I found out the other day, my block as 3 knock sensor locations along the
>length of the block though only the center one is used.

Mine has three - the boss between cylinders 1 and 2 is used.

-- 
Real Name: Bernd Felsche
    Email: nospam.bernie at perth.DIALix.com.au
     http://www.perth.dialix.com.au/~bernie - Private HP
----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org