AFM measurement/backpressure (was Turbo speed sensor)

[Bernd Felsche]

nacelp writes:

>> 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.

>If your only talking  aout one of the acoustic trash sensors.  Try working
>out an ionization type, and I'd bet your "idea" would change.

Not my idea, nor my engine.

> It is theoretically
>> possible that a single knock event can destroy a fragile engine.

>??, where did you hear that?.  That is in the realm of not knowing what your
>doing, tune up wise.  Theory based on what?

Theory based on the knowledge of pressures under knock.  If we
didn't make our engines so conservatively, the _fragile_ engine
which is just strong enough (with only a small factor of safety) to
handle only nominal combustion conditions would fail on the first
severe knock event.

>> 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.

>Sounds like your of the school, of as much timing as possible, rather then
>then as little timing as consistent with max HP.  The difference is what
>lets engine win races, or live on the street.

The timing envelope needs to be established before you can attempt
to set the timing for maximum BMEP. If you cannot safely advance the
timing that far - a typical situation - the a reduction in power is
evident. The original poster said that 0.5 degrees of retard was
worth 50 horsepower.

>he 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.

>Oh, well then the gm strategy of measuring accleration rates doesn't work??.

It just won't be as effective. GM are probably trying to save
themselves a sensor and encoder. Along with the associated interface
requirements.

I also said that measuring at the flywheel would be more difficult;
not impossible.

> 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).

>Retard with EGR??.  It's inert as far as combustion goes.  The delution, on
>higher rpms is being way over stated, or overthought here in my book.  The
>is a huge column of gas drawing across the exhust as it opens, and draws
>more raw fuel into the exhuast, then there is EGR at higher rpms.  At
>cruise, yes there is some, and at idle lots of self egring, but niether are
>boost conditions.

Yes; retard with EGR because the mixture is more volatile. There is
more energy in the mixture before compression increasing the
likelihood of knock.

The "huge column of gas drawing across the exhust as it opens" is
irrelevant here unless you open the inlet valve well before TDC on
the exhaust stroke, relying on the boost pressure to flush the
combustion chamber as the exhaust _closes_ - only feasible if you're
always operating under significant boost. The total EGR results is
that amount which is not exhausted at TDC, and any exhaust gas which
is drawn back with the exhaust valve still open as the piston moves
away from TDC.

With a higher exhaust backpressure possible under high-boost
conditions as described by the original poster, the amount of
exhaust gas drawn back in will increase if the exhaust valve is left
open for too long. If that weren't the case, then we could leave the
exhaust valve open for the entire intake stroke.

Intake only happens because the pressure in the cylinder is lower
than that "above" the valves. The amount drawn in through each valve
is determined by the pressure differential once the gases start to
flow. The higher the pressure in the inlet and exhaust tracts, the
greater the flow potential.

Flow dynamics resulting from valve opening/closing play a significant
role - the inertia of the gases increases with density under boost.

>> 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).

>Nope, just tune for best performance.

Easier said than done.

>Your acting like electronics is the end all to performance, and ignoring
>tuning.

The electronics is a tool to achieve optimum tune. It's the
"permanent" instrumentation that can be applied to extract maximum
performance. The more accurate your instrumentation, the greater the
potential to extract optimum power.

>There is nothing truly trusty about O2 sensors, they degrade with use, are
>affected, by temp, fuel, and back pressure.  So you need to worry about
>those considerations then also.

How are they affected by backpressure? Temperature is easy to
stabilise. Using compatible fuels is not a big problem either in
most cases. There are different types of sensors with different
compatabilities.

>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.

>Which again accounts for the backpressure (or lack of it) issue.

The simplification of reducing the AFR due to the amount of EGR is
just one possible way of applying the "knowledge". A separate EGR
mapping would (in hindsight) be more accurate.

-- 
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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