Turbo speed sensor ever used ?

[Seth]

For really high output engines, you don't have to go with as mild a cam
as you might think. Some Porsche turbo cams had durations in excess of
360 degrees. I have no idea what the powerband was like, and I can only
assume they relied on a resonating/standing wave tuning to make this
sort of cam timing work. But I bet it had a fair amount of overlap.

-Seth

Bernd Felsche wrote:
> 
> Espen Hilde writes:
> 
> >> mmm - Interesting, I'd assumed air flow measurement and MAP would be
> >> sufficient to determine volume of air entering the chamber. Are you
> >> suggesting AFM doesn't give an accurate enough figure due to backpressure
> >> at the turbo and that MAP in no way assists this - in the circumstance
> >> where we have an AFM and MAP ?
> 
> >The AFM dont know the backpressure and therefore the ecu cant predict
> >the right advance....we are wanting to be right on the edge of detonating
> >here....at a 700hp 2l turbo engine 0,5 degree wrong advance at max torque
> >is 40 hp lost.
> 
> The state of the turbo is defined by the operation of the engine.
> In theory, the ECU should be able to work it out given sufficient
> knowledge and the appropriate algorithm.
> 
> If the *backpressure* is so critical, then you should arrange for
> phasing the exhaust pressure pulses before the turbo so that the
> exhaust still "scavenges" the combustion chamber as the valve
> closes during overlap.
> 
> A reduction in overlap may also be worth considering. You don't
> really want internal exhaust gas recirculation if your sole quest if
> for maximum power.
> 
> >AFM and a MAP sensor on the exhaust could do the trick if the AFM is
> >fast enough.
> 
> Those sensors would not be fast enough and sufficiently resistant to
> the 500+C temperatures in the exhaust. If you're getting 700bhp from
> 2 litres, you're spinning at close to 10,000 rpm if not more. A
> pressure sensor needs to pick up the peaks and throughs at the
> exhaust valve to adapt the airflow.
> 
> Similarly, to accurately measure the airflow, you need a fast
> pressure measurement at the inlet as well. There are pulsations in
> the airflow even with (turbo-)supercharging. Transducers will
> typically not give you enough information at high speed.
> 
> Pressure sensors take about a millisecond to respond to a pressure
> change. So at 10,000 rpm, you get all of 6 samples per revolution -
> not very accurate in identifying the pulsations. (vis Motorola
> MPX5700 or MPX4250A) Those sensor need to be mounted remotely from
> the engine - else they become soup - so you need to have a pressure
> line to the ECU that introduces minimal delay. An oil-filled tube
> would improve the response time - you still need to allow for the
> speed of the wave propagation to determine precise phasing.
> 
> >How the algorithm or map for backpressure vs .boost should work beets
> >me....Ignition retard under accleration when backpressure is higher than
> >steady state...
> >Its complex......
> >Turbo speed VS. engine rpm wot table?
> >A backpressure table of %influens on volumertric filling at wot?
> >Use a valve on the exhaust pipe for adjusting backpressure under dyno
> >runand fill the tables....a hard day work?
> >Different engines responds different to backpressure at different rpms.
> 
> Because of the dynamics of the flow as noted above. Having no
> overlap would go a long way to solving the problem.
> As you don't have a long exhaust tract, you lose out on taking
> advantage of an "under-pressure" to help evacuate burnt gases from
> the combustion chamber as you have a turbocharger sitting in the
> way, a short distance downstream from the valve.
> 
>         If you had freedom in designing your cylinder head and
>         exhaust valves, I'd suggest that you have one exhaust valve
>         from each cylinder directed to the turbocharger, opening
>         first and a second (or third) valve opening later to a
>         turbo-bypass with the first closing slightly to prevent
>         return flow.  That permits a degree of overlap on the second
>         exhaust with the possibility of useful scavenging.
> 
>         With say three camshafts, a variable timing of the second
>         exhaust camshaft and hence the valve timing will also
>         alter the boost pressure - the duration of the second
>         exhaust opening need only be (guessing here!) about 100
>         degrees. The maximum duration would be determined by the
>         boost requirements and the limit of overlap.
> 
>         Also, the greater the advance on the second exhaust, the
>         greater the scavenging, but the lower the boost. Applying
>         traditional exhaust tuning techniques (resonance, etc) to
>         the bypass exhaust would mean more effective scavenging
>         during overlap as the bypass exhaust is only carrying a
>         portion of the full exhaust flow - between a quarter to a
>         third (again, guessing).
> 
> >> I've alwayus wondered what the relationshiup is between boost and less
> >> ignition advance - is it simply chamber filling, propensity to ping, or
> >> some other presumably non-linear relationship ?
> 
> >Exhaust backpressure is driving the exhaust temp to the roof..more residual
> >hotexhaustgasses .more prone to detonation.
> 
> The higher temperature will also contribute to detonation because
> the exhaust side stays hot.  That may be good for increasing boost,
> but not so good if you have to lose power by retarding the ignition.
> 
> If the static backpressure is too high and phasing the exhausts
> doesn't contribute to better throughflow, then it may be necessary
> to split the inlet and exhaust systems to equally-phased cylinder
> groups, necessitating multiple turbo-chargers.
> 
> --
> 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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