[Viper2000]

Originally Posted by Wolf_Rider

erm... where?

Try this from page 10 of the thread, complete with diagram.

The analysis linked to above only talks about mixture strength, and not the effects of that mixture strength on the engine.

During the lean cut, power initially falls away a bit slower than fuel flow because the design point was rich of stoichiometric, and therefore the temperature rise per unit fuel flow goes up. EGT would also initially rise. There might be incipient detonation, followed by misfiring as the mixture becomes too weak to support combustion (cylinder to cylinder variations in temperature and mixture strength mean that this behaviour will not affect all of the cylinders at the same time).

The lean cut will be less severe at lower power output under reduced positive g because fuel flow is less and so the fuel in the small chamber will therefore last longer; indeed at high altitude the engine might just go straight to rich cut. (Under negative g the small chamber empties much faster because fuel flow can escape from the holes in the small chamber which were provided to admit it.)

During the rich cut, power output falls because the Fuel:Air Ratio is approximately 2.4 times design point (which was already somewhat rich of stoichiometric for high power running). This lowers the combustion temperature considerably which kills the engine's BMEP.

However, combustion will continue because there is still oxygen and a source of ignition in the combustion chamber - the chemistry just changes so that it makes CO, H20, sundry cracked petroleum products, and of course some carbon (soot).

Boost shouldn't initially change during the cut unless engine rpm starts to fall, because the ABC doesn't know anything about the cut, and the supercharger pressure ratio is simply a function of its rpm; since it is geared to the engine.

Therefore, the chain of events is

1. Power reduction
2. Possible rpm reduction
   • Boost reduction

The rich cut is worse at lower engine power outputs because the fuel flow from the pumps is a fixed function of rpm, whilst the amount of fuel needed by the engine is a function of power.

At 30000' where the engine is only capable of producing about 500 bhp instead of nearly 1300 at its +12 psi FTH, the fuel flow during the cut would be 2.4*(1300/500) ~ 6.24 times that actually required by the engine. This is such a severe over-rich case that nett power output might fall to zero or even go negative (the prop will obviously windmill, so the engine won't stop; as long as it keeps turning the mags will keep going and so there will still be a spark and therefore recovery will follow).

Recovery time to float-controlled operation upon return to positive g would probably be longer at higher altitudes because the recovery depends upon the rate at which the carburettor can dispose of the excess fuel in the float chamber. This depends upon how much fuel can be carried away by the airflow through the venturi (which is mostly a function of ambient conditions & engine rpm; fuel temperature will also have a 2nd order effect).

At or below FTH, with the mixture about 2.4 times as rich as it should be, power will probably fall to about 1/3rd of the expected value, or perhaps a little less. I could do a cycle analysis if there is sufficient interest; the main unknown is the design point Fuel:Air ratio; I can't lay my hands on my copy of The Performance of a Supercharged Aero-Engine by Hooker et al at the moment...