Carb Heat in the Circuit

[ShyTorque]

This is now way off the original topic of carb heat, my apology for that.

IO540,

At 100,000ft a typical spark ignition engine or even a turbine would probably not be running at all.

However, to restate what I said, a simple carb does not measure anything. It just senses the difference between ambient and venturi pressures. The fuel jet is exposed to the depression in the venturi and is drawn out as an aerosol spray, mixed with the air and vapourised, either in the inlet manifold or partly in the cylinder. You could look at this from another viewpoint and say that fuel is pushed into the venturi by atmospheric pressure over venturi pressure.

More complex carbs, as you say, can be designed to compensate for altitude to some extent.

The simple carb doesn't know what altitude is, or what altitude it is operating at. Therefore, if the same differential pressure existed at your theoretical 100,000 feet, the same amount of fuel would be drawn out and into the engine because the fuel metering orifice / jet size is the same. However, because the density of the air at that altitude is a fraction of that prevailing at sea level, the mixture would become massively rich at such an altitude, there simply wouldn't be enough oxygen molecules to react with the fuel. The power output reduces with altitude because the airflow through the engine is less, although the engine is theoretically more efficient with increased altitude.

It isn't correct to generalise about when detonation might occur. Your quoted figures are undoubtedly correct for your engine as they will have been derived from complex experimentation by the manufacturer. It is a complex subject that depends on a number of factors such as cylinder size, combustion chamber shape, mixture strength, fuel quality, ignition timing, charge temperature, CHT etc. I agree it is definitely not the same as pre-ignition, I never said it was. It's not the same as auto-ignition either.

I can't give you an example of an engine that can be made to enter detonation at 50% power as I'm not an engine manufacturer, or someone with access to a library of such data. I do have some experience of piston engines, both building them, modifying them, using them and a while back, teaching student pilots about them. From my own experience, detonation may occur at any power setting if the conditions are suitable, certainly at smaller throttle openings than would give 75% power.

For example, in some car engines, you can hear the detonation "pinking" at very small throttle settings if the right conditions prevail. Opening the throttle further may actually result in the detonation no longer occurring.