Does throttle equal power on a turbocharged piston engine?

Question

On a normal piston engine powered airplane, throttle equals power. If I set the throttle to 50 %, I will get 50 % of the power the engine can develop at the current altitude, temperature, etc. In a flightgsimulator, I am getting full power and manifold pressure from 75 to 100 % throttle at sea level under ISA conditions. Is that normal ? I was expecting to get the same results as with a piston engine, i.e. that 50 % throttle produces 50 % power, and 100 % throttle produces 100 % power.

Answer 1

Throttle position in itself doesn't equal power on any piston engine. Power is a torque vs RPM, as in any engine.

Aircraft piston engines, whether turbocharged or not, use manifold pressure, the absolute pressure upstream of the throttle plate, given in inches of mercury like a barometer, as a proxy for torque, and the pilot will use a chart with curves that plot RPM and manifold pressure to derive horsepower being produced. In cars, if manifold pressure is being monitored, they reverse the concept and express it as vacuum (the differential between ambient and absolute pressure in the manifold), since the pressure in the manifold is normally lower than ambient, absent supercharging/turbocharging.

For a given atmospheric condition, manifold pressure will vary with throttle opening. Because power is a function of torque and RPM, 50% power will be achieved within a band of manifold pressures (throttle settings) and RPM values; for example, you may get 50% power at 20" of manifold pressure and 2000 RPM, or 19" of manifold pressure and 2100 RPM, or 21" of manifold pressure and 1900 RPM.

If the propeller is fixed pitch, you lose the ability to control RPM, and there will be one specific throttle setting that results in 50% power, the point where the manifold pressure and RPM add up to 50% of rated power for a given ambient condition. This setting however will vary with the atmospheric conditions and altitude, so you still don't end up with a throttle position that represents 50% in all conditions.

Turbocharging/supercharging doesn't really change things other than the fact that manifold pressures may be higher than ambient at maximum power, and certain manifold pressures can be achieved at high altitudes that a normally aspirated engine can't. As well, on a turbocharged or supercharged engine, you will have a maximum throttle setting that is a function of a manifold pressure redline, which can be less than physically wide open throttle.

On a PW-985 radial, which has a mechanical supercharger, the maximum throttle setting is at 36" of manifold pressure. Pushing the throttle all the way forward might get you 38 or 39", which will damage the engine. So in that case, you advance throttle to the MP redline and stop there, and you will be somewhere short of the throttle's physical travel limit, whereas on a normally aspirated engine you just push it until it stops.

It still comes down to the fact that X% percent power is achieved at various combinations of manifold pressure and RPM, and the throttle opening that results in a given manifold pressure will vary accordingly and there isn't a single universal setting except for wide open throttle.

Answer 2

As John K correctly explains, the relation between throttle position, or even manifold pressure, and torque or power is never linear. But I'd like to add one thing:

If the engine power, and the manifold pressure, stops increasing at 65% (or whatever other %) of throttle movement, you have an engine with manifold pressure red line, protected with automatic waste-gate (a “flat rated” engine, though that term only became common later with turbines).

The point of turbocharging an engine is that it can produce more power at high altitudes. So say your engine has turbocharged capable of producing 45 inHg manifold pressure up at 19,000 ft. The manifold pressure is still proportional to the density altitude, so at sea level, if you opened the throttle wide, the manifold pressure would rise to around 81 inHg. But if the engine was to handle that, it would have to be too heavy, and spark-ignition engines (unlike diesels) are limited to around maybe 55 inHg anyway because above that the air charge will get too hot and trigger preignition, which reduces the power and damages the engine.

So with such engine, you are told not to let it run over, say, 54 inHg manifold pressure, or the engine might be damaged.

The newer engines have automatic waste-gates in the turbo. Those are valves that will relief the pressure if it exceeds the red line. So if the engine hits that 54 inHg MP at 65% throttle, and you keep advancing the throttle, the waste gates will open and the manifold pressure will remain around 54 inHg, and the engine will not produce any more power.

Older engines did not have waste-gates, or only manually operated ones, so you had to be careful not to overboost (exceed the MP) the engine.

With big engines you are also likely to be told not to exceed some lower limit for more than some time. For example max 2 minutes above 49 inHg. That the pilots always have to watch for themselves.

Answer 3

Flight simulators used for aircrew training must replicate essential flight control behaviour, of which the throttle response is an important one. In flight sims this is implemented as follows:

• Measure the input parameters and associated output parameters (fuel flow), in multiple relevant circumstances. Such as in this Swiss research project on aeroplane emissions of a carbureted engine, in which they do state that "Piston engine fuel flow is constant for a given amount of power".
• Design system functionality based on the physics of the system. For the fuel system that includes fuel temperature & compressibility, pump behaviour, friction in the supply lines etc.
• Program the system functionality, then calibrate using the data points from the measurements.

A statement like “On a normal piston engine powered airplane, throttle equals power” would definitely need to be verified with measurements. Is the power setting really linear with fuel flow?

However, the behaviour you describe could be the effect of the characteristics shown in this answer: turbocharged and supercharged engines do have a non-linear relationship between max power and altitude.