Engine Power Settings, II

Reduced engine wear and fuel savings can be achieved by lowering RPM and increasing manifold air pressure (MAP) to maintain horsepower and thrust. Modern engines have approved power ranges for which this practice is allowed. These ranges are in the POH but can also be found in the engine manufacturer's handbook.

There advantages to be gained by running the engine oversquare, fuel savings and a longer engine life are the important ones. You as the pilot will have to learn this 'new' technique to realize these results.

Advantages

For example, in one particular aircraft (like the Piper Turbo Arrow IV PA-28RT-201T with a TSIO-360FB engine), the recommended combinations for 65% power at sea level and higher are:

As indicated above you can use a low RPM and a high MAP or a high RPM and a low MAP to achieve exactly the same power output from the engine. The low RPM / high MAP combination result in a more efficient cylinder charging and better combustion plus less friction. The high MAP also acts as a cushion in the cylinders, reducing engine stress. MAP is usually measured in inches of mercury ("Hg) rather than hecto pascals (hPa). Standard sea level barometric pressure is 29.92 "Hg or 1013.25 hPa.

But to enjoy all the advantages mentioned above you will need to select the lowest RPM and highest MAP combination for the power required and lean the mixture when in cruise.

Take off

On take-off do the same: 30" MAP and 2700 RPM and after passing 500' AGL reduce RPM to 2500 but leave the throttle wide open, this will reduce engine noise. Do not worry about the MAP this will reduce too as air pressure drops with 1" per 1000 ft, thus engine power reduces as well. This will not work with turbocharged engines.

Engine efficiency

Gasoline aviation engines have their best volumetric efficiency running at low RPM and high MAP, preferably at their full throttle height for their rated power. This will result in the best specific fuel consumption increasing range and endurance (and lowering your fuel bill too).

So, over square is not bad at all. Just remain within the engine limits set by the manufacturer and you will enjoy a greater range with lower fuel consumption and hourly cost to run the aircraft.

More background information

For readers wanting more information on this subject we refer them to the next series of articles:

The above articles are a must read for anyone with an engine and controllable propeller interested in keeping the engine in good shape for years (and flying hours) to come.

Written by EAI.