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.
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:
- 2200 + 32.8 inches (up to 6000ft) or
- 2300 + 31.1 inches (up to 10000ft) or
- 2400 + 30.0 inches (up to 14000ft) or
- 2500 + 29.2 inches (up to 18000ft)
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 hectopascals (hPa). International 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.
On take-off do this: set 30" MAP and 2700 RPM and after passing 500' AAL reduce RPM to 2500 but leave the throttle wide open, this reduction in RPM will reduce engine noise. Do not worry about the MAP, this will reduce also as air pressure normally drops with 1" per 1000 ft, lowering engine power automatically. Note that this will not work with turbocharged engines. The turbo controller will maintain the preset MAP.
After reaching cruise altitude, reduce RPM even further to 2200 or set the MAP / RPM as per POH recommendation (but choose the highest MAP and lowest RPM for the required power setting).
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 results in the best specific fuel consumption increasing range and endurance (and lowering your fuel bill too).
So, oversquare 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.