Because Pd170 is a piston engine, Al-450 is a turboprop engine. So one is an apple and the other is a pear.
Piston vs. Turboprop: Efficiency
Piston engines and turboprops are both internal combustion engines that must compress air, burn that air using fuel and expel the resulting exhaust gas. The thermodynamic properties that govern these engines result in interesting compromises between the two systems.
Generally speaking, turbine engines lose efficiency as they become smaller while piston engines become less efficient as they grow larger.
As an example, consider the following engines and their respective fuel flows at typical cruise power settings.
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Typical Piston and Turboprop Cruise Efficiencies
Before digging too deeply into the chart above, it is worth noting some details. The data above is representative, and non-specific to a particular aircraft, and represents typical values only.
Turboprop engine cruise data is more typically referenced with respect to torque output, but for the sake of direct comparison horsepower values are used.
Finally, the data above is for maximum continuous cruise, not for reduced power settings, this flight condition was selected to represent maximum performance of the engine.
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Lycoming IO-720
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Pratt and Whitney PT6A
Fuel Flow and Specific Fuel Consumption
Using a cursory review of the data above, the raw fuel flow values are misleading. The Lycoming engines appear to hold the advantage in terms of pure fuel flow, besting the PT6A-21 by at least 11 gallons per hour.
Closer inspection however reveals that the specific fuel consumption or Gal/Hr/HP value favors the IO-550 and the PT6A-60A. At the extremes of the power output ranges, efficiency is dramatically improved, the engines burn less fuel per hour to generate a given power output.
It is apparent that at some point larger and larger piston engines become less efficient relative to similarly sized turbine engines, and conversely, smaller turbines become less efficient than similarly sized pistons.
The exact crossover point where this happens is somewhat dependent on the particular installation and the mission profile of the aircraft.
Fuel Costs
Finally, discussion of cost would be incomplete without a discussion of fuel costs. Referencing AirNav.com average fuel price data as of December 2019, 100LL fuel (avgas) averages $5.06/gallon and Jet-A averages $4.68/gallon (see our article on “Why does jet fuel cost less than 100LL?”).
Applying these averages to the fuel burn data above in table 1 the
following direct fuel costs result:
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It is again evident that the turboprop engines command a premium with regard to direct fuel costs at the “low” end of the power range; however, that margin narrows as the power output of the turboprop engines increases.
This is a direct result of the improved thermal efficiency of turboprop engines. This fact again makes the operating costs of small turboprop engines difficult to justify in small aircraft such as the Beechcraft Bonanza or the Cessna 206 Stationair.
Ultimately for both types of engines, operating cost is managed by good engine operating habits and by proper maintenance and inspection intervals.
The most reliable piston engines and most reliable turboprop engines are those that are properly managed as the complex and expensive equipment that they are. While piston engines have a greater number of direct wear parts, it is generally simpler and less expensive to service those wear parts.
Replacing a cam or a valve is generally a straight forward exercise for a piston engine, while accessing and replacing a turbine blade is substantially more difficult and expensive proposition in a turboprop engine.
Piston vs. Turboprop: Performance
Piston engine performance and turboprop performance, while similar in some respects, is an illustration of the benefits of turbine engine operating characteristics. The performance envelope of a given engine is best described by cruise performance and takeoff performance.
In the case of both piston and turboprop engines, total power output is measured in horsepower, and the engine will be capable of generating maximum continuous power up to a “critical altitude;” above which, performance is reduced.
