Plane & Pilot
Tuesday, July 17, 2012

Engine Reliability

General-aviation piston engines are simpler and more reliable than other power plants...most of the time

Piston-powered aircraft most often come up to full power for takeoff and climb, then reduce to a more moderate cruise setting, usually 75, 65 or 55%. Mechanical engineers will tell you steady-state operation puts far less stress on the engine, even if that steady state is at relatively high power. On a typical 200 hp Lycoming IO-360-A1A for example, 75% power would be equal to a theoretical 150 hp, "theoretical" because few aircraft engines deliver rated hp at the stated manifold pressure and rpm.

Conversely, a car on the freeway can cruise along at 70 mph on probably more like 15% power, perhaps 30 hp from an equivalent 200 hp engine. That's because aircraft engines must propel the aircraft forward AND support the weight of the aircraft in flight. An auto engine obviously has only to drive the car forward. The tires support the weight.

Aircraft engines have a few other advantages, as well. They're typically lighter than car mills as most have no radiator and no muffler, and the engines are nearly always four- or six-cylinder opposed, compared to eight-, 10-or 12-cylinder car engines.

If you're into diesels, a growing contingent of general-aviation airplanes offer most of the advantages with little downside. Diesel fuel is everywhere on the motorways of Europe and much of the rest of the world, but it's not that popular on America's highways and interstates. Conversely, since aircraft diesel engines can use jet fuel, finding a place to refuel isn't that big a problem for an airplane.

Aircraft engines don't have all the aces, however. While the usual consequence of an engine failure in a car is limited to an expensive repair and some inconvenience, an inflight engine problem can be considerably more serious. For that very reason, aircraft engines employ two electrical systems and compression ratios well below those used in auto power plants. Lower compression equals lower cylinder pressure equals less stress on the cylinders, rings, lifters, valves, crank, pistons and all the other greasy bits.

Aviation companies have to deal with government approval on everything they do, and the costs of negotiating with sometimes fickle and contradictory FAA FSDOs can be staggering. This alone tends to discourage any form of innovation in aircraft engines, and it's one reason some aviation innovators will go FSDO shopping when they're looking to introduce new products. Car companies have only their customers to please, plus they can amortize any development costs over thousands (or even millions) of units, rather than mere hundreds of airplanes, in aviation.

Auto engines have a major advantage when it comes to overhaul or replacement. Aircraft engines and auto engines are very much the same, only different, intended for two dissimilar purposes. Personally, I'm happy that I could completely replace my car's engine for less than $15,000. Merely overhauling my Lycoming IO-360-A1A would cost over twice that.

Wonder if an Infiniti engine would fit in a Mooney?

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