On piston aircraft engines, part 1

If you’ve noticed, car manufacturers often brag on the technology or fuel efficiency of their engines. If you recognize phrases like “fuel-injected,” “variable valve timing,”  “double overhead cam,” or “turbocharged,” then the automotive industry’s marketing has succeeded— even if you don’t know what those things are you probably think of them as desirable.

Now forget most of that. The basic design of most aircraft piston engines are stuck solidly in the 1930s.

Take, for example, the 1975 Cessna 182P I often fly, N1298M. Its engine is a Continental O-470-U variant. No electronic ignition. No fuel injection. It weighs 390 pounds, makes 230 horsepower, and costs about $21,000 to overhaul (more if you replace your timed-out engine with a remanufactured or factory-new one).

Yep, that’s right. The engine in that airplane costs as much as many cars do, and yet from an efficiency perspective it’s terrible— 470 cubic inches to make only 230 horsepower! (In fairness, the O-470 is capable of more; in the 182 it’s derated to 230hp). By comparison, the Nissan Altima— hardly a supercar— has a 213 cubic-inch engine (well, 3.5L, really) that makes 270hp and can be completely replaced for about five grand. Now, in fairness, the Nissan engine is a much newer design. Maybe a better comparison is the engine from a 1975 Corvette, which made 205hp from 350 cubic inches and weighed about 325lbs. I won’t hazard a guess at the original cost, but overhauling a small-block 350 would cost maybe $1500 in parts today.

Behold the mighty O-470

Newer aircraft of course have somewhat more modern engines. For example, a 2012 Cessna 182 (identical in performance to the 1975 model I normally fly) uses a Lycoming IO-540-AB1A5 engine that still makes 230hp, but features fuel injection and a somewhat more modern design than the O-470. An overhaul for this engine will run you about $24K, while a brand-new one lists for just under $77K. (In 2013, Cessna stopped selling the piston 182 and moved to a new diesel engine, a topic I’ll have more to say about in part 2.) Another example: the Cirrus SR22G5, the latest version of the best-selling piston single, runs a fuel-injected Continental IO-550N that, apart from being fuel-injected, is still just as noisy, heavy, inefficient, and expensive as its predecessors.

Besides the expense, these engines require much more management than you might think. In flight, whether your engine is fuel-injected or carburated, you have to adjust the fuel-air mixture as you change altitude. You must also monitor the cylinder head temperatures (CHTs), and in some aircraft you have to adjust cowl flaps or other cooling devices. When was the last time you had to do that in your car? You don’t; in pretty much every car built since the late 1970s, a computer takes care of adjusting spark timing, mixture, and a number of other parameters to get the best performance or economy from the engine. All you do is press the accelerator. In a piston airplane, that’s a different story (something I’ll also talk more about in part 2).

The reasons for this sad state are many and complex, but the biggest two are easy to describe succinctly: reliability and cost.

Despite the fact that these engines use ancient technology, they are superbly reliable because their basic design is so mature. Engine and airframe manufacturers have 50+ years of data about their behavior, and when the possible consequences of an engine failure escalate from “pull over and call a tow truck” to “fall screaming out of the sky and die in a fireball,” you can see why that reliability is so desirable.

Cost is a multifaceted factor. First, it is exceptionally expensive to certify anything for aviation use. The FAA has a demanding and complex set of rules (known as “part 23”), backed by a fairly arbitrary process, for certifying things such as engines, propellers, and avionics. It’s prohibitively expensive for most new entrants to get a new engine and airframe combination certified. Manufacturers such as Cessna and Piper have little incentive to spend millions of dollars certifying new engine designs for their 50+-year-old airframe designs. Second, these engines are produced in very low volumes by modern manufacturing standards. In a really, really good year, Lycoming or Continental might sell a number of new engines measured in the low thousands (perhaps more, but it’s certainly fewer than 10K units/year). In that volume, it’s hard to see much improvement from scale, and given that these engines are largely hand-built, this is unlikely to change.

I haven’t touched on another drawback, one which really requires its own post: piston engines normally run on leaded fuel. This has several related consequences: economic (it’s more expensive because it’s a lower-volume product), environmental (duh), political (various satraps in California have tried several times to ban or legislate leaded aviation fuel out of existence), and technical. Some engines, such as the ones for the 182, can be made to run on ordinary auto gas (known as mogas), but higher-compression engines in larger airplanes need the lead to prevent pre-detonation, so we’re stuck with it for now.

Like the weather, the state of engine tech in general aviation is often discussed but there is little individual pilots and owners can do about it. Manufacturers, though, have a variety of tricks up their sleeve, which I’ll discuss in part 2.

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3 Comments

Filed under Aviation

3 responses to “On piston aircraft engines, part 1

  1. I’m interested in what you have to say about a) Diesel engine use in small planes, b) wankel engines, or c) very small jet engines or turboprop engines.

    • robichaux

      Excellent suggestions– those are all topics I plan to talk about in part 2, whenever I get around to writing it.

  2. You forgot one more reason: weight. The more complexity you add to the GA engine, the heavier it tends to get.

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