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Piston Engine Basics

It's not the engine in your father's Oldsmobile

By Marc E. Cook

Compared to automobile or motorcycle engines, aircraft piston engines are simple and, some say, crude. Yet as you are learning to fly, that jiggly old noisemaker ahead of the firewall holds both mystery and suspense. What's going on up there? Will it continue to run while I cross this ridge line?

You'll probably hear a lot about aircraft engines being one step up the food chain from your average lawn mower's or garden tractor's, and in the grossest of simplifications, that's true. Airplane powerplants are, save for a few rebels, simplistic, air-cooled, horizontally opposed, four-stroke internal-combustion devices with low operating speeds and low specific output. If you had to describe an automobile equivalent closest to the aviation average, you'd have to point to the venerable Volkswagen Beetle engine.

As with the People's Car, the vast majority of piston aircraft engines in service today use the Otto-cycle, invented by Nikolaus August Otto in 1876. Also called four stroke or four cycle, these engines contain a cylinder into which is fitted a piston; the piston acts on a crankshaft through a connecting rod. The crankshaft, which in most airplane applications is bolted directly to the propeller, translates the piston's linear (back and forth) motions to rotational work.

In the Otto-cycle scheme, there are four distinct cycles, differentiated by strokes of the piston inside the cylinder. On the first stroke, the piston moves downward, drawing fuel and air through a homeowner's nightmare of plumbing to the combustion chamber inside the cylinder. The second stroke sees the piston rising in the bore, compressing this mixture. Fuel in plain form is not particularly volatile — that is, it won't ignite with the slightest provocation. But compressed, it will. Typical aircraft engines attempt to compress this fuel/air mixture by a factor of between 6.5 and 8.5; this is called the compression ratio. Compression ratio is actually measured by determining the volume of the entire cylinder with the piston at the bottom of the stroke BDC (bottom dead center) to the volume with the piston at the top of the stroke TDC (top dead center). The total volume of all the cylinders measured at BDC is called displacement. So the 1.6-liter engine in your car has a displacement of 1.6 liters (about 96 cubic inches), and the Lycoming O-235 has a displacement of about 235 cubic inches.

Once the piston has compressed the mixture, a spark plug (or two, in aviation applications) lights off the mixture. The resulting explosion pushes the piston toward BDC and is called the power stroke. A final trip upward in the bore has the piston forcing the spent gases through the exhaust system and into the skies.

Movement of intake and exhaust gases into and out of the cylinder is managed by tulip-shaped valves placed at the top of the cylinder head. The valves are, in turn, activated by short rocker arms through long pushrods (you'll find them above the crankshaft on most Lycomings and below on Continentals). A camshaft, basically a steel rod with egg-shaped lobes along its length, activates the pushrods through film can-sized lifters (or hydraulic lash adjusters) in the engine case directly adjacent to the camshaft and rocker arms at the valve end of the pushrods.

To better understand the hardware layout, let's look at the Lycoming O-235 as used in the Cessna 152; other common types, like the Continental O-200 in the Cessna 150 and other versions of both marques' powerplants, share the same basic layout. Incidentally, these model numbers mean something. The O stands for opposed; the banks of cylinders are 180 degrees from each other, or flat, like the Beetle's engine. (Smarmy engineers sometimes call these 180-degree V engines, but what do they know?) The next number is the total displacement of the engine in cubic inches, rounded to the nearest 0 or 5. An I in the prefix denotes fuel injection. For Continentals, a TS prefix means turbocharged — or, "turbosupercharged" — and for Lycomings you'll find a T prefix. Presence of a G in the prefix declares a geared engine, in which the propeller turns more slowly than the engine itself; the vast majority of the popular engines are direct drive, however. These prefixes are additive, so a GTSIO-520 is a geared, turbocharged, injected, opposed, 520-cubic-inch engine. Suffixes to the displacement denote variations of the type. A Lycoming O-235-C2A is, for instance, a 115-hp version of the engine, while the O-235-F2A is one with 10 more horsepower.

So much for the numbers. Put simply, an internal-combustion engine makes power by converting heat into motion. The heat comes from the burning of fuel (combined with a lot of air, typically at a ratio of 15:1). Because they are air cooled, the cylinders employ fine fins — not like a 1959 Cadillac's — to help to promote transfer of heat produced in the combustion process to the airflow directed around them by the cowling and metal baffles around the cylinders.

The cylinder is made up of a cast aluminum head permanently — at least as far as the pilot is concerned — mated to a steel barrel that can be coated or treated with any number of processes.

If you compare the average airplane engine to the latest in cars coming from Germany, Japan, or Detroit, you'll be mighty disappointed. You won't find high-tech electronic fuel injection, overhead camshafts, stratospheric redline speeds, or engineer-pleasing high specific output. But the engines are designed to run at maximum rated power for a long time; 2,000 hours in an automobile is 110,000 miles, and the car is using on average about 20-percent power. Think about that when crossing the next ridge line during that cross-country trip.