Engine Layout: The Piston Bone Is Connected to the Rod Bone …

Thanks to near-perfect cars and the preponderance of non-farm, urban living, new pilots and builders typically have scant mechanical knowledge. Call this suboptimal when piloting or building a light aircraft, as a basic understanding of aircraft mechanics makes flying them safer, less expensive, and more enjoyable.

Thus, I’ve been tasked with generating an ongoing stream of mechanical tidbits for those just starting their aviation journey, and I thought to start with piston engines. They are, after all, at least a familiar concept, as that’s how most of us got to the airport, and electric airplanes are still future stuff.

Let’s note the core of an engine is a cylinder in which a piston slides up and down. The piston is connected to the crankshaft—that’s a shaft with offset “cranks,” think “crank” as in bicycle pedals—by the creatively named connecting rod. When the piston is pushed down by hot, expanding gas in the cylinder, this powerful motion is transferred by the connecting rod to the crankshaft, which then revolves. Bolt a propeller to the crankshaft and the propeller goes around, too. See, nothing to it.

The heart of all this motion is the combustion chamber. That’s the area above the piston, an area enclosed by the cylinder head. The cylinder head is fitted with an intake valve to admit air and fuel into the combustion chamber, a spark plug or two to light the air/fuel mixture on fire, and an exhaust valve to let out the combusted gases once the piston has moved to the bottom of the cylinder.

While not immediately obvious, the takeaway concept to a piston engine is an air pump, and air is its working medium. We only add fuel—gasoline or diesel—to the air and set it on fire to raise the air’s temperature. The rising temperature makes the air expand (see high school physics) and push down on the piston. Therefore, the more air a piston engine can process, the more power it can make. That’s why larger engines make more power than smaller engines: because larger engines process more air. Make the cylinder larger, or add more cylinders, or turn the engine faster, and you’ll get more air through and power out (plus weight and complexity, but that’s for later).

Another basic but important thought to remember is the fire triangle: fuel—oxygen—source of ignition—is key to having the engine keep on making its happy noise and us in the air. Best to keep the fire triangle in mind when operating, maintaining, or troubleshooting an engine.