Piston Engines - General — Page 34, Lesson 43

Let’s pick this up right where the power flows: from the crankshaft out to the propeller. Some engines have a gearbox fitted between those two. That gearbox is called a Reduction Gearbox, and its job is to reduce the speed of propeller rotation. Why do we need that? Think about the two ends of the system. The propeller operates efficiently at a comparatively low speed. The engine, on the other hand, must turn at high speed to develop its full power. So you have a conflict: the propeller wants to turn slowly, the engine wants to turn fast. The reduction gearbox resolves that conflict so both can operate efficiently. There are two typical types of reduction gearing you’ll see: Spur Gear and Planetary Gears. On the lower-powered engines, you don’t need that gearbox at all — the propeller is connected directly onto the crankshaft. Those are called Direct Drive engines. Now, how do we classify aero-engines? There’s a standard set of categories. They’re classified by Cylinder Arrangement, Type of Drive, Direction of Rotation, Cylinder Capacity, Cooling Method, Fuel System Type, and whether they are supercharged or normally aspirated. Let me make sure each of those is clear. Cylinder arrangement is how the cylinders are laid out — opposed, inline, radial, that sort of thing. Type of drive is whether it’s direct drive or geared. Direction of rotation is which way the crankshaft turns — left hand or right hand. Cylinder capacity is the displacement, the swept volume of the cylinders. Cooling method is how the engine gets rid of heat — air cooled or liquid cooled. Fuel system type is how fuel gets into the engine — carburetted or fuel injected. And supercharged versus normally aspirated tells you whether there’s a compressor forcing air in, or whether the engine just breathes naturally. Now, let’s look at a real example. Figure 2.17 shows the Textron Lycoming model AEIO 540 L1B5. That model number isn’t random — it’s a code that defines the engine. Let me decode it for you, letter by letter. AE stands for Aerobatic Engine. So this is built for aerobatic flight. I stands for Injected Fuel System. That means fuel is injected, not carburetted. O stands for Horizontally Opposed Cylinder Arrangement. The cylinders are laid out flat, opposing each other. 540 is the Cylinder displacement — 540 cubic inches. That’s the total swept volume of all the cylinders. L stands for Left Hand Rotation. The crankshaft turns to the left. 1B5 indicates Modifications from the basic model. So this is the basic design with a specific set of changes applied. This type of model numbering system is used by most manufacturers. And here’s a useful detail: if the letters G and S were included, it would imply the engine was geared and supercharged. So G for geared, S for supercharged — those letters tell you the engine has a reduction gearbox and a supercharger. So when you read an engine model number, you’re reading its whole identity: what it’s built for, how it breathes, how the cylinders are arranged, how big it is, which way it turns, and what modifications it carries. That’s the language of engine identification.