<< Page 10 Page 11: Page 12 >>   Piston Engines They can be classified into In-Line, Radial, and Rotary. (No, an airplane's "rotary" engine is not the same kind of design as a Wankel or Mazda "rotary" engine).   1)In-Line piston engines: This is the type of engine found in cars. The most natural way to arrange a bunch of cylinders is in a row of V’s (or just one V, in motorcycles and some small airplanes, or in a row of cylinders, the truest in-line engine) along the crankshaft. As you can see in the drawings below, as the pistons are pushed up and down through the cylinders, the central crankshaft (the main “axle”) is spun around (just like a bike’s pedal sprocket and gears are spun around by your legs going up and down while you bike). Here’s one piston’s cycle (and a prop): The pistons’ cycles are offset so that they do not all push the shaft around at the same time (the shaft would have to spin around on its own inertia the rest of the time). Instead, there are many relatively-evenly-spaced pushes and as little time as possible spinning around on inertia. I suppose a discussion of piston engines would not be complete without a discussion of the four-stroke cycle. In other words, a piston moves up and down because of the fuel combustion in it an in other pistons, and here’s how: On the first stroke (1, 2), the piston moves from the top of the cylinder downwards. The intake valve is open, allowing in fresh air (i.e. containing O2) and fuel. On the second stroke (3), the valve closes and the piston moves upwards, compressing and heating the air. On the third stroke (4, 5, 6), the sparkplug fires (or the diesel is injected, if it’s a diesel engine) and the fuel ignites, which heats the air and pushes the piston down, providing the power. Then, on the fourth stroke (7, 8), the piston moves back up, but the exhaust valve is open, so the burnt air and fuel are pushed out, leaving us at the first stroke again. These are called four-stroke engines because there is one power stroke every four strokes – but the power delivered in that stroke is so high (i.e. the piston is pushed down so hard by the heated air) that it powers the other three strokes on neighboring cylinders with plenty of power left over to turn the crankshaft. The shaft in the middle, spun around by the pistons, spins a propeller. above; The four-stroke cycle. Below, some in-line (or “V”) piston engines: a Jabiru 3300, a Lycoming 0235 V4, and a Lycoming V6: Below: The fastest piston-powered airplane is “Dago Red”, a highly-modified P-51 flown in the Reno Air Races. Its souped-up Merlin engine delivers over 4200hp, almost 3 times its original power output. Dago Red flies at almost 600 mph. The Merlin is liquid-cooled, and the cooling liquid is in turn cooled in a radiator that sits in the fairing visible in the belly of the P-51. Although mechanically simple and easy to be fit in a streamlined vehicle, in-line engines are the worst kind of engine to cool. Remember, the more power you want, the more fuel must be burnt, and the hotter the cylinders get, so the better a job you must do in cooling them. Air that has already cooled one piston will be a little warmer, so it will not cool the cylinder behind the first so efficiently, and will do an even worse job cooling the cylinder after that, and so on. In line engines today, since the 40’s, do not depend on air blowing by them to cool them: They are liquid-cooled (coolant fluid is pumped through thin pipes around the engine, absorbs the heat, and is cooled in a radiator). But this was not an option during the early days of aviation, which is why aviation pioneers came up with… 2) Radial Engines: The cylinders’ pistons work the same way an in an In-Line, but they are arranged in one plane, like flower petals around the shaft. They all are connected to the shaft at the same place, and are at different stages of the cycle, like before: Here, the blue circle in the middle is the crankshaft, and the red circle is the middle of the yellow part being spun around by the pistons. As the yellow part goes around, different pistons go into and out of their cylinders, so every piston fires at some point during any two full rotations of the crankshaft. The main advantage of this engine is that all the cylinders are at the front, bathed in fresh air. No cylinder is in air that has already been warmed by another cylinder in front of it. This means air-cooling is more effective than in In-Line engines. However, these engines are very wide, so an airplane with a radial engine has a fat nose, thus making it hard for it to be aerodynamic (single-radial-engine airplanes have a characteristic round nose, with the engine cylinders visible from the front). Because of streamlining, In-Line engines are now preferred, especially since liquid cooling has been developed. An interesting note is that some radial engines contain more than one plane of “petals”. Other than cooling issues, there is no reason why one could not stack two or more “layers” of radially-arranged cylinders around a single shaft. The 7-piston Wasp engine, for example, used in US Navy fighters during early World war 2, was developed into the Twin Wasp – two Wasps, one in front of the other, spinning one shaft. Twice the cylinders, twice the horsepower. Used in the B-24 and many late-WW2 naval fighter like the Corsair and Wildcat. Two of those (4 Wasps in total) made the Wasp Major, with four layers of cylinders, 4000hp, no fewer than 28 pistons. It was used in the Spruce Goose and the B-36. 3) Rotary piston engines: Air blowing by faster cools the cylinders better, so if they are spun through the air, a lot more cooling gets done, and a lot more power can be generated by the additional fuel that can be burnt. These engines work just like radial engines, except the shaft is fixed, so the cylinders spin themselves around as they work, and this in turn spins a propeller. Here is how a Gnome rotary engine works: A Gnome engine at 0, 90, 180 and 270 degrees. The engine spins around the red double-circle right in the middle. But so that the pistons go in and out, they are attached to the red circle just above it (so that their distances from the red circle change as they go around, and they are pulled in and out of the cylinder by being attached to this off-axis point). These engines are extremely complicated, mechanically. Lots of moving parts, lots of possibilities for failure, hard to start, and very difficult to throttle (speed up, slow down) due to the inertia of the pistons… They were popular during World War 1, but as better cooling systems were developed (like liquid cooling), engine designers turned to the more simple non-spinning piston engines in most piston-powered airplanes designed since 1920 or so. In addition, the development of turbochargers meant that more power could be gotten out of a small engine (as more air could be compressed into each cylinder, and thus more fuel could be burnt and more power extracted), and this also helped to lead designers back to simpler engines. Most importantly, turbochargers allowed piston aircraft to operate at much higher altitudes, compensating for the thinner air. (Note: an aero “rotary” engine is not at all the same concept as a Wankel or Mazda “rotary” engine. Wankel/Mazda rotary engines do not use cylindrical pistons, but an entirely different geometry to capture the push from the combustion of fuel. Because they can be lighter than a piston engine of comparable horsepower rating, these strange engines have been used in small aircraft in the past few years. But they are too rare and too complicated – and effectively not different enough from an in-line piston engine – to deserve being talked about here).