Piston Engines - General — Page 15, Lesson 21

I want to walk you through the general layout of piston engines and then the theoretical Otto cycle, because this is the foundation for everything else in powerplant. Let's start with engine configurations. Radial engines always have an odd number of cylinders. If you place further rows of cylinders behind the first, you get what are called Double and Triple Bank radials. These engines were very powerful, but they had real disadvantages: they were heavy, and because they were air-cooled, they presented a large frontal area. Now, most modern light aircraft use four or six cylinder engines arranged in the Flat, or Horizontally Opposed, configuration. That arrangement makes for a short, rigid engine that is easily streamlined. Now let's get into the theoretical Otto cycle. First, I need to define a stroke. A stroke is the linear distance that the piston moves in the cylinder. When the piston is at the top of the stroke, it's said to be at Top Dead Centre, or TDC. When it's at the bottom, it's at Bottom Dead Centre, or BDC. The piston is connected to a crankshaft. As the piston moves from TDC to BDC, the crankshaft rotates 180 degrees. The complete cycle takes 720 degrees, which is 4 times 180. And here's a key relationship: the stroke is equal to twice the crank-throw. Now, the internal diameter of the cylinder is called the bore. And there's a term in the text: an engine which has a bore equal to the stroke is known as over-square. I want to be careful here — the text states that an engine with bore equal to stroke is called over-square, so that's the term we use. These terms — stroke, bore, TDC, BDC — are used to explain the Otto cycle. Piston and valve positions are related to degrees of crankshaft movement, and position in relation to TDC and BDC. Now, the four strokes of the Otto cycle are: Induction, Compression, Power, and Exhaust. Let me walk you through the operation. When the piston is at TDC at the end of the compression stroke, an electrical spark is produced at the spark plug, and it ignites the fuel-air mixture. Now, it's really important you appreciate this: this does not result in an explosion of the mixture. It is a controlled burning. This event is called combustion. The combustion process takes place with the piston at TDC. The volume in the cylinder at that moment in time is constant. So combustion is said to take place at constant volume. Now, in the theoretical Otto cycle, there are five events: Induction, Compression, Combustion, Power, and Exhaust. Notice the difference — the four strokes are Induction, Compression, Power, Exhaust, but the five events add Combustion as a separate event, because combustion happens at constant volume at TDC. These events can be shown graphically by a valve timing diagram. The timing diagram shows the relationship between the events and degrees of crankshaft rotation. Each arc between TDC and BDC represents 180 degrees of crankshaft rotation. So let me tie this together. The stroke is the piston's linear travel, and it equals twice the crank-throw. The crankshaft rotates 180 degrees for each stroke, so the full four-stroke cycle is 720 degrees. The bore is the cylinder's internal diameter. And the five events — Induction, Compression, Combustion, Power, Exhaust — are laid out on the timing diagram, where each arc between TDC and BDC is 180 degrees of crankshaft rotation. Combustion is the controlled burning at constant volume at TDC, not an explosion. That's the theoretical Otto cycle in its entirety.