THE IDEAL OTTO CYCLE

A cycle is an idealization of what goes on in one of the devices that thermodynamicists call heat engines: that is, a gasoline or diesel engine, a jet engine, a steam engine, and so forth. All of these take some energy source and convert some of that energy into useful work. In the spark-ignition engine the energy source is a chemical fuel, usually gasoline, which is combined with oxygen from the air by burning to release heat. Expansion of the heated gases does the mechanical work.

For the spark-ignition engine the idealization is called the Otto cycle, after Dr. N. A. Otto who, in 1876, patented a stationary gas engine using approximately this cycle. In order to understand this ideal cycle, we must imagine a piston in a cylinder. The piston is connected to a crank by a connecting rod – see Figure 1.1. The crank rotates, and the piston travels up and down. There are two valves, an inlet and an exhaust valve, and an arrangement to open and close them. The idealized cycle is illustrated in Figure 1.2.

In Figure 1.2 we plot the pressure in the cylinder against the volume in the cylinder. Notice that the piston does not go quite all the way to the top of the cylinder; the piston is at the top of its travel at 0, 2 and 3, and there is a small space still above it, the combustion chamber.