In theory, the Otto combustion cycle is one in which combustion, caused by an electric spark, occurs at constant volume. The Otto cycle and its principles serve as the basis for modern gasoline engine design.

In the Otto cycle (view A of fig. 2-5), compression of the charge in the cylinder occurs at line AB. Spark ignition occurs at point B. Because of the volatility of the mixture, combustion practically amounts to an explosion. Combustion, represented by line BC, occurs (theoretically) just as the piston reaches TDC. During combustion, there is almost no piston travel. Thus, there is no change in the volume of the gas in the cylinder. This lack of change in volume accounts for the descriptive term, CONSTANT VOLUME. During combustion, there is a rapid rise in temperature followed by a pressure increase. The pressure increase performs the work during the expansion phase, as represented by line CD in the figure. The removal of gases, represented by line DA in the figure, is at CONSTANT VOLUME.

Theoretical Diesel (Constant-Pressure) Cycle

When discussing diesel engines, we must point out that there is a difference between the theoretical, or true, diesel cycle and the actual diesel cycle, that really occurs in an operating diesel engine.

The true diesel cycle may be defined as one in which combustion, induced by compression ignition, theoretically occurs at a constant pressure. (See view B of fig. 2-5.) Compression of the air (line AB) increases its temperature to a point that ignition occurs automatically when the fuel is injected. Fuel injection and combustion are controlled to give constant pressure combustion (line BC). This phase is followed by expansion (line CD) and CONSTANT-VOLUME REJECTION of the gases (line DA). In the true diesel cycle, the burning of the mixture of fuel and compressed air is a relatively slow process when compared with the quick, explosive type of combustion process of the Otto cycle. In the true diesel cycle, the injected fuel penetrates the compressed air, some of the fuel ignites, then the rest of the charge bums. The expansion of the gases keeps pace with the change in volume caused by piston travel. Thus, combustion is said to occur at CONSTANT PRESSURE (line BC).