10213 Air Standard Diesel Cycle

The practical cycles in diesel engines are based on the Diesel cycle. The air-standard Diesel cycle assumes that heat addition takes place at constant pressure, while heat rej ection occurs under constant volume. The cycle is shown on a p-v (pressure-volume) diagram in Figure 10.8.

The cycle begins with the intake of fresh air into the cylinder between 5 to 1.

FIGURE 10.8 p-v diagram of an air-standard Diesel cycle.

The intake valve is open between 5 and 1. The next process, 1 to 2, is the same as in the Otto cycle, when isentropic (constant entropy) compression takes place as the piston moves from the BDC to the TDC. With a sufficiently high compression ratio, the temperature and pressure of air reaches such a level that the combustion starts spontaneously due to inj ection of fuel near the end of the compression stroke. The heat is transferred to the working fluid under constant pressure during combustion in Process 2 to 3, which also makes up the first part of the expansion or power stroke. The isentropic expansion in Process 3 to 4

makes up for the rest of the power stroke. The exhaust valve opens at state point 4, allowing the pressure to drop under constant volume during Process 4 to 1. Heat is rejected during this process, while the piston is at BDC. The exhaust of the burnt fuel takes place during Process 1 to 5 at essentially constant pressure. The exhaust valve then closes, the intake valve opens, and the cylinder is ready to draw in fresh air for a repeat of the cycle.

The nominal range of compression ratios in CI engines is 13/1 to 17/1, and the air-fuel ratios used lie between 20/1 and 25/1. The higher compression ratio aided by the work produced during combustion results in higher efficiency in diesel engines compared to gasoline engines. Efficiencies in diesel engines can be as high as 40%. The diesel engine has a lower specific power than the gasoline engine. Diesel engines also have a broad torque range, as shown in Figure 10.9.

The maj or drawbacks of diesel engines include the requirement of stronger and heavier components that increase the mass of the engine and the speed limitation of the inj ection and flame propagation time. The improvements in diesel engines are directed toward reducing nitrogen oxides in the exhaust; and reducing the noise, vibration, and smell of the engine. Recent automotive diesel engines developed addressing the aforementioned issues made them excellent candidates for HEV applications.

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