Understanding Heat Addition in the Diesel Cycle

The Diesel cycle is an internal combustion engine thermodynamic cycle. It is often compared to the Otto cycle, but they differ in how heat is added and rejected.

Processes in a Standard Diesel Cycle

A standard ideal Diesel cycle consists of four main processes:

1. Isentropic Compression: The air is compressed without heat transfer. Volume decreases, and pressure and temperature increase.
2. Constant Pressure Heat Addition: Fuel is injected into the hot compressed air and combusts, adding heat to the system while the piston moves out, maintaining constant pressure.
3. Isentropic Expansion: The hot gases expand without heat transfer, doing work on the piston. Volume increases, and pressure and temperature decrease.
4. Constant Volume Heat Rejection: The remaining heat is rejected to the surroundings when the piston is at bottom dead center. Volume remains constant, and pressure and temperature decrease.

Heat Addition in the Diesel Cycle

In the Diesel cycle, the combustion of fuel occurs while the piston is moving away from top dead center. Fuel is injected over a period of time, and as it burns, the volume changes. However, due to the relatively slow rate of combustion compared to the piston movement and the continuous injection of fuel, the process is modeled ideally as occurring at constant pressure.

This constant pressure heat addition phase is a key distinguishing feature of the Diesel cycle compared to the Otto cycle (which has constant volume heat addition) or the Atkinson/Miller cycles (which have more complex heat addition processes).

Comparing Heat Addition: Diesel vs. Otto Cycle

• Diesel Cycle: Heat is added at constant pressure (Process 2-3). This is because fuel injection and combustion happen over a period as the volume increases.
• Otto Cycle: Heat is added at constant volume (Process 2-3). This is because ignition occurs when the piston is near top dead center, and combustion is assumed to be instantaneous, before the volume can change significantly.

Therefore, in a Diesel cycle, heat addition occurs at constant pressure.