The air-standard Diesel cycle is a thermodynamic cycle that models the operation of compression-ignition internal combustion engines. It differs from the Otto cycle primarily in the manner of heat addition. Understanding the specific processes of heat addition and heat rejection is crucial for comprehending the Diesel cycle's characteristics.

Diesel Cycle Processes Overview

The ideal air-standard Diesel cycle consists of four distinct processes:

• Isentropic Compression (Process 1-2): Air is compressed without heat transfer (adiabatically) and reversibly. During this stage, both the pressure and temperature of the air increase significantly.
• Constant Pressure Heat Addition (Process 2-3): Fuel is injected into the highly compressed, hot air, and ignition occurs. Combustion proceeds while the piston moves, allowing the pressure to remain constant as heat is added to the system. This is the defining characteristic of the Diesel cycle's combustion phase.
• Isentropic Expansion (Process 3-4): The hot combustion gases expand, doing work on the piston. This process is also adiabatic and reversible, leading to a decrease in both pressure and temperature. This is the power stroke of the engine.
• Constant Volume Heat Rejection (Process 4-1): Heat is rejected from the working fluid at a constant volume. This process typically represents the exhaust stroke, where exhaust gases are expelled and fresh air is prepared for the next cycle.

Heat Addition in Diesel Cycle

For an air-standard Diesel cycle, the process of heat addition is a critical distinction. Unlike gasoline engines (modeled by the Otto cycle) where combustion occurs almost instantaneously at constant volume, in a Diesel engine, fuel injection and subsequent combustion occur over a short period during which the piston moves. This allows the heat to be added at a nearly constant pressure.

Heat Rejection in Diesel Cycle

The heat rejection process in the air-standard Diesel cycle is modeled as occurring at a constant volume. This phase of the cycle represents the rapid release of heat from the system as exhaust gases are cleared and the cycle returns to its initial state before a new intake of air.

Therefore, based on the ideal processes of the air-standard Diesel cycle, heat addition occurs at constant pressure and heat rejection occurs at constant volume.