Short Answer:

The Diesel cycle is a thermodynamic cycle that represents the working of a compression ignition (CI) engine. It consists of four main processes—two are isentropic (compression and expansion), one is constant pressure (heat addition), and one is constant volume (heat rejection). In this cycle, fuel is injected into highly compressed air, and combustion occurs due to the heat of compression.

The Diesel cycle is used in diesel engines where air is compressed to a high temperature before fuel injection. This cycle is more efficient than the Otto cycle at the same compression ratio and is mainly used in heavy vehicles, trucks, ships, and industrial engines.

Detailed Explanation :

Diesel Cycle

The Diesel cycle is an air-standard thermodynamic cycle that explains how a compression ignition (CI) engine operates. It was developed by Rudolf Diesel in 1897 and is widely used in engines that operate on diesel fuel. In this cycle, only air is compressed during the compression stroke, and the fuel is injected directly into the cylinder near the end of compression. The high temperature of the compressed air causes the fuel to ignite automatically without the need for a spark plug.

The Diesel cycle is different from the Otto cycle because the combustion process in the Diesel cycle occurs at constant pressure, while in the Otto cycle it occurs at constant volume. This difference in heat addition method leads to variations in efficiency and power output between the two cycles.

Processes in Diesel Cycle

The Diesel cycle consists of four thermodynamic processes which form a closed loop on the Pressure-Volume (P–V) and Temperature-Entropy (T–S) diagrams.

1. Isentropic Compression (Process 1–2):
   In this process, air is compressed adiabatically (without heat transfer) by the upward movement of the piston. The pressure and temperature of the air rise significantly, while the volume decreases. The purpose of this step is to increase the air temperature to a level high enough to ignite the fuel when it is injected.
2. Constant Pressure Heat Addition (Process 2–3):
   Near the end of the compression stroke, fuel is injected into the hot compressed air through a fuel injector. The fuel begins to burn as it mixes with the air. During combustion, heat is added at constant pressure because the piston moves downward slightly, allowing the gases to expand. The temperature and volume increase considerably in this process.
3. Isentropic Expansion (Process 3–4):
   The high-pressure, high-temperature gases expand adiabatically, pushing the piston downward. This is the power stroke, where the engine produces mechanical work. During expansion, pressure and temperature decrease while the volume increases.
4. Constant Volume Heat Rejection (Process 4–1):
   After the expansion stroke, the exhaust valve opens, and heat is rejected at constant volume. The gas returns to its initial state, completing the cycle. The exhaust gases are then expelled from the cylinder, and the process repeats.

Representation on P–V and T–S Diagrams

• On the P–V diagram, the Diesel cycle consists of two curved isentropic processes, one horizontal constant pressure line, and one vertical constant volume line.
• On the T–S diagram, the isentropic processes appear as vertical lines (constant entropy), while the constant pressure and constant volume processes appear as inclined and horizontal lines, respectively.

These diagrams are helpful in understanding how the different thermodynamic properties like pressure, temperature, and volume vary during the engine operation.

Efficiency of Diesel Cycle

The thermal efficiency () of the Diesel cycle depends on both the compression ratio (r) and the cut-off ratio (), which is the ratio of the volume after combustion to the volume before combustion. The formula for the efficiency is:

Where:

•  = Thermal efficiency of the Diesel cycle
•  = Compression ratio (V₁/V₂)
•  = Cut-off ratio (V₃/V₂)
•  = Ratio of specific heats (Cp/Cv)

From this equation, the Diesel cycle is found to be less efficient than the Otto cycle for the same compression ratio, but since diesel engines can operate at higher compression ratios without knocking, their overall efficiency is higher.

Applications of Diesel Cycle

The Diesel cycle is used in many types of engines that require high torque and efficiency. Common applications include:

• Heavy-duty vehicles like trucks and buses
• Marine engines and ships
• Tractors and construction equipment
• Power generation units and industrial engines

These engines are preferred in areas where durability, fuel efficiency, and high power output are needed.

Advantages of Diesel Cycle

• Higher thermal efficiency compared to Otto cycle at high compression ratios
• Better fuel economy and lower fuel consumption
• More power and torque output
• Longer engine life due to lower operating speeds

Limitations of Diesel Cycle

• Diesel engines are heavier and costlier to manufacture
• Require stronger construction due to high pressure
• Produce more noise and vibration
• Emit more pollutants like NOx and particulates

Conclusion :

The Diesel cycle is a fundamental thermodynamic cycle used in compression ignition engines. It consists of four processes—isentropic compression, constant pressure heat addition, isentropic expansion, and constant volume heat rejection. The high efficiency of the Diesel cycle makes it ideal for heavy-duty applications. It provides better fuel economy and durability compared to the Otto cycle, which is why diesel engines are widely used in transportation, agriculture, and industrial fields.