diesel cycle explained: processes, Application and Efficiency

 The Diesel cycle is a thermodynamic cycle that describes the functioning of diesel engines, which operate on diesel fuel. It differs from the Otto cycle in its fuel type and combustion process. The Diesel cycle primarily consists of four processes: two adiabatic (compression and expansion) and two isochoric (heat addition and heat rejection), similar to the Otto cycle, but with key differences in how combustion occurs.

Key Phases of the Diesel Cycle:

1. Adiabatic Compression (Process 1-2):

   • The cycle begins with the piston in the cylinder moving down, drawing in air. The piston then compresses the air to a much higher pressure than in the Otto cycle, which raises the air temperature significantly.
   • Because diesel engines use a higher compression ratio (typically between 14:1 and 25:1), temperatures can reach around 500–700°C (932–1292°F) at the end of compression.

2. Constant Pressure Heat Addition (Process 2-3):

   • After reaching the maximum compression, a fuel injector sprays diesel fuel into the hot, compressed air. The fuel ignites spontaneously due to the high temperature, causing a rapid increase in pressure.
   • This process happens at almost constant pressure, unlike the Otto cycle where combustion occurs at constant volume.

3. Adiabatic Expansion (Process 3-4):

   • The high-pressure gases from combustion then expand, doing work on the piston as it moves down during the power stroke. This is similar to the power stroke in the Otto cycle but results in a greater expansion ratio and often more mechanical efficiency.

4. Isochoric Heat Rejection (Process 4-1):

   • After the expansion stroke, the exhaust valves open, allowing the exhaust gases to exit the cylinder at constant volume, reducing pressure and temperature before the cycle repeats.

Important Concepts:

• Compression Ratio: The Diesel cycle has a higher compression ratio compared to the Otto cycle, contributing to its efficiency. This results in higher thermal efficiency due to the greater temperature rise.

• Thermal Efficiency: The efficiency of the Diesel cycle can be expressed as

                                             $\eta =1-\frac{1}{r^{\gamma -1}}\cdot \frac{1}{{\rho }^{\gamma }}$, 

where $r$ is the compression ratio and $\rho$ is the cutoff ratio (the ratio of the cylinder volume after combustion to the volume before combustion).

• Ignition: Unlike Otto engines that use spark plugs, diesel engines rely on compression ignition, which means the fuel ignites automatically when injected into very hot compressed air.

Applications:

Diesel engines are widely used in heavy-duty vehicles, trucks, ships, and generators due to their efficiency and durability. They are also favored in applications where high torque is required.

If you’d like to explore any specific aspects of the Diesel cycle further, let me know!