Understanding the Diesel Cycle Combustion Process

The diesel cycle is a thermodynamic cycle that approximates the process occurring in a diesel engine. It consists of a sequence of four distinct processes.

Ideal Diesel Cycle Processes

An ideal diesel cycle is typically represented by the following four processes:

1. Process 1-2: Reversible adiabatic compression (Isentropic compression). Air is compressed, increasing its temperature and pressure.
2. Process 2-3: Reversible isobaric heat addition (Constant pressure combustion). Fuel is injected into the hot compressed air and burns, adding heat to the system while the piston moves, maintaining constant pressure. This is the combustion phase.
3. Process 3-4: Reversible adiabatic expansion (Isentropic expansion). The hot gases expand, pushing the piston and doing work.
4. Process 4-1: Reversible isochoric heat rejection (Constant volume heat rejection). Heat is rejected from the system at constant volume.

Focus on Diesel Combustion

In the diesel cycle, combustion is modeled as occurring at constant pressure. Here's why:

• Diesel engines inject fuel towards the end of the compression stroke, just before the piston reaches Top Dead Centre (TDC).
• The fuel is injected gradually over a short period.
• As the fuel burns, the heat release causes the working fluid (air and combustion products) to expand.
• However, because the piston is starting to move downwards during the combustion process, it accommodates this expansion.
• The rate of fuel injection and combustion is controlled such that the pressure inside the cylinder remains approximately constant during this heat addition phase.

This is a key difference compared to the Otto cycle (used in petrol engines), where a spark ignites a pre-mixed fuel-air charge rapidly at constant volume (piston near TDC).

Comparison: Diesel vs. Otto Combustion

Therefore, based on the ideal diesel cycle model, combustion takes place at constant pressure.

Pressure-Volume (P-v) Diagram Representation

On a P-v diagram:

• Process 1-2 (compression) is a curve upwards and left (increasing P, decreasing v).
• Process 2-3 (combustion/heat addition) is a horizontal line to the right (increasing v, constant P).
• Process 3-4 (expansion) is a curve downwards and right (decreasing P, increasing v).
• Process 4-1 (heat rejection) is a vertical line upwards (increasing P, constant v).

This visual representation clearly shows the isobaric (constant pressure) nature of the heat addition phase, which corresponds to combustion.

Conclusion on Diesel Combustion Pressure

In summary, the defining characteristic of the heat addition (combustion) process in the ideal diesel cycle is that it occurs while the pressure remains constant. This is achieved through controlled fuel injection as the piston begins to move downwards.

The combustion process in the diesel cycle occurs at constant pressure.

Revision Table: Key Concepts in Diesel Cycle

Additional Information on Diesel Cycle Thermodynamics

The efficiency of an ideal diesel cycle is given by the formula:

\eta_{diesel} = 1 - \frac{1}{r^{\gamma-1}} \left( \frac{r_c^{\gamma} - 1}{\gamma(r_c - 1)} \right)

Where:

• \eta_{diesel} is the thermal efficiency.
• r = \frac{V_1}{V_2} is the compression ratio.
• r_c = \frac{V_3}{V_2} is the cut-off ratio (ratio of volumes after and before the heat addition process).
• \gamma = \frac{C_p}{C_v} is the ratio of specific heats for the working fluid.

This formula highlights how both the compression ratio and the cut-off ratio influence the efficiency of the diesel cycle.

Real diesel engines deviate from the ideal cycle due to factors like friction, pressure drops during intake and exhaust, and non-instantaneous combustion and heat transfer.