In a Diesel cycle, the heat is supplied during compression from the external source at
Constant pressure
Diesel Cycle: Understanding Heat Supply
The Diesel cycle is a thermodynamic cycle that approximates the processes occurring in a Diesel engine. Unlike the Otto cycle, which assumes heat addition at constant volume, the ideal Diesel cycle is characterized by heat addition occurring at constant pressure.
An ideal Diesel cycle consists of four distinct processes:
- Isentropic Compression (1-2): Air is compressed without heat transfer, increasing its pressure and temperature. Work is done on the fluid.
- Constant Pressure Heat Addition (2-3): Fuel is injected and ignites due to the high temperature of the compressed air. Heat is added to the system at constant pressure. This is the power stroke where combustion occurs.
- Isentropic Expansion (3-4): The hot combustion gases expand, doing work on the piston. This process also occurs without heat transfer.
- Constant Volume Heat Rejection (4-1): Heat is rejected from the system at constant volume, typically when the exhaust valve opens, bringing the gas back to its initial state.
The question asks about the heat supply in a Diesel cycle. While the phrasing "during compression from the external source at" can be a bit confusing, it refers to the phase where heat (from fuel combustion) is added to the cycle. This heat addition in an ideal Diesel cycle takes place after the compression process is complete, and it occurs at a constant pressure.
Diesel Cycle Processes Summary
| Process | Description | Thermodynamic Characteristic |
|---|---|---|
| 1-2 | Isentropic Compression | No heat transfer ($Q = 0$), work done on the gas. |
| 2-3 | Constant Pressure Heat Addition | Heat supplied ($Q_{in}$), pressure remains constant ($P = \text{constant}$). |
| 3-4 | Isentropic Expansion | No heat transfer ($Q = 0$), work done by the gas. |
| 4-1 | Constant Volume Heat Rejection | Heat rejected ($Q_{out}$), volume remains constant ($V = \text{constant}$). |
As evident from the detailed processes of the ideal Diesel cycle, the heat supplied (or heat addition phase) occurs at Constant pressure. This is a defining characteristic that distinguishes the Diesel cycle from other internal combustion engine cycles like the Otto cycle.
Therefore, when considering where heat is supplied in the Diesel cycle, it is supplied at constant pressure after the air has been compressed to a high temperature.
Important Questions from Diesel Cycle
Which of the following medium is compressed in a diesel engine?
For the same maximum pressure and peak temperature, which cycle will be most efficient?
- Which of the following is compressed in diesel engine?
A diesel engine has a compression ratio of 18 and cut off takes place at 5% of the stroke. What will be cut off ratio?
The compression ratio of diesel engines normally have the range:
