What is the Diesel cycle, and how does it differ from the Otto cycle?

Short Answer:

The Diesel cycle is a thermodynamic cycle used in diesel engines, where air is compressed to a high pressure and temperature, and then fuel is injected, causing combustion at constant pressure. It consists of four processes: isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-volume heat rejection.

The main difference between the Diesel cycle and the Otto cycle (used in petrol engines) is the method of heat addition. In the Otto cycle, heat is added at constant volume, while in the Diesel cycle, heat is added at constant pressure. This difference affects the compression ratio, efficiency, and working of the engine.

Detailed Explanation:

Diesel cycle and its difference from Otto cycle

The Diesel cycle and the Otto cycle are two important air-standard cycles used in internal combustion engines. While both cycles have similar basic components (compression, combustion, expansion, and exhaust), they differ mainly in how the combustion (heat addition) occurs.

The Diesel cycle is used in compression ignition engines (diesel engines), whereas the Otto cycle is used in spark ignition engines (petrol engines). Understanding the difference between these two helps in comparing the performance, efficiency, and application of diesel and petrol engines.

Diesel Cycle – Process Description

The Diesel cycle has four ideal processes:

  1. Process 1–2: Isentropic Compression
    • Air is compressed adiabatically (no heat loss) inside the cylinder.
    • Pressure and temperature rise sharply due to compression.
    • This process requires high compression ratios, around 14:1 to 22:1.
  2. Process 2–3: Constant Pressure Heat Addition
    • At the end of compression, fuel is injected into the hot compressed air.
    • Fuel burns and adds heat at constant pressure.
    • Volume increases due to combustion.
  3. Process 3–4: Isentropic Expansion (Power Stroke)
    • High-pressure gases expand adiabatically, pushing the piston and doing work.
    • This is the main power-producing stroke.
  4. Process 4–1: Constant Volume Heat Rejection
    • At the end of expansion, the gases are cooled at constant volume, rejecting heat to the surroundings.

Otto Cycle – Key Difference

The Otto cycle, also with four processes, has the heat addition at constant volume (not pressure). In Otto cycle:

  • Fuel-air mixture is compressed and then ignited by a spark plug.
  • Combustion takes place almost instantly at constant volume.
  • Compression ratio is lower than Diesel cycle (typically 8:1 to 12:1).

Major Differences Between Diesel and Otto Cycles

  1. Combustion Type
    • Diesel Cycle: Constant pressure (compression ignition)
    • Otto Cycle: Constant volume (spark ignition)
  2. Compression Ratio
    • Diesel engines use higher compression ratios.
    • Otto engines use lower ratios due to knocking risk.
  3. Fuel Type
    • Diesel Cycle: Diesel fuel
    • Otto Cycle: Petrol (gasoline)
  4. Efficiency
    • Diesel engines are generally more efficient at part load due to higher compression ratio and lean burning.
  5. Ignition Method
    • Diesel: Fuel ignites due to high temperature of compressed air.
    • Otto: Requires a spark plug to initiate combustion.

Applications

  • Diesel Cycle: Trucks, buses, trains, ships, generators, heavy machinery
  • Otto Cycle: Cars, bikes, small power generators, lawnmowers

Efficiency Comparison

Although the Otto cycle can have higher theoretical efficiency for the same compression ratio, the Diesel cycle is used with higher compression ratios, which gives it better real-world efficiency. Also, diesel fuel has higher energy content and better fuel economy, especially in heavy-duty engines.

Conclusion

The Diesel cycle is a power-producing cycle used in diesel engines where heat is added at constant pressure. It differs from the Otto cycle, which adds heat at constant volume, commonly found in petrol engines. These differences affect compression ratio, efficiency, ignition method, and overall performance. Diesel engines are typically more efficient and powerful, making them suitable for heavy-duty applications, while Otto engines are lighter and simpler, ideal for smaller vehicles.