Four-Stroke Cycle in Diesel Engines

Four-stroke Cycle

• The diesel cycle is a type of four-stroke cycle, which consists of:
  1. Intake stroke: Air is drawn into the cylinder through the intake valve.
  2. Compression stroke: The intake valve closes, and the air is compressed by the piston.
  3. Power stroke: Fuel is injected into the cylinder, causing combustion, which pushes the piston down.
  4. Exhaust stroke: The exhaust valve opens, and the exhaust gases are released out of the cylinder.

Thermodynamic Analysis

• The diesel cycle is a constant pressure cycle, meaning that the combustion process occurs at a constant pressure.
• The cycle can be divided into four processes:
  1. Isochoric compression (compression stroke): The air is compressed at constant volume.
  2. Constant pressure heat addition (power stroke): Fuel is injected, and combustion occurs at constant pressure.
  3. Isentropic expansion (power stroke): The hot gases expand, pushing the piston down.
  4. Isochoric heat rejection (exhaust stroke): The exhaust gases are released, and the cycle repeats.

Fuel Injection

• In a diesel engine, fuel is injected into the cylinder at the end of the compression stroke.
• The fuel injection process is timed to coincide with the top dead center (TDC) of the piston.
• The injection process is typically achieved through a fuel injector, which sprays fuel into the cylinder at high pressure.

Engine Efficiency

• The diesel cycle is more efficient than the Otto cycle (used in gasoline engines) due to:
  • Higher compression ratio, which leads to higher thermal efficiency.
  • Constant pressure heat addition, which allows for more efficient energy conversion.
• The efficiency of a diesel engine is typically measured by its brake thermal efficiency (BTE), which is the ratio of the engine's output power to the input energy.

Compression Ratio

• The compression ratio is the ratio of the cylinder volume when the piston is at the bottom dead center (BDC) to the volume when the piston is at the top dead center (TDC).
• A higher compression ratio allows for:
  • Better fuel efficiency, due to increased thermal efficiency.
  • Increased power output, due to more efficient energy conversion.
• However, a higher compression ratio also increases the engine's mechanical stress and knocking tendency.

Four-Stroke Cycle

• The diesel cycle consists of four strokes: intake, compression, power, and exhaust.
• Intake stroke: air is drawn into the cylinder through the intake valve.
• Compression stroke: air is compressed by the piston.
• Power stroke: fuel is injected, causing combustion, which pushes the piston down.
• Exhaust stroke: exhaust valve opens, and exhaust gases are released out of the cylinder.

Thermodynamic Analysis

• The diesel cycle is a constant pressure cycle.
• The cycle consists of four processes: isochoric compression, constant pressure heat addition, isentropic expansion, and isochoric heat rejection.
• Isochoric compression: air is compressed at constant volume.
• Constant pressure heat addition: fuel is injected, and combustion occurs at constant pressure.
• Isentropic expansion: hot gases expand, pushing the piston down.
• Isochoric heat rejection: exhaust gases are released, and the cycle repeats.

Fuel Injection

• Fuel is injected into the cylinder at the end of the compression stroke.
• Fuel injection is timed to coincide with top dead center (TDC) of the piston.
• Fuel injection is achieved through a fuel injector, which sprays fuel into the cylinder at high pressure.

Engine Efficiency

• The diesel cycle is more efficient than the Otto cycle due to higher compression ratio and constant pressure heat addition.
• The efficiency of a diesel engine is measured by its brake thermal efficiency (BTE), which is the ratio of the engine's output power to the input energy.

Compression Ratio

• The compression ratio is the ratio of the cylinder volume when the piston is at the bottom dead center (BDC) to the volume when the piston is at the top dead center (TDC).
• A higher compression ratio leads to better fuel efficiency and increased power output due to increased thermal efficiency.
• However, a higher compression ratio also increases the engine's mechanical stress and knocking tendency.