Working of Petrol Engine & Diesel Engine
Detailed step-by-step operational explanation of Four-Stroke Petrol Engines (Otto Cycle) and Two-Stroke Diesel Engines.
Q. Describe the working of a Petrol Engine and a Diesel Engine.
1. Petrol Engine (Four-Stroke Engine)
Definition: A petrol engine is an internal combustion engine designed to run on volatile fuel (gasoline) using spark ignition. It works on the Otto cycle, named after the German engineer Nikolaus Otto who created the first working prototype in 1876.
A typical four-stroke petrol engine undergoes four successive processes (strokes) per operating cycle, corresponding to two full revolutions of the crankshaft:
- Suction / Intake Stroke: The inlet valve opens while the exhaust valve remains closed. The piston moves downward from TDC (Top Dead Center) to BDC (Bottom Dead Center). This movement creates a low-pressure area inside the cylinder, drawing the air-fuel mixture into the combustion chamber. At BDC, the intake valve closes, sealing the mixture.
- Compression Stroke: Both inlet and exhaust valves remain tightly closed. The piston moves upward from BDC to TDC, driven by the momentum of the flywheel. The trapped air-fuel charge is compressed into a small volume at the top of the cylinder, raising its temperature and pressure.
- Power / Combustion Stroke: As the piston reaches TDC, a high-voltage electric spark from the spark plug ignites the compressed air-fuel mixture. Rapid chemical combustion releases immense heat energy. The expanding hot gases exert high pressure, forcing the piston downward from TDC to BDC. This stroke produces useful mechanical work.
- Exhaust Stroke: The exhaust valve opens while the inlet valve stays closed. Driven by the flywheel inertia, the piston moves upward from BDC back to TDC, forcing the burnt exhaust gases out through the open exhaust port. At TDC, the cycle completes and is ready to repeat.
2. Diesel Engine (Two-Stroke Engine)
A two-stroke diesel engine completes its full thermodynamic cycle (intake, compression, power, and exhaust) in just two piston strokes, corresponding to one single revolution of the crankshaft.
Working Mechanism
Unlike petrol engines, a diesel engine relies on high compression to ignite fuel spontaneously (compression ignition) without a spark plug:
- Intake & Compression Stroke (Upward Stroke): As the piston moves upward from BDC to TDC, it draws fresh air into the crankcase through a transfer port. Simultaneously, the piston compresses the air inside the cylinder above it to extremely high pressure and temperature.
- Power & Exhaust Stroke (Downward Stroke): When the piston reaches the top of its stroke (TDC), diesel fuel is injected under high pressure. The hot compressed air causes the fuel to ignite spontaneously. The expanding combustion gases drive the piston downward to BDC (power stroke). As the piston moves down, it uncovers the exhaust ports, allowing burnt gases to escape. A scavenging pump assists in clearing waste gases and introducing fresh air.
Key Point for Board Exams:
- Petrol Engine: Uses air-fuel mixture, operates on the Otto cycle, uses a spark plug for ignition, and typically completes a cycle in 4 strokes.
- Diesel Engine: Compresses air alone, operates on compression ignition (no spark plug required), fuel is directly injected, and a two-stroke version completes a cycle in 2 strokes (1 revolution of the crankshaft).
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