In a two-stroke petrol engine, the inlet port is opened by the piston:

Explanation:

Two-Stroke Petrol Engine:

• A two-stroke petrol engine is a type of internal combustion engine that completes a power cycle in two strokes of the piston during only one crankshaft revolution. In this type of engine, the inlet port, transfer port, and exhaust port play crucial roles in ensuring the proper intake of the air-fuel mixture, transfer of the mixture to the combustion chamber, and expulsion of exhaust gases.
• The opening and closing of the inlet port in a two-stroke engine are controlled by the movement of the piston itself. Unlike in four-stroke engines, there are no valves in a traditional two-stroke engine; instead, ports are used. The timing of these ports is critical for the efficient operation of the engine.

30° to 40° before BDC:

• The inlet port begins to open when the piston is moving towards the bottom dead center (BDC), which allows the fresh air-fuel mixture to enter the crankcase. This occurs slightly before the piston reaches BDC, typically in the range of 30° to 40° before BDC, to ensure that the mixture has enough time to flow into the crankcase under the influence of the pressure difference created by the piston's motion.

Working Mechanism:

1. Induction Phase: As the piston moves downward during its power stroke, it simultaneously uncovers the inlet port. This movement creates a low-pressure area in the crankcase. The fresh air-fuel mixture is then drawn into the crankcase through the inlet port due to the pressure difference.

2. Timing of Inlet Port Opening: The inlet port opens slightly before the piston reaches BDC, as specified in the range of 30° to 40° before BDC. This ensures that the air-fuel mixture starts entering the crankcase at the optimal time, maximizing the engine's efficiency and power output.

3. Transfer and Exhaust Phases: Once the piston starts moving upward after reaching BDC, it compresses the air-fuel mixture in the crankcase. At the same time, the transfer port and exhaust port open to allow the fresh mixture to enter the combustion chamber and the exhaust gases to exit, respectively.