In four-stroke diesel engine, which valves are closed during the expansion stroke?

Explanation:

Four-Stroke Diesel Engine: Expansion Stroke

A four-stroke diesel engine operates through four distinct strokes: intake, compression, power (expansion), and exhaust. Each stroke has a specific function and involves the opening and closing of the intake and exhaust valves at appropriate times to facilitate the engine's operation. During the expansion stroke, also known as the power stroke, the engine converts the energy from the combustion of fuel into mechanical work.

During the expansion stroke in a four-stroke diesel engine, the following events occur:

• At the beginning of the expansion stroke, the piston is at its top dead center (TDC) position, and both the intake and exhaust valves are closed. This ensures that the combustion chamber is sealed.
• The fuel, which was injected at the end of the compression stroke, ignites due to the high temperature and pressure inside the combustion chamber. This ignition causes a rapid increase in pressure, forcing the piston downwards.
• As the piston moves downwards towards the bottom dead center (BDC), it converts the energy from the expanding gases into mechanical work, which is transmitted to the crankshaft.
• Throughout this stroke, both the intake and exhaust valves remain closed to ensure that the expanding gases exert maximum pressure on the piston, maximizing the efficiency of the power stroke.
• As the piston reaches the BDC, the expansion stroke ends, and the exhaust stroke begins, where the exhaust valve opens to expel the burnt gases.

Therefore, during the expansion stroke, both the intake and exhaust valves are closed to allow the combustion gases to expand and push the piston downwards efficiently.