What is valve timing diagram?

Short Answer:

A valve timing diagram is a graphical representation that shows the exact timing of the opening and closing of the inlet and exhaust valves of an internal combustion engine. It indicates the position of the piston and the crank angle at which these events occur during the engine’s four-stroke cycle.

The valve timing diagram helps to understand the breathing process of the engine, showing how air-fuel mixture enters and exhaust gases leave the cylinder. Proper valve timing improves engine efficiency, power output, and fuel economy while reducing emissions.

Detailed Explanation:

Valve Timing Diagram

A valve timing diagram is an important tool used in the study of internal combustion engines. It shows the relation between the movement of the piston inside the cylinder and the opening and closing of both inlet and exhaust valves. The diagram represents the different strokes of the engine cycle — suction, compression, power, and exhaust — and shows when each valve opens and closes with respect to the crankshaft rotation.

The timing of valve operation is not exactly at the top dead centre (TDC) or bottom dead centre (BDC) positions. Instead, the valves open and close slightly earlier or later than these points to improve the efficiency of the engine. This advanced and delayed valve operation ensures better filling of the cylinder with the air-fuel mixture and complete removal of exhaust gases.

Purpose of Valve Timing Diagram

The main purpose of a valve timing diagram is to show the correct sequence and timing of valve operations. It helps engineers and mechanics to understand how efficiently the engine breathes and how effectively it performs the suction and exhaust processes. The diagram also helps in determining the overlap period when both the inlet and exhaust valves remain open for a short time, which helps in better scavenging and cooling of the combustion chamber.

Valve Timing in Four-Stroke Petrol Engine

In a four-stroke petrol engine, the valve timing is arranged to achieve maximum power and fuel efficiency. The timing of valve events in a typical four-stroke petrol engine is as follows:

  1. Inlet Valve Opens:
    The inlet valve opens before the piston reaches the top dead centre (TDC) at the end of the exhaust stroke, usually about 10° to 20° before TDC. This allows the air-fuel mixture to enter smoothly into the cylinder.
  2. Inlet Valve Closes:
    The inlet valve closes slightly after the piston passes the bottom dead centre (BDC), about 30° to 40° after BDC, ensuring the complete filling of the cylinder with the air-fuel mixture.
  3. Exhaust Valve Opens:
    The exhaust valve opens before the piston reaches the bottom dead centre (BDC) on the power stroke, generally about 35° to 45° before BDC. This allows exhaust gases to start escaping early.
  4. Exhaust Valve Closes:
    The exhaust valve closes after the piston reaches the top dead centre (TDC), around 10° to 15° after TDC, ensuring complete removal of the burnt gases from the cylinder.

Valve Overlap

Valve overlap is an important concept shown in the valve timing diagram. It is the short period during which both the inlet and exhaust valves are open at the same time. This usually happens when the exhaust stroke is ending and the intake stroke is beginning. The overlap helps in improving engine breathing by allowing fresh charge to enter while exhaust gases are still leaving. This improves volumetric efficiency and reduces engine temperature.

Advantages of Correct Valve Timing

  1. Improved Power Output:
    Proper timing ensures that the maximum amount of charge enters and the exhaust gases leave completely, resulting in higher engine power.
  2. Better Fuel Efficiency:
    Correct timing reduces fuel wastage and ensures complete combustion of the air-fuel mixture.
  3. Smooth Operation:
    Balanced timing minimizes knocking and vibration in the engine.
  4. Reduced Emissions:
    Efficient combustion and exhaust result in lower emission of harmful gases.
  5. Longer Engine Life:
    Correct timing reduces stress on engine parts and helps in smoother operation, increasing the lifespan of the engine.

Importance of Valve Timing Diagram

The valve timing diagram helps engineers analyze and adjust the timing of valve operation for maximum efficiency. Even a small change in timing can affect engine performance significantly. For example, if the inlet valve opens too late, the cylinder may not fill completely, reducing power output. Similarly, if the exhaust valve closes too early, it may trap gases inside the cylinder, reducing efficiency. Therefore, designing a correct valve timing diagram is very important for achieving the desired performance and durability of the engine.

Conclusion:

In conclusion, a valve timing diagram is a graphical representation that shows when the inlet and exhaust valves open and close during different strokes of an engine. It helps in understanding and controlling the breathing process of the engine. Proper valve timing improves power, efficiency, and smoothness of operation while reducing fuel consumption and emissions. Hence, the valve timing diagram plays a vital role in the design and analysis of internal combustion engines.