Valve Timing Diagram of Two Stroke and Four Stroke Engine

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Valve Timing Diagram
Valve Timing Diagram

While repairing your bike at a garage, If you have heard about valve timing of engine, don’t confuse. because here in this article i am going to explain you about valve timing diagram. In this article i will explain you valve timing diagram of two and four Stroke engine. so lets begin.

What is Valve Timing Diagram(VTD)?

It is a graphical representation of opening & closing of inlet and exhaust valve of the engine with respect to crank angle. It shows position of inlet and exhaust valve which is open or closed. As we are aware about, opening and closing of valve is depend on movement of piston between TDC & BDC. The relation between position of piston and valves is represented graphical diagram. This graphical diagram is known as Valve timing diagram.

Why VTD is Important?

Valve timing diagram provide following advantage over the ideal valve timing diagram.

  • It allowing a greater mass flow into the engine during Suction stroke. and it will help to develop maximum maximum power.
  • It help to push out the combustion product (flue gases) of previous stroke from combustion chamber.
  • It will reduce the pumping power required for Exhaust stroke.
  • It will help to improve Efficiency of engine and reduce the specific fuel consumption.

Valve timing diagram provide all above benefits so the 2 stroke or 4 stroke engine are designed according to Valve timing diagram.

How does 4 Stroke Engine Works?

How Does Two Stroke Engine Works?

VTD for 4-Stroke Engine 

Theoretical Valve Timing Diagram

Valve Timing Diagram
Theoretical Valve timing Diagram for Four Stroke engine

4 Stroke engine consist of Suction, Compression, Expansion(Power), Exhaust stroke. According to theoretical valve timing Diagram, Inlet valve open at the TDC (beginning of suction stroke) and it closes at the BDC (End of Suction stroke). During compression stroke both valve (Inlet & Exhaust) are remains closed. The spark (In case of petrol engine) or fuel ( in case of diesel engine) is injected at TDC (end of compression stroke). During expansion (power) stroke, both valve (inlet & exhaust) are remains closed. The exhaust valve is open at BDC (beginning of exhaust stroke) and it closes at TDC (end of exhaust stroke)

Actual Valve Timing Diagram for Petrol and Diesel Engine

Valve Timing Diagram

Inlet Valve

  • In case of petrol engine, the inlet valve is open before 15 degree of TDC while in diesel engine Inlet valve open 25 degree before TDC.
  • In case of petrol engine, the inlet valve is closes after 30 degree after BDC while in diesel engine Inlet valve closes 25 degree after BDC.
  • Here one question arises that, Why Inlet valve is closes after BDC? The answer is, When the piston reaches BDC and start its compression stroke, the charge continue to move into the cylinder due to its K.E. To take its advantage the inlet valve is closed after BDC. The K. E. of the charge produces a ram effect which forces more charge to be admitted into the cylinder.

Exhaust Valve

  • In both engine (petrol and diesel engine) the exhaust valve is open 50 degree Before BDC.
  • In case of petrol engine the exhaust valve is closed 20 degree after TDC while in diesel engine exhaust valve closed 15 degree after TDC.
  • In IC engines, ideally the exhaust valve must open at BDC, but practically the exhaust valve is programmed to open, just before reaching BDC. if exhaust valve is open before the BDC, the pressure inside the cylinder drops before the exhaust stroke starts and some products escape. As a result, it becomes easier for the piston to remove the exhaust gases during the exhaust stroke and piston power is saved. There is a overall gain in engine efficiency in this process. Thus ,the exhaust valve is timed to open before the piston reaches BDC.
  • As we already know that inlet valve is Open Before TDC and exhaust valve is closes after BDC. here for some specific time period both valve are open and it help to improve volumetric efficiency also help for scavenging. The scavenging is the process of removing the combustion gases from combustion chamber with the help of free charge intake.
    It’s helps to develop engine power and efficiency.

Spark Injection/ fuel Injection

  • Due to time lag between the supply of spark and ignition of charge, the spark is provided few degree before the end of compression stroke called ignition advance.
  • in case of petrol engine, the spark is provided 35 degree before TDC.
  • In case of diesel engine, the fuel injector start injection of fuel before 5 degree of T.D.C. and stop the injection of fuel 25 degree after T.D.C.

VTD for 2-Stroke Engine 

Valve Timing Diagram
  • In diesel engine, Fuel need to be injected few degree before 10 degree of T.D.C. for better evaporation and mixing of fuel and air and the fuel injection closes after 15 degree of T.D.C.
  • In both petrol and diesel two stroke engine, the Exhaust port open 60 degree before B.D.C. and Closes 60 Degree of B.D.C.
  • In both petrol and diesel two stroke engine, the transport port open 45 degree before B.D.C. and Closes 45 Degree of B.D.C.

The valve timing is changes according to requirement of speed.

Watch this video for better understanding

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