Valve Timing Diagram of IC Engine.

Experiment No. – 7

Name of the Experiment: –

Valve Timing Diagram of IC Engine

Valve timing :

In a piston engine, the valve timing is the precise timing of the opening and closing of the valves. In an internal combustion engine these are usually poppet valves and in a steam engine they are usually slide valves or piston valves

Valve overlap:

With traditional fixed valve timing, an engine will have a period of “valve overlap” at the end of the exhaust stroke, when both the intake and exhaust valves are open. The intake valve is opened before the exhaust gases have completely left the cylinder, and their considerable velocity assists in drawing in the fresh charge. Engine designers aim to close the exhaust valve just as the fresh charge from the intake valve reaches it, to prevent either loss of fresh charge or unscavenged exhaust gas. In the diagram, the valve overlap periods are indicated by the overlap of the red and blue arcs.

Construction of four stroke petrol engine:

In four stroke petrol engine the entire cycle is completed in four strokes. In this the cycle is completed in two revolution of crank shaft. Each stroke consist of 180* and cycle is completed in 720*of crank shaft rotation.

Four stroke engine diagram shown in figure.

Working of four stroke petrol engine:

There are four strokes to complete the cycle:

1) Suction stroke:

In this stroke the piston is start moving downward direction. During this stroke the suction valve is opened and exhaust valve is closed. Pressure is dropped in the cylinder and the mixture of fuel and air is sucked inside the cylinder.

2) Compression stroke:

In this stroke the piston moves to upward direction. Inlet and exhaust valves are closed in this stroke. Pressure and temperature increases and just before the end of power stroke an electric spark is produced by spark plug and fuel is ignited.

3) Power stroke:

In this stroke the inlet and exhaust valves are closed. The high pressure developed by the burnt gases forces the piston to move downward towards BDC. Power is obtain during this expansion of burnt gases.

4) Exhaust stroke:

In this stroke the piston moves in upward direction. Exhaust valve is opened and inlet valve is closed. Burnt gases are escaped from the cylinder. Then exhaust valve is closed.

Table for camshaft rotation

Cylinder no.	Inlet valve			Exhaust valve
	Open	Close	Duration	Open	Close	Duration
1	358°	100°	102°	255°	358°	103°
2	85°	200°	115°	340°	85°	115°
3	273°	13°	100°	158°	265°	107°
4	178°	285°	107°	67°	180°	113°

 Table For crankshaft rotation

Cylinder no.	Inlet valve			Exhaust valve
	Open	Close	Duration	Open	Close	Duration
1	358°	205°	207°	148°	354°	206°
2	175°	36°	221°	315°	175°	220°
3	181°	24°	203°	317°	165°	208°
4	358°	206°	208°	135°	359°	224°

The chain reaction…

Describing the action of the piston within the cylinder generally start with the piston in the bottom dead center position (BDC), at the end of its travel downward before it’s travel upwards. With the piston, having covered the intake ports, travels upwards, compressing the trapped air from its original volume to a fraction of it, generally anywhere from 17:1 to 23:1 (gas engines are about 9:1) to about 42 bars (~600psi). The air’s volume reduction crushes the air molecule together and as a result, they build up heat.

Slightly before top dead center (TDC), around 10 degrees, injection of atomized fuel begins, it burns, ignited by the high temperature from to the air being compressed. Injection last about 30 degrees, this varies depending on load. The fuel continues to burn and expands which increases the pressure and temperature within the cylinder. Around 15 degrees after TDC, combustion is complete and maximum pressure is reached within the cylinder, this stage is commonly called P_max. P_max is anywhere between 180 bars to 195 bars (~2500 – 2800 psi); this is why Diesel engine construction is so robust, it is to withstand this pressure.

Camshafts

Depending on the design, this functionality is implemented using one or more camshafts to open and close the valves according to the engine’s four stroke cycle. Since precise timing (which is known as valve timing) is required to achieve this task, camshaft is connected to the engine’s crankshaft. The exact connection might differ from manufacturer and model but a general overview (using two camshafts) looks like this:

The connection of the crankshaft and the camshaft(s) is achieved using a so called timing belt and the ideal timing based on the crankshaft’s rotation is a result of the camshaft gears which are connected to the crankshaft using the aforementioned timing belt.

Three stages which are identical for both opening and closing of the valves.

• Ramp
  The stage when the valve will start opening (for opening) or has just finished closing (for closing cycle).
• Flank
  This part of the cam is the factor of how fast or slow the valve will start opening or closing depending on the side.
•  Nose
  The point where the valve reaches its maximum lift. This part also determines how deep will be that lift.
  Although the terminology might differ, the operation remains the same.
  
  

Duration is the time required (measured in degrees) from the beginning of opening/closing to reaching the maximum/minimum lift of a valve. This is one of the most important characteristics among camshaft technical specifications. Now, the overlap is an interval when both intake and exhaust valves are neither closed nor open.

Conclusion: –

To be written by student.