THEORETICAL AND ACTUAL

A valve timing diagram is a graphical representation of the opening and closing of the intake and exhaust valve of the engine, The opening and closing of the valves of the engine depend upon the movement of piston from TDC to BDC, This relation between piston and valves is controlled by setting a graphical representation between these two, which is known as valve timing diagram.

The valve timing diagram comprises of a 360 degree figure which represents the movement of the piston from TDC to BDC in all the strokes of the engine cycle, Which is measured in degrees and the opening and closing of the valves is controlled according to these degrees.

WHY DO WE NEED VALVE TIMING DIAGRAM?

The normal engine completes around 100000 cycles per minute, as we know there are number of processes involve in a single cycle (from the intake of the air-fuel mixture to the exhaust of the combustion residual) of a internal which makes it necessary to be equipped with an effective system that can enable

§ Synchronisation between the steps of a cycle of the engine from intake of air-fuel ratio to the exhaust of combustion residual.
§ Complete seizure of the combustion chamber at the instant at which the combustion of air-fuel mixture takes place as the leakage can cause damage to the engine and can be hazardous.
§ Provide engine with a mixed air and fuel or air in case of diesel engine when required ( at the time of suction) which is the necessity of the engine.
§ Provide the exit for the combustion residual so that the next cycle of the engine can take place.
§ Ideal timing for the opening and closing of the inlet and outlet valve which in turn protect the engine from defects like knocking or detonation.
§ A high compression ratio required to combust the fuel especially in case of diesel engine by overlapping the closing of the valve.
§ The cleaning of engine cylinder which in turn maintain the quality of combustion and decreases wear and tear inside the cylinder.
§ The study of the details of the combustion that is required for the modification of the power of the engine.
So due to these reasons a engine weather it is 2-stroke or 4-stroke is designed according to the valve timing diagram, so that the movement of piston from TDC to BDC is provided with the ideal timing of opening and closing of the intake and exhaust valves respectively.

VALVE TIMING DIAGRAM FOR 4-STROKE ENGINE (PETROL AND DIESEL)

As we all know in 4-stroke engine the cycle completes in 4-strokes that are suction, compression, expansion and exhaust , The relation between the valves (inlet and outlet) and piston movement from TDC to BDC is represented by the graph known as valve timing diagram.

THEORETICAL

Suction Stroke-

The engine cycle starts with this stroke, Inlet valve opens as the piston which is at TDC starts moving towards BDC and the air-fuel mixture in case of petrol and fresh air in case of diesel engine starts entering the cylinder,till the piston moves to BDC.

Compression Stroke-

After the suction stroke the piston again starts moving from BDC to TDC in order to compress the air-fuel (petrol engine) and fresh air (diesel engine) which in turn raises the pressure inside the cylinder which is essential for the combustion of the fuel.
§ The inlet valve closes during this operation to provide seizure of the chamber for the compression of the fuel.

Expansion Stroke-

After compressing the fuel, The combustion of the fuel takes place which in turn pushes the piston which is at TDC towards BDC in order to release the pressure developed by the combustion and output is obtained .
Note – In petrol engine combustion takes place due to the spark produced by the spark plug.
§ In petrol engine the air and fuel charge enters the cylinder during suction stroke.
§ In diesel engine combustion occurs due to the high compression provided by the compression stroke which is responsible for raising the temperature inside cylinder upto auto-ignition temperature of the diesel and air charge.
§ In diesel engine the fresh air enters inside the cylinder during suction stroke and the fuel is sprayed by the fuel injectors over the air.

Exhaust Stroke-

After expansion stroke the piston which is at BDC starts moving towards TDC followed by the opening of exhaust valve for the removal of the combustion residual
§ Exhaust valve closes after the piston reaches TDC.

ACTUAL OR PRACTICAL PROCESS

In suction stroke of 4-stroke engine the inlet valve opens 10-20 degree advance to TDC for the proper intake of air-fuel(petrol) or air (diesel) ,which also provides cleaning of remaining combustion residuals in the combustion chamber.
§ When the piston reaches BDC the compression stroke starts and again the piston starts moving towards TDC ,The inlet valve closes 25-30 degree past the BDC during the compression stroke,which provide complete seizure of the combustion chamber for the compression of air-fuel(petrol engine)and air(diesel engine).

§ During the compression stroke as the piston moves towards TDC ,combustion of fuel takes place 20-35 degree before TDC which provides the proper combustion of fuel and proper propagation of flame.
§ The expansion strokes starts due to the combustion of fuel which in turn releases the pressure inside the combustion chamber and provide rotation to the crank shaft,The piston moves from TDC to BDC during expansion stroke which continuous 30-50 degree before BDC.
§ The exhaust valve opens 30-50 degree before BDC which in turn starts the exhaust stroke and the exhaust of the combustion residual takes place with movement of the piston from BDC toTDC which continues till the 10-20 degree after the piston reaches TDC.
As we can see in the entire cycle of engine valves overlap 2 times i.e. closing of both valves during compression stroke and opening of both valves during exhaust stroke.

VALVE TIMING DIAGRAM FOR 2-STROKE ENGINE

In 2-stroke petrol engine as we all know the engine cycle completes in 2-strokes i.e expansion stroke and compression stroke, The fuel intake and combustion residual exhaust occurs respectively during these 2 strokes.

THEORETICAL VALVE TIMING

Expansion stroke-

At the beginning of the expansion stroke the piston which is at TDC starts moving towards BDC due to the combustion of compressed air-fuel (petrol engine) and (diesel sprayed charge in diesel engine) during compression stroke and the power output is obtained.
§ The air-fuel(petrol engine) and air (diesel diesel) enters through the inlet port during the expansion strokes as the piston moves from TDC to BDC during this stroke.
§ The expansion stroke continuous till the piston reaches BDC.