VALVE TIMING OF 4-STROKE PETROL AND DIESEL ENGINE

VALVE TIMING OF 4-STROKE PETROL (SI) AND DIESEL ENGINE (CI)

Valve Timing of Petrol Engine (SI) 

Valve timing is the regulation of the points in the cycle at which the valves are set to open and close. We can easily explain valve open and closing through the valve timing diagram. The exact moment at which the inlet and outlet valve opens and closes with reference to the position of the piston and crank shown diagrammatically is known as the valve timing diagram. It is expressed in terms of degree crank angle.

If we consider the ideal cycle, inlet, and exhaust valves open and close at dead centers. But in an actual cycle, they open or close before and after dead centers. Valve timing has to be adjusted according to the speed of the engine.

A typical valve timing diagram for the slow and high-speed petrol engine is shown in fig.

 

	Low-Speed Engine	High-Speed Engine
TDC- Top Dead Centre	-	-
BDC- Bottom Dead Centre	-	-
IVO- Inlet Valve Open	10 degree TDC	10 degree TDC
IVC- Inlet Valve Close	20-25 degree after BDC	40-50 degree after BDC
EVO- Exhaust Valve Opens	25 degree before BDC	45 degree before BDC
EVC- Exhaust Valve Close	5 degree after TDC	20 degree after TDC
IGA- Ignition Advance.	15 degree before TDC	40 degree before TDC

Reasons for actual valve timing:-

1.     Mechanical Factor: -The clearance between cam, tappet and valve should be slowly taken up and valve slowly lifted, in the beginning, if noise and wear are to be prevented. For the same reason the valve cannot be closed suddenly, else it's going to bounce on its seat. hence the valve opening and closing periods are spread over a considerable number of crankshaft degrees. As a result, the opening of the valve should begin ahead of the time at which it is fully opened (i.e. before dead centers). The same reasoning applies for the closing time and the valves should close after the dead centers.

2.     Dynamic Factor: -  The actual valve timing is set taking into considering the dynamic effects of gas flow.

Intake Valve Timing:

Intake Valve Open :

To begin with, inlet valve timing, let us see both low speed and high-speed SI engines fig below

As we can see in the above figure that for both low speed and high speed engine the intake valve open 10 degrees before the arrival of the piston at TDC on the exhaust stroke. This is to ensure that the valve will be fully open and the fresh charge starting to flow into the cylinder as soon as possible after TDC. Due to the effect of inertia and time required to attain the full opening, the inlet valve opens somewhat earlier than at TDC, so that by the time the piston reaches TDC the inlet valve is fully open.

Intake Valve Close :

The closing of the inlet valve takes place during the start of the compression stroke (i.e. when the piston move from BDC to TDC after finishing suction stroke and the starting of compression stroke). If the inlet the valve is allowed to close exactly at BDC then less charge than the capacity enters during the suction stroke so the inlet valve closing is delayed to 20°-25 degrees after the crank reaches the BDC position during slow speed and 40-50 degrees after the crank reaches the BDC position during high speed.

Exhaust valve Timing:

Exhaust Valve Open :

The exhaust valve opens at the end of the expansion stroke. The exhaust valve opening is done before the piston reaches the BDC so as to provide more time for all the burnt gases to escape. The opening of the exhaust valve is necessary because if there are some burnt up gases left in the cylinder it may affect the cylinder walls and the spark plug. So the exhaust valve is opened 30-35 degrees before BDC for slow speed and 45-50 degrees before BDC for high speed.

Exhaust Valve Close :

The exhaust valve closing is also important to let out all the burnt gases. The time between the exhaust valve opening and the exhaust valve closing determines the amount of burnt gases that escape. Usually, the exhaust valve is closed 8°-10°after the piston reaches the TDC position. An important phenomenon in the valve timing diagram is the angle of overlap.

Angle of Overlap :

The angle of overlap is the angle at which both the inlet valve and the exhaust valves remain open. Thus it can be seen that from the diagram the angle of overlap during slow speed is 5+8=13. The crank position at which ignition occurs is also indicated in the valve timing diagram. The ignition is provided 38°-40°before TDC during compression.

Valve Timing of Diesel Engine (CI) 

The four-stroke CI engine is similar to a 4-stroke SI engine except that a high compression ratio is used in the former, and during the suction stroke, air alone, instead of a fuel-air mixture, is inducted. Due to ti high compression ratio, the temperature at the end of the compression stroke is sufficient to ignite the fuel which is injected into the combustion chamber.

IVO- Inlet Valve Open	10 -25 degree before TDC
IVC- Inlet Valve Close	30-50 degree after BDC
EVO- Exhaust Valve Opens	30-45 degree before BDC
EVC- Exhaust Valve Close	15-30 degree after TDC
FIS – Fuel Injection start	10-15 degree before TDC
FIC – Fuel Injection Close	15-25 degree after TDC