 Hello students, I am Dr. Prakash Sarunke. Let us talk on valve timing diagrams in IC engines, the first session. At the end of this session, students will describe the term valve timing diagrams and will be able to explain effect of changes in the valve timings on the operation of the four stroke petrol engine. So before going for valve timing, we must understand the operation of four stroke SI means spark ignition engine. To understand the valve timings, initially we must know the working of four stroke IC engine. In this engine, operating cycle is completed in four strokes of piston or two revolutions of crankshaft. The four strokes are suction, compression, expansion and exhaust for suction stroke. The engine cycle starts with suction stroke. The inlet valve will open as the piston starts moving from top dead center position TDC towards bottom dead center position. And the air fuel mixture will be sucked inside the cylinder throughout the stroke. This is suction stroke. After the suction stroke, the piston reverses the direction and starts moving from BDC to TDC which will compress the air fuel mixture during its upward movement. Now this action causes rise in the pressure inside the cylinder. Then it will create high pressure which is essential for the combustion of the fuel. The inlet valve is closed at the beginning of the stroke and remain closed so that compression will occur. The next stroke is expansion stroke. At the end of compression stroke, the combustion of the fuel takes place due to ignition by spark and results in further rise of pressure and temperature. The charge or products of combustion will push the piston which is at TDC towards BDC and output power is obtained. During this stroke, both intake as well as exhaust valve remain closed. Then comes the exhaust stroke. After expansion stroke, the piston which is at BDC reverses and starts moving towards TDC or top dead center. At this time, opening of exhaust valve will take place so which is required for the removal of these residual gases from the cylinder. So exhaust valve will close after the piston reaches top dead center so that all the residual gases are removed. Now valve timing of force to move. Now what do you understand by the term valve timing? Yes, think for some time. Valve timing? Yes, you are right. Valve timing is the control of the points or timings in the operating cycle of engine at which valves are set to open or close. I mean both inlet or exhaust. And a valve timing diagram is a graphical representation of the timing at which opening and closing of both intake and exhaust valve of the engine take place. The opening and closing of the valve of the engine depends upon the movement of piston from TDC to BDC and vice versa. The valve timing diagram comprises of a 360 degree figure which represents the movement of the piston from TDC to BDC and back. On this diagram expected opening and closing of the valve are marked in degrees. This is the theoretical valve timing diagram. Now you can see here so this is TDC and this is the BDC so this is the position from which piston moves. So this is represented in 360 degree of crank rotation. Now theoretically as the piston starts moving from TDC to BDC the inlet valve will open and suction will start. So that will end here at BDC. Then inlet valve will close here and compression will start. The piston will start moving. So the compression will end here. At that time both inlet as well as exhaust valves are closed. So then here spark will be produced and combustion will take place and that will push again the piston back. So this is the power stroke or it is expansion. So products of combustion will push and up to this point it will be going on. Now here again exhaust valve will open and the piston will reverse the direction and the products of combustion will be driven out so this is the exhaust valve. As the piston moves up to top dead center so this 180 degree they are completed and exhaust valve will close here and again the cycle will repeat. So this is the theoretical valve timing diagram. Now actual valve timings differ. Let us try to understand. As described earlier in the idea cycle opening and closing occurs at dead centers. However there is deviation in the actual cycle. There are two factors mechanical and dynamic so which cause the deviation. Mechanical factor. Vals are opened with cam and tapet mechanism which are required to be operated slowly so that they should not be jerk. Similarly abrupt closing may cause, closing opening may cause bounce of valves on the seat. So both operations should be smooth and to smooth to avoid wear and noise. So these are the mechanical factors. The second factor which cause the deviation are the dynamic factors. So quantity of air fuel admitted inside the cylinder is affected by the duration of opening, the speed of engines and dynamics of fluid flow. So that affects in turn the volumetric efficiency. And because of that again say we have to change the valve timings. So let us see. Intake valve timings for both low and high speed SI engines, spark ignition engines. Inlet valve is normally opened 10 degrees before the piston reaches top dates interposition. This will ensure that the valve will be slowly opening and it will be fully open at top dates interposition and press charge will move inside completely and immediately. Now press charge will be sucked inside continuously throughout the suction flow. Even though piston reverses direction due to momentum and inertia the flow of charge continues to flow inside. This is called RAM effect. To take advantage of this effect which will enhance volumetric efficiency, intake valve is kept open even after BDC. Theoretically it is supposed to close at BDC. For low speed engines normally valve is open for 10 degrees after BDC. While for high speed the duration could be up to 60 degrees after BDC. That is one third part. So if you see this actual valve timing diagram you will find that the inlet valve opens. Before BDC. So that here the suction instead of starting from here. So it will start here and it will continue throughout this. And after BDC also that RAM effect is there. So that inlet valve will be open and it will be closed later. Similarly now say the compression instead of starting from here will start from here. So it will continue. So now both valves are closed. So this will continue up to top dead center position. Here there will be ignition say in the spark ignition. And expansion or expansion stroke will start. So that process of combustion will push. No expansion stroke should continue up to this. However exhaust valve will be opened earlier. And say you will find that the expansion stroke from here onwards itself. The next that is exhaust will start. Exhaust will start. And say that will continue. So further after TDC also. So for high speed engine the RAM effect is higher and to take advantage of it. Intake valve may be open up to 60 degree after BDC. For variable speed engine the valve timing is adjusted for best performance between the two. Exhaust valve timing. Exhaust valve is said to open before BDC. Otherwise pressure inside the cylinder will be considerably higher. Increasing the work required to expel the gases outside. So that is changed. Too early opening causes loss of useful expansion work. And tradeoff is done to achieve the net gain. For low speed engine valve is opened 25 degrees while for high speed once opening is at up to 55 degrees before BDC. Exhaust valves are closed after TDC to take advantage of inertia of moving gases thus increasing the volumetric efficiency. For inside there is a RAM effect. Similarly for outside say the charges will be moving on say even though the piston is reversed. The valves are closed 10 degrees for low speed and 15 degrees for high speed engines after TDC. So if we see the valve timing for low and high speed engines these are drawn separately here. Earlier diagram only one diagram were described. Now these are separately for low and high speed. So which are showing the relative differences say here the angles and different that you can see. So this is for SI engine and four stroke. So today we shall stop here. I have taken the references from the book of IC engine Mathur Sharma and say this web site mechanicalbooster.com from the Atowile page. Say that figure is figures of valve timing diagrams are taken. I thank to both of them. So I thank you also for patient.