 Well friends, today let us continue our discussion on wall timing diagrams in IC engines. The second section. At the end of this session, students will describe the term wall timing diagrams and will be able to explain effects of changes in the wall timings of the on the operation of the four stroke diesel engine and two stroke engine. The function of CI engine, four stroke CI engine, it is similar to that of SI engine except the differences discussed below. CI means compression ignition. The same four strokes are there suction, compression, expansion and exhaust. However, in CI engine, only fresh air enters inside the cylinder during suction stroke and the fuel is injected at the end of compression stroke inside the cylinder. In case of SI engine, we have said air fuel mixture is sucked inside. For CI engine, fuel used is diesel and the compression ratios are higher, much higher. In diesel engine, combustion occurs due to high pressures and temperatures prevailing at the end of compression stroke, which are higher than atto ignition temperature of the diesel and air charge. In case of SI engine, the spark is needed. Here, the temperatures are higher than the atto ignition, self-ignition temperature. So, there is no necessity of any external semi source. Due to higher pressures, CI engines is heavier than SI engine. Similarly, thermal efficiency of CI engine is greater than that of SI engine naturally because of higher compression ratio. Now, what timing for CI engines? Consideration for what timing in CI engines are similar to those of SI engines. Diesel and dynamic both factors are considered here. Objective is to enhance volumetric efficiency and facilitate the efficient operation of suction and exhaust. Now, inlet valve here opens 25 to 30 degree before TDC, top dead center, and closes 25 to 50 degrees after BDC. Well, exhaust valve opens 45 to 50 degree before BDC and closes 15 to 30 degree after TDC. So, with the same consideration, so we are not going in details because already we have discussed in detail last time. And the valve timing diagram looks like this here. As you can see here, this is the TDC is a TDC. The inlet valve opens earlier, so suction, so we start earlier and it will continue from here. Then the compression will start, so which will end here at the top dead center position. Already inlet valve is closed, exhaust valve is closed only. Then comes the expansion, expansion it will continue up to this instead of up to BDC because exhaust valve opens much earlier and exhaust stroke starts from here. So, this will continue even after say this to take the effect that momentum or I mean inertia say will drive the exhaust charges outside ok. So, that is the say valve timing diagram for CI engine for stroke compression ignition engine. Now, there is a term valve overlap, what is valve overlap, can you tell me, we have seen yes ok. It is the term which refers to a period when both the inlet and exhaust valve remain open at the same time. It may be around 15 to 30 degrees of that crank, so that is the time. If the overlap is excess, it may cause flow of burn gases through inlet valve and except of fresh charges through the exhaust valve without doing useful work. So, it should be optimum, it should be optimum. Now, after studying the four stroke engine say today we shall continue our discussion on operation of two stroke engine. So, before discussing the valve timing diagram say let us study the operation. Using two stroke engine the engine cycle complex in two strokes or only one revolution of the crankshaft. The two strokes are expansion stroke and compression stroke. As seen in figure two, we shall see the figure. Suction of air and fuel takes place in the crank case through the inlet valve. So, that inlet or suction occurs inside the crank case instead of cylinder instead of cylinder as piston moves towards top dead center and by the end of compression. So, combustion occurs at top dead center and expansion stroke starts pushing piston downwards. During the downward travel piston uncovers the exhaust port first and exhaust starts. So, cylinder transfer port is open, transfer port is one which transfer the charge from crank case to the cylinder. So, the transfer port is open and gases from crank case will rush inside the cylinder from opposite side as shown in the figure we shall refer the figure. Now, due to typical shape of the piston and of the transfer ports incoming charge moves to top of the cylinder and helps scavenging of the residual gases. During upward travel of the piston initially transport port is closed, later exhaust port are closed and compression stroke starts and cycle then is repeated. The cycle is completed in two strokes, so let us try to understand this with figure. So, this is almost end of compression stroke say by that time this piston will uncover this inlet port. So, inlet of either fresh air in case of CI engines or fresh air and fuel mixture will take place inside the crank case in case of SI engine. So, that is the inlet that is the engine. So, this is position say this thing now say here combustion will occur at the TDC and this piston will be driven I mean downwards downwards. So, it will move downwards during its downward motion here exhaust port will be open and exhaust will start exhaust will start. After some degrees say here say this is the some in transfer port transfer port P for transfer port. So, I mean crank from crank case here you can see so as the piston moves further say that gases from the crank case say they will rush inside and due to this shape of pistons say they will drive they will move to upwards upper part of the cylinder and it will help scavenging the residual gases or exhaust from the exhaust one. So, as this moves downwards and reverses say during its upward tower first exhaust port will be I mean transfer port and exhaust port say they will be I mean closed and compression will begin. So, thus in two stroke only that expansion and compression the cycle will be completed. Now, before completion of the compression stroke the inlet now let us see the actual valve timings. Before completion of the compression stroke the inlet port opens 10 to 20 degrees before the top dead center position of the piston that we have seen in the up here. The inlet port closes 15 to 20 degrees after TDC during the expansion stroke during the expansion stroke. So, as the piston moves down so inlet port will be closed during expansion stroke exhaust port opens 35 to 60 degrees before piston reaches the bottom dead center position BDC. So, which in in terms starts the exhaust of the combustion residual and transfer port opens 30 to 45 degrees before BDC and it helps scavenging process scavenging process that we have seen say in that diagram. So, when the piston moves towards TDC from BDC the transfer port closes again 30 to 45 degrees after BDC and stops the scavenging process then say that exhaust port will be covered and during the movement of piston from BDC to TDC exhaust port closes 35 to 60 degrees after BDC and pressure inside the combustion chamber increases due to start of the compression stroke and the cycle repeats again. The air fill mixture in case of petrol engine and air in case of diesel engine is transported to the cylinder during the opening of the transfer port. If we see the wall timing diagram of two stroke petrol engine so this is the top dead center position. Now here it is a cycle is complete only in two strokes so you can see here ideally the this is the expansion stroke starts here and it should have completed here but here exhaust port opens. Now here instead of wall it is called as port okay exhaust port opens and exhaust starts. So at somewhere here this is TPO means transport port opens transport port opens and you say that will have scavenging so scavenging that is driving out the exhaust gases. So it will continue say up to here now transfer port closes so scavenging will be I mean finished but that exhaust is still going on because exhaust valve is open and then here at this point say both walls are closed so the charge which is admitted inside the cylinder from the transfer port so that will be compressed that will be compressed. So this compression will continue say like say this will continue now inlet port so that will close here inlet port opens here so that inlet means here the inlet inside the crank case so that will say start here from here to here so this is the inlet position okay. So that way so this is the wall timing or you can say port timing diagram for two stroke petrol engine in two stroke petrol engine somewhere here the spark is ignited so before the top date center now this is wall timing diagram of two stroke diesel engine the timings and say port openings all these are almost similar similar of course they will change slightly from engine to engine and its speed and all these things. But instead of that spark ignition here there is a fuel injection and fuel injection is not instantaneous so that supply continues so fuel injection starts at this point and it ends up to continues up to this here it ends here okay so this but everything else so that is seen. Now references for this session are taken from the book of Matsura and Sharma then web pages mechanicalbooster.com and say this figure is taken from this source for that IC engine operation of two stroke engine. I thank to all these same sources and I thank you all say for the patient listening so thank you.