 Hello everyone, Myself P.V. Ekhande working as an assistant professor in Department of Mechanical Engineering at Valchiyan Stop Technology, Swalapur. So today we will have the discussion related to the two-stroke engines. At the end of this session, students will be able to understand the working of two-stroke engines. In this first we will have the discussion related to the introduction of two-stroke engine, its parts. Then we will have the discussion related to the working of two-stroke SI engine cycle. Then working of two-stroke CI engine cycle. That means in SI engine, petrol engine will be considered and in CI engine, diesel engine will be considered. Now introduction to the two-stroke engines. In two-stroke engines, one cycle is completed in one revolution of the crankshaft. So as compared to the four-stroke engine where the cycle is completed in two revolutions of the crankshaft, so there the four strokes are completed independently and the cycle is completed in two revolutions of the crankshaft. Whereas in case of the two-stroke engine, the four strokes or the four processes that is suction, compression, expansion and exhaust, these four processes are carried out simultaneously or that is in combination. Working of the two-stroke engine for that when we consider this figure, the parts or the construction wise the difference as compared to the four-stroke engine is like this. In case of the four-stroke engine, there are walls provided for entry of the charge and exiting the burnt gases. That means there is a provision of suction wall at the cylinder head portion as well as the exhaust fall at the cylinder portion which is provided somewhere here and at the central location there will be spark plug in case of the SI engine and there will be fuel injector in case of the CI engine. But in case of the two-stroke engine, it consists of cylinder which is provided with the ports instead of the walls. So that is the main difference between four-stroke and two-stroke engines. In case of the four-stroke engine, the walls are provided which are mounted on the cylinder head portion whereas in case of the two-stroke engine the ports are provided. So ports are nothing but the openings which are provided on the periphery of the cylinder portion. So there is an inlet port which is allowing the entry of the fresh charge in the crankcase portion that is the mixture of the petrol and air which is taken from the carburetor so that will be the portion, this is the portion where the entry of the fresh charge will be there. Then the exhaust port is provided somewhere here which is a port or the opening provided on the periphery for exhausting the gases after burnt gases after doing its work on the piston portion and there is a third port which is called as a transfer port which is actually provided on the periphery of the cylinder and connected to the crankcase. So this is the path through which the charge will be taken from the crankcase portion after its partial compression in the cylinder portion. So these three ports are provided on the periphery in case of the two-stroke engine and in case of the four-stroke engine there is a valve provided for the suction of the charge as a suction valve and another valve is provided as an exhaust valve for exhausting the gases. So this is regarding the construction of the two-stroke engine. In case of the SI engine there will be a spark plug, in case of the CI engine there will be a fuel injector which is used for the diesel engines. Now when we consider the working of the two-stroke engine, in two-stroke engine as told there will be a combination of four strokes while completing the cycle. So when we consider these two figures in this there is a different position for the piston. So first stroke or the one stroke will consider where the piston will be at the top position and another stroke which will be considering there will have the consideration of the piston at the bottom most position. Now in one stroke what happens? Suppose the charge which has been taken in the cylinder portion during the previous stroke that will be compressed and at the end of the compression the charge will be at very high pressure and high temperature in the combustion chamber or the chamber in between the piston top surface and the cylinder head. So this high pressure, high temperature charge will get ignited because of the generating the spark with the help of the spark plug. So due to this what happens there will be expansion of the burnt gases. So sudden blasting effect will be there which causes the movement of the piston in the downward direction. So that is the generation of the power to the engine. So this expansion of the gas will have the forcing of the piston in the downward direction. Now during this movement that is from top dead center to the bottom dead center position the fresh charge which has been taken in the crankcase from the transfer port in the previous stroke will get compressed to some extent that is called as a partial compression. So as it moves in the downward direction there will be partial compression taking place to the fresh charge at certain distance this exhaust valve will get opened. After that there will be opening of the transfer port. So exhaust port will get opened first because of that the burnt gases will pass through this exhaust port through the silencer to the atmosphere whereas after certain movement of that there will be opening of the transfer port and through the transfer port there will be entry of the fresh charge which is taking over here. So at a time there will be two ports in the open condition. So transfer port is in the open condition as well as exhaust port is also in the open condition. Now due to this what will happen the charge which is coming through this transfer port to inside the cylinder portion so this charge will be deflected first because of the shape which is provided for the piston top surface in the upward direction. So this upward direction movement of the piston upward direction of the charge which is taken inside the cylinder portion will force the burnt gases towards the transfer port but during this certain amount of the fresh charge will get wasted along with the burnt gases. So this is the topic of the SI engine because of that only the efficiency of the two stroke engine is less as compared to the four stroke engine but that can be minimized by scavenging effect means first this will be forced in the upward direction because of the shape provided and then the burnt gases will be forced towards the exhaust port towards the silencer because of the movement of the fresh charge. And when this piston moves in the upward direction during the second stroke what will happen? First the transfer port will get closed and this compression will start after that exhaust port will also get closed and there will be silt at portion in the top position and this movement of the piston in the upward direction will compress the charge and as it moves in the upward direction this transfer port will get opened and because of the opening of this transfer port because of uncovering of the portion over here by the piston the entry of the fresh charge will be in the crankcase portion means here the combination of suction is going on as well as compression on the top portion is going on. So this piston moves in the upward direction compression will get completed and again there will be generation of the spark plug with the help of the spark plug. So this is the working of the two stroke petrol engine. In case of the two stroke diesel engine the difference will be of there is a fuel injector instead of the spark plug and when we consider the entry of the fresh charge in the crankcase that charges air alone there will not be any mixture. The air will be taken as a charge inside this the air which will be compressed during the compression stroke. So in that portion there will be injection of the fuel and the fuel will get run on its own because of the high pressure and the high temperature developed by the compression process to that air. So air in that when the fuel is ignited it will get ignited automatically. So that requires the certain self ignition temperature that can be achieved with the help of the high compression ratio which is provided for the compression ignition engine. Pause the video and think of this question. In how many revolutions of the crankshaft one cycle completes in two stroke engine. So in this what will happen as compared to the four stroke engine the two stroke engine cycle will get completed in only one revolution because of the combination of the four strokes over there. So one revolution of the crankshaft is required for completion of the one cycle of the two stroke engine whether it is a PSI engine or the CI engine.