Two And Four-Stroke-Cycle Diesel Engine Operation

Two And Four Stroke Cycle Diesel Engine Operation Points : Two-And Four-Stroke-Cycle Diesel Engine Operation, Two-Stroke-Cycle Engines, Four Stroke Cycle Engines, The word cycle refers to a series of events that repeat themselves. Cycle in relation to diesel engines refers to the series of events that must occur in an engine for it to operate. The somewhat separate but closely related events, which must occur are intake, compression, power, and exhaust. For each cylinder in a two-stroke-cycle engine, all four events occur in one revolution of the crankshaft. For each cylinder in a four-stroke-cycle engine, all four events occur in two revolutions of the crankshaft. Two-Stroke-Cycle Engines The exhaust valves are closed as the piston moves upward on the compression stroke (Fig). Fuel injection begins approximately 23 crankshaft degrees before top dead center (BTDC) and ends 6° BTDC (Fig. The power stroke begins at TDC as the fuel and air in the cylinder ignite, and begin to expand. This expansion forces the piston downward, which in turn causes the crankshaft to rotate. When the piston has moved approximately halfway down the cylinder, the exhaust valves open and in doing so release what pressure remains in the cylinder. At the same time, a positive-displacement roots-type blower forces fresh air into the cylinder through the sleeve intake ports and out the exhaust valves. This process is called scavenging. About 44 percent of the total working cycle is needed to remove the exhaust gases and bring in fresh air. A two-stroke cycle diesel engine requires a blower for scavenging and will not operate without one. The blower must be capable of pumping a large quantity of air at a pressure of 2 to 7 psi [14 to 48 kPa] into the cylinder to replace the exhaust with fresh air. An added benefit of scavenging is that it cools the engine. Positive-displacement blowers operate with little mechanical friction and are lubricated by the engine’s lubrication system.

As the piston begins its upward travel, it moves past the intake ports, closing them approximately at 48° after bottom dead center (ABDC). The exhaust valves are completely closed at approximately 117° BTDC. This is the beginning of compression. The piston continues to move upward compressing and thereby heating, the air in the cylinder. Once again fuel injection begins at approximately 23 BTDC, and the process repeats itself.

With each downward piston movement there is a power stroke, and with each upward piston movement a compression stroke. The intake and exhaust stroke may be considered a part of the power and compression stroke and begins after completion of the power stroke as the exhaust valves open. The intake and exhaust stroke ends after the piston closes off the inlet ports of the cylinder liner on the compression stroke. Four Stroke Cycle Engines Because a four-stroke engine has intake valves rather than intake ports in the cylinder sleeve, we will find a considerable difference in the way four-stroke engines operate as compared with two-stroke engines (Fig).

As the piston moves downward from TDC, the exhaust valves close while the intake valves remain open. For this reason, fresh air rushes into the cylinder to fill the void left by the piston (Fig.). The piston reaches BDC and starts moving upward again. At approximately 430 ABDC, the intake valve is closed and compression begins. The piston moves upward, compressing and heating the air in the cylinder as it does so. At approximately 28° BTDC, fuel injection begins and, because the air in the cylinder is very hot. The fuel ignites as the piston moves up and past TDC. beginning its downward travel.

This downward travel after the fuel ignites is the power stroke, and it continues until the piston has moved downward to approximately 53° before bottom dead center (BBDC), at which time the exhaust valves open. At this point there is enough pressure in the cylinder to force exhaust gases from the cylinder into the exhaust manifold. As the piston reaches BDC and starts moving upward, the exhaust valve remains open and the upward movement of the piston continues to force exhaust gases from the cylinder and into the exhaust manifold. There is period as the piston nears TDC when the intake valves open, and for just approximately 53 crankshaft degrees, both valves remain open so that the cylinder is completely charged with fresh air. This is called valve overlap, and it ensures that the cylinder is purged of all exhaust gases before the intake stroke starts.