Two Stroke Engine
The fundamental di erence between the four stroke engine and the two stroke engine is the way in which the induction and exhaust process takes place. In the four stroke engine there are separate strokes for the induction and exhaust processes. In the two stroke engine however, both the induction and exhaust processes take place with the same stroke. The process that involves both induction and exhaust is called scavenging, or simply a gas exchange process.
The two stroke engine can be either made into a spark ignition or compression ignition engine. The smallest engines used in two stroke engines are compression ignition engines. The engines are usually used in models and their power output does not exceed 100 W. The other type of two stroke engine with power output of up to 100 kW is spark ignition engine. Some of these engines output high power relative to their weight and bulk. Some applications of these engines are in motorcycles, chain saws and small generators.
A two stroke engine . Some of the important parts of this engine are the exhaust, inlet, and crankcase port, and spark plug. The de ector is also an important design of the engine. The inlet port is where the charge is drawn from. The charge is a mixture of mainly air and fuel but may contain some exhaust. The exhaust port is where the exhaust leaves the piston, and the crankcase port provides the mixture. The combustion process for the two stroke engine goes through various processes. Following are the steps for combustion:
1) At 60 before hitting BDC the piston uncovers the exhaust port (EO), and the exhaust leaves the cylinder chamber while attaining atmospheric pressure. This is the end of the power stroke.
2) At 5-10 later the inlet port (IO) will open and the charge that was compressed by the crankcase will ow into the main chamber and mix with some exhaust residual. Some charge will leave the exhaust port. The de ector will aid in a way that it will divert the cross ow of charge from the inlet port into the exhaust port.
3) At about 55 after BDC, with the piston moving up, the inlet port will now close (IC). There will be some back ow of charge from the inlet port into the crankcase.
4) At 60 after BDC the exhaust port will close (EC) and the piston will now compress the charge through its upward movement.
5) At 60 before TDC the crankcase port will open (CO) and allow charge to ow into the crankcase. The charge will ow into the crankcase since the pressure in the crankcase is below the ambient pressure.
6) When the piston is within 10-40 before TDC the charge will be compressed enough to be at a high temperature. Then combustion will follow with ame initiation from the spark plug. In this process work is done by the engine on the air and fuel mixture. The power stroke starts when the piston hits TDC and continous until the exhaust port opens in step represents the steps talked above for the two stroke engine.
Two undesirable aspects of the two stroke engine is the interfacing between the charge and the exhaust gas, and the passage of the charge into the exhaust system. Scavenging is the simultaneous process by which the charge enters the cylinder chamber and the exhaust leaves the chamber. Even though both gases will always interface with each other, there are ways of designing the piston and cylinder in order to minimize the cross ow from the inlet port to the exhaust port. Figure 4.3 shows various scavenging systems that will decrease the cross ow. In the cross scavenge design the charge is diverted upwards by a de ector on the piston face. The second scavenge system is the loop scavenging design. This design has the inlet port just below the exhaust port, therefore creating a circular type gas ow throughout the cylinder. A modi ed design to the loope scavenging design is the Schnurle loop scaveng design. In this arrangement the "inlet ports are located symmetrically around the exhaust ports" . This arrangement will create a `U' gure type ow in the cylinder. The last two arrangements are mainly for diesel engines. The inlet ports are at the bottom and the exhaust ports at top. The piston design is such that the formation of swirl is developed.
The fundamental di erence between the four stroke engine and the two stroke engine is the way in which the induction and exhaust process takes place. In the four stroke engine there are separate strokes for the induction and exhaust processes. In the two stroke engine however, both the induction and exhaust processes take place with the same stroke. The process that involves both induction and exhaust is called scavenging, or simply a gas exchange process.
The two stroke engine can be either made into a spark ignition or compression ignition engine. The smallest engines used in two stroke engines are compression ignition engines. The engines are usually used in models and their power output does not exceed 100 W. The other type of two stroke engine with power output of up to 100 kW is spark ignition engine. Some of these engines output high power relative to their weight and bulk. Some applications of these engines are in motorcycles, chain saws and small generators.
A two stroke engine . Some of the important parts of this engine are the exhaust, inlet, and crankcase port, and spark plug. The de ector is also an important design of the engine. The inlet port is where the charge is drawn from. The charge is a mixture of mainly air and fuel but may contain some exhaust. The exhaust port is where the exhaust leaves the piston, and the crankcase port provides the mixture. The combustion process for the two stroke engine goes through various processes. Following are the steps for combustion:
1) At 60 before hitting BDC the piston uncovers the exhaust port (EO), and the exhaust leaves the cylinder chamber while attaining atmospheric pressure. This is the end of the power stroke.
2) At 5-10 later the inlet port (IO) will open and the charge that was compressed by the crankcase will ow into the main chamber and mix with some exhaust residual. Some charge will leave the exhaust port. The de ector will aid in a way that it will divert the cross ow of charge from the inlet port into the exhaust port.
3) At about 55 after BDC, with the piston moving up, the inlet port will now close (IC). There will be some back ow of charge from the inlet port into the crankcase.
4) At 60 after BDC the exhaust port will close (EC) and the piston will now compress the charge through its upward movement.
5) At 60 before TDC the crankcase port will open (CO) and allow charge to ow into the crankcase. The charge will ow into the crankcase since the pressure in the crankcase is below the ambient pressure.
6) When the piston is within 10-40 before TDC the charge will be compressed enough to be at a high temperature. Then combustion will follow with ame initiation from the spark plug. In this process work is done by the engine on the air and fuel mixture. The power stroke starts when the piston hits TDC and continous until the exhaust port opens in step represents the steps talked above for the two stroke engine.
Two undesirable aspects of the two stroke engine is the interfacing between the charge and the exhaust gas, and the passage of the charge into the exhaust system. Scavenging is the simultaneous process by which the charge enters the cylinder chamber and the exhaust leaves the chamber. Even though both gases will always interface with each other, there are ways of designing the piston and cylinder in order to minimize the cross ow from the inlet port to the exhaust port. Figure 4.3 shows various scavenging systems that will decrease the cross ow. In the cross scavenge design the charge is diverted upwards by a de ector on the piston face. The second scavenge system is the loop scavenging design. This design has the inlet port just below the exhaust port, therefore creating a circular type gas ow throughout the cylinder. A modi ed design to the loope scavenging design is the Schnurle loop scaveng design. In this arrangement the "inlet ports are located symmetrically around the exhaust ports" . This arrangement will create a `U' gure type ow in the cylinder. The last two arrangements are mainly for diesel engines. The inlet ports are at the bottom and the exhaust ports at top. The piston design is such that the formation of swirl is developed.
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