Two Stroke Engine VS Four Stroke Engine
The distinctive features differ between two stroke engines and four stroke engines are the simple structure and few movement part. Two stroke engines have no air valve, and the structure can be simplified. Besides, two stroke engine operating need to ignition once a week (while four stroke rotation need ignition once in two weeks), and it has more explosive. Two characteristics make two stroke engine simple portable and low manufacturing cost.
Main parts of the engine include: Crankshaft, connecting rod, piston and cylinder block.
Crankshaft
The function of the crankshaft is to turn a piston movement of up to down transfer moving in circular, and transfer the energy produced from the cylinder block burning used to rotational torque to pass out from the engine.
Connecting rod
The big head and little head of the connecting rod are divided to connect the crankshaft and piston.
The piston
Two stroke engine pistons can be designed as single or double loop. The main advantage of monocyclic design is that the frictional resistance is low, lightweight, large power, high speed and light vibration. While double loop design has more reliable sealing and long working lift, which has a slightly limit for stall.
Cylinder block
Cylinder bore parts can accommodate the piston, and the cylinder wall has an air inlet and outlet. The cylinder wall scavenging passage can lead the mixed gas from the bottom of the crankcase into the combustion chamber at the top of the cylinder block.
What's more, the difference between the two stroke engine and four-stroke engine is the carburetor structure, two stroke engines are usually adopts the model of pump carburetor, and this structure can guarantee the normal work of the engine in all angles, especially suitable for flexible flip angles. So considering two stroke engines has simple structure, large power output and other characteristics of high speed, many hand-held power devices are chosen the two stroke engine as their power. But due to its own structural features, the two stroke engine's insufficient combustion, fuel consumption, noise and exhaust emission index is all higher than four stroke engines. European and American countries continuously introduce more stringent environmental regulations; also restrict the further promotion of two stroke engine products to come out in the market now.
This kind of engine is just like its name, the two stroke engine including ascending and descending two working cycles. And it also contains two basic working principles: cross scavenging passage and loop scavenging passage. Loop scavenging passage is the most common form by far, the top of the piston can be flat or slightly convex. Generally, there are two cylinders scavenging passage in the inner vent, and the working principle and cross scavenging passage are similar, but the gas flow controlling is different.
Two stroke engine working cycle
Up trip
Piston begins to move upward after passing the bottom dead center point, when go through the air inlet it will form certain vacuum negative pressure inside the crankcase, and then bring in fresh mixture of oil and gas from the carburetor to the crankcase. This process continued until the piston finished the upward stroke, and even due to suction gas inertia can be maintained to the piston top dead center. Before the piston reaches to the top dead center, mixed gas will be light in the ignited electrode spark. Explosive combustion pressure drives piston to do downward movement.
Down trip
When the piston downward exhausts outlet, the combustion gas will discharge from the vent. As the piston to continue downward, extruding the crankcase of inhaled gas mixture of oil and gas through scavenging passage to get into the combustion chamber at the top of the piston. After going past the bottom dead center, the piston begins to upward, close the scavenging passage and exhaust. The piston will continue upward to compression of the gas in the combustion chamber. Until now, the whole working cycle has finished.
Four stroke engine working cycle
Two stroke engine pistons are doing the power trip every time after it got past the top dead center, while the four strokes engine does a power trip at every two weeks. In order to ensure the four schedules go smoothly, the airplane engine was designed with intake and exhaust door. The advantage of the four-stroke engine is that it can ensure efficient combustion at different rotation speed. Torque is bigger in low speed, due to it has increased structural components, the weight and complexity have also increased, but the structure principle of reliability has proved to be great in the practical for many years.
Four-stroke engine working cycle starting from the intake stroke, when the piston downward to the bottom of the cylinder, inlet valve will open; the fuel air mixture was drawn from the carburetor to the cylinder block, until the inlet valve to close. During this trip, the valve kept being closed.
Compression stroke
Compression stroke is the second work schedule, starting from the piston moving up just after the bottom dead center, the exhaust valve to keep close and the inlet valve also closed at the same time. The piston upward compresses gas mixture in order to get the largest power. The greater compression, the bigger deflagration pressure after compression will have. And it has also done a function of preheat mixed gas and increases the efficiency of the combustion.
Combustion stroke (power stroke)
Combustion stroke is the third work cycle. The combustion chamber inlet valve and the exhaust valve closed, cylinder of compressed gas mixture of oil and gas was ignited by the Spark Plug electrode, and which will control the open and timely cooperate of the air valve.
Exhaust stroke
Finally stroke is the exhaust stroke, after that you have complete the entire work cycle.
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