The Parts and Operation of Two Stroke Engines

A two-stroke engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during a single revolution of the crankshaft. This differs from a four-stroke engine,  wherein four strokes of the piston are needed to complete a power cycle during two crankshaft revolutions. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke occur simultaneously, with the intake and exhaust functions happening at the same time.

Two-stroke engines have a high power-to-weight ratio, power being available in a short range of rotational speeds called the power band. Compared to four-stroke engines, two-strokes have fewer moving parts. The main parts of the average two-stroke engine are the fuel injector, cylinder & cylinder head, spark plug, crank, crankshaft, crankcase, connecting rod, ports (inlet, transfer, and exhaust), piston, and piston rings.

As stated, there are two cycles involved in the function of a two-stroke engine. Let’s take a look at each. The first cycle is the suction and compression cycle. During this, the piston moves from the bottom center to the top center, and the inlet, transfer, and exhaust ports are all closed. The charge above the piston is compressed, causing the spark plug to ignite and create a power stroke. The power is then transferred to the crankshaft with the help of the connecting rod. There is also a partial vacuum created within the crankcase. This vacuum opens the inlet port and allows the mixture of fuel and air inside.

The second cycle is the power and exhaust stroke. During the second cycle, the piston moves down from the top center and the inlet port is closed. The downward movement of the piston forces the fuel-air mixture and the charge from the crankcase out through the transfer port. The exhaust port is open, meaning most of the exhaust gas leaves the cylinder. Any remaining gas is forced through the exhaust port by the pressure of the fuel-air mixture flowing downward. The exhaust gas is then pushed out by the next charge.

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