A 2-stroke engine is a type of small internal combustion engine that uses two different piston strokes to complete one operating cycle. During this cycle, the crankshaft rotates once while the piston goes up and down once to fire the spark plug.


What Is a 2-Stroke Engine?

In a 2-stroke engine, it only takes one piston stroke to complete the combustion cycle. There’s a compression stroke, then an explosion of compressed fuel. On the return, the exhaust is pushed out of the cylinder by the fresh fuel moving in. The spark plugs fire for each revolution. Engine power is produced every two piston strokes, which is how these engines got their name.

These engines have a few advantages over 4-stroke engines. They are lightweight, often weighing up to 50% less, and provide more torque at higher RPMs. Two-stroke engines also feature a simplified design, making them easier to maintain. A unique feature of 2-stroke engines is that they require you to premix your oil and fuel, while 4-stroke engines do not.

2-Stroke Engine Parts List

The parts of a 2-stroke petrol engine include:


  • Fuel injector
  • Cylinder
  • Cylinder head
  • Spark plug
  • Crank
  • Crankshaft
  • Crankcase
  • Connecting rod
  • Ports — inlet, transfer and exhaust
  • Piston
  • Piston rings


How Does A 2-Stroke Engine Cycle Work?

2-Stroke engines function through a simple two-cycle process.

1. First Stroke (Suction and Compression)

During this cycle, the piston moves from the bottom center to the top center, and all three ports — inlet, transfer and exhaust — are closed. The charge above the piston is compressed, and the spark plug ignites the charge and creates a power stroke. That power is transferred with the help of the connecting rod to the crankshaft.

There is also a partial vacuum created in the crankcase, which opens the inlet port and allows the fuel-air mixture inside.

2. Second Stroke (Power and Exhaust Stroke)

During the second cycle, the piston moves down from the top center, and the inlet port is closed. The downward motion of the piston pushes the fuel-air mixture, and the charge from the crankcase comes out through the transfer port.

Because the exhaust port is open, most of the exhaust gas leaves the cylinder. The exhaust gas that remains is pushed through the exhaust port by the pressure of the fuel-air mixture flowing downward. Then, with the help of a fresh charge, the exhaust gas is pushed out.

The parts of a 2-stroke petrol engine work this way, and the parts of a 2-stroke diesel engine work similarly except it has a fuel injector instead of a spark plug.

Common Functions of 2-Stroke Engines

The size and power-to-weight ratio for 2-stroke diesel engine parts and functions make them ideal for smaller-scale applications. You can typically find them on:


  • RC toys
  • Dirt bikes
  • Chainsaws
  • Small watercraft
  • Landscaping tools


The reduced number of parts of a 2-stroke petrol engine, the simple design and the lack of oil sump mean these engines are more reliable in cold temperatures. This feature makes them also suited for use in machines like snowmobiles and snowblowers.


Contact Prime Source Parts and Equipment Today

Whether you need parts for a 2-stroke marine diesel engine or a 2-stroke petrol engine, Prime Source Parts and Equipment can help. Contact us today for more information about our 2-stroke engine parts and small engine services.

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