To best understand the difference between a two-stroke engine and a four-stroke engine, it is first best to understand what a ‘stroke’ is. In essence, it is the cycle of a reciprocating engine, which operates through a basic forward and backward motion. Each completion of a cycle, once backwards and once forwards, is called a stroke.
In the form of a four-stroke engine you have four separate cycles of operation: intake, compression, combustion and exhaust. Here’s a brief overview of them all:
Intake: The first cycle of an engine, the intake is controlled through the process of the piston and a valve in the engine cylinder, which control the flow of fuel and air into the engine.
Compression: With the valve closed and the fuel/air mix in the cylinder, the piston fires up the cylinder to compress the gases into the engine.
Combustion: Ignited through the spark plug, the compressed gas combusts (or explodes) forcing the piston back down the cylinder.
Exhaust: With the valve opening again to let in fuel and air, the piston clears the chamber of spent fuel, gas and oil to resume the process.
At every step in the process, the movement of the cylinder drives the crankshaft connected to the engine, which in turn drive the wheels. This is the most stripped back and basic of operations that a four-stroke engine uses to create forward motion.
A two-stroke process is essentially the same, with the difference being the integration of the intake and compression cycles with the combustion and exhaust processes. The other main difference is that, thanks to its mode of operation of firing twice per revolution instead of once, a two-stroke will produce more power than a four-stroke.
Lighter, simpler and cheaper, two-strokes are preferred for use in garden equipment, mopeds and outboards. However, with a dedicated oil reserve allowing for better lubrication, four-strokes have a longer lifecycle, though special two-stroke premix oils from the likes of Fuchs and Mobil optimise this.