The primary difference between a two-stroke engine and a four-stroke engine is the timing of firing. How often they fire tells you how they convert energy and how quickly it occurs. They're also known as two-cycle and four-cycle engines.
What is a stroke?
To understand the difference between the two engines, you must know what a stroke is. It takes four processes to burn fuel with each process being one stroke. The difference between the two-stroke and four-stroke engine is in how fast those processes occur.
The first stroke is intake, which is when the piston moves down the cylinder and a mix of air and fuel enter the combustion chamber.
The second stroke is compression, which occurs when the piston moves back up the cylinder. The intake valve is closed and the gases are compressed.
The third stroke is combustion, which occurs when the compressed gas is ignited by a spark from the spark plug.
The fourth stroke is exhaust, which occurs when the piston goes back up the cylinder while the exhaust valve is open.
The differences between two-stroke and four-stroke engines
The first difference is the fact that the spark plugs fire once every revolution in a two-stroke engine, while firing is once every other revolution in a four-stroke engine. A revolution is one set of four strokes. Four-stroke engines allow each stroke to happen independently. A two-stroke engine requires the four processes to occur in a downward and upward stroke, which gives the two-stroke its name.
Another difference is that two-stroke engines don’t need valves because intake and exhaust are part of the compression and combustion of the piston. Instead, there is an exhaust port in the combustion chamber.
Two-stroke engines don’t have a separate chamber for the oil, so it must be mixed with the fuel in proper amounts. The specific ratio depends on the vehicle and is found in the owner’s manual. Two of the most common ratios is 50:1 and 32:1 with the 50 and 32 referring to the amount of gasoline to one part of oil.
A four-stroke engine has a separate compartment for the oil and doesn’t require mixing. This is one of the easiest ways to tell the difference between the two types of engines. Another method of identifying the two is by the sound. Two-stroke engines are often loud with a high-pitched rumble while the four-stroke engine emits more of a soft purring noise. Two-stroke engines are often seen in lawnmowers while four-stroke engines are what you’ll find in vehicles.
This article originally appeared on YourMechanic.com as What Is the Difference Between a 4-Stroke and 2-Stroke Engine? and was authored by Joyce Morse.