What Is the Difference Between a 4-Stroke and 2-Stroke Engine?

What is engine stroke?

Most newer cars, trucks and SUVs have engines that are very fuel efficient. In order for any engine to work correctly, it must complete the combustion process, which includes four individual strokes of the connecting rod and piston inside the combustion chamber in a four-stroke engine, or two for the two-stroke engine. 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.

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 comprising one stroke. Listed below are the four individual strokes that are involved in the four-stroke process.

  • The first stroke is intake stroke. The process of operating an engine begins with the intake stroke, where the piston is pulled in a downward direction. This allows a mixture of fuel and air to enter the combustion chamber through the intake valve. During the starting process, power to complete the intake stroke is supplied by the starter motor, which is and electric motor attached to the flywheel, which spins the crankshaft and moves each individual cylinder.

  • The second stroke is compression. A they say, what goes down, must come up. This is what happens during the compression stroke, which occurs when the piston moves back up the cylinder. During this stroke, the intake valve is closed, which compresses the stored fuel and air gases as the piston moves towards the top of the combustion chamber.

  • The third stroke is combustion. This is where power is created. Once the piston reaches the top of the cylinder, the compressed gases are ignited by the spark plug. This creates a small explosion inside the combustion chamber which forces the piston back downward.

  • The fourth stroke is exhaust. This completes the four-stroke combustion process, as the piston is pushed back upward by the connecting rod, while the exhaust valve opens up and releases burnt exhaust gases out of the combustion chamber.

The stroke is considered one revolution – so when you hear the term Revolutions Per Minute, this means that it’s one complete engine cycle – or four individual strokes per revolution. So, when an engine is idling at 1,000 RPM, it means that your engine completes the four-stroke process 1,000 times per minute, or about 16-times per second.

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, and fire 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 buzz, while a four-stroke engine emits more of a soft humming noise. Two-stroke engines are often seen in lawnmowers and performance off-road vehicles (like motorcycles and snow mobiles), while four-stroke engines are what you’ll find powering on-road vehicles and high performance large displacement engines.


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