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Two-Stroke Engines – What makes a Two-Cycle engine different?

man cleaning autumn leaves next to two riders on 2-stroke engine bikes
A two-stroke or two-cycle engine is an internal combustion engine that is often found in smaller, lower power engines such as scooters, dirt bikes, jet skis, smaller outboard motors and lawn equipment such as lawn mowers and chain saws. Two-stroke engines contain many (but not all) of the same components of four-stroke engines common in automobiles, but they have significant differences that allow for different performance and require different types of lubrication.
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What is the difference between a two stroke and four stroke engine?

The key mechanical difference between a two-stroke engine and a four-stroke engine is that

  1. Two-stroke engines do not have valves. Many of the other components of a four-stroke engine (piston, cylinder, spark plug) are also part of the two-stroke engine. However, the fact that a valve is not needed allows for two-stroke engines to be lighter and often better for smaller engine applications such as scooters and lawn equipment.
  2. Two-stroke engines fire every revolution of the piston (four-stroke engines fire every other revolution), which can result in increased power from a smaller engine. 
  3. Two-stroke engines are often smaller, lighter and simpler than four-stroke engines and deliver a higher power-to-weight ratio.


How Two-Stroke Engines Work

In a two-stroke engine, the spark plug fires every revolution, which is different from four-stroke engines. Similar to a four-stroke engine, fuel and air are mixed in the cylinder. The fuel/air mixture is compressed and pushed toward the spark plug by the piston – this is known as the compression stroke.


As the fuel/air mixture is compressed in the piston, it creates a vacuum that opens a reed valve that pulls air, fuel and oil from the carburetor. The spark plug ignites the mixture and the piston is driven down by the explosion of the spark plug.


As the piston returns to its lower position, the exhaust gases are released from the cylinder and the cycle start all over again with fresh fuel entering the chamber. This is known as the combustion stroke.


Two-Stroke Oils - Can I use regular oil in a 2 cycle engine?

Because oil is part of the mixture that is burned in the operation of a two-stroke engine, special oils are required for two-stroke engines. The crankcase is acting to pressurize the chamber with the fuel/air mixture and the fuel is mixed with oil. Because of this mixture of fuel and oil, thinner viscosity oils are often required for two-stroke engines.


Disadvantages of Two-Stroke Engines

Two-stroke engines often have advantages for use in small engines; but they do have issues that make them not as effective in other engine operations. Some of the disadvantages of two-stroke engines are:

  • Fuel use is not as efficient in two-stroke engines – fuel economy is significantly reduced as compared to four-stroke engines.
  • Two-stroke engines produce significantly more emissions and pollution as a result of oil combustion and release of hydrocarbons from the burning of fuel with the oil. The combustion of oil also can result in an oily smoke cloud from the two-stroke engine.
  • The cost of running a two-stroke engine is more expensive than running a four-stroke engine. This is a result of the reduced fuel economy and the fact that oil is being burned with the gasoline in a two-stroke engine. Typically about 4 ounces of oil is burned per gallon of gasoline in a two-stroke engine.
  • The fact that engine lubrication is so different in a two-stroke often leads to reduced engine life.


Two-stroke engines have a wide variety of uses for smaller engine applications such as smaller vehicles such as ATV’s, jet skis and lawn equipment. They can deliver a higher power-to-weight ratio that is very effective for certain equipment. If you have a two-stroke engine, make sure you check out the Castrol Oil Selector to find the right two-stroke oil for your equipment.