Engines run a number of things in our everyday lives. People use engines to transport themselves to work, mow their grass, pump water, and make energy. These engines all fall into two main categories: two-stroke engines and four-stroke engines. Both of these engines are great for their own reasons, but they work differently. I am here to help you explore how they are different and how that can be used to our advantage.

To start off, an engine is defined as “a machine with moving parts that converts power into motion.” This is traditionally done when a fuel, such as gasoline, is ignited and harnessed to spin a shaft. That rotating motion and energy is then used to complete the tasks we have created for engines to do. These tasks often help determine where two-stroke and four-stroke engines are used. Two-stroke engines are typically used for smaller applications like remote controlled cars, small dirt bikes, lawn tools, and chainsaws. These two-stroke engines are typically light in weight. Four-stroke engines, on the other hand, are typically used in lawnmowers, go-karts, larger dirt bikes, and even combustion vehicles. 

Now let’s explore how a four-stroke engine works. A four-stroke engine is designed where the piston sits in a sleeve, with a shaft attached to the piston, and a set of valves at the top of the piston chamber. The term “four-stroke” comes from the number of strokes the piston makes when undergoing a power combustion cycle. A stroke is defined as a full up or down motion of a piston. The four strokes include: the intake stroke, compression stroke, power stroke, and exhaust stroke. 

The intake stroke is the stroke where the piston is moving down into the engine. Simultaneously, valves at the top of the piston bore open and allow a gasoline/air mixture into the cylinder. The vacuum created by the downward motion of the piston aids in drawing the mixture in. 

In the compression stroke, the valves close and the piston is pushed back up to the top of the cylinder. This action compresses the gasoline/air mixture and excites it chemically. 

Next is the power stroke. This stroke is the one that most people think of when envisioning an engine. The power stroke uses either a glow plug and compression (diesel) or a spark plug (gasoline) to ignite the fuel. This violent explosion drives the piston back down the cylinder spinning the crankshaft. 

Finally is the exhaust stroke. In this stroke the piston returns to the upper position and pushes out all of the leftover fumes through an open valve. The cycle then repeats over and over. 

On the other hand, two-stroke engines are much less complicated. This is because they have fewer strokes and less moving parts. Although, a two-stroke engine completes the actions of all four strokes in just two. 

A two-stroke engine begins by pulling air into an area under the piston. This happens because two-stroke engines use ports and not valves to control the entrance and exit of fluids. When the piston is in the compression phase, air is being pulled into the engine from below the piston. When the fuel ignites and the piston is pushed down the cylinder, the air from the bottom flows around the piston until it is above it. The exhaust is then pushed out of the exhaust port that is now open due to the piston’s position. The engine then repeats this cycle.

Now that you see how two and four-stroke engines are different, it is easy to wonder which engine is better. This depends. Two-stroke engines are much stronger at high RPM, and four-stroke engines are stronger at low RPM. Two-stroke engines wear out faster because they run faster than four-stroke engines. Four stroke engines are quieter than two-stroke engines but two-stroke engines are easier to fix. These are just a few of the factors that come into play when choosing between a two or four-stroke engine. 

After learning about the difference between these two varying engine technologies, I hope it is easier to appreciate the science and engineering that went into creating these wonderful machines. Think about where you use engines everyday and how they benefit you. Engines are so cool!