The Difference Between 2-Cycle and 4-Cycle Engines

You probably have some knowledge of how internal combustion engines work. They involve pistons, cams, explosions, and more, but do you know the differences between 2-stroke and 4-stroke engines?

Learn more by continuing to read or by watching the animated video below!

The history of automotive and engine design has been marked by the existence of two main types of gasoline powered combustion engines: the 2-cycle and 4-cycle engine.

The name of each of these engine types suggests exactly how they operate. In order for any engine to complete combustion and thus drive their respective vehicles or machines, there needs to be combustion of the gas-air mixture and expulsion through the exhaust ports. The main differences in these two engine types is just how they approach this process.

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2-cycle engines complete the combustion and exhaust cycle in only two strokes of the piston, with 4-cycle engines taking four strokes of the piston. Simple enough, but there are other intricate differences to these designs.

4-Stroke Engine

For a given 4-cycle engine, there is a power stroke where the piston is in the down position. This is caused by the combustion of the air-fuel mixture in the cylinder. There’s then an exhaust stroke where the piston moves up and pushed the gas exhaust out of the engine. After these two strokes that do positive work on the driveshaft, there’s an intake stroke with the piston moving down to draw the fuel and air in followed by a compression stroke to start the entire process all over again.

If you take a closer look at the entire process, you can see that the entire 4-cycles of the engine are powered by the initial combustion stroke in the cylinder as well as the other cylinders in an engine, if any, that drive the crankshaft. This also means that for every single combustion in a cylinder, there’s enough power imparted on the piston to allow it to rotate the crankshaft twice.

2-Stroke Engine

Two-cycle engines have much simpler mechanics as they basically consolidate all the different processes of a 4-stroke engine. There is a power stroke which also releases the exhaust after a certain distance of travel in the cylinder. Following this, there’s an intake and compression stroke that initially draws in new fuel and air followed by a completion of compression of the mixture.

These are the basic mechanics of how these two engine types work, but each has its own set of advantages and disadvantages when it comes to implementing them in any machine. These differences have led to the more widespread use of 4-stroke engines.

Advantages and Disadvantages of Each

If we look first at efficiency of a given engine, the 4-cycle engine achieves great efficiency due to the fact that nearly no fuel is wasted in the intake cycle. In other words, the fuel is drawn into a close system having no other outlet. On the other hand, in a 2-stroke engine, since the fuel is drawn in during a combined intake and compression stroke, there will always be some inefficiency of fuel burn due to leakage out the intake port upon compression. Ultimately, this makes 2-stroke engines less fuel efficient in general.

To compensate for this, many modern 2-stroke engines use fuel injection to bring their efficiency back up to levels on par with 4-stroke engines. Generally, though, a 4-stroke engine can always be made much more efficient.

All that said, efficiency isn’t everything when it comes to engine design. While 4-cycle engines are more efficient, they can also weight over 50% more than similar 2-stroke engines, bringing overall effectiveness down. The increase in weight is due to more complexity in the motor’s design, which ultimately leads to more moving parts in 4-cycle engines. All this complexity means that when it comes to repair work, 2-cycle engines are usually easier to fix.

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The last main difference between these two engine types is how oil is added to the engine. Two-stroke engines usually require that the oil be mixed with the fuel in a ratio of around 50 to 1 or 20 to 1. If you’ve ever owned gas-powered lawn equipment, you probably know this process well. This ultimately makes oil application fairly reliable, but it also means increased initial fuel cost and modification.

4-cycle engines use normal gasoline, like your car. In this design, the oil is injected from a reservoir maintained separately from the fuel. This eases the burden of fueling up the engine initially and increases overall ease of use.

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When all things are considered, the drawbacks of the 4-stroke design are overcome by their advantages in efficiency and power compared to 2-stroke engines. There have been automobiles made with 2-stroke engines in the past, but these designs mostly died off in the early 1980s as governments started cracking down on emissions. You can still find the trusty 2-stroke engine in marine craft and other smaller engines, such as lawn equipment.

Trevor is a civil engineer (B.S.) by trade and an accomplished writer with a passion for inspiring everyone with new and exciting technologies. He is also a published children’s book author and the producer for the YouTube channel Concerning Reality.

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