Understanding 2-Stroke Engines – How They Work

Two stroke engines are simple in terms of the number of moving parts. Otherwise, these engines can be difficult to understand. Many of the methods these machines use is old fashioned. As a result, they have some major inefficiencies. It can be difficult for someone new to engines to understand how these engines work because so many events are happening at the same time.

The Six Events of a 2-Stroke Engine

Two stroke engines have six events that occur to make the engine function. All six of these events happen in one full rotation of the piston from the Top Dead Center of the cylinder to the Bottom Dead Center of the cylinder and back to the Top Dead Center of the cylinder again. The challenging concept of understanding a two stroke engine is I that many of these events are happening at the same time, because a new cycle is starting while the previous cycle is about half way through. It is easiest to just look at each event individually to start to understand the 2-Stroke cycle.

Intake

This event is where the air/fuel/oil mixture is brought into the secondary compression area, above the piston. It is important to note that 2-stroke engines rely on the oil being mixed into the air/fuel mixture. This is done based on engine specifications. Some engines have external oil reserves that are added as the engine runs. Other engines require the user to mix in oil directly into the gas reservoir at a specified ratio. Either way, the oil is critical to lubricating the 2-stroke engine and without it the engine will overheat and can cause terrible damage to the cylinders and pistons.

Compression

This occurs as the piston moves up through the cylinder. The cylinder head compresses the air/fuel/oil mixture into the cylinder head. As the piston moves up and compresses the mixture it heats up and makes combustion easier. The strokes and events in a 2-stroke engine are largely dependent on where the piston is and what ports are opened or closed. During the compression stroke all of the ports are closed in order to compress the mixture. This requires good seals in order to not lose pressure.

Timed Ignition

Timed ignition is one of the most important elements of any gas powered internal combustion engine. The spark plug needs to function just slightly ahead of the piston reaching top dead center in the cylinder. This early firing allows the small amount of time needed for the fuel mixture to ignite just as the piston reaches top dead center. Timing this correctly allows for the most energy to be created for the power stroke.

Power

The power stroke is why we make the rest of this happen. This is the stroke that is the result of the explosion from ignition. The force of the piston moving down is transferred into energy that eventually turns our propellers and runs our boats. The piston goes from the top of the cylinder towards the bottom of the cylinder.

Exhaust

Exhaust is the process of the spent air/fuel/oil mixture being expelled by the piston passing the exhaust port. The piston moving past the port effectively opens it and the expanding gasses start to escape. It is at this point that there is some overlap between the power stroke and the transfer event.

Transfer

The transfer event is the event that distinguishes 2-strokes from 4- stroke engines. This event occurs as the piston continues to move down through the cylinder. This motion pushes the gasses that were brought in under the piston head and transfers them into the secondary compression area. This is where the intake starts to come back into play. The concept of scavenging the area above the piston head come into play and continues to force out the exhaust. While scavenging there is inevitable some air/fuel/oil mixture that escapes from the exhaust. This causes increases emissions that is unavoidable in 2-stroke engines.

Summary

Those are the very basic events of the 2-stroke engine. There is a little more to all of these events. Two strokes are an interesting challenge to work on. They have a simplicity, but that simplicity results in some crazy failures. When things go wrong with a 2-stroke engine they go bad in really bad ways. The piston is the main failure point of a 2-stroke engine. Problems come up from too little oil, or too much oil. They also tend to get large amounts of carbon build up, which can cause ignition problems. All in all, 2-stokes need to be maintained more meticulously than 4-stroke engines. But they are light weight, fast, and are fairly reliable when maintained.