2-Stroke Engine, How It Works

Ok, so it's obvious that a 2-stroke makes 2 strokes of the piston to every power stroke while a 4 stroke makes 4, but it wasn't until I had to work on these engines that I really started to understand how they work. While the 4 stroke motor is the new driving force for R&D and motocross nowadays and the 450 4-stroke dominates the race track, the 2-stroke still has it's place in my opinion. The simple design, and significantly fewer parts make it much easier and cheaper to work on and hence can be more reliable. It also starts easier when warm, and the motor can be tuned to put the power where it's needed. Because the 2-stroke has twice as many power strokes as the four stroke, the motor can put out nearly twice the power for the same cylinder volume as a four stroke.(that's why 250 cc 2-strokes race against 450 4-strokes). Because of the inherent inefficiencies of the design it is not quite twice as much power, but it still has a better power to weight ratio of the four stroke. As can be seen in the illustration above, as the piston travels downward the exhaust port is uncovered allowing the pressurized exhaust gas to escape. A fresh fuel/air charge that has been slightly compressed by the downward movement of the piston travels into the cylinder through the transfer port. This charge enters under pressure and helps to push out the exhaust gasses. The crankshaft continues to rotate and the piston moves upward, covering the transfer port and exhaust port. The piston compresses the new fuel/air mixture and creates a low pressure area in the crankcase. As the piston continues to move upward, it uncovers the intake port. A fresh fuel/air charge from the carburetor is drawn through the intake port because of the low pressure in the crankcase. Reed valves that are flexible reeds react to the change in pressure to allow the fuel/air mix to enter the crankcase. As the piston nears the top of the cylinder, the spark plug fires igniting the compressed mixture. Many things can be done to make this process more efficient and/or tune the power to different needs, such as a different shaped or sized pipe, different stiffness of reeds or a dual reed, different length and/or size of silencer etc. etc. As you can see in the illustration, the shape of the pipe is very important to reacting to the exhaust gasses and sending a secondary pulse back towards the cylinder as the compression stroke begins. Different sizes and shapes can tune the power. Obviously a lot more could be written about these other components.

As you can see, the 4 stroke is making 4 strokes for every spark of the spark plug, or every power stroke. Also obvious by this comparison is how many more parts are necessary for the 4 stroke's operation. the 4 stroke uses exhaust and intake valves that are mechanically opened and closed at the appropriate times. Much can be done with the timing of these valves etc. to change the power characteristics. I thought these illustrations were pretty cool, and a good way to show the differences between these two designs. I believe that although the four stroke is said to be the future for dirt bikes and it does have many advantages over the 2-stroke in different areas, there will always be a place for the 2-stroke because of it's simple design and great off-road power capabilities and superior power to weight ratio.

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