2 strokes...
Robin, 20 years ago, what you learned was the cycles of the 2 stroke engine. Unlike a car, which is 4 stroke, intake, compression, power and exhaust, the 2 stroke combines two of those to create a spark on every revolution of the piston.
Every half revolution of the flywheel, a critical point occurs when the position of the magnets changes the polarity of the current flowing through the generating coil. The CDI module detects the change of polarity and signals at the right time, to create spark to the coil. The MPEM sends the signal to tell the coil, which plug gets the spark.
Although I was certain that both cylinders didn’t fire at the same time, I felt the need to research the 2 cycle engine to put to rest any questions you may have about the operation of this engine. The easy way to know that the engine didn’t fire both cylinders at the same time, is due to the relation of the piston on the crankshaft. If they both rose and fell at the same time, then I could see spark at the same time. Of course, that isn’t feasible because the opposing forces would tear the engine apart.
The 4 stroke engine has 4 cycles. The intake, compression power and exhaust. The spark plug fires every other revolution of the piston. The 2 stroke has 2 cycles. The plug fires every revolution. The below is a brief summary of the cycles of the 2 stroke motor and how it works
Fuel Intake
As the piston finally bottoms out, the intake port is uncovered. The piston's movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel, as shown here:
The Compression Stroke
Now the momentum in the crankshaft starts driving the piston back toward the spark plug for the compression stroke. As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum opens the reed valve and sucks air/fuel/oil in from the carburetor.
Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle. It's called a two-stoke engine because there is a compression stroke and then a combustion stroke. In a four-stroke engine, there are separate intake, compression, combustion and exhaust strokes.
You can see that the piston is really doing three different things in a two-stroke engine:
• On one side of the piston is the combustion chamber, where the piston is compressing the air/fuel mixture and capturing the energy released by the ignition of the fuel.
• On the other side of the piston is the crankcase, where the piston is creating a vacuum to suck in air/fuel from the carburetor through the reed valve and then pressurizing the crankcase so that air/fuel is forced into the combustion chamber.
• Meanwhile, the sides of the piston are acting like valves, covering and uncovering the intake and exhaust ports drilled into the side of the cylinder wall.
It's really pretty neat to see the piston doing so many different things! That's what makes two-stroke engines so simple and lightweight.
http://science.howstuffworks.com/two-stroke5.htm