Part 2 - 2 Stroke Process

The 2 Stroke Engine

I’m not going to go on forever about how 2 stroke engines work but a basic understanding is required. It can be broken down into 3 sections, induction/exhaust, compression and combustion. The example below shows how this ‘system’ works. Only one half of the crankcase is pictured so you can get a good view of what’s going on. You don’t have to understand what everything is we’ll cover that as we go on. 

The first to cover is INDUCTION/EXHAUST. A fuel/air mixture is drawn into the engine. The fuel won’t burn on its own, petrol needs oxygen to burn so that’s why it’s a mixture.

 

 

A valve called a REED VALVE (1) allows the mixture to enter the engine but doesn’t allow it to leave. Basically it’s a one way valve. The mixture is then drawn/forced into the cylinder (2). The cylinder is exactly what it sounds like, a cylindrical void.

 

The mixture enters the cylinder through passages called Tranfer Ports. For the Piaggio engine there are 5 ports, 4 either side of the piston and 1 larger one opersite the exhaust port.

The piston acts as a kind of plug between the crankcase and the cylinder but we’ll get into that abit later on. The reason for so many ports is to maximise how much fuel and air mixture can be transferred.

Now putting the piston back in place you can see how the mixture is now contained within the cylinder. This is were combustion occurs (3).

 

This is the next step the COMBUSTION process. The piston moves towards the cylinder head, squeezing the mixture as the space becomes smaller. When the piston reaches its limit the spark plug ignites the mixture and the fuel burns, it doesn’t explode as some believe.

 

The rapid increase in temperature causes the pressure with in the cylinder to increase. This pressure exerts force against ever surface of the cylinder, including the piston. Because the piston is a movable component the pressure forces the piston back down the cylinder. This is where the power is produced so to speak.

We’ve missed out the exhaust process at this time because its abit more complicated that whats already been covered. So just to go over whats happening inside this engine. Fuel and air has to enter the crankcase, transferred into the cylinder, squashed (compressed) and then ignited to produce pressure which is then converted into motion. That’s it. If you don’t have the correct mixture, don’t have a spark, don’t have enough compression or your piston isn’t free to move than you won’t get any power its as simple as that.

Now we’ve covered the very basics lets get into the mechanical process of the engine.