Part 7 - Suck, Squeeze, Bang, Pop!

Suck, Squeeze, Bang, Pop!

This is something I was told many years ago, and it the basic principle of how this whole system works. Suck is the fuel/air mixture being drawn into the engine. Squeeze is the fuel/air mixture being compressed, Bang is pretty obvious and Pop is the exhaust gases leaving the engine. We haven’t covered the exhaust process yet as it needs a more in depth explanation.  So lets stop messing around and get on with how this whole thing actually works……….

Stroke

Stroke is the name given to the motion of the piston, it also describes the distance the piston travels in one direction but we’ll get to that. The Piaggio engine is a 2 stroke engine, this means that the engine completes all 4 stages (suck.squeeze, bang and pop) in 2 strokes i.e the piston moves up once (1st stroke) and down once (2nd stroke).

 

 

The picture above shows the piston in the cylinder, the con-rod and crankshaft have been removed for simplicity. When the piston is at the bottom of its ‘stroke’ this is called BDC (Bottom Dead Center). When the piston moves from this position to its highest position this is called a stroke.

The picture above shows the piston at the top of its ‘stroke’ this is called TDC (Top Dead Center). This is a far as the piston can go and has to return back down the cylinder. DON’T become confused with UP and DOWN, the Piaggio cylinder is banked at 12° from the horizontal (its practically laying flat) No matter how the cylinder is positioned we still call it up and down the strokes so BDC and TDC remain the same. 

The distance between TDC and BDC is called the stroke and a 2 stroke engine completes 2 stroke to produce power. Its as simple as that, 4 stroke engine like most car engines complete 4 strokes but that’s a different kettle of fish.

Now that we have sorted what a stroke is we can move onto the operations of the engine in whole. The engine when running continuiously goes through the 2 strokes again and again. Well start with the piston at TDC, pictured below.

To keep things in perspective lets imagine we’re viewing the engine as its running. With a 2 stroke engine the spark plug fires when the piston is at TDC. Due to the pressure build up within the cylinder the expansion of the gases forces the piston down the cylinder.

 

As the piston decends the piston uncovers the EXHAUST PORT. This allows the exhaust gases to escape from the cylinder ready for the fresh fuel/air mixture to enter the cylinder.

 

As the piston reaches BDC the piston uncovers the transfer ports, this allows the fuel air mixture to enter the cylinder, the ports have been designed to direct the mixture up towards the spark plug.

 

One of the problems of the 2 stroke design is that the exhaust port is still uncovered as the fuel/air mixture enters the cylinder. Some of the fuel/air mixture is forced out of the cylinder as the piston begins to climb back up the cylinder. We’ll get to this later.

 

The piston continues to climb and in doing so closes/blocks off the transfer ports, stopping the flow of mixture into the cylinder.

 

Due to the design of the exhaust mixture that was lost is forced back into the cylinder just before the piston closes off the exhaust port. We’ll get to the exhaust design later.

 

As the piston climbs towards TDC the pressure with the cylinder begins to raise. This is due to the volume within the cylinder shrinking.

 

Once the piston has reached TDC the spark plug fires and the mixture rapidly burns causing a massive temperature and therefore pressure increase, this in turn forces the piston back down the cylinder, and the whole process starts again.

Now we have a basic understanding of how pressure within a cylinder force the piston down the length of the stroke from TDC to BDC we can take a sneak peek into how this is transferred to the rear wheel through the a mechanism known as torque. You may have heard of torque as a force applied offset to a pivotal centre. We’ll go into the ins and outs of torque later, but just remember that fundamentally it’s a force. 

By increasing the number of cylinders we can apply for force more frequently. One thing to keep in mind is that even though the peak force is high the average torque over an entire rotation is what matters. There’s little point having a peak torque figure for only 20 degrees of the crank rotation when the crank is to turn a full 360 to complete 1 cycle. So to this end, torque figure supplied by manufacturers is a total torque over the entire 360 degrees of rotation and is (for the most examples) calculated. More on this later, but here is a simple graph demonstrating this. 

 















There are many other factors that justify using more than one cylinder but to simplify it, having more cylinder is like having more legs, like a horse vs say a man. This is a massively oversimplified example but it’’ll do for now, and we’ll cover this is more detail later.