Part 8 - The bottom end

The Bottom End

The ‘Bottom End’ is the rotating assembly, which we’ve already covered briefly. This includes the crankshaft and some include the con-rod, which we will here. The crankshaft sits inside the casing and runs in bearings which is covered later. The pictures below show the relationship between the crankshaft, con-rod and piston linked together with the crank pin and the wrist pin. These ‘pivots’ are highlighted below.

 

TDC

 

90° rotation

 

180° rotation or BDC

 

270° rotation

As you can see the piston climbs and falls up and down the cylinder (the cylinder isn’t shown for clarity). The piston is linked to the crankshaft by the con-rod. This transfers the movement of the piston (linear – up and down) to the crankshaft which rotates. You can see how con-rods direction changes as the crankshaft turns, this will become important in a later explanation.

The rotational angle of the crankshaft is known as the crank angle and this is important in relation to when the spark plug fires.

There is a vital function of the engine that isn’t one component but a function of the piston and the casing. It is basically a pumping action that the piston exerts on the volume of fuel/air mixture within the casing. The pictures below show how this works. The piston is at TDC and the casing is already ‘charged’ (filled) with fresh fuel/air mixture. 

When the piston descends the fuel/air mixture is squashed i.e compressed slightly. When the piston falls low enough it uncovers the transfer ports and the slightly compressed mixture has a void to fill within the cylinder. This causes the mixture to fill the cylinder. 

 

The blue arrow above shows the flow of mixture into the cylinder.