Engine Cylinder Design and Function - Part 3

Cylinder wear


Either 2 or 4 stroke engines have 1 or multiple cylinders which have 2 functions 1 – house the pumping process of the engine and 2 – act as a guide for the piston. What a lot of people seem to ignore is that the cylinder is a bearing surface. Piston rings are one half of the bearing and the cylinder is the other.

Building of this you could think of the cylinder being the ‘outer race’ and the rings as the bearing material, but this is of little consequence. What matters if the first fundamental rule of cylinders, they need protection.

Protection comes in the form of an oil film, a thin coating of oil on the inside surface of the cylinder. Without this the rings would quickly heat up due to friction between the 2 and melt, yes melt!

Heat Seize

This is usually known as a heat sieze and can really ruin your day.

 
This tends to happen in 2 strokes more than 4 strokes due to the oil control and management of a 2 stroke design, but can and does happen with 4 stroke engines.

However there is something puzzling, why then do the piston show score marks? Has the piston come into contact with the cylinder wall? Does the piston touch the cylinder or not?

These are some good questions, so we’ll break them down to understand what’s going on. 

In an ideal world the piston shouldn’t come into contact with the cylinder wall, we won’t go into the function of the piston yet (that’s another page) but it shouldn’t. Unfortunately it does, however not at the top of the piston but at the bottom. The picture above shows wear marks at the top and not the bottom, so what’s going on?

Most of the heat that is transferred to the piston is through the piston crown (the top surface facing the head) All of the heat generated through friction in the rings also transfers to the piston. So most of the heat the piston encounters is in the top quarter of the piston. 

This causes the piston to grow (expand) and unfortunately this expansion is radial i.e outwards towards the cylinder. Hence when a heat seize occurs the piston usual collides with the cylinder and you get nasty score marks around the piston ring region.


Now with all that sorted lets go back to what we mentioned earlier. I stated that the piston makes contact with the cylinder at the bottom section of the piston. This region is called the piston skirt for obvious reasons. The piston can rattle around slightly in the cylinder as it pivots on the wrist (gudgeon) pin as the con-rod throws from side to side. 

This is unfortunately unavoidable with the current design and is something that designers have learned to live with. This action is called piston slap, and is the main factor of the majority of cylinder wear.

Larger stroke engines suffer more as a result of larger perpendicular (side-to-side) motion of the con- rod and thus the piston itself. This side to side slaping of the piston against the cylinder wall causes the cylinder to wear in an oval shape, and this oval-ing usually occurs further down in the cylinder, away from the head.