The four stroke engine liners do not require any ports and are relatively simpler in construction as compared to cylinder liners for two stroke engines discussed previously. Instead of ports, a 4-S engine contains valves and the valve timing is controlled by the cams. The upper end of the liner forms a flange of sufficient strength to support it. A flange is secured between the cylinder cover and the jacket or engine and the lower part of the liner is free for thermal expansion.
The liner thickness gives adequate strength to resist the internal gas load but at the same time another factor has to be kept in mind that is the heat transfer capacity. If the liner is made too thick, it would hinder the smooth transfer of heat and hence it would to thermal stresses in the liner. So the thickness has to be kept at an optimum value which is neither too weak nor to thick for effective heat transfer.
In four stroke engines the valves are used for scavenging and exhaust of the residual gases. Unlike two stroke engines, it requires four strokes of the engine to complete the four processes of the engine cycle namely scavenging, compression, power and exhaust.
Thus the valve is placed on the cylinder head and no cut marks are made on the cylinder liner. The sketch below shows a pictorial description of such a cylinder liner and all the parts are clearly labeled therein. Just take a close look at the sketch before studying further, as that would help you to gain a better idea of what is being talked about.
Cylinder Liner of 4-S Engine
Scavenging in 4-S Engine
In an internal combustion engine, scavenging refers to the complete removal of exhaust gases from the cylinder. As you must have noticed that in four stroke engine cylinder liners, there are no scavenge ports. The scavenging is only done through the inlet valve, efficient scavenging is necessary for good combustion and it is required for the very first working cycles of the engine.
The arrangement for passage of the scavenging air should be such that it should assist cooling of the cylinder, piston and valves. This can be achieved in the most effective manner in the four stroke engines due to the presence of valves.
Scavenging is mainly carried out by the pumping action of the piston during two of it strokes. The piston expels exhaust gases on its upward stroke and draws in air on its subsequent downward stroke. During this process there is a period of overlap during which exhaust and inlet valve are both open at the same time. Provided that the turbocharger supplies air at a pressure greater than that in the exhaust manifold, a flow of cooling air will pass through the cylinder during this period.
In this type of liner design, the scavenging is positive and very efficient, even in very high speed four stroke engines.