Engine Valve

Small in size, great importance in Engine Performance and Service Life...

In its original configuration, the four-stroke engine relies entirely on the piston's motion to draw in fuel and air through inlet valves, and to force out the exhaust gasses through exhaust valves.

As the piston descends on the intake (inlet valve open) stroke, a partial vacuum is created within the cylinder which draws in the fuel/air mixture.

The intake valve then closes, the piston ascends, and the mixture is compressed and ignited, causing the piston to descend again.

As the exhaust valve opens, the piston forces the exhaust out.

This was the technique used in early four-stroke engines. It was soon discovered, however, that at rotational speeds approaching 100 revolutions per minute (RPM) or greater, the exhaust gasses could not change direction quickly enough to exit past the exhaust valve by the piston's motion alone.

At high rotational speeds, consistent flow through the intake and exhaust ports is maintained by allowing the intake and exhaust valves to be open simultaneously at top dead center. The momentum of the exhausting gas maintains the outward flow and initiates the induction flow. The trick is to close the exhaust valve before much fresh mixture is drawn into the exhaust port. After ignition of the fuel/air charge, as the piston approaches bottom dead center, it becomes less useful to retain the hot, high-pressure gasses within the cylinder. To this end, the exhaust valve is typically opened at about twenty degrees of crankshaft rotation before bottom dead center. This allows the development of gas momentum in the exhaust port before the piston rises to push the gas out. The advantage of the extra time available for more complete exhaust of the cylinder outweighs the loss of the slight potential power in the still expanding gas.

As the piston ascends through the exhaust stroke, the intake valve will be opened, also approximately twenty degrees before top dead center. Ideally, the momentum of the exhaust flow will cause a lower pressure within the cylinder, pulling the fuel/air mixture in more easily. Consequently, both valves may be open simultaneously for a total of more than forty-five degrees of rotation, a technique called valve overlap. Under ideal conditions, the fresh fuel/air charge will push remaining exhaust gasses out the cylinder before the exhaust valve closes, leaving only a clean fuel/air mixture. Aiding the exhaust flow in this way is called scavenging. The disadvantage is lower fuel efficiency owing to the loss of fresh mixture into the exhaust port which can, however, serve the useful purpose of cooling the exhaust valve, an important consideration at high engine speeds, such as are experienced during races.

Accomplishing maximum volumetric efficiency for a given engine is not a formulaic process. You have to use the best equipment you have in order to get the most out from your engine.

Valve Guide

Guides are the critical parts since they have close effect on valve performance. They are cylinderic parts, leaving just enough space for the valves slide/move in. Defective or worn guides will prevent the valve work properly or cause engine oil to escape into the combustion chamber.

Valve Cotter

Cotters are small parts which hold the valves from their tip end though specially machined cotter keys.