Inlet and Exhaust Valves of Main engine

         

Air inlet and exhaust valves of the mushroom type are always used in four stroke engines and sometimes in two stroke engines.

To handle large flow rates, they are of large size. Both open into the cylinder, so that the greater the gas pressure in the cylinder, the more firmly are the lids pressed against their seats. Therefore the springs employed to close the valves require being strong enough to keep the lids on its seat during the low- pressure period of the cycle of the engine.

It is not unusual to find exhaust valves having smaller diameter than inlet valves.

The reasons are:

a)      Exhaust valves open against higher pressures within cylinder.

b)      Exhaust gases assist in expelling the gases through open exhaust valve, unlike the inlet valve.

c)      Being smaller assists in keeping them cool which is important as exhaust valves operate at higher temperatures.

However, large engines can have them of same size. As inlet and exhaust valves withstand different thermal loads, they might differ in material also. Also, exhaust valves require cooling. In large engines it is better to duplicate inlet and exhaust valve. It gives better gas and airflow resulting in reasonable sized valves, better volumetric efficiency, better scavenging, cooler piston and liner and better performance. The figure on the following pages illustrate typical valves and shows common terminology. Typically the valve seat is angled at 45* for diesel engines, although some valves use a narrower 30* angle from the horizontal. The 30* angle allows less restriction across the seat and flow can start sooner and end later. Valve guides, typically make of Cast iron, guide the valve stems, which tend to wear against the valve guides due to angularity of up and down motion of the valve contribution by rocker arm action.

It is important that the clearance between the valve and the valve guide i.e. valve guide clearance, be within the manufacturer's specification.

The valve seat must be smooth, not only to prevent leakage, but also to allow for good heat transfer. As valve seats are prone to damage, burning and distortion, Exhaust valves have replaceable valve seat inserts, as shown in solid black in the figure on following page 66.

As the inserts are ground away and exceed dimensional limitations desired, they are replaced with new inserts.

If a valve had to be repaired or replaced the entire head might have to be removed because the valves are installed solidly in the head. Heads can be removed on small engines but on large engines head removal is certainly difficult. Valve cages are used to overcome this difficulty. An example is illustrated in the figure on next page. A copper gasket is inserted between the combustion chamber and the cylinder and the head in the valve cage. This copper gasket prevents both leakages from the combustion chamber and carbon build up