VALVES :
Inlet and exhaust valves are fitted to each cylinder of the four-stroke engine; the inlet valve admits the petrol/ air mixture, and the exhaust valve releases the combustion products, which are vented via the exhaust. A typical poppet valve is shown in Figure.
VALVE TIMING :
The opening and closing of the valves in an engine in relation to the movement of the piston and flywheel is called valve timing. Very old, slow speed engines had the following valve timing:
Inlet valve – opens at t.d.c. and closes at b.d.c.
Exhaust valve - opens at b.d.c. and closes at t.d.c.
Modern engines operate at high speeds, so to allow for this the valve timing is modified to :
Inlet valve — opens just before t.d.c. and closes well past b.d.c.
Exhaust valve — opens well before b.d.c. and closes just after t.d.c.
VALVE OPERATING MECHANISMS :
Two types of valve operating mechanisms are used in engines :
SIDE VALVE MECHANISM :
A cam mounted on a rotating camshaft operates the tappet, which further actuates the valve against a spring. The valve moves in a valve stem guide and is seated on valve seats when closed a valve spring, fitted between a section of a block and a valve retainer locked with bottom end of the valve stem, acts to close the valve when the cam moves in the closing position.
OVERHEAD VALVE MECHANISM :
It has two additional moving parts, i.e., a push rod and a rocker arm. The cam lifts the valve lifter, which actuates the rod. The push rod, in turn, causes the rocker arm to rotate about a shaft. The other end of the rocker arm pushes down the valve against the pressure of the valve spring.
VALVE CLEARANCE:
A clearance of about 0.3 mm is given to ensure that the valve fully closes. The clearance is measured with a feeler gauge when the follower is on the back of the cam.