Engine Valve

Valves:

Inlet and Exhaust valves are fitted to each cylinder of the four-stroke engine; the Inlet valve admits the air, and the exhaust valve releases the combustion products, which are vented via the exhaust. A typical poppet valve is shown in Picture. It is usually a flat disk with a long rod known as valve stem. The engine normally operates the valves by pushing on the stems with cams determines the valve lift and when and how quickly (or slowly) the valve is opened.

valve materials:

(i) Inlet valve: Nickel steel alloy, stellite facing

(ii) Exhaust valve: Silicon-chrome alloy steel, sodium filled valves

Valve burn (overheating) is a major problem. It causes excessive valve were and defective sealing as well as engine knocking, It can be solved by valve cooling systems are used. These hollow valve stems are partially filled with sodium and act as a heat pipe.

Parts of valve:

1) Valve head,

2) valve Face,

3) Valve margin,

4) Valve Grove,

5) valve stem,

6) Valve Lock,

7) Seating face angle,

8) Neck,

9) Valve dis,

10) Shaft end

Valve Timing:

The opening and closing of valves in an engine in relation to the movement of the piston and flywheel is called valve timing. It is controlled by the camshaft. Very old, slow speed engines had the following valve timing:

Inlet valve: Opens at T.D.C and closer at B.D.C.

Exhaust Valve: Opens at B.D.C and closes at T.D.C.

Modern engine 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.

At the end of the exhaust stroke and the beginning of the suction stroke, both the valves remain open for certain degrees. This period during which both the valve remain open is called valve overlap. Engines that always run at high speed, Such as race car engines, have considerable overlap in their valve timings. This is required to achieve high volumetric efficiency.

There are some new ways vary valve timings in modern engines. In some Honda engines, VTEC (Variable Valve Timing and Lift Electronic Control) is used. It is an electronic and mechanical system. It follows this cam. At low engine speeds, this rocker is a piston locks the extra rocker to the two rockers that control the two intake valves.

Valve lead: This is where a valve open so many degrees of crankshaft rotation before either T.D.C or B.D.C

Valve lag: This is where a valve closes so many degrees of crankshaft after T.D.C or B.D.C