Engine Valves are categorized as per application viz. Inlet or Exhaust. Inlet Valve can normally be recognized by its Head Dia which is bigger then that of the Exhaust Valve. Number of Valves per cylinder is as follows:
1) 1 In / 1 Ex: In majority of cases this is the combination.
2) 2 In / 1 Ex : Usually to provide more
intake, increased power with a
3) 2 In / 2 Ex : Surface Area of pots is
increased by using two smaller valves rather than one big valve. Used in very
high H. P. Engines, or
for very high speed / low noise / high torque application for improved efficiency with a highly compact Engine design.
4) 2 In / 2 Ex. / 1 Idle : Latest design from Volkswagen for the same reason as mentioned above with an added valves which acts only during idle running.
MAJOR CATEGORIES OF VALVES:
MONO - METAL VALVES: It is the least complicated of all valve constructions. Reduces manufacturing cost.
BI-METAL VALVES: Usually used for making Exhaust Valves. Since the temperatures & working conditions is severe at exhaust port, austenitic steels like 21-4 / N or higher grade is the preferred material choice. There are two major disadvantages of using this in single metal construction.
A) Austenitic Steels are not easily hardenable. Hence the Valve Tip has to be either Stellited or a hardened piece has to be welded at the tip end. Both the operations are relatively expensive compared to induction hardening.
B) The severe working conditions are only near the Valve Seat area & the Valve Neck portion to some extent. The portion of Valve in valve Guide i.e. the Valve Stem, does not have to sustain these extreme conditions. Hence the use of costly austenitic steel is not justified. The solution was to make a Bi-metal Valve. The most reliable of making Bi-metal Valves is by Friction Welding Process. Hence Bi-metal Valves reduces the cost of Valve.
SHROUDED VALVES: The “shroud” or the projection just above the Valve Seat increases the swirling effect inside the Combustion chamber as the valve is usually rotating inside the chamber. This improves the efficiency. The valve shape is complicated & is difficult & costly to manufacture.
SUPER ALLOY VALVES: Recent developments in very high performance engines require very high performance valves where the conventional Valve Steels could not satisfy the required parameters. Hence newer material were invented like Nimonic 80 A, Pyromet 31, Inconel - X751, Nitronic - 20, etc. Most of these Super Alloys are Nickel based with various other alloying elements including Iron (Fe) present only in small percentages to enhance the performance.
SOME TECHNICAL SPECIFICATION ABOUT VALVES: