Design of Engine Valves P M V Subbarao

Design of Engine Valves P M V Subbarao Professor Mechanical Engineering Department An Extended /Applied Fluid Mechanics….

Geometrical Nomenclature for Valves The instantaneous valve flow area depends on valve lift and the geometrical details of the valve head, seat and stem.

Size of Valve head

Maximum Flow Velocity Location of Crank

Pseudo Flow Velocity • Pseudo Flow Velocity is a parameter to study the effect of valve geometry and the rate change of cylinder volume on air flow into cylinder. • PFV is the ratio of rate of change of cylinder volume to instantaneous minimum valve flow area.

Flow Kinematics through Valves

Valve Timing for Better Flow

Valve Lift in General (Design) Practice : Inlet • The maximum valve lift is normally about 12% of the cylinder bore. • Inlet valve typically occurs 10 to 250 BTC. • IVO should occur sufficiently before TC so that cylinder does not dip early in the intake stroke. • IVC usually falls in the range 40 to 600 after BC. • IVC should occur sufficiently after BC, to provide more time for cylinder filing under conditions where cylinder pressure is below the intake manifold pressure at BC. • IVC one of the principal factors that determines the amount of air taken in high-speed engines.

Valve Lift in General (Design) Practice : Exhaust • EVO strongly effects the engine thermal efficiency, as it affects the effective expansion ratio. • The goal of EVO is to reduce cylinder pressure to close to manifold pressure as soon as possible. • EVC typically falls in the range 8 to 200 after TC. • EVC regulates the quantity of exhaust gases that flow back into the combustion chamber in SI engines. • It also regulates the amount of cylinder burned gases exhausted. • EVC timing should occur sufficiently far after TC so that the cylinder pressure does not rise near the end of the exhaust stroke. • Late EVC favors high power at the expense of low-speed torque and idle combustion quality.

Mach Number For an ideal and calorically perfect gas:

Mass of Fresh air : Intake Process Effect of Sonic Conditions and Irreversibilities Isentropic Subsonic Flow Frictional Subsonic Flow Sonic Efects

Exhaust Gas Flow Through VALVES

Flow Rate Capacity of A Valve Passage • The mass flow rate through a poppet valve is described by the equation for compressible flow through a variable area restriction. • This equation is derived form a one-dimensional isentropic flow analysis. Instantaneous mass flow rate through a valve