Introduction :-
Every cylinder of an automobile engine has one inlet valve for admission of charge and one exhaust valve for escape of burned gases.
However, in some modern cars two inlet valves and two exhaust valves are being provided per cylinder to achieve high volumetric efficiency in high speed engines. The most common mushroom type poppet valve used in automotive engines is shown in Figure below. There are very strict requirements on the geometry, material of construction and speed of opening and closing of valves as these are exposed to very high temperatures (especially exhaust valve) and pressure drop through them affects the volumetric and overall efficiency the engine.
Every cylinder of an automobile engine has one inlet valve for admission of charge and one exhaust valve for escape of burned gases.
However, in some modern cars two inlet valves and two exhaust valves are being provided per cylinder to achieve high volumetric efficiency in high speed engines. The most common mushroom type poppet valve used in automotive engines is shown in Figure below. There are very strict requirements on the geometry, material of construction and speed of opening and closing of valves as these are exposed to very high temperatures (especially exhaust valve) and pressure drop through them affects the volumetric and overall efficiency the engine.
 |
| Poppet Valve |
The common valve mechanism to operate the valves are
1. Valve mechanism in engine block
2. Valve-operating mechanism for valve in cylinder head
3. Hydraulic valve lifter
4. Mechanism with eccentric rocker arm
1.Valve operating Mechanism for Valves in the Engine Block (L-T- and F-head Design)
When the cam rotates, it lifts the tappet which in turn lifts the valve to open position, connecting the valve port with the combustion chamber. the valve tappet clearance between the valve lifter and the valve stem allows for expansion of valve stem and other parts as the engine becomes heated. It can be adjusted by turning the tappet-adjusting screw. In non-adjustable type, the tappet clearance is adjusted by grinding the bottom of the valve steam.
 |
| Valve operating Mechanism for Valves in the Engine Block |
2.Valve-operating Mechanism for Valve in Cylinder Head (Overhead Valves of I and F-head design)
The cam lifts the valve lifter which actuates the push-rod. This action rotates the rocker arm and pushes the valve stem to open the valve.
3.Hydraulic Valve Lifter
In order to avoid valve-tappet clearance so that minimum noise is produced, a hydraulic valve lifter is used. The tappet acts as an oil reservoir which receives lubricating oil. A piston is connected to the push rod and moves in a cylinder which is connected to the oil reservoir through a ball-check valve. When the cam lifts the tappet, the ball-check valve closes the oil drain from cylinder to reservoir and causes the entire tappet unit to lift the push rod to open the valve.
4.Eccentric Rocker Arm
The position of an eccentric held in the rocker arm is controlled by a oil-operated plunger and a spring and ensures zero valve-tappet clearance
No comments:
Post a Comment