What is an Engine valve?
Functionally, they perform similarly to many other types of valves in that they block or pass flow, however, they are a purely mechanical device that interfaces with other engine components such as rocker arms in order to open and close in the correct sequence and with the correct timing.
The term engine valve may also refer to a type of check valve that is used for air injection as part of the emission control and exhaust gas recirculation systems in vehicles. This type of engine valve will not be addressed in this article.
Engine valves are common to many types of combustion engines, whether they run off a fuel such as gasoline, diesel, kerosene, natural gas (LNG), or propane (LP). Engine types vary by the number of cylinders which are the combustion chambers that generate power from the ignition of fuel.
They also vary by the type of operation (2-cycle or 4-cycle), and by the design placement of the valves within the engine [overhead valve (OHV), overhead cam (OHC), or valve in block (VIB)].
What is Overhead Valve?
An overhead valve (OHV) engine is a piston engine whose valves are located in the cylinder head above the combustion chamber. This contrasts with earlier flathead engines, where the valves were located below the combustion chamber in the engine block.
The camshaft in a traditional OHV engine is located in the engine block. The motion of the camshaft is transferred using pushrods and rocker arms to operate the valves at the top of the engine.
An overhead camshaft (OHC) engine also has overhead valves; however, to avoid confusion, Overhead valve engines that use pushrods are often called “pushrod engines”. Some early “intake over exhaust” engines used a hybrid design combining elements of both side-valves and overhead valves.
How does engine valve work?
The valve which allows mixture into the cylinder is the inlet valve; the one through which the spent gases escape is the exhaust valve. They are designed to open and close at precise moments, to allow the engine to run efficiently at all speeds.
The operation is controlled by pear-shaped lobes, called cams, on a rotating shaft, the camshaft, driven by a chain, a belt, or a set of gears from the crankshaft.
Where the camshaft is mounted in the engine block, small metal cylinders tappets sit in channels above each cam, and from the tappets, a metal pushrod extends up into the cylinder head. The top of each pushrod meets a rocker arm which bears against the stem of a valve, which is held in a raised (closed) position by a strong coiled spring the valve spring.
As the pushrod rises on the cam it pivots the rocker arm, which pushes the valve down (open) against the pressure of its spring. As the cam lobe rotates further, the valve spring acts to close the valve. This is called an overhead-valve (OHV) system.
Some engines have no pushrods; the valves are operated more directly by single or double camshafts in the cylinder head itself the overhead-cam system.
As there are fewer moving parts between the camshaft and the valve, the overhead-cam (OHC) method is more efficient and produces more power for a given engine capacity than an engine with pushrods, because it can operate at higher speeds. With either system, there must be some free play in the operating gear, so that the valve can still close completely when parts have expanded through heat.
A pre-set gap tappet clearance is essential between the valve stem and the rocker arm or cam, to allow for expansion. Tappet clearances vary widely on different cars, and faulty adjustment can have serious effects.
If the gap is too large, the valves open late and close early, reducing power and increasing engine noise.
Too small a clearance prevents the valves from closing properly, with a consequent loss of compression. Some engines have self-adjusting tappets, which are hydraulically operated by the engine oil pressure.
Engine Valve Nomenclature
Most engine valves are designed as poppet-style valves because of their up and down popping motion and feature a conical profile valve head that fits against a machined valve seat to seal off the passage of fluids or gases. They are also called mushroom valves because of the distinctive shape of the valve head.
The two primary elements are the valve stem and the valve head. The head contains a fillet that leads into a seat face that is machined at a specified angle to match the machining of the valve seat to which it will match. The seating of the valve face to the valve seat is what provides the seal for the valve against combustion pressure.
The valve stem connects the valve to the mechanical elements in the engine that operate the valve by creating a force to move the stem against the seating pressure provided by a valve spring. The keeper groove is used to hold the spring in position, and the tip of the valve stem is repeatedly contacted by a rocker arm, tappet, or lifter that actuates the valve.
Types of Engine Valves
There are 3 different types of engine valves as follows:
- Poppet valve
- Sleeve valve
- Rotary valve
1. Poppet Valve
It is also known as a mushroom valve because of its shape. It is used to control the timing and quantity of gas flow into an engine. This is the most widely used valve in an automobile engine. The poppet valve is given the name because of its motion of popping up and down.
It consists of a head and a stem. The valve face usually with an angle of 30° to 45° is ground perfectly, since it has to match with the valve seat for perfect sealing. The stem has a spring retainer lock groove and its end is in contact with the cam for up & down movements of a valve. In exhaust, a pressure differential helps to seal the valve. In intake valves, the pressure differential helps open them.
2. Sleeve Valve
The sleeve valve as the name implies, that it is a tube or sleeve that fits between the piston and the cylinder wall in the cylinder of an internal combustion engine, where it rotates/slides.
Ports on the side of the sleeves come into alignment with the cylinder’s inlet and exhaust ports at the appropriate stages in the engine’s cycle.
The inner surface of the sleeve forms the inner cylinder barrel in which the piston slides. The sleeve is in continuous motion allows and drives out the gases by virtue of the periodic coincidence of port cut in the sleeve with ports formed through the main cylinder casting.
Advantages: These valves are simple in construction and are silent in operation. There is noise because there are no noise-making parts like valve cams, rocker arm, tappets valves, etc., Sleeve valve has less tendency of detonation. Cooling is very effective as the valve is in contact with water jackets.
3. Rotary Valve
There are many types of rotary valves. The figure shows the disc-type rotary valve. It consists of a rotating disc that has a port. While rotating, it communicates alternately with the inlet and exhaust manifolds.
Advantages: Rotary valves are simple in construction and are manufactured at cheaper costs. They are suitable for high-speed engines. These valves have fewer stresses and vibrations. Rotary calves perform smooth, uniform, and noise-free operations.