Engine valves are the major part of an engine. They ensure the proper supply of air and fuel to the combustion chamber. They also help to expel the exhaust gases from the engine. This article explains the engine valve function, types, working, and applications.
What is Engine Valve?
The engine valve is a mechanical component that allows the fluid to flow into and out of the combustion chamber or cylinder while the engine is operating. The engine valves work in the same way as other valves (i.e., they pass or block the fluid flow).
Engine valves are connected to the camshaft. The movement of the camshaft regulates the opening and closing of the suction and exhaust valves. The proper operation of the camshaft ensures the proper opening and closing of the valves.
Function of Engine Valves
The cylinder head provides safety to the engine valves. The main function of the engine valve is to allow the fluid to flow into and out of the cylinder. The air helps to ignite the fuel. The produced power during the combustion of the air-fuel mixture is utilized to push the piston upward and downward.
The engine valve has the following two types:
The austenitic stainless steel is most commonly used for the construction of inlet valves. This material is a heat and corrosion-resisting material. Nickel-chromium alloy steel is also used for the construction of the inlet valve.
The exhaust valve is also known as the discharge valve. It uses to discharge exhaust gases. Silichrome steel is usually used for the construction of the exhaust valve. Silichrome steel is an alloy of chromium and silicon.
The valves employed in the car engines are also known as mushroom or poppet valves. The valve head has a precision ground surface with enough clearance to prevent a thin edge.
The angular surface is grounded at a 30° or 45° angle to the valve head to match the valve seat angle in the cylinder head. The spring retainer lock groove is located at the valve stem end.
Working Principle of Engine Valve
The working of the engine valves is highly dependent on the motion of the piston, crankshaft, and camshaft. The camshaft regulates the opening and closing of the inlet and exhaust valves. An engine valve works in the following way:
- For the suction stroke, the piston moves from TDC to BDC. During this stroke, the piston transfers its motion to the crankshaft via a connecting rod.
- The crankshaft further transfers its motion to the camshaft.
- As the camshaft receives motion, it pushes the intake valve downward and opens it. When the intake valve opens, the fluid starts to enter the cylinder. During this process, the exhaust valve remains closed.
- The piston reciprocates to perform its compression stroke when the fluid is transferred according to the requirements.
- During the compression stroke, the camshaft again pushes the intake valve and closes it.
- After compression, the piston performs a power stroke. During the power stroke, the intake and exhaust valves remain closed.
- As the power stroke completes, the piston moves to perform the exhaust stroke. During the exhaust stroke, the camshaft pushes the exhaust valve and opens it while the intake valve remains closed.
Read More: Working of Engine Piston
Types of Engine Valves
The engine valve has the following types:
- Poppet valve
- Sleeve valve
- Rotary valve
- Reed valve
1) Poppet Valve
The poppet valve is also called a mushroom valve. It is known as a poppet valve because of its popping upward and downward motion. This type of exhaust valve uses to regulate the timing and amount of gas going to the engine. The poppet valve is mainly used in the automobile engine.
This type of valve has a stem and a head. Typically, the valve face must be matched to the valve seat at an angle of 30° to 45° for a perfect seal.
The valve stem contains a spring retainer locking groove—the end of the stem contacts the cam, allowing the valve to move upward and downward.
For the exhaust valve, the pressure differential uses to seal the valve. For the intake valve, the pressure difference assists in opening the valve.
Read More: Different types of Engines
2) Sleeve Valve
As the name suggests, the sleeve valve is a sleeve or tube installed between the cylinder wall and piston of an IC engine cylinder where it slides or rotates.
The internal area of the sleeve creates an internal cylinder barrel in which the piston rotates/slides. The sleeve continuously moves and expels gas by periodically adjusting the opening cut in the sleeve to the port produced by casting the main cylinder.
Advantages of Sleeve Valve
- The sleeve valve has a simple design.
- They have a silent operation.
- This type of exhaust valve doesn’t contain noise-making parts such as tappet valves, rocker arms, and valve cams.
- They have less detonation capacity.
3) Rotary Valve
The rotary valves have multiple types. The disc-type valve is one of the most famous types of rotary valve. This valve contains a spinning disc that contains a port. When the disc rotates, it alternates between the intake manifold and the exhaust manifold.
Advantages of Rotary Valve
- These valves have low construction costs.
- They have a simple construction.
- These valves are ideal for high-speed engines.
- They produce very low vibration and have fewer stresses.
- They have noise-free, uniform, and smooth working.
4) Reed Valve
The reed valve opens and closes the fluid flow in the same direction at different pressures on each side. It is a type of check valve. It has a mechanical rod. This rod hinged at one end, covering the channel and allowing air or charge to flow in only one direction.
Valve Timing Diagram
The valve timing diagram is a schematic diagram representing the precise timing for closing and opening the intake valve and exhaust valve. The valve timing diagram also shows the piston motion in the cylinder.
On the valve timing diagram, the TDC or top dead center is represented by 90 degrees relative to the x-axis. 270 degrees relative to the x-axis is known as the BDC or bottom dead center.
Ideal or Theoretical Valve Timing Diagram
The ideal valve timing diagram represents the correspondence of the valve timing to the movement of the piston.
- As the piston reaches at TDC, the camshaft pushes the intake valve and opens it (IVO). During this process, the exhaust valve remains closed.
- When the intake valve opens, the air starts to enter the engine cylinder.
- As the piston reaches at BDC, the intake valve closes (IVC).
- After the suction process, the piston performs the compression stroke. During this stroke, the piston compresses the air-fuel mixture according to the requirements.
- After the compression process, the piston travels to perform the power stroke. During the power stroke, the air-fuel mixture burns, and the generated power pushes the piston back to the BDC.
- As the piston reaches the BDC, the exhaust valve opens (EVO), and the exhaust gases are discharged.
- As the piston reaches back to TDC, the exhaust valve closes.
In the case of the overhead poppet valve, a slight clearance is kept between the valve stem and the rocker arm, while in cases of the straight poppet valve, a slight clearance is kept between the valve stem and valve tappet. This slight clearance is known as the valve tappet clearance.
The valve tappet clearance permits the valve stem to expand as the engine warms up.
As the engine gets hot, the valve will not close properly without adequate clearance, resulting in loss of power and valve lifting. Therefore, it is best to have more clearance than the requirements, which may increase valvetrain noise slightly.
Engine Valve Construction Material
The engine valve is one of the most important and most stressed parts of the IC engine. The valves must have the ability to endure extreme combustion chamber pressures, extreme temperatures, and repeated continuous mechanical and engine dynamic stress to ensure the reliable working of the engine.
The intake valve of the IC engine has reduced thermal stress due to the cooling effect of the air-fuel mixture flowing through the intake valve during the suction cycle. However, the exhaust valve has higher thermal stress because it is located in the exhaust path of the gases during the exhaust cycle.
During the exhaust stroke, the exhaust valve is opened and doesn’t make contact with the cylinder head. Therefore, the combustion surface has lower thermal mass, and the quick temperate change ability of the valve head increases.
Due to the low operating temperature, intake valves are most commonly made from materials such as tungsten steel, nickel, or chrome. The exhaust valve bears higher temperature and pressure than the intake valve. Therefore, cobalt-chromium alloys, silicon chromium, or nichrome are used to construct the exhaust valves.
Valve surfaces exposed to high temperatures can be reinforced by brazing a cobalt-chromium alloy, Stellite, to the valve surface.
Tribaloy, titanium, and stainless-steel alloys are other materials used in the manufacture of engine valves.
Coatings and surface treatments can also be used to improve the mechanical quality and wear properties of engine valves.
What are the engine valves used for?
The engine valves have two categories:
- Inlet Valve: This engine valve is used to suck the atmospheric air into the engine cylinder.
- Exhaust Valve: This valve is used to expel the exhaust gases into the atmosphere.
What causes engine valve damage?
Many things can cause burnt valves, but one of the most common reasons is ignoring leaking seals and guides or failing to fix other compression problems. Combine those problems with a cooling system or EGR (exhaust gas recirculation) issue, and you are extremely likely to burn the valves.