What is an Engine Valve and What are Its Types?

An engine is one of the most remarkable inventions that enables us to perform seemingly complex tasks without hassles. However, a series of complex components play vital roles while allowing the engine to convert fuel into power. 

One of the primary components of an engine is an engine valve that works with utmost precision and flawlessly governs the flow of gasses. It involves a set of intriguing mechanisms that ensures the seamless functioning of the engine.

Engine valves regulate the movement of air and fuel in and out of a combustion chamber of an engine. They play an essential part in the combustion process of engines with their intricate designs and precise mechanisms. 

Below discussed are the two key components of an engine valve that jointly perform the task of gas flow.

1. Inlet Valve

An inlet valve of an internal combustion engine allows the entry of air-fuel mixture into the cylinder with precise timings. They close tightly to firmly seal the combustion space, thereby preventing the backflow of gasses. Since inlet valves do not get subjected to excess heat, they are made of special nickel-chromium alloys.

2. Exhaust Valve

Exhaust valves are another crucial component of an internal combustion engine. These valves open to allow the exit of burnt gasses from the combustion chamber. Then, they close firmly to shut the combustion chamber, thereby preventing any possible gas leakage. Besides, exhaust valves consist of silicone-chromium alloy that provides resistance to high temperatures.

You can find several types of engine valves in combustion engines. Here are a few of them.

1. Poppet Valve

Poppet or mushroom valves are commonly found in car engines for their remarkable features. Poppet valves with their mushroom shapes, incredibly regulate the entry of gasses into an engine. Under this type, there are two essential parts of engine valve, namely 

• Head: The valve’s head is perfectly positioned at a 30–45-degree angle that ideally fits with the valve seat, creating a firm seal.

• Stem: A valve stem consists of a groove in the form of a spring retainer that moves up and down to be in contact with the cam. 

2. Sleeve Valve

Sleeve valves are tube-like structures lying between a piston and cylinder wall that regulates the flow of air and fuel inside the cylinder. These sleeve valves have small holes or ports in their sides that promptly line up with the holes of a cylinder. As a result, these holes allow the movement of air and fuel.

3. Rotary Valve

Rotary valves work with a rotating disc that comprises small openings called ports or holes. This rotating disc aligns with the inlet allowing inward movement of gasses and fuel. Correspondingly, this valve aligns with the exhaust manifold to expel gasses and liquid. Rotary valves are popular for their smooth and quiet operations.

4. Reed Valve

Reed valves are a special kind of valve commonly found in internal combustion engines. Unlike rotary valves that consist of rotating discs, a reed valve comprises hinged stripes of metal that control airflow. 

The pressure of fuel and air pushes open these strips covering the inlet port to enter the engine. Simultaneously, the exhaust port remains closed, thus preventing gas from escaping.

Here is a step-by-step explanation of the workings of an engine valve:

Step 1: Initially, power is supplied to the crankshaft, an essential component of every machine. This facilitates the conversion of up and down piston motion into a rotational motion.

Step 2: In the next step, a connection is made between a camshaft and a crankshaft. This camshaft now moves to control the opening and closing of its engine's valves.

Step 3: At this stage, a suction stroke initiates an up-and-down motion in an engine's piston, causing its camshaft to push down the inlet valve. As a result, air and fuel enter an engine's combustion chamber.

Step 4: In this step, as the piston moves up to create a compression force, the camshaft movement leads to inlet valve closure. Besides, a spring force keeps this valve closed.

Step 5: This step involves a power stroke that makes the combustion of air and fuel mixture push down the piston. At this stage, the inlet and exhaust valves remain shut to maintain pressure that prevents gasses from escaping.

Step 6: This step commences with an exhaust stroke that moves the piston in an upward direction, thereby pushing the burnt gasses out of the combustion chamber. Besides, the exhaust valve opens to expel these gasses into an engine's exhaust system.

This cycle repeats as intake valves open again to allow fresh air and fuel inside.

Here are the two valve operating mechanisms.

1. Side Valve Mechanism

In a side valve mechanism, the valve lies on top instead of the sides. While an engine is running, a camshaft rotates to ensure a cam pushes against a part called a tappet. Furthermore, this tappet pushes against a spring which moves the valves.

The cam reaches the highest point to push open this valve. Lastly, the cam continues rotating while the tappet moves down to close the valves again.

2. Overhead Valve Mechanism

As the name suggests, in an overhead valve mechanism, the valves lie above a combustion chamber. Furthermore, a camshaft turns to move the tappet upwards. Then, this tappet moves a push-rod and one end of a rocker's arm. As a result, the other end of this rocker's arm moves downwards. Furthermore, the valve opens against the tension of a spring.

With a movement of the cam to its highest point, this valve opens fully, allowing fuel and air movement. The cam continues its rotational motion, which moves the tappet down, closing the valve due to the tension force of the spring.

Most engines in recent times use poppet-type valves, mainly because of its up and down motion when closing and opening the valve. The structure of a modern engine valve is divided into two parts:

• Stem: This is the upper part of the valve. The top part of a stem is also known as the tip, which stays in contact with the camshaft. A keeper groove comes right below the tip in the stem.
• Head: It is the lower part of the valve. It consists of a fillet, designed according to the port of an engine. Moreover, a seat face is also fitted to the port. Its function is to block the flow of the substance.

Besides that, a combustion face is placed within the cylinder, which is responsible for handling all the combustion reactions here. 

source:wikimedia

This diagram helps to get a detailed idea of the accurate timing of opening and closing of the exhaust and inlet valves. The movement of the piston inside the cylinder mainly determines the design of a valve timing diagram. 

The top dead centre is the top of the circles or the 90 degrees of the x-axis. The bottom or 270 degrees of the x-axis constitutes the bottom dead centre. 

An ideal or theoretical valve diagram shows the following activities:

• When the piston reaches the top dead centre, the inlet valve opens (IVO) opens. 

• Then, when the piston reaches the bottom dead centre, the IVO closes. 

• Wherever the fuel is compressed or burned, it follows a compression stroke.

• While fuel is burning, the piston is pushed against the bottom dead centre, and the power stroke expands.

• The exhaust valve opens once the piston reaches the bottom dead centre. 

• It then follows the exhaust stroke once the piston pushes the gases out of the exhaust valve.

• After the exhaust valve closes, the piston then reaches the top dead centre. 

The materials used for making inlet and exhaust valves are different, owing to their differences in functionality. Inlet valves allow the entry of fuel and air so it does not get hot. Contrarily, exhaust valves undergo pressure and heat owing to the combustion process.

An inlet valve is made of materials like tungsten or nickel steel. These materials are not capable of withstanding high temperatures. Contrarily, exhaust valves are made of cobalt-chromium and silicon-chromium alloys since they are resistant to heat and pressure. 

To conclude, an engine valve plays a critical role in the smooth functioning of engines. Over the years, there have been significant developments in the manufacturing and technological sector of engine valves. Modern cutting-edge technology has given rise to engine valves that are nothing short of masterpieces and significantly improve user experience.