How Valves Work in an Engine?

How Valves Work in an Engine

How Valves Work in an Engine – Engine valves are mechanical components that allow or limit the flow of fluid or gas to and from the cylinders or combustion chambers during engine operation. They function similarly to many other types of valves in that they block or pass flow, but they are purely mechanical devices that interface with other engine components such as rocker arms to open and close in an accurate sequence and timing.

 

What Is the Function of Valves in An Engine?

The cylinder head houses the engine valves. The engine valve’s primary function is to allow air into and out of the cylinder. Air is used to assist in the ignition of the fuel, which pushes the pistons up and down.

Engine valves are classified into two types: intake valves and exhaust valves. Of course, the intake valve allows air to enter and the exhaust valve allows air to exit. The more air you take in and out of the engine, the more efficient it will be thus generating more power. This is why engine valves are so important to engine performance.

The piston in the cylinder moves up and down. The valves are located at the top of the piston stroke. Depending on the manufacturer, the number of valves varies. Because the piston is at the bottom of the cylinder, the intake valve opens to allow air into the cylinder and then closes to make the cylinder airtight to establish compression. The exhaust valve will open after the piston has completed the compression and ignition strokes. Then it shuts down. However, you may be wondering how the valve is opened and closed. A camshaft is a shaft that drives all of the valves. To see more information about camshaft, visit here

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Various Parts of an Engine Valve

Because of its up-and-down popping motion, most engine valves are constructed as poppet-style valves, with a conical profile valve head that fits against a machined valve seat to close off the passage of fluids or gases. Because of the unique shape of the valve head, they are also known as mushroom valves. The nomenclature for the various parts in a typical engine valve is shown in the following figure.

How Valves Work in an Engine
Various parts of an engine valve (Reference: thomasnet.com)

 

The valve stem and valve head are the two most important components. The head has a fillet that leads to a seat face that is machined at an angle to agree on the machining of the valve seat it will be matched to. The seal for the valve against combustion pressure is provided by the seating of the valve face to the valve seat.

By providing a force to move the stem against the seating pressure generated by a valve spring, the valve stem connects the valve to the mechanical mechanisms in the engine that operate the valve. The spring is held in place by the keeper groove, and the valve is actuated by a rocker arm, tappet, or lifter repeatedly contacting the tip of the valve stem.

 

Working Principle of an Engine Valve

The following steps are the working procedure of an engine valve:

  • The crankshaft of the engine receives the power initially.
  • The engine’s camshaft is driven by the crankshaft.
  • During the suction stroke of the engine, the camshaft moves, pushing the inlet valve down.
  • Because of its spring force, the camshaft allows the inlet valve to close when the piston follows the compression stroke.
  • Following the compression stroke, the piston moves into a power stroke with both valves closed.
  • Following the engine’s power stroke, the exhaust stroke occurs, in which the camshaft pushes and opens the exhaust valve.
  • The piston pushes the exhaust fumes out, and the cycle continues.

 

Valve Timing Diagram

The valve timing diagram is a schematic that depicts the proper timing of the opening and closing of exhaust and inlet valves. The movement of the piston inside the cylinder is depicted in a valve timing diagram.

The top of the circles, or 90 degrees relative to the x-axis, is referred to as the top dead center, or TDC, in a valve timing diagram. The bottom dead center, or BDC, is represented by 270 degrees on the x-axis.

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Ideal or Theoretical Valve Timing Diagram

The ideal valve timing diagram depicts how the valve’s timing should correspond to the piston’s movement.

How Valves Work in an Engine
Ideal or theoretical valve timing diagram (Reference: themechanicalengineering.com)

 

  • When the piston approaches the top dead center, the inlet valve opens (IVO).
  • When the piston approaches the bottom dead center, the inlet valve closes (IVC).
  • The piston then moves to the compression stroke, which compresses and burns the fuel.
  • Following the expansion or power stroke, the gasoline burns, pushing the piston back to the bottom dead center.
  • The exhaust valve is opened when the piston approaches the bottom dead center (EVO).
  • The piston now moves to the exhaust stroke, pushing the exhaust gases through the exhaust valve.
  • The exhaust valve closes when the piston hits the top dead center after the exhaust stroke (EVC).

Actual Valve Timing Diagram For 4 Stroke Petrol Engine

The ideal or theoretical valve timing diagram differs from the real valve timing diagram and has the following steps: 

  1. Suction Stroke
  2. Compression Stroke
  3. Expansion Stroke and
  4. Overlap
How Valves Work in an Engine
A valve timing diagram for 4 stroke petrol engine (Reference: themechanicalengineering.com)

 

In what follows, various steps are described.

 

Suction stroke

When the pressure inside the cylinder drops below atmospheric pressure, the suction stroke begins roughly 15 degrees before the top dead center in a four-stroke petrol engine. The ideal suction stroke is completed when the piston reaches the bottom dead center. However, even when the piston begins to move higher, the inlet valve remains open for around 30 degrees in this situation. After BDC, the inlet valve closes 30 degrees. As a result, the real suction stroke is longer than the ideal one.

 

Compression stroke

After the BDC, the compression stroke begins at 30 degrees and continues until TDC, when the fuel is burned with the help of a spark plug. The actual compression stroke differs from the ideal compression stroke by 30 degrees. Both valves remain closed during this stroke.

 

Expansion stroke

In a four-stroke engine, the expansion or power stroke is the only usable stroke. The piston is forced down by the combustion of the fuel, which aids in the movement of the flywheel. Both the intake and output valves are closed during the expansion stroke. Around 50 degrees before the BDC, the expansion stroke comes to a halt.

 

Exhaust stroke

In a four-stroke petrol engine, the exhaust stroke begins around 50 degrees before BDC, when the exhaust valve is opened, and ends 20 degrees after TDC when the exhaust valve is closed. The piston pushes the exhaust gases out of the exhaust valve during this stroke.

 

Overlap

Both the inlet and exhaust valves are open for a 40 to 45-degree range. This is referred to as overlap. Both the suction and exhaust strokes share this time. During overlap, the fuel and exhaust gases inside the cylinder are pushed out of the cylinder through the exhaust valve. This aids in better cylinder cleaning, which is necessary to keep the cylinder’s inner walls clear of carbon deposits.

 

Types of Engine Valves

Apart from engine valves being classified by function (intake versus exhaust), there are various types of engine valves based on design and materials. The following are the most common types of engine valves:

  • Monometallic engine valves
  • Bimetallic engine valves
  • Hollow engine valves

Monometallic engine valves are made of a single material that is used to make both the valve stem and the valve head. These engine valves have strong heat resistance as well as excellent anti-friction properties.

Bimetallic engine valves, also called bimetal engine valves, are created by friction welding two distinct materials together to make a valve with austenitic steel on the valve head and martensitic steel on the valve stem. The austenitic steel on the valve head provides high-temperature resistance and corrosion resistance, while the martensitic steel on the valve stem provides high tensile strength and abrasive wear resistance.

Hollow engine valves are a type of bimetallic valve that has a sodium-filled hollow chamber. As the valve temperature rises, the sodium liquefies and is circulated by the valve’s motion, which helps disperse heat from the hotter valve head. Because the martensitic stem material conducts heat better than the austenitic head material, the hollow shape allows for more heat transfer through the stem than solid valves. Hollow valves are particularly well adapted to current engines, which require more power from smaller, denser engine designs with higher exhaust gas temperatures than solid valves can handle.

Several factors contribute to the higher exhaust temperatures, including:

  • A goal to diminish greenhouse gas emissions by using a lean-burn combustion technique
  • Higher compression ratios and higher combustion pressures in engine designs provide greater efficiency.
  • Integrated manifold designs with turbocharger support for increased engine performance from smaller engines

There are a variety of engine valve designs to choose from. Sleeve valves are made up of a tube or sleeve that sits between the cylinder wall and the piston, and they slide or rotate like other engine valves thanks to a camshaft. The sleeve valve’s movement enables ports cut into the sleeve to align with similar ports in the cylinder wall at certain moments during the engine cycle, acting as a basic engine intake and exhaust valve without the complications of rocker arms and lifters.

How Valves Work in an Engine
Various Types of Engine Valves (Reference: kishanauto.com)

 

Engine Valve Materials

Engine valves are one of the most strained components of internal combustion engines. Engine valves must be able to withstand repeated and continuous exposure to high temperatures, high pressure from the combustion chamber, and mechanical stresses and loads from the engine dynamics to provide reliable engine operation.

Because of the cooling effects of the incoming air/fuel combination that flows through the intake valve during the intake cycle, intake valves on internal combustion engines are subjected to reduced thermal stress. On the other hand, Exhaust valves are subjected to higher amounts of thermal stress due to their location in the path of exhaust gases during the engine’s exhaust cycle. Furthermore, because the exhaust valve is open and not in contact with the cylinder head during the exhaust cycle, the smaller thermal mass of the combustion face and valve head has a higher potential for a quick temperature change.

Because of the lower operating temperatures, intake valves are usually composed of materials like chrome, nickel, or tungsten steel. More heat-resistant metals, such as nichrome, silicon chromium, or cobalt-chromium alloys, may be used in higher-temperature exhaust valves.

Valve faces exposed to high temperatures are occasionally strengthened by welding Stellite, a cobalt and chromium alloy, to the valve face.

Stainless steel, titanium, and Tribaloy® alloys are among the other materials utilized to make engine valves.

Coatings and surface treatments can also be used to improve the engine valves’ mechanical qualities and wear characteristics. Chromium plating, nitride coating, phosphate plating, and swirl finishing are examples of this.

 

FAQs about Engine Valves

  • What are the Types of Engine Valves?

The following are the three types of engine valves: Valve with a poppet. The valve on the sleeve Valve with a rotary action.

 

  • What is the Name of the Valve that is in an Engine?

The valves are located in the engine’s head and are responsible for allowing air and/or fuel into the cylinders to be combusted (intake valves) and allowing the exhaust from that combustion to exit the cylinders (exhaust valves).

 

  • What Causes Engine Valve Damage?

Burnt valves can be caused by a variety of factors, but one of the most common is failing to address leaking seals and guides, as well as other compression issues. When those issues are combined with a cooling system or EGR (exhaust gas recirculation) malfunction, the valves are very likely to be burned.

 

  • How Much Does it Cost to Replace a Car Valve?

Determine how much you should pay to have your vehicle repaired. A valve adjustment costs between $246 and $336 on average. Labor costs are expected to range from $220 to $278, with parts costing between $26 and $58.

 

  • What Causes Valves to Float?

When the valve train loses control, valve float happens. It is brought on by a lack of Valve Spring Pressure.

 

  • How Long Do Engine Valves Last?

In a dirt track or drag racing engine, an exhaust valve that may endure for years and 150,000 miles or more in a typical passenger vehicle engine might only last a season or two.

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