Valve Timing and Working

Valve Timing

Before we discuss valve timing, we shall know about valves and why are they used. Valves are a small component in any engine yet has the most important function to perform.

In short, any automobile out there we see breathes with the help of valves. Valves are small mechanical components having a head and a long tail that is located on the top of the cylinder namely inlet valve and an outlet valve powered by the camshaft through a spring action.

Inlet valves are used to allow the fresh air-fuel mixture in a gasoline engine and fresh air only in a diesel engine to enter the cylinder, whereas exhaust valves are used to allow the burnt gases to move out of the cylinder which is then later released into the atmosphere.

The time manipulation of both the valves for maximum efficiency ( volumetric ) and power is termed as valve timing.

As discussed earlier, automobiles work on 4 stroke engine ie

The Suction Stroke- Piston reciprocates from Top Dead Center (TDC) to Bottom Dead Center (BDC)

The Compression Stroke- Piston reciprocates from Bottom Dead Center (BDC) to Top Dead Center (TDC)

The Power Stroke-Piston reciprocates from Top Dead Center (TDC) to Bottom Dead Center (BDC)

The Exhaust Stroke-Piston reciprocates from Bottom Dead Center (BDC) to Top Dead Center (TDC)

Working

Now when the piston is reaching Top Dead Center (TDC) in the exhaust stroke the inlet valve opens a few degrees before the piston position at Top Dead Center (TDC) during the exhaust stroke.

After reaching the Top Dead Center (TDC) piston moves downwards to Bottom Dead Center (BDC) creating a suction stroke to allow the charge to enter the cylinder via an opened inlet valve.

After reaching Bottom Dead Center (BDC) in suction stroke when piston again tent to move upwards to Top Dead Center (TDC) for compression stroke the inlet valve closes just after the piston starts moving upwards for the compression stroke.

The piston after the compression stroke is pushed downwards due to the power generated by combustion, also the power stroke from Top Dead Center (TDC) to Bottom Dead Center (BDC) and just before reaching Bottom Dead Center (BDC) in the power stroke the exhaust valve also opens due to which a small amount of power is also lost and then again when the piston moves from Bottom Dead Center (BDC) to Top Dead Center (TDC) after the power stroke for the last stroke ie exhaust stroke the exhaust valve is kept open till the piston again start moving downwards for the new suction stroke to allow the burnt gases to move out.

This cycle keeps on running which forms how an engine breathes.

Valve Overlap

When the piston starts to move downwards from Top Dead Center (TDC) to Bottom Dead Center (BDC) in the suction stroke both the inlet and outlet valves are opened simultaneously for a very short period of time.

This time in which both the valves remain open is called valve overlap.

Types of Valves

Monometallic Valves- Made up of single material.

Bimetallic Valves- Made up of 2 materials.

Hollow Engine Valves- Consist of a hollow cavity filled up with sodium.

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