Sectional Model of Valve Timer of 4-Stroke Engine

Fawaz Khalid H.Zia-ur-Rahman Usman Ali Sher Talha Hanif

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Introduction Valve overlap External Combustion Engine Parts Ported Engine Working

Contents

Valve Timing In a piston engine, the valve

timing is the precise timing of the opening and closing of the valves. In an internal combustion engine these are usually poppet valves and in a steam engine they are usually slide valves or piston valves.

Introduction

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The inlet valve is made to open 10degree to 30degree before the piston reaches the Top Dead Center (TDC) during Suction Stroke and is allowed to close only after 30degree to 40degree after the piston reaches and leaves the BDC in the beginning of compression stroke

The exhaust valve is made to open 30degree

to 60degree before the TDC in the exhaust stroke and allowed to close only after 80 to 10 in0 the beginning of the suction stroke

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With traditional fixed valve timing, an engine will have a period of "valve

overlap" at the end of the exhaust stroke, when both the intake and exhaust valves are open. The intake valve is opened before the exhaust gases have completely left the cylinder, and their considerable velocity assists in drawing in the fresh charge. Engine designers aim to close the exhaust valve just as the fresh charge from the intake valve reaches it, to prevent either loss of fresh charge or unscavenged exhaust gas. In the diagram, the valve overlap periods are indicated by the overlap of the red and blue arcs. Key:

TDC = Top dead centre BDC = Bottom dead centre IO = Inlet valve opens IC = Inlet valve closes EO = Exhaust valve opens EC = Exhaust valve closes

Valve Overlap

Many two-stroke cycle and all wankel

engines do not have a camshaft or valves, and the port timing can only be varied by machining the ports, and/or modifying the piston skirt (two stroke applications). However, some supercharged two-stroke diesel engines (such as the Wilksch aero-engine) do have a cylinder head and poppet valves, similar to a four-stroke cycle engine.

Ported Engine

Cam Camchaft Follower Inlet valve Outlet valve Fly wheel

Parts

A cam is a rotating or sliding

piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice versa. It is often a part of a rotating wheel (e.g. an eccentric wheel) or shaft (e.g. a cylinder with an irregular shape) that strikes a lever at one or more points on its circular path.

Our cam is made of wood

Cam

A camshaft is a shaft

to which a cam is fastened or of which a cam forms an integral part.

Our camshaft is made of wood.

Cam Shaft

A inlet valve is a valve typically used to control the timing and quantity of gas or vapour flow into an engine. It consists of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The portion of the hole where the plug meets with it is referred as the 'seat' or 'valve seat'. The shaft guides the plug portion by sliding through a valve guide. In exhaust applications a pressure differential helps to seal the valve and in intake valves a pressure differential helps open it

Inlet and Outlet valve

A flywheel is a rotating

mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia and thus resist changes in rotational speed

Fly wheel

The Camshaft is the "heart" of the gasoline

engine.  Your engine will not perform to it’s highest potential unless the cam is precise to provide performance at the speeds that you require. Everything else in your engine may be performing tip-top, the carburetor and ignition systems for example.

You are familiar with the strokes of the four cycle engine, intake, compression, power and exhaust.

Working

Each stroke is one half of a revolution or

180° of crank movement. All four strokes (4 x 180° = 720° ) represent two complete revolutions of the crank.

The purpose of the cam is to operate the valves in the correct sequence in relation to piston movement.

The cam is connected by a 2-1 ratio sprocket or gears, so it only turns once to the crank’s two turns (Two crank degrees equal one cam degree). 

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In order for a diesel engine to operate, all of

its components must perform their functions at very precise intervals in relation to the motion of the piston. To accomplish this a camshaft is used.

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A camshaft is a long bar with egg-shaped eccentric

lobes, one lobe for each valve and fuel injector. Each lobe has a follower as shown on Figure 10. As the camshaft is rotated, the follower is forced up and down as it follows the profile of the cam lobe. The followers are connected to the engine's valves and fuel injectors through various types of linkages called pushrods and rocker arms. The pushrods and rocker arms transfer the reciprocating motion generated by the cam shaft lobes to the valves and injectors, opening and closing them as needed. The valves are maintained closed by springs.

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As the valve is opened by the camshaft, it compresses

the valve spring. The energy stored in the valve spring is then used to close the valve as the camshaft lobe rotates out from under the follower. 

The camshaft is driven by the engine's crank shaft through a series of gears called idler gears and timing gears. The gears allow the rotation of the camshaft to correspond or be in time with, the rotation of the crank shaft and thereby allows the valve opening, valve closing to be timed to occur at precise intervals in the piston's travel.

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