In four stroke internal combustion engine,the "Poppet Valve" performed the opening of the cylinder to inlet or exhaust manifold at the correct moment.Generally the face of valve is ground at 45 degree but in same cases it is ground at 30 degree also.It is not important to have a same angle of face in inlet and exhaust valve of same engines.To make it in right order,the valve may be reground after some use.There is some margin provided to avoid sharp edges.The groove,retain the valve spring which aids in keeping the valve pressed against the seat when closed and thus seal the combustion space tightly.In close position,the valve face,fits the accurately matched ground seat in the cylinder block.Generally replaceable ring inserts are used for exhaust valve seat.
The inlet valves are made from plain nickel,nickel chrome or chrome molybdenum.Where as exhaust valves are made from nickel chrome,silicon chrome steel,high speed steel,stainless steel,high nickel chrome,tungsten steel and cobalt chrome steel.
Valve Mechanism of a Side Valve:-
Poppet valve has following main parts,
01) Cam Shaft
03) Cam Follower
05) Adjusting Screw
07) Valve Spring
08) Valve Stem
09) Valve Stem Guide
10) Valve Face
With the help of these parts,valve performs it's operation very accurately in internal combustion engine.
The cam actuates the movement of the valve through the tappet.The replaceable valve stem moves up and down in the valve stem guide.This movement is obtained by rotation of camshaft and cam,which generally runs at the half the engine speed.
The valve spring,keeps the valve pressed against its seat and ensure a leakage proof operation and also bring back the valve very quickly during its closing.
When the engine is started,it gets heated up gradually there by causing the valve stem to expands.A valve tappet clearance is always provided to allow the expansion of valve stem and other parts.This clearance value depends upon the length of the valve,its material and the operating temperature of the engine.The tappet valve clearance can be adjusted by rotating the adjusting screw.
Where adjusting screw is not provided to vary the clearance,it can be increased by grinding the bottom of the valve stem and face or by using longer valve.Due care must be taken because even a slightly insufficient clearance may lead to the valve not properly resting against its seat as the engine gets heated causing increased noise level and loss of power.
The clearance provided in exhaust valve is slightly more than that of inlet valve.This is due to slightly more expansion in exhaust valve because of higher temperature of hot exhaust gases produced during combustion.
Valve Mechanism Of Overhead Valve:-
In this mechanism,a push rod and a rocker arm,to push the valve against the spring pressure is required.The rocker arm rotates about the rocker arm shaft under the force exerted by the push rod.The clearance in this case is kept between the rocker arm and the valve stem and can be adjusted by the screw adjuster.A suitable gear train or chain is provided to convert the drive from crankshaft to camshaft.
In high speed engines,the frequency of vibration of the valve spring coincides with normal operating frequency of the valve which leads to resonance and increasing the surging effect.To overcome this problem,now a days compound spring are used.Compound spring means "One spring within the other of different natural frequencies".
Valves are the most important part of every engines,So due care must be taken in selection and maintenance of valve.
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