VALVE GEARS

Rotorua Morning Post, Volume 2, Issue 256, 21 June 1932, Page 2

VALVE GEARS

OPERATION AND USE OF POPPET TYPE. DELICATE ADJUSTMENTS. Apai't from the sleeve valvc, the j use of which is limited to a few well known makes of car, the poppet valve is in general use for petrol ■ engines. Two sueh • valves are provided for each cylinder, each Ijeing of mushropm shape, with a circular head to control the port leading to or from the cylinder in question. The valve is opened by, a cam to admit or release gases, and is subsequently closed by means of a spring. Broadly speaking, there are two ways in which poppet valves are ordinarily arranged, these being (a) side-hy-side in the cylinder bloc;; (b) inverted in the cylinder-head. The first arrangement is the more usual, the head then being a relatively simple casting containing combustion chamhers, water spaces and sparking plugs. When the second system is used the head must be made of deeper section and must accommodate the valves and ports in addition to its other duties. Valves which are placecl side-by-side in the block can conveh'iently be operated by tappets which, in turn, are actuated by a camshaft placed in the crankcase. There is a clearance between each tappet and valve stem (to allow the expansion with heat), which is adjusted hy means of a screwed head fitted to each tappet. side-by-side in the cylinder block; (b) advantage of simplicity and low rnanufacturing cost. Little Disparity. At one time side-valve engines could not be made to give such a high power output per litre as the overhead valve type, but so much has been learned about combustion-chamber design dufing recent years that the disparity is now only small. There are two ways in which overhead poppet valves can be operated. One of these is really an elaboration of the side-valves system, as the camshaft remains in the crankcase and each cam operates its valve through the medium of a sliding tappet, a light push rod, and an overhead rocker. To enable high speeds to be reached without , valve bounce or undue wear it is essential that the moving parts should be made as light as possible. Consequently, tubular push rods and scientifically lightened rockers are commonly employed. The adjustment for clearance is usually located at one end of the rocker. In order to reduce the weight of the reciprocating parts of the valve gear to a minimum, an overhead camshaft carried on the cylinder head is used to work the valves of many racing and aero-engines in addition to a certain nurnber of touring cars. There is no question but that this system is scientifically correct where high speeds and a maximum power output are desired. On the other hand, it must be very carefully made and fitted to avoid noise from the drive used to connect the camshaft to the crankshaft.