TECHNICAL TERMS

Sun (Auckland), Volume 1, Issue 92, 9 July 1927, Page 23

TECHNICAL TERMS

THE PARTS OF YOUR CAR SIMPLE EXPLANATIONS To those motorists who are not altogether familiar with the various technical terms and expressions in connection with the car, such words as choke, strangler, hammered rings, etc., augur more of violence than any unit of their motor. A floating bush might mean anything but what the words imply. Such terms as worm and worm wheel, while known to bear no relationship to the garden variety, are, in the majority of cases, hedged in mystery as to their exact meaning, writes F. J. Berry in the “Daily Guardian.” Yet all these terms apply to definite units or functions of the motor of today, and are capable of simple explanation. Perhaps the most confusing of all is in connection with the ignition timing. A mechanic will probably advice you to give the magneto another tooth. This is not a job for a dentist, but merely refers to advancing the ignition point by disconnecting the timing wheel of the magneto and rotating it forward one tooth of the gear wheel. Where vernier couplings between the magneto and drive are fitted this operation may be done by simply loosening the magneto from its holding-down band and after drawing it away sufficiently to free the coupling, rotate it in the direction of its travel one notch of the coupling. The choke, or more correctly, the venturi tube, is a fitting in the carburettor placed above the jet and scientifically designed so as to give the

greatest suction or. to be more exact, the lowest pressure in the induction system at that particular point. This ensures a very complete vaporisation of the fuel passing through the jet orifice. The strangler is another fitting of the carburettor, and consists of a simple butterfly valve fitted on the outer side of the main air opening. When this valve is closed it strangles the air entering the main air passage, and so the suction of the engine pulls strongly on the jet and so gives a very rich mixture so desirable when starting up in cold weather. The hot spot of the induction system really explains itself. The hot gases from the motor are brought into contact with the induction manifold by passing through the exhaust pipe and then around the centre of the indLic.tion manifold above the carburettor. This fitting prevents undue condensation of the gases passing to the cylinders when the motor is cold. Many parts of the car are said to float. This, of course, has no bearing on their weight or their behaviour in water, but merely that they “float” in relation to some other part. Take for example axles. There are three types of floating axle —the full floating, threequarter floating, and semi-floating. The full-floating axle bears no stresses other than those imposed by the power used in driving. Bearings inside the housing support the axle for its full length and so all stresses are absorbed by this housNeither do the axles support the rear road wheels. These are px-ovided with a separate pair of bearings fitted in the wheel hub housing.

In this full floating type either axle may be withdrawn by simply removing the wheel caps and using a suitable pulling device. Further, the axle has no bearing on

the alignment of the wheels. In the threequarter floating type the inner ends of the axle are similar to the full floating type, but only one bearing supports the wheels, so that the outer ends of the axle support the wheels and take portion of the shocks and stresses imparted by driving. They therefore have a bearing upon the wheel alignment. The axles in this type may, however, be withdrawn without removing the wheels. The semi-floating axles have tho driving wheels keyed to the sha‘ft. The axles themselves run in outer bearings and so take the load and stresses transmited via the wheels when driving as well as the power transmitted. The wheels depend almost entirely upon the axle for alignment, and they must be removed before the axle can be withdrawn. Compression ratio is the term used to explain the degree to which the gases in the cylinder are compressed before being fired by the spark. For instance, if when the • piston of ihe cylinder were at bottom dead centre it had sucked in a charge of gas equal to one cubic foot and then on reaching the topmost position the volume was onlv one-quarter of a cubic foot, the compression ratio would be said to be four to one.

Present-day touring engines vary in their compression ratio from four to six to one.

Many racing engines, however, have ratio in the order of eight to one. but these must be run on special fuels in which alcohol is in a large measure used, otherwise the efficiency of the motor could not be obtained. If ordinary petrol were used in a motor of fhis kind, it would detonate when fired and not burn in the ordinary manner.