Motor Road Transport For Commercial Purposes
(Liquid Fuel, Steam, Electricity)

SELECTION OF A VEHICLE 39 piston head and resultant stress in the connecting rod, and if a bearing or the gudgeon pin is worn, a knock will make itself heard. This can, of course, be corrected to a large extent if the timing is regulated by the driver. Undoubtedly the reason why some manufacturers fit their chassis with fixed-point ignition is the fear that if the other type were supplied its application would be abused, causing undue wear in the engine. The argument is, however, weak, for the simple reason that misuse is equally possible with the fixed-point system. Variable Ignition a Necessity for High Engine Efficiency. In the opinion of the writer, the importance of simplicity in the design of a chassis throughout cannot be too strongly emphasized, but the few additional parts required for the variable type of ignition are an absolute necessity if high engine efficiency is called for. Imperative as simplicity in design is, tliere is yet a limit beyond which it cannot be carried unless some vital factor is sacrificed, and ignition is a case in point. With the carburettor as designed to-day, it is practically an impossibility to provide a correctly proportioned mixture for varying loads and engine speeds which will allow rapid and complete combustion to take place in the cylinder. It will be found that at some speeds the vapour will be too rich, and at others too weak, imperfect combustion resulting in both cases. The reason in the first instance is lack of oxygen, and in the second the slow rate of burning. These different mixtures will manifestly possess varied rates of flame circulation, and it is of great importance for maximum thermal efficiency to be able to alter the timing of the spark accordingly; for instance, to advance it for the burning of a weak mixture, and to