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

SELECTION OF A VEHICLE 37 discussed from time to time, and because this system is capable of abuse by those who handle it without proper understanding, that of fixed type of ignition has become unwarrantably popular with both commercial motor designers and users. It may be advisable before illustrating the advan- tages gained by the employment of the variable type of ignition over the fixed point pattern, to show in simple language for the benefit of those whose experience in this matter is not wide, what takes place in the engine when the spark control lever is moved. Many men when they are being taught to drive a car are told to accelerate by opening the throttle and advancing the spark lever, and should knock- ing or thumping in the engine be heard, to retard the lever. This method is good as far as it goes, but unless both. owner and driver are taught to understand something of the theory of the case, fixed type of ignition, with all its drawbacks, will appear, owing to the greater simplicity, to be the better method of the two. Early Ignition Desir able. With the ordinary four-stroke internal combustion engine, as even the novice will probably know, when the piston is near the top of the compression stroke a spark occurs at the plug and the “ explosion ” drives the piston downwards, so constituting the firing or power stroke in the cycle. The complete burning of the charge takes an appreciable period, and for this reason it is desirable to make the spark occur early, namely, while the piston is still on the up- stroke. It will be easily realized that if the “ explo- sion ” strikes the piston head—the only movable part at the moment in a gas-tight chamber—while it is