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

38 MOTOR ROAD TRANSPORT ascending, the effect will be far more severe than if the piston was already on the downward stroke. If the crankshaft is turned over slowly so that the flywheel possesses only a minimum of stored energy, and the spark passes while the piston is on the up- stroke, it will not be allowed to finish, its travel, but will be pushed down again owing to the increased heat and subsequent expansion of the gas on burning. The crankshaft will consequently be turned in the reverse direction ; this is what is implied by the term back-fire. When the shaft, however, is rotating at a higher revolution rate, a greater amount of energy is stored by the flywheel, and this will prove sufficient to overcome the resistance of the expansion until the dead centre is passed and the piston is ready to begin the downward path. With the engine running under a light load or at high revolution rate, it will readily be seen that if the spark is made to ignite the charge before the piston reaches the top of the stroke, the amount of power which will be given off by the engine will be in excess of the amount which would result with a lat er firing point. The Fixed-point System. Fixed-point ignition, as the name implies, does not allow the timing of the spark to be controlled from the driver’s seat. It is set at an average point; conse- quently, when the engine is running under a com- paratively light load the ignition is too much retarded for efficiency, whereas inversely it is advanced in too great a degree, with the result that in order to avoid engine-knocking a change of gear ratio must be made. Obviously, the lighter the load on the engine when running under power, the earlier should the combustion of the vapour be begun, and vice versa, for in the latter case there will be considerable pressure on the