EASY STARTING ENGINES

Wairarapa Times-Age, 2 September 1938, Page 5

EASY STARTING ENGINES

MANY FACTORS IN EVIDENCE. Why are some engines so easy to start? Primarily good petrol is the explanation, because, before the engine can start, there must be an easily ignitable vapour of air and petrol in the cylinders. But there are many other factors. There must be sufficient suction set up by the pistons to draw in an air-petrol vapour from the jet and carburettor. To achieve this, the pistons and their rings must be a reasonably good fit in the cylinder, the exhaust valves must be properly closed, and the inlet valve stems must be a good enough fit in their guides to prevent air being drawn past them instead of through the carburettor. Secondly, all suction must be directed to the carburettor, as any leakage has a serious effect on easy starting. If the throttle is opened wide there is not sufficient suction on the jet to draw up the petrol in the quantity necessary to provide an ignitable mixture. The engine must be rotated fast enough to create the necessary suction. Finally, the ignition must be healthy: a good supply of current to the plugs is vital, and the plugs themselves must be reasonably clean and their gaps adjusted to the setting laid down by the makers. Most cars have some form of choke whereby'an extra-rich mixture can be made, and it is possible to overdo the use of the choke, and so get such a lot of petrol _yapour into the cylinders that ignition cannot take place until further air has been introduced. Figures published show that air petrol vapour can be ignited only over a very small range of proportions, and that a petrol engine will work only over a range of 8 to 20 parts of air to one of petrol by weight. Outside this range a start cannot be made, and that is why, if the choke or strangler has been misused, the engine has to be rotated for so long before it will fire again. In other words, air has to be pumped through until the mixture strength becomes correct again.