IMPORTANT TEST.

King Country Chronicle, Volume XXXII, Issue 4640, 16 May 1938, Page 3

IMPORTANT TEST.

HIGH VOLTAGE IGNITION. NOTEWORTHY DISCOVERIES. Recent research on the internal combustion engine has been directed to the behaviour ,of the ignition spark. It was initiated in England, but has been carried a stage further by the scientists attached to a wellknown United States company, and some rather startling things have been disclosed.

These upset the principles governing ignition voltage and sparking plug gap which have been laid down for the guidance of motor engineers. The mistakes made when the rules were laid down were due to a belief that the characteristics of a petrolair mixture were much the same as coal gas, whereas it is now known that they are entirely different. Coal gas was used in the tests because petrol was found to be a very inconvenient fuel in the type of laboratory tests carried out. Wide Plug Gap. Sparking plug gaps are set to .025-.030 inch and the voltage of the coil is sufficient to provide a spark that will bridge this gap. Recent experiments have been carried out in the belief that a much wider gap is desirable, especially at low engine speeds, and this would now appear to be confirmed. With wide sparking plug gaps, however, it has been found necessary to employ a current of much higher voltage, and this has necessitated the design of special coils. One of these special coils was acquired by a well-known English motor engineer, and he gives his opinions, formed after a thorough test, in one of the foremost technical journals. He fitted the coil to a 1938 light car with a very high compression ratio (early 7 to 1), a low rotary mass (the reciprocating parts are rather light), and a generous valve timing overlap—an engine designed for attaining high revolutions and excellent from 15 m.p.h. upwards in top gear, but not very pleasant at speeds below that figure. In its original state, it was chronically incapable of an even “tick-over” because of the absence of the heavy flywheel fitted to its forerunners. The special high voltage coil he fitted gave an output of 20,000 volts at the high tension terminal, with the original sparking plugs were set to the wide gap of .045 inch.

Result of Tests

Knowing the normal characteristics of the engine, the engineer, after fitting the new coil and resetting the plugs, said: — “With the high voltage coil and wide plug gaps, it amazed me. Its vigour and general ‘joie de vivre’ when the accelerator was ‘stepped on’ were in no way impaired; on the contrary, there was an improvement. Furthermore, the erstwhile tendency to snatch, which is the motoring term for shying and pawing the ground when asked to do any top-gear, lowspeed tricks, had disappeared to such an extent that, on a level road, one could get out of the car, walk alongside, and then step in and open out again without changing down or touching the clutch pedal.” He then goes on to describe his surprise at the fact that even a considerable increase in the size of the spark should effect such a change in a form of misbehaviour which is generally attributed to a combination of high compression, a 20 degree overlap, and light reciprocating parts, which rob the engine of the stability given by mass inertia. He also states that he found it possible to use a much leaner idling mixture, so that great improvements could be anticipated in low speed driving economy. At speeds above 30 miles an hour, there appears to be little improvement in any direction, but as tractability at low speeds in traffic driving is such an important thing, and its introduction into that form of small, high “revving” power unit so desirable, he considers the progress so far made in the development of high voltage coils of the greatest importance.