IGNITION SPARK.

Hawera Star, Volume XLVII, 19 November 1927, Page 12

IGNITION SPARK.

AUTOMATC CONTROL. It is not- surprising that an increasing number of car makers is taking the control of the ignition spark out of the hands of the car driver, and making the advance and retard operations automatic. For the spark lever is probably one of the least used filaments on a. motor vehicle. Many drivers allow the lever to remain in the fully advanced position irrespective of speed, road conditions, or grades. Others leave it where it happens to be when they take tlu wheel, while a minority use the lever intelligently, and get, more efficient operation of the eingine. in effect, the spark lever enables the driver to adjust the spark to coincide with piston speed and the class of fuel used. There is no hard and fastrule regarding ignition timing, owing to the fact that piston speed and the degree of inflammability of the mixture of petrol and air are variable factors. Economy of fuel and a better delivery of power wiii result if the engine is run with the greatest amount of spark “advance” it will take without knock, ing.

The explosive mixture comprising petrol and air is comparatively slow burning, although the fact does not seem to be generally recognised by motorists. The mixture takes a definite period of time from when the spark occurs until it gives its full driving power. This period depends upon the strength, quantity, and degree of compression of ' the mixture. It follows, then, that, at any given piston speed (engine revs.) there must be a definite point in the stroke which is the most favourable for the piston to take the full power of the explosion. The timing lever, which operates a simple piece of on the magneto, or in the. case of coil ignition on the distributor, allows the man at the wheel to control the spark so that it will take place at the most favourable position of the stroke. When the piston is travelling at high speed the spark should be sent on ahead, so that the full force of the explosion will be available precisely at the right moment. f The spark, of course, occurs at some point before top dead centre, and by experience a driver learns just how the spark should be timed for all running conditions.

IGNITION FAULTS. Who no oar is difficult to start, or when the engine appears to be firing unevenly, it is generally thought that a reliable test for the sparking plugs is to place them on top of the cylinder block, depressing the starter button and noting if sparks appear at the plug points. It is well to remember, however, that this test shows only that the magneto is functioning and that at least some electrial energy is being delivered t-o the points. It does not in any way follow that because, a weak spark will bridge a G m.m. gap in air it will do so under compression. A leak may be present in the high tension cable, also, which will not affect the free spark in air, hut when the plug is subject to the compression in the engine cylinder the spark may take the easier path from cable to earth