"Soot Proof" Plugs.
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A Few Ignition Wrinkles which Drivers Should Note.
The widespread belief that the formation of soot in an engine cylinder is a necessary evil is a fallacy that has entailed the expenditure of immense sums of money in the aggregate on the purchase of socalled non-sooting sparking plugs. The number of different makes of plugs which are said to prevent this evil is simply appalling, and still they come. It must be admitted that many of these devices undoubtedly tend to lessen ignition troubles. A sootproof plug may be a good thing, but a sootproof mixture, and an oiling system that confines its attention to the lubrication of the bearing surfaces rather than to the filling of the combustion head with carbonizing oil, are much to be preferred.
Petrol engines, when fed with good mixture and properly lubricated, will run indefinitely without the formation of soot and without plug troubles. The formation of soot on a plug is a danger signal ; it means that fuel is being drawn away through the exhaust box, and that the engine's power is being curtailed. Further, it means that the cylinders are being seriously fouled, and possibly that a large amount of lubricating oil is being employed. Probably, there are many non-sooting plugs which would keep the engine running under the conditions named, but they could only be tolerated at the expense of loss of fuel and power, and future bills for the cleansing of the cylinders and piston-heads. It is hoped that this article will bring an appreciation of the fact to those drivers who do not now realize that soot-depositing conditions in an engine cylinder are not a necessary evil, but an abnormal state. The proper thing to do is to remove the cause. Soot-proof plugs may be all right, and their employment is recommended as an extra precaution against the occasional deposition of soot due to some unusual cause, but such plugs cannot. and will not cure a chronic condition of sootiness.
While on the subject of plugs, it may be as well to touch upon a curious belief which is widely current in regard to the cleansing of plugs when they become foul. Unfortunately, one often sees the most-minute care being exercised in the brightening of the metal of the sparking points, but the driver will ignore the foul condition of the insulating surfaces between the centre electrode and the plug shell. As a matter of fact, the presence of soot on the ter_ minal points is of no importance whatever, unless there should be enough present to form a bridge between the points. The matter which really demands close attention on a sooty plug is whether there is a layer of soot upon the surface of the insulation, and over which layer the electrical discharge can find a path and leak to such an extent that the voltage fails to build up to that required for the passing of a spark between the terminals.
It is often the insulation that requires cleansing, rather than the points, and occasionally it may be necessary to take the plug apart in order to allow of its being thoroughly cleansed ; a mere wiping of the porcelain or mica surface may not be sufficient.. We would also warn, owners against a too-common practice of using a piece of emery cloth for the cleaning of the porcelain ; nothing of an abrasive nature should be used, as it destroys the glaze and materially injures the insulation of the part.
There is still one more point relating to plugs with which we wish to deal. In spite of all that has been said and written on the subject, there are still many motorists who believe that the condition of the sparkplug can be determined by testing its ability to pass a spark between its points when in the open air. This test is about as foolish a one as that of applying a test water-pressure of 50 lb. per sq. in. to a steam boiler which is intended to stand up to a steam pressure of 300 lb. to the sq. in. The fact that a plug will spark when outside the cylinder is no guarantee that a spark will pass when the plug is in its proper position and there is a compression within the cylinder which may amount to anything from 50 to 100 lb. per sq. in., as the added resistance in the air gap between the sparking points, caused by the pressure of the gas, is often enough to make the electrical discharge pass by preference through defects in the insulation, which defects are not sufficiently serious to show at atmospheric pressure.