MOTORING NOTES

Waipa Post, Volume 48, Issue 3472, 29 May 1934, Page 2

MOTORING NOTES

SPARKING PLUGS. .IMPORTANCE TO CAR. No matter how good a sparking plug may be it will not give satisfactory service if it is of an incorrect type. Generally speaking, the motorist ..fails ito realise that there can be a' great deal of difference between sparking plugs—not. only .as regards quality but also in the heat characteristics of the plugs themselves. Heat Rauge and Qharaeteristies. Engines in use to-day vary widely in type from, say, the large and 1 elativeiy “woolly” engine to .the small high compression, high speed type. The heat conditions inside the cylinders of these engines vary accordingly. ‘ In addition to the different designs of engine, the conditions under which they are operated, e.g., high speed—a great deal, of traffic driving—or continued heavy haulage, all have their effect on the suitability or otherwise of the sparking plug. No one make or type of plug has yet been evolved which will give satisfactory service under all these varying conditions and in consequence each manufacturer has to provide a series of different types, each designed to give optimum results when used under a given range of conditions. The two essential factors to be satisfied are (1) That the plug shall keep sufficiently hot that it will burn off any oil or carbon which may tend to form on the insulator. (2) That it will keep sufficiently cool and it will not cause pre-ignition.

The former requirement is necessary on account of the fact that carbon is a. good electrical conductor and, yjearly, if a layer of this substance is formed over the surface of the insulator, a short circuit would result and the plug would cease to fire.

Pre-ignition implies that the points of the plug become so hot (through their inability to get rid of their heat sufficiently rapidly) that they may eventually become incandescent and cause ignition of the charge before the spark fires it. When pre-ignition occurs (and this must not be confused with detonation or pinking which happens after the spark has passed normally) very high pressure will be imposed on the piston before it reaches its top centre position. Pre-ignitioji is usually manifest by severe thumping in the engine—by overheating and maybe eventually stoppage.

Time also has an influence on the performance of the sparking plug for it is desirable that the temperature shall rise sufficiently rapidly to prevent fouling when the engine is cool. The method in which a plug loses its heat is very largely via the following path; the heat travels up from the centre point through the insulator, through the copper gasket into the body of the plug, and thence either into the air or cooling water. The shape and size of the insulator determine the heat range of the plugs for a given engine (i.e. a given set of cooling conditions.) It is advan- , tagecus that the rate of heat flow away from a plug be such that it balances the heat given to it, causing the temperature to rise to a safe maximum and stay there, i.e. that the plug does not get hotter and hotter as time goes on. The important point to bear in mind is, that every large plug manufacturer has a range of different types graduated as their heat characteristics. A modern trend is to employ a 14 m/m type of plug in certain engines of recent design in place of the more orthodox 18 m/m type, the smaller plug is capable of being made with a wider heat range, that is, it heats up quickly but its temperature does not exceed a given maximum if the car is driven hard. It is useless to use a 14 m/m plug in a'n engine made to fit the 18 m/m size, by employing an adaptor, for the presence of the two threads will entirely upset the heat flow from the plug body to the cooling water. Practical Selection of Suitable Type. Racing and sports car engines develop a relatively much greater amount of power per cylinder than their normal prototypes. Consequently a greater amount of heat has tt be handled by the sparking plugs of these engines. Therefore, it is necessary to use a plug which will get rid ol its heat quickly, i.e. a cool running plug. If these cars be driven slowly in traffic it is well known that they are liable to suffer from “oiling-up.” This is due to the fact that under partial throttle conditions the amount of heat passing through the plug is insufficient to raise its temperature to the degree required to burn off the carbon. If on the other hand an ordinary or hotter running plug is used, trouble may be experienced from pre-ignition when the engine is opened up to full throttle.

The above example was given to illustrate that different conditions may demand a change in plugs. It is proposed to give below an analysis of the different factors which can affect the performance of a plug and, naturally/ also that of the engine. It ip. hoped that the information will be sufficient to allow the motorist to decide intelligently for himself whether the plugs he is using are giving him the most satisfactory service and, if not, in which direction a change

should be made. The car manufacturers themselves give recommendations for their engines, but it must be appreciated that, these can cover only an average set of conditions. The initial recommendation, however, is obviously the most suitable starting point from which to make a selection. Persistent oiling and sooting up is an indication that the plug is operating at too low a temperature (e.g. a sports car type of plug in a touring engine) and a change should be made to a hotter running plug.

Pre-ignition or severe over-heating. Overheating may be due to pi’e-igni-tion, which can he detected by a rapid falling off in power after the car has been driven fast for some time. If t his condition is experienced, and it. is further shown up in rapid burning away of the points, a change should be made to a cooler plug. High compression. If your car is fitted with a head giving a higher compression ratio than normal, it may require a change to a cooler running plug. Excess of traffic driving. If a great deal of traffic driving is done, especially in a high compression car, then the plugs may keep so cool that, trouble from sooting-up occurred. A change to a hotter running type of plug will overcome this in most cases. Regular driving at high speed. If this habit is indulged in owing to the presence of modern arterial roads, then the temperature of the plug may in time become too hot, although it might give quite satisfactory service under more modern conditions. If a loss of power is* experienced, a change to a cooler plug should give an improvement, hut clearly more care must be given to avoid oiling up when the car is used in town driving. Ethyl petrols. The march of progress has been towards higher compression ratios and, to meet this, the oil companies are marketing antiknock fuels. The most popular of this type are the ethyl petrols which are now extensively used. These are high class petrols to which a minute quantity of ethyl fluid (which contains tetra-ethyl lead) has been added. When this is burnt there is sometimes a tendency for lead salts to be deposited on the sparking plugs. Whilst these are practically nonconductors at normal temperatures, an increase in temperature may result in the fluxing of them on the insulator, under which conditions they may become conductive. If trouble is experienced from these lead deposits—don’t blame the petrol, hut change to a cooler plug when no further trouble should result.

Deterioration of Sparking Plugs, Sparking plugs quite definitely wear out, even though the motorist may say they are working properly. Petrol contains minute traces oi impurities, such as sulphur and iron. The former definitely leads to corrosion of the electrodes with subsequent widening of the gap. Recognised No. 1 spirits contain absolutely the minimum percentage of sulphur, but cheap petrols may contain an appreciable quantity. This explains why a set of plugs may seen to have a shorter life, especially if different qualities of fuels have been used. Another source of deterioration is due to the iron co'ntent of the petrol. This is extremely minute, but its presence may sometimes be observed especially on porcelain plugs as a brownish colouration on the insulator. This coating has the effect of becoming a conductor at high temperature, and may lead to missing when the engine is running fast, although at normal speeds quite satisfactory sparking may occur. It is generally impossible for the motorist to remove this coating without damaging the insulator. The rate at which it builds up, however, is very small indeed and, in general, it only assumes serious proportions when the life of the plug is finished.

Rapid burning away of the electrodes may, as pointed out above, bo due to high temperature resultant on the use of a plug of unsuitable heat characteristics.

A common methods of testing whether the ignition system is functioning is to remove a sparking plug and place on top of the cylinder head, observing whether a spark passes • when the engine is cranked. It should be realised that although a spark may pass in these circumstances, it may fail to do so when the plug is in the cylinder. The reason for this is that a lower voltage is needed to be supplied across the gap in free air, but a much higher voltage is needed when the air across the gap is compressed, as occurs, when the plug is in the cylinder. The higher the compression the greater is the “breakdown voltage.” Now if there is a coating of a partial conductor over the insulator it is quite possible that it is not sufficiently conductive ‘to sidetrack the current at the relatively low voltage needed to pass a spark in free air, but its resistance might be less than that of the gap when the air is compressed. In these circumstances it is possible for a short circuit to occur when the plug is in the cylinder, but for no visible evidence of this to be apparent when tested in free air. Maintenance and Adjustment. Deteriorated plugs, even though they appear to be operating satisfactorily may, in reality, be the cause of a drop in engine power and efficiency. The process of deterioration is a very gradual one and cannot be detected easily. It probably pays to renew

the plugs at intervals of, say, from from 10,000 to 15,000 miles. As plugs in bad condition can appreciably increase the fuel consumption, the initial outlay will, in all prqbability, be regained rapidy. It is recommended that the plugs be kept "in correct adjustment throughout their life. Too wide a gap renders starting difficult and leads to misfiring. The gap should be set to the recommendations in the maker’s instruction book, and these should lie between the following limits. For high compression engines r. cioser gap is required on account of the difficulty the spark has in passing across the denser charge. For this type of engine the gap should be from 16 to 20 thousandths of an inch. For more normal engines, from 2o to 25 thousandths. These distances should not be “guessed” hut should be set to a suitable gauge which often may he obtained gratis from the various plug manufacturers. When adjusting the gap the centre electrode should never he bent, hut the adjustment should be carried out by the bending of the wide-wire at. tached to the plug body, preferably with a small pair of pliers.