SPARKING PLUGS

Timaru Herald, Volume CXL, Issue 20261, 9 November 1935, Page 10

SPARKING PLUGS

IMPORTANCE OF SELECTION It is not so very many years ago that the selection of sparking plugs was merely a matter of knowing the correct size and buying an approved make. With the gradual increase In compression ratios and the consequent stepping up of the power and speed of engines, a considerable number of new factors have arisen which render the choosing of suitable sparking plugs a matter deserving of a little thought and study. Once the fundamental facts have been grasped, however, it is a comparatively simple matter not only to diagnose the cause of plug failure, but also to accurately select the correct plug for the particular conditions under which the engine operates. A condition that is necessarily incidental to an increase in power from an engine is an increase in the temperature during the burning of the fuel. Extra heat must be dissipated, and this factor is one of the main reasons why a sparkling plug which suits a lowcompression engine will be unsuitable for the greatly-increased power output from the high-compression engine. A sparkling plug relies mainly upon its porcelain insulator to carry away the intense heat at the tip to the metal portions of the cylinder. Porcelain is used because of its high insulation properties, and its ability to stand up to high temperatures. Being a good electrical insulator, porcelain naturally is not a good conductor of heat, and it Is therefore necessary to design the insulator in such a manner that it can cope with the heat to which the plug is subjected. This has given rise to the somewhat broad classification of sparking plugs Into three types, cold, normal, and hot. A cold plug is one in which the heat path Is short, enabling the heat to be transmitted rapidly from the electrode to the landing on the cylinder. This type of plug is necessary for high-eom-pression engines, or for those engines generally which are called upon to do heavy slogging work. In a hot plug the heat path is long. Heat generated at the electrodes cannot get away rapidly, and as a result the electrodes and porcelain heat up and remain hot. Such a plug is still used in medium to low-compression engines, and for some high-compres-sion motors on very light duty. Leading sparking plug manufacturers have increased considerably the range of sparking plugs during the last few years, until today it is comparatively simple to select plugs which will function hot enough to burn off any oil or carbon lodging on the insulator and yet not reach such a temperature as i to cause pre-ignition or burning of the electrodes. Sparking plugs fail prematurely for one of two main reasons. Either the plug has been running too cool or too hot. Evidence of too cool running is the fouling of the insulator with car-

bon and soot. Evidence of hot running is rapid gap growth, blistered or cracked insulators, or pre-ignition. Many faults in engine operation, such as excessive petrol consumption, pre-ignition, and carbon deposits, can be traced to the incorrect selection of sparking plugs. In fact some of the high-compression engines now in use are so sensitive to the heat characteristic of sparking plugs that merely one

step too hot or too cold will noticeably affect the performance of an engine. From the condition of the porcelain tip of a sparking plug, the indication can be secured of the carburettor setting. If a new set of plugs is installed, and after a few hours run are removed for examination, the colour of the porcelain tip will indicate the carburettor setting. If the porcelain is black, the mixture is too rich. On the other hand, if the porcelain is white the setting is too lean. The correct setting is indicated by a light tan stain right at the tip of the plug. Considering sparking plug operation in general, it is quite evident that a great deal of research and thought is behind the construction and design of plugs for modern engines. Wherever sparking plug failure has occurred, it will be found almost invariably that it is the application of the plug rather than the plug itself which is at fault. In other words, either a too hot or too cold sparking plug has been chosen for the particular engine.