Fouled Plugs improve Engine Performance
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Low-tension Ignition System Employs Surface-discharge Sp'arking Plugs. Condenser Used for Current Storage
TWO important claims made for the Smitsvonk low-tension condenser-discharge ignition equipment are that a fouled sparking plug improves performance and that little consideration need be had for the type of fuel used.
It is well known that most faults with high-tension ignition are caused by poor insulation in some part of the circuit. The electrical pressure in the secondary circuit of an ignition coil may be anything from 8,000 to 20,000 volts, and a particularly high standard of insulation efficiency must be reached if leaks and breakdowns are to be prevented.
Efficiency Drops Off
With high-tension coil ignition. efficiency tails off as engine speed increases,' as the higher the speed the less time has the primary winding of the coil to build up. With a magneto, the opposite applies, as in this case the higher the speed the hotter the spark.
With both high-tension coil and magneto ignition, a contact breaker is responsible for breaking down the primary circuit, the sudden collapse of which induces a current of high voltage in the secondary winding. It is this secondary current which is led away to the distributor—the unit responsible for feeding each plug in correct sequence.
In the Smitsvonk low-tension system the current, which may be obtained from a coil, magneto, or other source, is stored in a condenser, and unlike a high-tension ignition circuit, it is the distributor which is responsible for controlling the current-flow and not a contact breaker. • A vital component in the equipment is the sparking plug. This is of the surface-discharge type, as opposed to that generally employed to-day, in which the discharge takes place across two point electrodes.
"De Ingenieur." of Holland, in dealing with the system, shows a circuit diagram of the Smitsvonk low-tension layout. In this. the primary circuit is broken by a contact breaker, which causes a high-tension current to flow in the secondar. winding in the usual manner. IncteWd of this current being led direct to the distributor, it is fed into a condenser of pre-determined capacity, we a rectifier.
The release of energy from the condenser is controlled by the distributor, n12 the design of which plays a vital part in the overall efficiency of the system. Low-tension current will pass only when almost direct contact is made between the rotating arm and the fixed contact, there being less tendency for the current to jump from one contact to the other and affect ignition timing.
Accurate timing is achieved in the Smitsvonk system by making the contact areas of the rotating arm and fixed surfaces narrow, so that they represent an arc of less than 10 degrees taken on the axis of rotation. The wear on the contact surfaces is relatively heavy and to compensate for this they are given a greater axial length than is normal in a high-tension distributor.
In the case of high-revving units, which make a heavy demand on ignition current, two condensers are employed, one for storage and the other connected to the distributor.
No claims are made that the surfacedischarge sparking plug is a new conception. It is true, however, that it has not been developed for the reason that its application has been mainly to hightension current supplies for which it is not entirely suited.
With any electric spark, the voltage drops appreciably once ionization of the path taken by the spark has occurred. The ionieation of an open gap, such as in a high-tension sparking plug, requires a relatively high voltage, and the tension needed depends on the conditions of the medium across which the spark has to jump.
In the surface-discharge sparking plug, where the spark gap is supported by a layer boiween the electrodes over which the spark sweeps, these initial processes take place on this surface layer, independent of gas pressure. It was found that only a relatively low
tension was needed, provided that the space between the electrodes was kept small and •the resistance low. This was achieved by making the surface layer semi-conductive.
Electrode wear in such a type of plug is considerable and, therefore, the electrodes are in the form of concentric rings. The spark wanders along the entire length of the spark path, and always chooses its crossing point at places where the resistance is lowest.
Electrode wear at a certain part of the path will automatically cause he spark to cross at another point, so that the whole length of path. eventually becomes worn out before the plug ceases to function satisfactorily.
Replaceable Elements The sparking element is made as a separate unit-and it can easily be fitted into different sizes of plug body. Instead of replacing a complete plug, the -old clement is replaced by a new one.
It is claimed that, irrespective of the type of unit concerned, only one kind of sparking plug is required. A coil or magneto for use in the system differs materially from the high-tension version.
A coil, for instance, would have a .different quantity of iron and copper in its construction, whilst the disposition of the metals would not be the same. The requirement is a constant output 'over the full range of engine revolutions, and as no tension has to be built up, the efficiency of such a coil can readily be made higher than its hightension counterpart.
No contact breaker is required in the case of a magneto, because as the current is produced it is shunted into the storage condenser.
An important claim made for the system is that a wide range of fuel can be ignited. Oil-engine fuel and those having a high concentration of tetraethyl lead, or those known to deposit heavy impurities do not, it is said, affect the efficiency of spanking. Provided that a suitable condenser be chosen even bituminous fuels such as tar can be ignited.
A most interesting development suggested is the unit construction of a surface discharge sparking plug and a fuel injector for spark-ignition oil engines.