Combined semi-surface discharge candles. This type of candle differs from the standard plug with the insulator cone protruding from the body and the internal threshold at the bottom of the body.
During normal operation of the plug, the spark jumps from the center electrode to the side electrode by the shortest path. However, when carbon deposits build up on the insulator cone, it's a spark, looking for the easiest way to mass, first it "creeps" on the well-conducting layer of the sediment, to jump at the point furthest from the body. It is a place located in the zone, in which the mixture has free access to the electrodes of the candle. As a result, the ignition of the fuel in the cylinder can proceed smoothly. The current flow through the insulator removes the carbon deposit, even before the candle tip reaches the self-cleaning temperature. Cleaning the insulator makes, that the spark discharges are already taking their normal route.
Combined semi-surface discharge candles may have one side electrode (np. W20EP-S11 by DENSO, BU6EA-11 and BKR6EK firm NGK) or a few (np. Ultra X by Beru, VTYC by Champion, NGK's BKUR5ET). Multi-electrode candles are characterized by lower electrode consumption. Thanks to this, it is not necessary to periodically adjust the spark gap, and the magnitude of the ignition voltage is almost constant over the time between spark plug changes.
An example of a multi electrode candle is the Super candle 4 from Bosch, characterized by the use of specially shaped side electrodes, silver-plated center electrode with pointed end and aluminum oxide insulator.
Thanks to the thinning of the side electrodes and so many possibilities for spark-over, the wear of each electrode is significantly reduced. Thus, the ignition voltage remains almost constant throughout the service life of the plug. The advantage of Super candles 4, compared to traditional ones, first and foremost is resistance to spark loss under extremely difficult conditions, np. when the candle is covered with carbon deposits, when there is a strong leaning of the mixture under rapid acceleration. The disadvantages are that the flame front is covered by four side electrodes and the center electrode has a cooling effect, which may be manifested by unstable engine idling. The new Bosch spark plug is mainly intended for the used car market with a conventional ignition system. The electrode gap is factory set and does not need to be adjusted. The thermal value of candles has been graded only in two sizes "78" and "56", which cover the entire range of required heat for 90% common engines. The heat value "78" combines the previous values "7" and "8", while "56" – values "5" and "6". The life of the candle is estimated at approx 60 000 km.