Suppression of RFI from ignition systems.

Transcription

1 Suppression of RFI from ignition systems Ron Hughes/Tom Walker Nowadays there are many far more complex systems that can be affected by the RFI (Radio frequency Interference) than simply our televisions, including mobile phones, navigation aids and even Engine Management Systems; so suppression is of far more significance than it was in the 1950 s In those days, suppression was usually incorporated in the plug cap or HT pickup on multi-cylinder engines, or in gadgets that were inserted in the plug lead and guaranteed not only to suppress but also to improve the voltage and spark produced Those that did not believe that suppression did not adversely effect the spark and chose to fix the HT lead to the plug with the time honoured spring clip or brass tab and knurled nut risked prosecution, as well as the wrath of neighbours Time has moved on and suppression is incorporated in the majority of sparking plugs, some HT leads are constructed so as to suppress, and plug caps usually contain a suppressor, so it is almost impossible to circumvent the Law, so much so that prosecutions are unheard of RFI is created in the microseconds as the spark jumps the gap from the positive plug electrode to ground and suppression is achieved by introducing a small resistance into the HT circuit Initially, in most cases, the resistance comprised a section of carbon placed in the plug or plug cap As ignition systems have become more sophisticated, so have the resistances devices for suppressing RFI For example, in the 1990 s Champion developed a suppressor coded the SAC-9 with almost undetectably low resistance and virtually total reliability, and, with the development of Capacitive Discharge systems alternative wire-wound resistors were designed for sparking plugs and NGK for instance, have wire-wound resistors in their high performance plug leads However, with the majority (I suspect) of our classics still ignited by current supplied from a magneto (and mostly very well) the mere mention of a resistance (no matter how small the manufacturers claim it to be) in the path of our precious HT current sends us into a minor panic In fact, as I looked deeper into this matter it became clear to me that there are several other factors, which are within our control, of far greater significance to the efficient ignition of our fuel/air mixture than the RFI suppressor Pressure The pressure created in the cylinder has a marked effect on the ease with which a spark is generated How often have we had a plug spark quite happily whilst lying on the cylinder head, yet the engine refuses to start with the plug in the cylinder head, or fires initially but stutters and cuts out when we try to accelerate because the increased filling of the cylinder/s creates higher pressures, which our meagre voltage cannot overcome (see Fig1)

2 Plug gap With the shortage of HT from our magnetos, a wide gap is hard to jump and for most cases a small gap, say 0015 will be more reliable BUT, a small gap can be more prone to fouling if the engine does not start cleanly, especially in cold conditions (see Fig2) Temperature and choice of plug range Explanations of plug heat ranges have appeared in previous Jampots but the importance of correct plug selection cannot be stressed too highly A plug that is too cold (hard) ie a racing plug, will foul in an engine running with the electrode too cool; whereas a plug that is too hot (soft) will overheat in an engine working hard and cause pre-ignition, detonation and erosion of the electrode Paradoxically, racing engines, designed not to over heat under racing conditions, will usually run quite happily on plugs designed for tourers in all but the longest racers

3 Certainly, the use of a hard racing plug or one from a highly-developed electronic system, will not improve the performance of a Model 16 AJS (see figs3 and 3a) Air/fuel ratio and Ignition timing Careful attention to carburettor and ignition settings are important to make maximum use of the available voltage (see Figs 4 and 5)

4 Speed It is well known that a magneto is not so generous with its volts at low revolutions, hence most of our starting problems and where coil ignition systems score, but the magneto improves with speed, which is just as well because the voltage required to fire a plug also increases with speed (see Fig6) Dwell angle and Ignition timing All systems, whether magneto, Coil or Capacitive Discharge, require some time after sparking to regenerate the voltage for the next spark, provided by the dwell angle when the points are closed (or in the gap between the electronic pick-ups) So points gap is important since the wider the gap, even if the timing is correct, the smaller the dwell angle and the shorter will be the time for the voltage to build up With a multicylinder set-up the dwell angle is less than with our four-stroke singles and, since it is the time that the points are closed, rather than the angle that provides the regeneration of voltage it figures that there is a limit of engine speed at which the time is too short for effective voltage generation and we run out of sparks Considering this fact, it is amazing that Honda managed to race with magnetos as long as they did They must have been some magnetos! Plug Selection From the foregoing the moral is very simple; the manufacturers of plugs and engines have been very careful to select the most effective match - have faith and go with it!

5 Sources of information Manufacturers are only to willing to provide technical data, much of it on the Internet, (it is in their interest to avoid complaints about their products due to misuse) and I am indebted to Federal Mogul for permission to reproduce diagrams from their literature about their Champion range