Automotive Ignition Transfer Efficiency 2002-01-2839
Measurement and analysis of typical automotive spark ignitions operating up to 30Kv and 30 to 100ma show the electrical-to-plasma (energy) transfer efficiency to be very low, significantly less than one-percent (1%).1 The reason for this low energy transfer efficiency is high resistance in the driving circuit that is in series with the rather low resistance of the ignition spark. The largest components of resistance are in the ignition coil and the high voltage cables.
The latest evolution of ignition systems has been the elimination or severe shortening of the high voltage cable and the incorporation of one ignition coil attached directly to the individual spark plug. While this has eliminated much, and in some cases all, of the transfer losses attributed to the cables, the system as a whole is still very inefficient in the electrical-to-plasma conversion efficiency because the spark coil and spark plugs still have many times the resistance of the spark discharge channel.
These systems will therefore not meet the needs of future lean burn and alternative fuel engines requiring higher energy discharges to effectively ignite the fuel mixtures.2, 3, 4, 5 A further complication is the present requirement to reduce the power consumption of the ignition system.
There are two basic approaches to increasing the electrical efficiency of ignition systems. Either will reduce average power consumption. The first is to design the coil circuit with low resistance, which would require upgraded electrical components to handle higher currents. The second and most practical is to use a peaking capacitor between the coil output and the spark plug. With the second alternative, energy transfer efficiencies approaching 50% can be achieved using existing ignition components.1
This paper will present a general analysis of conventional and modified ignition systems and the data from measurements of an ignition system with a peaking capacitor. Recommendations for increasing general ignition efficiency and effectiveness will also be included.