Examination of the Factors that Influence the Durability of Railplugs 940201
A new type of ignitor, the railplug, shows promise of extending the dilution limits for spark ignition engines. While much of the effort expended in our study of railplugs has focused upon demonstrating their effectiveness, it is recognized that railplug durability is presently not acceptable for production engine applications. The goal of the present study was to examine the factors that affect durability. The results of two types of investigations are reported. The effects of rail materials, pressure, delivered energy, and voltage at constant delivered energy on electrode erosion rates were studied for repeated firings in air at constant pressure. Railplug durability in a four-stroke SI engine was also evaluated, including examination of the effects of delivered energy, current pulse characteristics, and materials. This research was not intended to demonstrate improved railplug durability, but rather was intended to develop information to guide future efforts to improve railplug durability. The erosion rate studies showed 1) that several materials are promising candidates for center rails, and 2) that use of a high voltage firing circuit should significantly improve durability. The engine tests used a low voltage firing circuit and 1) indicated that future durability tests should be done using the capacitance that results in fastest combustion, 2) reaffirmed that decreased delivered energies increase durability, and 3) isolated two separate failure mechanisms that depend upon the delivered energy, with the division appearing to be a delivered energy in the range of 1.0-1.4 J. It is concluded that future efforts to improve railplug durability should focus upon 1) additional studies of center rail materials, 2) use of a high voltage firing circuit, 3) the lower delivered energy levels, and 4) most importantly, improved railplug geometries.