Analysis of Factors that Affect the Performance of Railplugs 2005-01-0252

As natural gas engines are designed to operate leaner and with increased boost pressure, durability of the spark plugs becomes problematic. Among the various new ignition devices that have been considered to solve some of the problems facing spark plugs, railplugs appear to hold clear advantages in some areas. There are two types of railplugs: coaxial rail and parallel rail. This paper reports the results of an experimental study of various parameters that affect the performance of parallel railplugs. Their performance was quantified by the distance that the arc traveled along the rails from the initiation point. Travel along the rails is thought to be an important performance metric because rail-travel limits excessive local wear and produces a distributed ignition source which can potentially reduce mixture inhomogeneity induced ignition problems. Parameters considered included rail shape, rail divergence angle, initiation gap distance, and electrical polarity, the extent to which the rails were enclosed, and delivered energy. Arc travel distance was measured as a function of pressure from 100 kPa to 1800 kPa for capacitor charging voltages from 122 Volts to 221 Volts. Delivered energies ranged from 0.4 to 3.4 J. The effect of an externally applied magnetic field on arc travel was also investigated. The cross-sectional shape was found to be important to arc travel. When the facing sides of the rails had a flatter or rounded profile the arc tended to travel along the outer edges of the rails rather than along the rail axis. Smaller rail divergence angles tended to enhance arc travel. It was found that very small initiation gap distances (less than about 0.5 mm) tended to impede arc travel. Arc travel distance decreases with increasing pressure. Arc travel was less for fully enclosed railplugs then for an unenclosed plug, however, a partially enclosed designed showed promise.