Initial Studies of a New Type of Ignitor: The Railplug

Initial investigations of a new type of high energy ignitor for I.C. engines are described. The ignitor is a miniaturized railgun, or “railplug.” The railplug produces a relatively large mass of high velocity plasma. These characteristics may be advantageous for initiating combustion in a number of different applications. Unlike a plasma jet ignitor, the railplug plasma is driven not only by thermodynamic expansion, but by electromagnetic forces as well. Four experimental railplug designs were evaluated using schlieren and shadowgraphy visualization to examine plasma movement and shape. Railplug current and voltage were also measured. One railplug consisting of two unenclosed parallel rails was used to demonstrate the electromagnetically induced motion of the plasma at ambient conditions. Schlieren photos showed that the plasma plume moves strongly in the direction of the electromagnetic Lorentz forces. Two transparent railplugs, one with short rails (24 mm) and the other with long rails (35 mm), were tested to show plasma motion and structure inside the plug cavity. Plasma exit velocities in excess of 300 m/s were observed for the transparent railplug with short rails in air at ambient conditions. Schlieren imaging of the transparent railplugs proved that proper matching of the railplug with the electronics is essential. In the long transparent railplug, a collapse of the current while the plasma was still in the plug cavity resulted in generation of eddy currents which rapidly decelerated the plasma preventing it from reaching the plug exit. A fourth railplug was used to ignite lean (ϕ=0.85) quiescent methane-air mixtures at 3.3 atm total pressure in a constant volume bomb. For the bomb experiments, schlieren imaging and pressure histories were obtained and compared with ignition using a conventional spark plug and surface gap ignitor with the same energy as the railplug. Rates of pressure rise in the combustion bomb were much greater for the railplug compared with the conventional spark plug or the surface gap ignitor; the time to peak pressure was approximately cut in half by the railplug.