Performance of a Plasma Jet Igniter

The main advantage of jet igniters lies in their ability to provide distributed ignition sources that are capable of initiating and enhancing combustion in lean mixtures. This is achieved principally by two mechanisms: the provision of a high concentration of free radicals, enhancing ignition at lower temperatures, and of the extended highly turbulent igniting surface, yielding larger flame front areas and hence increased burning rates. The paper describes experimental and analytical procedures that were developed to determine the performance of a jet igniter with particular emphasis on the fluid mechanic effects. This consists of the evaluation of the penetration depth of the jet as a function of the plasma plug geometry, and of the rate of burning in the jet-ignited charge, as well as the corresponding turbulent and laminar flame burning velocities. To de-emphasize the thermochemical effects, the plasma medium and energy were maintained without change for all the tests. Thus the only variables in our study pertained just to the geometry of the igniter, that is, the volume of the cylindrical plasma cavity and the area of the orifice discharging the jet. The results are based on high speed schlieren cinematography of the combustion process in a cylindrical bomb, and simultaneous pressure transducer measurements. The penetration depth is shown to be proportional to the square root of the characteristic length – the ratio of the volume of igniter cavity to its orifice area. The capability to ignite lean mixtures is demonstrated by the repeatibility of results attained with the use of methane-air mixtures at an equivalence ratio of 0.6, initially at atmospheric pressure and room temperature. Under these conditions the observed enhancement of the rate of combustion could be accounted for entirely by the fact that the flame surface was enlarged due to ignition by a highly turbulent jet. While the laminar burning velocity was well within the range reported in the literature, the corresponding ‘turbulent’ burning velocity was about three times greater, the ratio between the two remaining nearly constant throughout the process.