Calorimetry of Ignition Sparks

The thermal energy transferred to noncombusting gases by a spark at pressures from 1 to 7 atmospheres was measured using a pressure-rise calorimeter. The energy transfer efficiency (from electrical to thermal energy) was determined for conventional coil ignition systems, a plasma jet ignitor, and an ultra-short pulse (USP) ignitor. For the coil systems, the efficiency was strongly dependent on pressure and gap size, but weakly dependent on the type of spark plug. The efficiencies varied from 5% to 65%, with increases in pressure and gap size giving higher values. Electrode diameter and gas composition also had an effect on the efficiency. For a constant current, variable duration discharge system, the efficiency decreased by a factor of three as the energy increased from 12 to 75 mJ. The efficiency of the USP (breakdown discharge) system was slightly higher than for a conventional coil system. The energy efficiency for the plasma jet ignitor was fairly constant for electrical energies from 0.2 to 1 J.