UC Berkeley

Spark plugs have been igniting combustible mixtures like those found in automotive engines for over a century, and the principles of the associated ignition techniques using thermal plasma (inductive or capacitive sparks) have remained relatively unchanged during that time. However, internal combustion engines are increasingly operating with boosted intake pressures (i.e. turbo- or super-charged) in order to maintain power output while simultaneously reducing engine size and weight, and they are also operating with increased recirculated exhaust gas dilution to reduce the production of harmful nitrogen oxides. This “downsizing” to increase fuel economy compounded with diluting to decrease emissions leads to challenges in both obtaining traditional ignition and promoting sufficiently fast combustion under this operating paradigm. In conjunction with appropriate electrode design, transient non-thermal plasma can exploit certain non-equilibrium chemistry and physics to bypass these challenges and ultimately promote more reliable ignition and faster combustion. Applied and fundamental experimental investigations of two different advanced ignition techniques are presented: 1) corona discharges igniting gasoline/air/exhaust mixtures in a boosted direct- injection single cylinder research engine and 2) repetitively pulsed nanosecond discharges igniting methane/air mixtures in a constant volume chamber. The engine experimental results show significant decreases in fuel consumption and nitrogen oxide emissions under boosted operation, and both experiments demonstrate more robust ignition and faster flame development. The constant volume chamber results in particular raise important questions about the relative contributions of chemistry and transport to the experimentally observed combustion enhancement. These results highlight the critical importance of electrode design in advanced ignition techniques—the shape and position of electrodes greatly influences the hydrodynamics of developing flame kernels into fuel-air charges. While this work demonstrates that non-thermal plasma ignition is a promising solution to both increase fuel economy and decrease emissions of future automotive engines, much work remains to be done to understand the beneficial coupling between the detailed non-thermal plasma chemistry and the hydrodynamics associated with these real ignition devices.