Pulse train ignition with passively Q-switched laser spark plugs

Leaner burning and downsizing are two concepts pursued by engine developers to reduce fuel consumption and emissions. Both approaches lead to increasing challenges concerning ignition, as these concepts are typically associated with an increase in flow velocity and degree of turbulence as well as raised pressure at the moment of ignition. In this context, the use of miniaturized passively Q-switched laser spark plugs with pulse train ignition is considered as a promising alternative to conventional spark plugs. However, the application of these passively Q-switched laser spark plugs inevitably leads to the question of optimum pulse train parameters. For a better understanding, this study deals with improved flame formation by passively Q-switched laser pulse train ignition under engine-like conditions. The entire ignition process is investigated with a special focus on interactions of consecutive pulses. Therefore, three methods are combined: energy transfer measurements from laser pulse to plasma with high temporal and spatial resolution show the breakdown process depending on different pressures and fluid mixtures. The temperature decrease in the induced plasma is analyzed with measurement strategies for temporal high-resolved plasma spectroscopy especially adapted to passively Q-switched lasers. In combination with high-speed schlieren measurements, the changing local ignition conditions during pulse train ignition are demonstrated. The experiments show how consecutive pulses interact and contribute to the ignition in case of a gas flow. The used prototypes of laser spark plugs are provided by Robert Bosch GmbH.