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The influence of early induction stroke direct injection on late-cycle flows was investigated for a lean-burn, high-tumble, gasoline engine. The engine features side-mounted injection and was operated at a moderate load (8.5 bar brake mean effective pressure) and engine speed (2000 revolutions per minute) condition representative of a significant portion of the duty cycle for a hybridized powertrain system. Thermodynamic engine tests were used to evaluate cam phasing, injection schedule, and ignition timing such that an optimal balance of acceptable fuel economy, combustion stability, and engine-out nitrogen oxide (NOx) emissions was achieved. A single cylinder of the 4-cylinder thermodynamic engine was outfitted with an endoscope that enabled direct imaging of the spark discharge and early flame development.

Combustion issued from an eight-hole, direct-injection spray was experimentally studied in a constant-volume pre-burn combustion vessel using simultaneous high-speed diffused back-illumination extinction imaging (DBIEI) and OH* chemiluminescence. DBIEI has been employed to observe the liquid-phase of the spray and to quantitatively investigate the soot formation and oxidation taking place during combustion. The fuel-air mixture was ignited with a plasma induced by a single-shot Nd:YAG laser, permitting precise control of the ignition location in space and time. OH* chemiluminescence was used to track the high-temperature ignition and flame. The study showed that increasing the delay between the end of injection and ignition drastically reduces soot formation without necessarily compromising combustion efficiency. For long delays between the end of injection and ignition (1.9 ms) soot formation was eliminated in the main downstream charge of the fuel spray.

Laser plasma ignition has been pursued by engine researchers as an alternative to electric spark-ignition systems, potentially offering benefits by avoiding quenching surfaces and extending breakdown limits at higher boost pressure and lower equivalence ratio. For this study, we demonstrate another potential benefit: the ability to control the timing of ignition with short, nanosecond pulses, thereby optimizing the type of mixture that burns in rapidly changing, stratified fuel-air mixtures. We study laser ignition at various timings during single and double injections at simulated gasoline engine conditions within a controlled, high-temperature, high-pressure vessel. Laser ignition is accomplished with a single low-energy (10 mJ), short duration (8 ns) Nd:YAG laser beam that is tightly focused (0.015 mm average measured 1/e₂ diameter) at a typical GDI spark plug location.