High-Speed Fuel and Flow Imaging to Investigate Misfires in a Spray-Guided Direct-Injection Engine.

Abstract

Simultaneous high-speed fuel fluorescence and particle image velocimetry measurements are combined with spark discharge measurements for the first time to identify the cause of misfire and partial burn cycles in a spray-guided spark-ignited direct-injection engine. Spark ignition is studied for stratified charge operation under a range of external dilution levels (0% - 26% nitrogen). Available spark energy is characterized for a wide range of well-controlled homogeneous gas-phase fuel-air mixtures (Φ = 0.0 - 2.9) and flow conditions (|V| = 1 - 10 m/s) that are characteristic for the spark energy under stratified charge conditions. Under stratified charge conditions, the engine operates with an optimized end-of-injection and spark timing that provides stable engine operation with the occurrence of rare misfire and partial burn cycles. Fuel concentration and flow field measurements are analyzed closest to the spark plug and in the entire field of view (38 mm x 30 mm) within the combustion chamber to diagnose the role of fuel distribution, flow field, and spark energy on misfire, partial burn, and well-burning cycles. Results show that abnormal spark behavior is not the cause of the misfire and partial burn cycles and a model is used to show that all cycles have sufficient electrical spark energy to ignite the flammable mixture nearby the spark plug. The high-speed fuel and flow images reveal that a flame kernel is developed for all cycles. For all errant cycles, flame propagation is too slow due to either lean and/or diluted mixtures and the flame kernel is not able to catch up to the fuel cloud that travels away from the spark plug. As a result, the flame kernel is surrounded by lean fuel-air mixtures that are insufficient to further support adequate flame kernel growth, leading to a misfire or partial burn. This work demonstrates the increased need for precise fuel injection and atomization control as well as consistent in-cylinder flow patterns that provide favorable mixtures for flame kernel development throughout the entire spark event.