High-speed imaging of spray-guided DISI engine combustion with near-TDC injection of E85 for ultra-low NO and soot

Using E85 fuel, this work demonstrates the use of near-TDC injection and highly turbulent combustion to achieve ultra-low emissions of NO and soot from a DISI engine with a spray-guided, stratified-charge combustion system. Despite the low NO emissions, both combustion efficiency and stability remain relatively high. One striking aspect of engine operation with near-TDC injection is the need to initiate the spark prior to fuel injection. Using E85 fuel, such early spark timing leads to stable ignition. For gasoline, however, such early spark timing leads to misfire under the conditions studied, thereby preventing the use of near-TDC fuel injection to achieve low exhaust emissions of NO. High-speed imaging shows that the early spark timing for E85 allows the spark plasma to develop prior to interacting with one of the fuel jets adjacent to the spark plug, and this contributes to misfire-free operation. The spark-spray interactions generate a very fast spread of either weak or thin flames throughout a large fraction of the piston bowl prior to the main combustion event. The short delay between the near-TDC fuel injection and the combustion event are believed to enhance the mixing rates during the combustion compared to the use of earlier injection, which may contribute to the low NO emissions. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.