History Effect on Particulate Emissions in a Gasoline Direct Injection Engine

Soot formation in internal combustion engines is a combination of complex phenomena. Understanding the formation mechanism that influences particulate emissions can help to make gasoline direct injection (GDI) engines comply with increasingly stringent emission standards. It is generally accepted that the deposition of liquid fuel wall films in the combustion chamber is a significant source of particulate formation in GDI engines. The injection timing, which can help avoid interaction between the pistons and fuel spray, has been identified as the parameter with the greatest influence. Traditionally, the start of injection (SOI) sweeps one can find in the literature are carried out by changing the timing one value at a time. To quantify the influence of SOI, variations in our study were carried out in a novel way using cycle-to-cycle parameter control. Instead of motoring or turning off the engine between different SOI variations, the motor was run continuously with combustion and SOI sweeps carried out online in a series of preprogrammed perfectly deterministic SOI sequences to provide evidence of so-called history effects on particulate number (PN). The variation in SOI produces a change in engine combustion and liquid fuel impingement, leading to a state that acts as a precursor for the next state. The different preprogrammed sequences provided excellent data repeatability between engine runs but very different results, depending on the order in which the SOI timings were set. In-cylinder combustion was visualized with an endoscope connected to a high-speed camera. Two SOI timings were chosen (based on piston deposit level data from stationary measurements) to investigate the history effect of preceding conditions on PN. The results show that the preceding engine states influence PN formation and emission that is established as history effect in the study. The history effect is pronounced and was most noticeable under impinging conditions such as early injection timings like -340 crank angle degrees (CAD). History effect was also found to depend on the duration and SOI of the preceding state. More importantly, the history effect depends on how SOI is varied, which in turn influences PN emissions. In the cycle-to-cycle variation of SOI, PN levels at relatively later injection timing of -250 CAD resulted in similarly high levels at an early injection timing of -340 CAD