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.
