This study explores an Adaptive Injection Strategy (AIS) that
employs multiple injections at both low and high pressures to
reduce spray-wall impingement, control combustion phasing, and
limit pressure rise rates in a Premixed Compression Ignition (PCI)
engine. Previous computational studies have shown that reducing the
injection pressure of early injections can prevent spray-wall
impingement caused by long liquid penetration lengths. This
research focuses on understanding the performance and emissions
benefits of low and high pressure split injections through
experimental parametric sweeps of a 0.48 L single-cylinder test
engine operating at 2000 rev/min and 5.5 bar nominal IMEP. This
study examines the effects of 2nd injection pressure, EGR, swirl
ratio, and 1st and 2nd injection timing, for both single heat
release and two-peak high temperature heat release cases.
In order to investigate the AIS concept experimentally, a
Variable Injection Pressure (VIP) system was developed. The VIP
system is capable of both low and high pressure injections (~300
bar and ~1200 bar respectively) through one injector in the same
cycle. For both the single heat release and TSC experiments,
optimal operating conditions were found. The single heat release
cases tended to have better fuel economy and lower emissions than
the TSC cases. However, the peak pressure rise rates (PRR) for the
single heat release cases were typically above 6 bar/deg whereas
TSC peak PRR were typically under 3 bar/deg. Further, for the
single heat release cases, it was found that high EGR rates
sufficiently suppressed the first stage of combustion allowing the
combustion phasing to be controlled by the second injection. The
TSC combustion phasing of both heat releases could be controlled
with injection timing and EGR. Emission and engine performance
trade-offs were observed over the injection timing ranges for all
cases. Variable pressure injection was also compared to a highly
dilute (~67% EGR) low temperature combustion (LTC) and was found to
produce slightly higher emissions while maintaining the same engine
performance at a lower EGR rate (~55%).