Turbocharged spark-ignition (SI) engines, owing to frequent engine knocking
events, utilize retarded spark timing that causes combustion inefficiency, and
high turbine inlet temperature (Trb-In T) levels. Fuel enrichment is implemented
at high power levels to prevent excessive Trb-In T levels, resulting in an
additional fueling penalty and higher CO emissions. In current times,
fuel-enrichment reductions are of high strategic importance for engine
manufacturers to meet the imminent emissions regulations. To that end, the
authors investigated the divided exhaust period (DEP) concept in a 2.2 L
turbocharged SI engine with a geometric compression ratio of 14 by decoupling
blowdown (BD) and scavenge (SC) events during the exhaust process. Using a
validated 1D engine model, the authors first analyzed the DEP concept in terms
of pumping mean effective pressure (PMEP) and engine knocking (KI) reduction.
Subsequently, the authors examined the effectiveness of the DEP concept using a
“low-restriction exhaust flowpath” and varying late intake valve closing (LIVC)
duration.
First, using DEP, significant PMEP and KI reductions benefits were observed at
high power engine conditions along with a large increase in Trb-In T from the
early blowdown event. Subsequently, use of a low restriction exhaust flowpath
and a shortened LIVC duration further elevated the DEP benefits, including
Trb-In T reduction that facilitated enrichment reduction. At 4,000 RPM/20 bar
BMEP, ~70% lower PMEP and a 2.2 point increase in ITEg were noted relative to
the base engine. However, the 2,000 RPM peak torque engine condition was
compromised using DEP, due to knock limitation and deteriorated stock
turbocharger performance. Finally, DEP design integrated with an off-the-shelf
(new) turbocharger system remedied the low-end torque challenges and
demonstrated a notable enrichment reduction and thermal efficiency benefits at
the full load engine curve including the 200 kW rated condition.
