In order to avoid the high CO and HC emissions associated with
low temperature when using high levels of EGR, partially premixed
combustion is an interesting possibility. One way to achieve this
combustion mode is to increase the ignition delay by adjusting the
inlet valve closing timing, and thus the effective compression
ratio.
The purpose of this study was to investigate experimentally the
possibilities of using late and early inlet valve closure to reduce
NOx emissions without increasing emissions of soot or
unburned hydrocarbons, or fuel consumption. The effect of
increasing the swirl number (from 0.2 to 2.5) was also
investigated. The combustion timing (CA50) was kept constant by
adjusting the start of injection and the possibilities of
optimizing combustion using EGR and high injection pressures were
investigated. Furthermore, the airflow was kept constant for a
given EGR level.
The engine used in the experiments was a single-cylinder
heavy-duty direct injection diesel engine equipped with an
electronically controlled unit injector with variable needle
opening pressure. Its geometry is based on the Volvo D12C
production engine with the compression ratio lowered from 18.5 to
17. The engine is also equipped with a fully flexible pneumatic
inlet valve system, enabling both early and late inlet valve
closing times and varied swirl levels.
The results show that late or early IVC can be used to achieve
partially premixed combustion (and thus low soot and NOx
emissions) without a fuel consumption penalty.