As regulations regarding vehicle emissions and fuel consumption become
increasingly stringent, the development of hybrid power systems is accelerating,
primarily due to their benefits in fuel efficiency and reduction of pollutants.
Hybrid engines are specially designed to operate optimally at mid to high speeds
and loads. But for low-speed low-load conditions, due to the relatively low
in-cylinder tumble intensity and lower injection pressure, the fuel-air mixture
tends to deteriorate, resulting in an increase in particle number. To enable the
engine to reach optimal RPM and load quickly during frequent start-stop cycles,
hybrid engines typically set a higher startup engine speed and establish fuel
rail pressure more quickly compared to traditional engines. Yet hybrid engines
still encounter challenges of soot generation during cold start conditions.
Especially in urban driving conditions where the hybrid engine frequently
experiences startups and idling, the soot generation problem becomes severe.
Understanding the soot generation process under these specific conditions is
critical for hybrid engines. This research aims to investigate soot formation
process during cold-start and low-speed low-load conditions by examining the
in-cylinder spray development and combustion processes. To achieve this, an
optical engine has been employed, designed with a combustion system indentical
to that of a cutting-edge hybrid engine. Backlit imaging method was used to
capture the movement of spray droplets, which provides a clear visualization of
how the fuel interacts with the piston top and in-cylinder airflow. High-speed
color photography was used to record the flame propagation process and the sooty
flame development from the bottom of the combustion chamber. By focusing on
these processes, this research finds that for low-speed low-load conditions, the
combustion of fuel film, which generates pool fire, is the main source of carbon
soot. While for cold-start conditions, combustion of rich pockets in the chamber
space is the primary source of soot. Insights gained from this research could
inform the design of more efficient hybrid engines that minimize soot emissions,
ultimately contributing to cleaner air and more sustainable transportation
solutions.