In diesel engines with a straight intake port and a lipless
cavity to restrict in-cylinder flow, an injector with numerous
small-diameter orifices with a narrow angle can be used to create a
highly homogeneous air-fuel mixture that, during PCCI combustion,
dramatically reduces the NOX and soot without the addition of
expensive new devices.
To further improve this new combustion concept, this research
focused on cooling losses, which are generally thought to account
for 16 to 35% of the total energy of the fuel, and approaches to
reducing fuel consumption were explored. First, to clarify the
proportions of convective heat transfer and radiation in the
cooling losses, a Rapid Compression Machine (RCM) was used to
measure the local heat flux and radiation to the combustion chamber
wall. The results showed that though larger amounts of injected
fuel increased the proportion of heat losses from radiation, the
primary factor in cooling losses is convective heat transfer. Next,
3D simulations were used to predict the cooling loss behavior over
the entire combustion chamber, and in conjunction with local heat
flux measurements on an actual engine, an analysis was performed to
determine where cooling losses are significant and when these
cooling losses occur. The results showed that because of the
convective heat transfer from the reversed squish flow while the
piston is descending, the cooling losses were greatest along the
side wall of the cavity to the squish region.
Based on the findings above, a piston cavity was designed that
suppresses reversed squish flow. The shape of this shallow-dish
open-chamber cavity suppressed reversed squish flow including local
flow, resulting in reduced fuel consumption.
