Homogeneous charge compression ignition (HCCI) is an autoignition
combustion process with a lean or dilute fuel/air mixture. It can provide both good fuel
economy and very low emissions of nitrogen oxides (NOx) and particulates. Therefore,
it is considered to be one of the most promising internal combustion engine concepts for
the future. However, there are some obstacles that must be overcome before the
potential benefits of HCCI combustion can be fully realized in commercialization,
including combustion phasing control, operation range extending (high levels of noise,
UHC and CO emissions), cold start, and homogeneous mixture preparation. All these
HCCI characteristics have been summarized in Section 1. To overcome these obstacles,
many effective technologies have been carried out and these technologies will be
reviewed in Section 2 according to different fuel properties. HCCI can be applied to a
variety of fuel types and the choice of fuel will have a significant impact on both engine
design and control strategies. Some chemical components have the ability to inhibit or
promote the heat release process associated with autoignition. Typical generalized
diesel-fuelled HCCI combustion modes include: early direct injection HCCI, late direct
injection HCCI, premixed/direct-injected HCCI combustion and low temperature
combustion. Mixture control (mixture preparation), including charge components and
temperature control in the whole combustion history and high pre-ignition mixing rate,
is the key issue to achieve diesel HCCI combustion. The high octane numbers of
gasoline fuels mean that such fuels need high ignition temperatures, which highlights
the difficulty of autoignition. The main challenge for gasoline HCCI operation is focus
on the obtaining sufficient thermal energy to trigger autoignition of mixtures late in the
compression stroke, extending the operational range, and the transient control. In
addition, alternative fuel can save the fossil fuel and reduce the CO2 emission, therefore
it has been got more attention in recent years. And to understand fundamental theory of
HCCI combustion process, the primary reference fuel is the best choice due to the better
understood chemical kinetics. All these fuels will be also introduced in the Section 2.
Advanced control strategies of fuel/air mixture are more important than simple
“homogeneous charge’’ for the HCCI combustion control. Further, it is impossible to
get an absolutely homogeneous mixture in the operation of practical HCCI engines.
Modest inhomogeneity in fuel concentration or temperature appearing in mixing can
affect the autoignition and combustion process. And stratification strategy also has the
potential to extend the HCCI operation range to higher loads. The thermal stratification
can be caused by wall heat transfer and turbulent mixing during the compression stroke
for a low-residual engine. This thermal stratification causes the combustion to occur as
a sequential autoignition of progressively cooler regions, slowing the rate of pressure
rise. For engines with high levels of retained residuals, incomplete mixing between the
fresh charge and hot residuals could also contribute to the thermal stratification. Apart
from the thermal stratification, more researches are about the charge or compositional
stratification. The charge stratification is focus on the different injection strategies,
while the compositional stratification means that all the EGR, internal or external,
changes the composition of the charge therefore forming the different compositional
stratification. These stratification combustion characteristics have been reviewed in
Section 3. Finally, a summary for the progress of HCCI combustion and future research
direction has been shown in Section 4.
Keywords: Fuel, chemistry, mixture stratification, homogeneous charge
compression ignition (HCCI), combustion control, auto-ignition, operation range,
mixture preparation, gasoline, diesel, fuel surrogate, natural thermal stratification,
charge and compositional stratification, low temperature combustion (LTC),
combustion mechanism, chemical kinetics.
