In order to meet the emission requirements of the China VI regulations on natural
gas (NG) engines, the China VI compliant NG engines generally adopt the
equivalent combustion technology route with high-pressure exhaust gas
recirculation (HP-EGR). However, the HP-EGR introduction mode heavily relies on
engine exhaust pressure, which has negative impact on engine pumping work. In
regards to this issue, study on an alternative EGR technology is very important
to achieve high EGR introduction ability with low pumping work.
In this research, an experimental study on an equivalent-NG engine used in
extended-range hybrid vehicles was carried out. The influence of
high-low-pressure EGR (HLP-EGR) technology on engine combustion, performance,
and emission characteristics was analyzed. The potential of HLP-EGR in improving
engine economy and reducing emissions was explored. The test results showed that
the HLP-EGR achieved a better combustion quality, due to better intake air
mixing uniformity and lower intake manifold temperature in comparison with the
HP-EGR. As a result, the combustion phase was advanced and the pumping loss was
significantly reduced. Finally, the brake specific fuel consumption (BSFC) of
50% and 100% engine loads were reduced by 1% and 3.9%, respectively. The
increase in combustion efficiency and in-cylinder combustion temperature
slightly reduced carbon monoxide (CO) and CH4 emissions at cost of
higher nitrogen oxide (NOx) emissions. Specifically, an energy balance analysis
was conducted. The pumping losses, heat transfer losses, and exhaust energy
losses in HLP-EGR mode mainly contributed to the BSFC reduction in comparison
with the HP-EGR mode.
