Impact of natural gas injection strategies on combustion and emissions of a dual fuel natural gas engine ignited with diesel at low loads

Previous studies have shown that diesel ignited natural gas dual fuel engine is multi-point ignition, so a more uniform mixture of natural gas and air is conducive to engine combustion and emissions. To improve engine combustion and emission by improving mixture uniformity, the effects of natural gas injection timing, direction and position on combustion and emissions were studied experimentally on a 6-cylinder turbocharged intercooled diesel/natural gas dual fuel heavy-duty engine at low loads. The natural gas injection timing changes from 220 °CA BTDC (before top dead center) to 420 °CA BTDC. Natural gas injection nozzle lengthened by 2 cm (changing position) and downward injection (changing direction) were studied. The experiments kept the total fuel quantity and diesel injection strategy unchanged. The results show that advanced natural gas injection timing can improve mixture uniformity and engine combustion and emission, but too early injection timing will involve a part of natural gas scavenging and a little part of natural gas is concentrated at the bottom of the combustion chamber, making engine combustion and emission performance worse. The optimum injection timing of this engine is about 300 °CA BTDC. Pmax (maximum of cylinder pressure) has good linear relationship with PHRR2 (the second heat release rate peak) under the change of injection timing. Natural gas injection nozzle lengthened by 2 cm (B strategy) makes the combustion and emissions worse. Natural gas downward injection (C strategy) can improve the combustion and emissions. When λ is 1.9 and compared with original injection strategy (A strategy), the Pmax of C strategy rises 5.3%, and CO and THC decrease 12.9% and 19.9% respectively. In addition, the effect of natural gas injection timing on mixture uniformity was carried out by simulation and there came similar conclusions with previous studies.