Study on Auto-Ignition Characteristics of High Pressure Methane Jet for Compression Ignition Engine Application 2018-01-0274
Natural gas has been considered as an alternative fuel for a heavy duty diesel engine with its lower pollutant and carbon dioxide emissions than its counterpart. However, due to the high auto-ignition temperature of methane, this alternate fuel has been mainly used in spark-ignited engine with relatively lower compression ratio, losing full potential of achieving high efficiency. To overcome these limitations, high-pressure direct injection of the natural gas in a compression ignition engine has been proposed, and there have been several attempts to understand physical behaviors and ignition of methane jet. In this study, auto-ignition characteristics of high-pressure methane jet were investigated both through the experiment and the multi-zone modeling to suggest the applicability to such engine. In the experiments, constant volume combustion chamber equipped with a single-hole, gasoline-direct-injector was used to understand the jet behavior and the ignition characteristic of the high-pressure (~90 bar) methane injection. To visualize the shape of jet and combustion, shadowgraph/schlieren photography and natural luminosity imaging were carried out. From the experimental results, the penetration distance of 15~25 mm is achieved in ~1.5 msec and the first apparent ignition of methane jet occurs around 1.3 to 5.5 msec after start-of-injection, as background temperature decreases from 1300 K to 1150 K in the background pressure of 50 ± 5 bar. In the multi-zone modeling, jet penetration model based on Abramovich concentration and velocity gradient profile was adapted for overall penetration calculation, while the species transfer by convection and the GRI 3.0 mechanism were adapted for calculating the chemical reaction kinetics and validated against the experimental result. The model was used to study the effects of fuel and background conditions (pressure, temperature and composition) on ignition characteristic of high pressure methane jet, and the applicability to a compression ignition engine was evaluated based on the results.