Numerical Study of Intake Manifold Water Injection on Characteristics of Combustion and Emissions in a Heavy-Duty Natural Gas Engine 2019-01-0562

The performances of heavy-duty natural gas engines have been limited by combustion temperature and NOx emissions for a long time. Recently, water injection technology has been widely considered as a technical solution in reducing fuel consumption and emissions simultaneously in both gasoline and diesel engines. This paper focuses on the impacts of intake manifold water injection on characteristics of combustion and emissions in a natural gas heavy-duty engine through numerical methods. A computational model was setup and validated with experimental data of pressure traces in a CFD software coupled with detailed chemical kinetics. The simulation was mainly carried out in low-speed and full-load conditions, and knock level was also measured and calculated by maximum amplitude of pressure oscillations (MAPO). The results show that the quantity of injected water does not have a negative effect on water spray and film at an appropriate position, but an increase in the quantity of injected water leads to a negative effect on charging efficiency and a decrease in IMEP. However, the knock in natural gas engines can be suppressed after water injected into intake manifold. The simulation results illustrate that, at the same MAPO index, the spark timing can be advanced from 16.3 °CA BTDC to 22.3 °CA BTDC while keeping a relative low level of NOx emissions, and the indicated thermal efficiency increases by 0.6% when the water-to-natural-gas mass ratio is 0.5. Intake manifold water injection technology is also an efficient way to reduce NOx emissions in natural gas engines. Compared with the original emissions, a decrease of 40% in NOx emissions can be achieved under a higher mass ratio of water to natural gas, at 0.6. These indicative results can be helpful for the application of water injection technology in practical heavy-duty natural gas engines to reduce emissions and increase thermal efficiency in the future.