Experimental And Numerical Study On Fuel Injection Strategy Of Diesel/natural Gas Dual Fuel Engine

In order to solve the problem of increasing environmental pollution and shortage of oil resources,researchers are constantly looking for new clean alternative fuels.However,natural gas has a high auto-ignition temperature and is difficult to compress for ignition.Therefore,the ignition source is necessary to assist ignition for applying of conventional engine.Diesel/natural gas dual fuel engines can solve this problem better.While,there are also some drawbacks exist in this concept,such as low thermal efficiency and high level of unbumed methane emission at low engine load conditions.The influence of different injection strategies and EGR on combustion performance and emissions is investigated at low engine load conditions by engine bench test and three-dimensional numerical simulation method.The result showed that:(I)There were no obvious effect on NOx emissions with increasing injection pressure.However,it can effectively reduced CO and CH4 emissions and increased indicated thermal efficiency.CO and CH4 emissions increased and NOx decreased by increasing EGR rate.20%EGR can achieve the best compromise between combustion and emission performance with constant injection pressure under single injection strategy.(2)When the diesel injection timing was advanced,the indicated thermal efficiency and NOX emissions increased first and then decreased,while the maximum pressure rise rate,CO and CH4 emissions reduced initially and then increased with pre-injection strategy.When the pre-injected diesel ratio increased,combustion rate of natural gas-air mixture increased,CO and CH4 emission decreased,and NOx emission increased2When the pre-injected diesel timing was approach the TDC,the combustion process showed a distinct four-stage heat release characteristic.(3)The proportion of premixed combustion of the pre-injected diesel and the velocity of flame propagation of natural gas had considerable influence on CH4 emission.When the pre-diesel injection timing is far from TDC,the pre-injected diesel played a premixed role,and the start of combustion was controlled by the second injected diesel.A relatively high indicated thermal efficiency can be obtained under this injection strategy.