The Combustion Chamber Design For The Natural Gas / Diesel Dual Fuel Engine

In the background of the energy crisis and the environmental problems, seeking the alternative fuel of the traditional engine is an effective solution. Among the alternative fuels of engines, the natural gas is the most fascinating due to its low cost, abundant reserves and clean emissions. The natural gas/diesel dual fuel engine is a kind of natural gas engine.It can both substitute diesel and overcome the natural gas ignition instability and so on. In recent years it has become a hot research topic, the research are mainly focus on solving the problem of high HC emission, low thermal efficiency in low load and knock in high load.As is known to all, the combustion not only relates with the chemical kinetics factor, but also relates with some physical factors, such as the velocity distribution, the structure of combustion chamber, the turbulence distribution and so on. Therefore, this paper designed some new combustion chambers to change turbulent flow in cylinder by the commercial software STAR-CD in low load in this paper in order to study the influence of the combustion chamber shape on duel fuel engine. The operating point is 1335r/min, 218 N/m, the replacement rate of 90%. It can explore the impact of shape of combustion chambers, solve the main problems existing in low load and provide a reasonable combustion chamber of the dual fuel engines.In order to solve the problem of high HC emissions and low thermal efficiency in low loads, the combustion process of the dual fuel engine and the main reason from both macro and micro aspects have been deply studied.The results show that: the combustion stage of diesel and natural gas coincide with each other. From the macroscopic point of view the problem in low load is due to the mixture is too lean, the natural gas remaining percentage ratio is too large and close to 30 percent, the center of heat release rate of natural gas delay and have a long combustion duration. From the microcosmic point of view, the flow in cylinder is weak and it goes against to the spread of diesel ignition and natural gas flame itself in the combustion process.In order to improve the flow in the cylinder, the design idea of the combustion chamber of the dual fuel engine is proposed and the corresponding combustion chamber is designed. The design scheme is as follows: the bottom part of 2# combustion chamber with octagon dumbbell, 3# combustion chamber with eight petal dumbbell and 4# combustion chamber with four angle dumbbell are designed into the same big boss, which occupy the central flow dead zone.The upper part of these combustion chambers are designed into the same big squeeze gas area compared with the original engine to enhance squeeze flow and reverse squish flow.The difference between four combustion chambers are the squish shape.The squeezing gas edge shape of 3# and 2# are different and the squeezing gas edge number of 2# and 4# are different to compare the squish shape of dual fuel engine effects. 5# combustion chamber with open-end is developed by the United States team and it is suitable for gasoline / diesel dual fuel engine. 5# combustion chamber is to verify whether other dual fuel engine combustion chamber is suitable for natural gas / diesel dual fuel engine or not.5# combustion chamber has a smallest squish area.It compares with the original engine, 2#, 3# and 4# to research squeezing effect of gas area.In order to explore the influence of the shape of combustion chamber on combustion and emission characteristics of natural dual/diesel fuel engines, several different combustion chambers were simulated and calculated. The simulation results show that the shape of combustion chamber has a direct effect on the cylinder flow.The flow in cylinder of 2#, 3#, 4# combustion chamber improves significantly.4# combustion chamber has the best effect, 2# and 3# combustion chamber are better than the original machine 1#,5# combustion chamber is the worst. In-cylinder flow has an important influence on the remaining percentage ratio and the distribution of the fuel in the cylinder.The pilotdiesel of 4# combustion chamber distributes evenly and natural gas remaining percentage ratio is under 1%.Of all the combustion chamber the cylinder maximum explosion pressure of 4# is highest. Compared with the original engine the center of heat release rate of natural gas of 4# goes 7 °CA in advance and 4# has greatly improved the mixing, but NO emissions are higher. Followed by are 2# and 3#. The remaining percentage ratios of natural gas of them reach 7%. 2# and 3# also contribute to the combustion of Natural gas-air mixture to a certain extent and NO and Soot emissions are improved compared with the original one. In a word, 2#、3# and 4# combustion chambers are suitable for dual fuel engines.As for us which one to choose, we have to depend on our own needs. The remaining percentage ratio of natural gas of 5# combustion chamber is close to 60%, which is not suitable for natural gas / diesel dual fuel engine.