Simulation Study On Pressure-time Controlled Natural Gas Multi-point Injection System

As an important material basis for the survival and development of human society,energy is closely related to the development of the national economy and the lives of the people.Compared with gasoline and diesel,natural gas leads to less environmental pollution as a vehicle and marine fuel,so research on natural gas engines is receiving increasing attention.At present,the existing multi-point gas injection systems on natural gas engines controls gas injection quantity mostly based on time.The injection pressure of the gas injection valve is basically constant,and the gas injection duration of the gas machine is different under different loads.During the duration of the injection,the air mixed with the gas enters the cylinder through the intake valve and at other time,what enters the cylinder is pure air.The injection duration is very short under a small load which is less than 20 CA° or even shorter and this will lead to a kind of uneven mixture of air and gas.In order to improve the mixing of gas and air,this paper proposes a pressure-time based control model.With the method of controlling the injection quantity based on time and pressure,taking a 12V190 natural gas engine of Shengdong Group Company as the research object,the intake process and compression process are simulated in CONVERGE to research the effects of bent pipe outlet and the start and end time of injection on gas injection and mixture,as well as to make initial optimization of the injection valve injection strategy.In this paper,four different bent pipe outlet positions are set,and the effects on gas injection and mixing under two different load conditions are studied.The simulation results show that when injection start time is the same,under the small load condition the farther the exit position of the bent pipe outlet is from the intake valve,the easier it is to diffuse back to the intake manifold and in the case of heavy load,the closer the exit position is,the more likely it is to use natural gas to scavenge.In the case where it is ensured that the gas does not flow back to the intake manifold and scavenge,the selected bent pipe outlet is at the middle position 3.Based on the selected bent pipe outlet position 3,the effects of different injection start and end time on the gas injection and mixing processes,respectively,were investigated.The results show that the later the injection start time is,the shorter the gas backward diffusion time is.In order to avoid the gas flow back to the intake manifold,the injection start time should not be earlier than 360 ℃A.It also shows that the later the injection end time is,the easier it causes incomplete intake and in order to avoid residual natural gas in the intake port,the injection end time must be no later than 465 ℃A.The earliest injection start time and the latest end time under the small load condition are selected to determine the longest injection duration.Two aspects of the injection strategy were studied:the effects of pressure fluctuations in the common rail pipe and double injection valve injection on mixing.Under the premise that the injection duration and the injection amount are the same,when the injection pressure is different,although there is a certain difference in the intake passage at the initial and end stages of the intake process,the mixing of the gas and air in the cylinder is not significantly different.Compared with single injection valve injection,the double injection valve injection strategy can reduce the common rail injection pressure,extend the total injection time and concentrate a large amount of natural gas into the intake port within the crankshaft angle where the turbulent flow energy is relatively strong which will promote the mixing movement of gas and air.