Simulation Study Of Phasing Control In High-pressure Air Jet Controlled Diesel Pre-mixed Compression Ignition

Premixed charge compression ignition(PCCI)combustion in diesel engines can achieve low NOx and PM emissions simultaneously,meanwhile maintaining high thermal efficiency.While the control of combustion phasing is the key problem for realizing full-load PCCI combustion.To directly control the phasing of PCCI combustion,a novel method called high-pressure air jet controlled compression ignition(JCCI)is proposed in this work.The basic principle of high-pressure air JCCI is firstly to adjust the compression ratio,which makes sure that the auto-ignition of the pre-mixture at critical conditions cannot occur.Then,high-pressure air is jetted to ignite the in-cylinder pre-mixture,and thus achieving the precise phasing control.This thermodynamic process with "low-compression ratio,high pressure-rising ratio,and high-expansion ratio" is defined as compound thermodynamic cycle.To realize compound thermodynamic cycle,compound engine concept is also proposed.Compound engine-a kind of combustion-pneumatic hybrid engine-has compressor and power cylinders,and the compressor cylinder can be converted to power cylinder via variable valve technology.It can be operated as combustion,air motor,combustion + air motor and regenerative braking modes to accomplish vehicle start,low-speed operation,acceleration,cruising,braking and so on,which can improve the overall fuel economy and emissions further.This simulation work focuses on the effects of high-pressure air jet on the pure gas under the constant volume condition,and the combustion process of high-pressure air JCCI in the power cylinder.Firstly,a three-dimensional(3D)computational fluid dynamics(CFD)model of a constant volume chamber(CVC)was established to investigate the process of high-pressure air jet compression through the check valve.The effects ofjet pressure,jet temperature and the outlet diameter of check valve on the temperature in the local region in CVC were analyzed.The simulation results demonstrated that the maximum temperature rising in the local region is increased with a higher jet pressure,a higher jet temperature and a larger outlet diameter of check valve.A higher jet pressure has higher mass flow rate and leads to faster mixing rate of high-pressure air and the gas in CVC.A higher jet temperature and a larger outlet diameter of check valve can result in a larger temperature-rising region.Then,a 3D CFD model of the power cylinder in the compound engine was established to investigate the process of high-pressure air JCCI combustion.It has the characteristics of intensified low-temperature reaction and two-stage high-temperature reaction.The first-stage high-temperature reaction is the spontaneous combustion of the in-cylinder pre-mixture in the local region caused by high-pressure air jet compression While the second-stage high-temperature reaction is affected by the mixing of high-pressure air and in-cylinder pre-mixture.The nature of high-pressure air JCCI combustion is to compress the mixture in the in-cylinder local region.The mixture in this region satisfies the spontaneous combustion condition,and thus the auto-ignition occurs.The simulation results from the effects of jet parameters on high-pressure air JCCI combustion and emissions indicated that the variations of SOC and CA50 with the start of jet(SOJ)timing are linear,which reveals that the combustion phasing is directly controlled by high-pressure air jet.Meanwhile,with other fixed jet parameters,there is an optimum SOJ timing for obtaining the shortest burning duration and highest combustion efficiency.Too short jet duration would make SOC retard too much,or even misfire.When it is long enough,SOC is the same with various jet durations.Because of the faster mixing rate of "low-temperature"high-pressure air and in-cylinder pre-mixture,a higher jet pressure leads to a slower late-stage chemical reacltion rate,and therefore a longer burning duration.While a higher jet temperature can make more fuel involved in the first-stage high-temperature reaction and also accelerate the second-stage high-temperature reaction rate,which is beneficial for improving the overall combustion efficiency.The NOx and soot emissions are ultra-low in the high pressure air JCCI combustion mode,while the THC and CO emissions are slightly high.The in-cylinder NOx emission is only affected by the first-stage high-temperature reaction,and it can be decreased with a higher jet pressure,a lower jet temperature and a shorter jet duration.Since the diesel fuel is early injection with high injection pressure to form the pre-mixture,and the in-cylinder mixture is also mixed with the high-pressure air,so the soot emission is almost close to zero.Because the in-cylinder overall temperature is low due to the mixing effect of the low-temperature high-pressure air,especially in the second-stage high-temperature reaction,so the THC and CO emissions are relatively high.While they can be decreased with a lowerjetpressure and ahigher jet temperature.