Experimental Investigation on Space-dispersed Double-wall Jet Combustion System for DI Diesel Engine

GUO Peng-jiang(郭鵬江)，GAO Xi-yan(高希彥)

(Institute of Internal Combustion Engine，Dalian University of Technology，Dalian 116024，Liaoning，China)

Introduction

The DI diesel engine has found a wide application because of its higher thermal efficiency，better durability and reliability.The DI diesel engine is of significant importance due to its highest effectiveness in settling the hothouse effect caused by CO2and utilizing the petroleum resources.Since the diesel engine emission can cause atmospheric pollution，the emission of NOx，the smoke intensity and PM should be greatly reduced［1－2］.

The air absorption of the traditional diesel enginespray is of quite slow speed，so the fuel begins to burn before sufficiently mixed with air due to the short ignition lag phase.Thus in this way，a carbon smoke is produced in the centre of spray because of the high mixture concentration，and high concentration NOxis produced in the outer zone of spray because of the ideal mixture ratio［2］.In the process of fuel injection，there is a large quantity of carbon smoke in the spray core adjacent to the flame front area，and the fuel incessantly rushes into the mixture zone to interact with the flame［3－6］.And the highly diffused combustion leads to the heavy emission of carbon smoke which needs to be oxidized with sufficient oxygen at a high temperature.The mix intensity is weak at the diesel engine spray tip beside which is the highly concentrated NOxzone.The highly concentrated NOxzone moves downwards over time and accordingly，the high temperature at the spray downside leads to the production of much NOx［7］.Specifically，the high temperature at the spray downside is caused by the gas retention:the momentum and the mix intensity are reduced by colliding and mixing with surrounding air so that the ability of diluting the mixture with surrounding air is subsequently weakened，which causes the gas retention at the spray downside.

1 Space-dispersed Combustion System of Double-wall Jet

A space-dispersed double-wall jet combustion system was developed for DI diesel engine.NOxis reduced by depressing the burning temperature，and the particulate emission is reduced using the premixing combustion and double-wall jet techniques.The reduction of NOxand PM is realized to solve the“two difficult problems of diesel engine”.The combustion system features low cylinder pressure，quick mixing，long combustion lagging period and low combustion temperature.In the double-wall jet technology，the guide bellied arcs and little shoulders are set on the surface of chamber wall.The liquid fuel bunches are sprayed from a multi orifice fuel injector to impact against the surface of chamber wall;and then reflected from the guide bellied arcs and little shoulders to form a hierarchical wall jet flow，as shown in Fig.1.The reflected fuel bunches are distributed on the jet shoving zone and concave zone of combustion chamber to form a big swirl flow of spray and expand to a larger area.The fluidic bunches on the wall can entrain more air to produce more combustible mixture in limited combustible lagging period，generate more kindling points and shorten the duration of combustion.After the spray bunch combusts，the upper layer of and the lower layer of spray bunches mutually radiate and absorb heat.Under the action of centrifugal force，the imperfectly atomized oil drops of the upper layer spray bunch can continue to burn in the lower layer spray bunch;and under the action of antidromic shoving flow in the piston，the incompletely combusted products of the lower layer spray bunch can continue to burn in the upper layer spray bunch.Moreover，the reflection achieves better splitting effect than the spray，and at the same time more homogeneous mixture can be obtained at a rapid speed due to impaction and diffusion.

The double-wall jet technique is based on the following three ideas:

1)Spray bunch guiding

A“wall-guiding”method is used to keep the liquid fuel bunch impinge against the chamber wall to generate the phenomenon of adhering，spreading out，redounding，crushing and splashing.After the spray bunch impinges against chamber wall，a majority of it is reflected into the chamber room，and a minority of it forms a fuel film on the wall.The wall guiding can protect from producing the carbon smoke due to high fuel concentration in the fuel bunch centre of the traditional diesel engine，and can avoid high NOxconcentration of waste gas in the outer zone of spray bunch.Moreover，the wall-guiding can be used to promote the fore spray mixing and make the flow structure of the flame resemble that of the steady spraying flame.

Fig.1 Schematic diagram of double-wall jet combustion chamber and generating principle of mixture

2)Space dispersing

The double-wall jet technique belongs to a spacedistributed spray method.For DI diesel engine，one of the main reasons for the generation of carbon smoke is the interaction of liquid fuel or high concentration mixture with high air-fuel ratio and flame.A mass of carbon smoke can be generated when the fuel is sprayed into the combusted region.The space-dispersing method can avoid the unfavourable interaction of liquid fuel and flame，improve the utilization rate of air，increase the local AF ratio and decrease the emission of carbon smoke.

3)Stratification technique

The spray bunch reflected from the chamber wall can mix in a hierarchical and rapid way.And the guide bellied arcs and little shoulders can make the fuel bunches form the jets in their fair share，respectively.

2 Experimental Research

2.1 Experimental Facility and Methods

According to the characteristics of space-dispersing double-wall jet combustion system，an experimental optimization research was carried out for the effect of structural parameters of combustion system on its performance，combustion and emissions.The specific parameters of diesel engine are listed in Tab.1.All the experiments were conducted on the engine bench.The main test equipment used in experiment includes DW250 electric eddy current dynamometer，F(xiàn)GA-4100 auto exhaust gas analyzer，F(xiàn)BY-3 bosch smoke intensity indicator，AVL Indiset 620 combustion analyzer.The optimized structural parameters of the engine combustion system include chamber structure，swirl ratio of cylinder head，included angle of jet orifice，number and diameter of jet orifice，pressure and timing of fuel injection.The effects of all these parameters on the engine performance and combustion were researched to get the best performance index.The effect of injection timing on engine performance and combustion were also researched.The optimized space-dispersing doublewall jet combustion system has the best power，fuel economy and emission performances.The comparable result shows the feasibility and superiority of space-dispersed double-wall jet combustion system for DI diesel engine based on the low temperature premixing combustion.

Tab.1 Parameters of Y4100zl diesel engine

2.2 Optimization ofStructuralParametersof Space-dispersed Double-wall Jet Combustion System

As mentioned above，the optimized structural parameters of double-wall jet combustion system are mainly combustion chamber shape，swirl ratio of cylinder head and injection conditions.

2.2.1 Effect of Combustion Chamber Shape

Two types of the combustion chambers are designed for the test of double-wall jet combustion system.The structures of combustion chambers are shown in Fig.2，and their specific parameters are listed in Tab.2.

Fig.2 Diagram of two types of combustion chambers for double wall jet combustion system

Tab.2 Structural parameters of two types of combustion chambers for double wall jet combustion system

It can be seen from Fig.2 and Tab.2 that the bores(Φ)of two combustion chambers are different，but their depths are essentially the same.The compression ratio of combustion chamberⅡis bigger than that of combustion chamberⅠ，and the distance from jet orifice to chamber wall of combustion chamberⅡis less than that of combustion chamberⅠ.In the case of the same injection pressure and jet-hole diameter/cone angle，the penetrativity of combustion chamberⅡ is stronger than that of combustion chamberⅠ.

Fig.3 shows the changes of NOx，carbon smoke and fuel consumption under full load at low speed(1 400 r/min)，medium speed(2 100 r/min)and high speed(3 000 r/min)，respectively.In order to optimize the two types of the combustion chambers，the static state oil supply time of 12°CA BTDC，swirl ratio of 1.76 and fuel injector with six 0.21 mm jet orifices and 158°included angle of jet orifice(the injector is shortened to 6-21-158)are selected.The injection pressure rises up with the increase in rotating speed.The NOxcontent is relatively low due to the longer penetration length of combustion chamberⅡwhich leads to the increase of fuel film thickness and the decreased fuel returned into the combustion chamber.The fuel film on the chamber wall keeps absorbing heat and then vaporizing in the combustion process，resulting in the prolongation of the combustion duration and the deterioration of carbon smoke and fuel consumption rate in the diffuse combustion.Given those circumstances，we selected the combustion chamberⅠ.

Fig.3 Effect of combustion chamber shape on NOx，smoke and BSFC

Fig.4 Effect of included angle of jet orifice on NOx，smoke and BSFC

2.2.2 Effect of Included Angle of Jet Orifice

The combustion chamberⅠis selected for the experiment.6-21-152 and 6-21-158 fuel injectors are experimentally compared with the swirl ratio of airway being 3.0，static state fuel supply time being 10°CA BTDC.The results of load-characteristic test at the speeds of 1 400 r/min，2 100 r/min and 3 000 r/min are shown in Fig.4.It can be seen from Fig.4 that the two injectors have little difference in performance.The included angle of jet orifice determines the proportion of the fuel bunch reflected from the guide bellied arcs and the little shoulders，and the fuel distribution at the sho-ving flow zone and the concave zone.Due to the fact that there is little difference between the test results，both these two injectors can be adopted.

2.2.3 Effect of Swirl Ratio

Fig.5 －6 show the effect of swirl ratio on the engine performance.

Combustion chamberⅠand 6-21-158 fuel injector are selected for the experiment with static state fuel supply time being 10°CA BTDC and two swirls ratios being 1.76 and 3.0.The swirl ratio has an important effect on the double-wall jet combustion system in the following way:on the one hand，if the swirl ratio is too strong，the fuel film area in the circumferential direction of combustion chamber may expand，which is disadvantageous to the reflection of fuel bunch;on the other hand，if the reflected fuel bunches are over concentrated，the equivalent ratio of local fuel to air in the combustion chamber may increase and the local temperature rises，resulting in high emission of NOxand carbon smoke.

Fig.5 shows the effect of swirl ratio on external characteristics of engine.If swirl is too strong，the NOxcontent will be comparatively high in the whole range of rotating speeds;and at the low rotating speed，the air inflow velocity and the shoving flow velocity of the combustion chamber are relatively low with weak injection pressure，which leads to relatively less fuel-airmixing energy;if the swirl ratio is increased，the fuelair-mixing energy may be enhanced with sufficient combustion and high NOxcontent;and at the high rotating speed，the swirl flow is too strong to over concentration the distribution of reflected jets，resulting in high local temperature and increased NOxcontent，and what's more，the fuel film area also expand to lead to the decrease in economic efficiency.

Fig.6 shows the results of load-characteristic test at the rotating speed of 3 000 r/min.The test result shows that the carbon smoke and the fuel consumption rates of two swirls have little differences under high.

Load and the strong swirl causes the deterioration of carbon smoke and fuel consumption under low load.Based on the above analysis，the selected swirl ratio is 1.76.

Fig.5 Effect of swirl ratio on external characteristic of engine

Fig.6 Effect of swirl ratio on BSFC and smoke at 3 000 r·min －1

2.2.4 Effect of Diameter and Number of Jet Orifice

The jet orifice diameter has a great effect on the wall-jet combustion system.If the jet orifice diameter is increased，the penetrative distance and the penetrative intensity may increase under the same injection pressure.The spray tip interacts with the air to reduce its momentum and keep gas be in the high temperature area and continuous high temperature be in the downstream zone.Thus the high concentration NOxis produced due to a low mixing intensity at the spray tip.The spray tip is controlled using the wall jet method，and the reflected fuel bunches are recombined in the combustion chamber to avoid the production of high concentration NOx.Therefore，the penetrative of fuel bunch and the NOxcontent may increase with the increase in the jet orifice diameter.In addition，if the jet orifice diameter is augmented，the thickness of fuel film may be increased while the amount of fuel returned to the wall，the highest combustion temperature and the NOxdischarge are decreased.At the same time，the evaporation time and the combustion duration are prolonged，and the fuel consumption and the carbon smoke are deteriorated.In order to keep the constant oil supply，the number of the jet orifices needs to be reduced.Combustion chamber I is selected for the experiment with the swirl ratio being 1.76，the static state oil supply time being 12°CA BTDC，and the 6-21-158 and 5-25-158 injectors are compared.Fig.7 shows the effect of diameter/number of jet orifice on external characteristics of engine such as NOx，carbon smoke and fuel consumption.In the whole range of the rotating speeds，the 6-21-158 injector has relative low oil consumption and emission of carbon smoke.At low rotating speed，the fuel injection pressure is relatively low，the thickness of fuel film injected by 6-21-158 injector decreases，and the most of fuel is reflected to the combustion chamber to be sufficiently combusted，thus producing a high concentration NOx.The 6-21-158 injector is selected through the analyses.

2.2.5 Effect of Injection Pressure

Fig.7 Effect of diameter/number of jet orifice on NOx，smoke and BSFC

Combustion chamberⅠand 6-21-158 injector are selected for the experiment with the swirl ratio in flow passage being 1.76.The effect of injection pressure on engine performance is researched.In general，the penetrative distance of fuel bunch increases with the injection pressure.If the injection pressure is continuously increased，the diameters of atomized fuel drops are decreased and the flying resistance exerted on those fuel drops increases，so the penetrative distance is shortened.The Φ9.5 and Φ10.5 plunger pistons are selected in the experiment.If the diameter of plunger piston is reduced，the production of fuel film on combustion chamber wall may be decreased，the fuel consumption is lowered， in the meanwhile， and the amount of fuel in the premixing combustion is reduced to lead to the decreased emission of NOx.Fig.8 shows the results of injection pressure on external characteristics of engine when static state fuel supply time is 12°CA BTDC.We can conclude that Φ9.5 plunger piston is better than Φ10.5 plunger piston.

Fig.8 Effect of injection pressure on NOx，smoke and BSFC

Fig.9 Effect of injection timing on performance of double-wall jet DI diesel engine

2.3 Effect of Injection Timing on Performance of Double-wall Jet DI Diesel Engine

Injection timing affects the proportion of premixing combustion.If injection timing is advanced，the combustible lagging period is prolonged，the amount of fuel participating in the premixing combustion is increased，the combustion temperature rises，and the NOxdischarge is increased;the carbon smoke emission and the fuel consumption are improved due to less diffusive combustion.The structural parameters of combustion system described in Section 2.2.5 are used here.The Φ9.5 plunger piston is selected in order to decrease the injection pressure.And the effect of injection timing on external characteristics of engine is also researched in the experiment.From Fig.9 it can be seen that the effect of injection timing on double-wall jet combustion system conforms to the general rules of conventional engines.At low rotating speed，the carbon smoke emission at the static state fuel supply timing of 13.5°CA BTDC is higher than that at 12°CA BTDC because the injection timing is advanced and the flow in the cylinder is relatively slow，the flying resistance exerted on the spray decreases so that the thickness of fuel film on the surface of combustion chamber wall and the carbon smoke emission increase.Advance of injection timing makes temperature in cylinder higher，which is helpful for shortening atomization time and for improving oil consumption.As shown in Fig.10，at rated point(3 000 r·min－1/267 N·m)，the ignition time defers with the retardation of injection timing，the centroid of heat-release-rate curve moves backwards and the pressure and mean combustion temperature in the cylinder are decreased.

Fig.10 Combustion analysis of different injection timings of double-wall jet combustion system at rated point(3 000 r·min －1/267 N·m)

2.4 Performance of Optimized Double-wall Jet Combustion System of DI Diesel Engine

After thorough consideration，we select the structural parameters of combustion system in Section 2.2.5 and 12°CA BTDC static state fuel supply timing.An experiment is conducted under the total working condition of double-wall jet combustion system of DI diesel engine.It can be seen from Fig.11 that the maximum smoke intensity emerges at the low rotating speed and maximum peak load，which is 2.1 BSU;the minimum oil consumption is 210.3 g/(kW·h)at the 1 700 r·min－1/203 N·m;and the NOxcontent decreases with the increase in rotating speed.For the optimized diesel engine，the NOxconcentration is 487 PPm，the smoke intensity is 0.95 BSU，and the oil consumption is 218.9 g/(kW·h)at 2 100 r/min and under full load;the NOxconcentration is 369 PPm，the smoke intensity is 1.09 BSU，and the fuel consumption is 248.2 g/(kW·h)at 3 000 r/min and under full load.

Fig.11 Performance of optimized double-wall jet combustion system

2.5 Comparison of Optimized DI Diesel Engine and Prototype

The comparison between the optimized DI diesel engine and the prototype further proves the practicability and superiority of the double-wall jet combustion system.The comparison results of external characteristics are shown in Fig.12.In order to ensure that the double-wall jet diesel engine has the same combustion phase as the prototype，which means keeping the same fuel consumption，the static fuel supply timing of original engine is postponed to 9 BTDC.It can be seen from Fig.12 that NOxis reduced from 712 PPm to 487 PPm at the rotating speed of 2 100 r/min，and from 593 PPm to 369 PPm at the rotating speed of 3 000 r/min.

Fig.12 Comparison of the external characteristics of optimized DI diesel engine and prototype

Smoke intensity is obviously reduced from 3.67 BSU to 2.1 BSU at low rotating speed.And the smoke intensity is slightly increased by 0.2 BSU at high rotating speed.The fuel consumption is decreased from 240.5 g/(kW·h)to 225.4 g/(kW·h)at low rotating speed，which is about 6.3%.It can be seen from Fig.13 that the cylinder pressure of optimized diesel engine is obviously decreased from 115 bar to 108 bar compared with prototype，which is about 6%.

3 Conclusions

1)The basic presumption of space-dispersed double-wall jet combustion system is that the relatively low compression ratio is used to lower the explosion pressure and the highest combustion temperature，thereby decreasing the NOxemission and combustion noise，and prolonging the combustible lagging period.The double-wall jet technique is helpful to accelerate the combustion and the fuel-air-mixing speed，increase the premixing combustion amount and decrease the diffusive combustion，so the fuel can be injected beside the TDC and more homogenous mixture can be obtained to eliminate the theoretical mixture ratio and rich oil zone.The technique is helpful to control the burning temperature，increase the low temperature premixing combustion，and realize the hierarchical space-distributed mixing and combusting to improve the particulate emission.

Fig.13 Comparison of the optimized DI diesel engine and the prototype at 2 000(r/min)/307(N·m )

2)Combustion chamberⅠ，1.76 swirl ratio in flow passage，6-21-158 injector，Φ9.5 plunger piston and 12°CA BTDC static state fuel supply timing are chosen based on the optimized structural parameters of double-wall jet combustion system.The optimal performance can be achieved.

3)For the optimized diesel engine，NOxis 487 PPm(decreasing 31.6%compared with prototype)at the rotating speed of 2 100 r/min and 369 PPm(de-?creasing 37.7%compared with prototype)at the rotating speed of 3 000 r/min.Smoke intensity is obviously reduced speed from 3.67 BSU to 2.1 BSU at low rotating.Fuel consumption is changed from 240.5 g/(kW·h)to 225.4 g/(kW·h)at low rotating speed，which is about 6.3%.

4)Compared with prototype，the cylinder pressure of optimized diesel engine is obviously decreased from 115 bar to 108 bar，which is about 6%.

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