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DOI: 10.1243/09544070JAUTO376

2007 221: 845Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile EngineeringT Wang, Z Peng, S-L Liu, H-D Xiao and H Zhao

Optimization of stratification combustion in a spark ignition engine by double-pulse port fuel injection

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845

Optimization of stratification combustion in a sparkignition engine by double-pulse port fuel injectionT Wang1, Z Peng2*, S-L Liu1, H-D Xiao1, and H Zhao1

1State Key Laboratory of Engines, Tianjin University, People’s Republic of China2Department of Engineering and Design, University of Sussex, Brighton, UK

The manuscript was received on 5 June 2006 and was accepted after revision for publication on 30 March 2007.

DOI: 10.1243/09544070JAUTO376

Abstract: The potential of lean burn in a spark-ignition (SI) engine with optimized fuelinjection was experimentally investigated and numerically simulated. The experiments werecarried out on a production SI engine which has a port fuel injection (PFI) system. The previousport electronic fuel injection system was modified and the technique of double-pulse fuelinjection (DFI) was employed. By regulating injection timings and proportions of DFI, theair–fuel mixture stratification was significantly improved and the expected lean burn wasimplemented. The experimental results showed that the reduction of fuel consumption withDFI could be above 10 per cent over quite a wide load range, compared to single fuel injection.With optimized fuel injection timings and double-pulse proportions, the ideal engine perform-ance and emissions can be achieved with a two to three times higher air–fuel ratio (AFR) thansingle fuel injection.

With numerical simulation, the effects of mixture stratification formed by different fuelinjection amounts and timings were analysed using a phenomenological model. The mixturein the cylinder was divided into different regions that distribute spherically around the sparkplug and consist of a central region of stoichiometric air–fuel mixture and a gradually leaneroutside region. Simulation results demonstrated that the improvements in fuel economy andemissions with DFI were mainly attributed to increased stratification zones and a reduced AFRgradient in the stratification zones.

Keywords: lean burn, charge stratification, SI engine, air–fuel ratio, emissions

1 INTRODUCTION engines: large cyclic variations, long combustionduration, and unreliable ignition of the mixture insome cycles. Large proportions of excess air/EGRLean-burn combustion has been widely developedwhen mixed with the combustible air–fuel mixtureas one of the main ways for spark ignition (SI)would also lead to higher HC and CO emissions [2].engines to improve fuel economy and carbon dioxide

Some effective technologies have been developedemissions [1]. This will diminish the green-housefor overcoming the problem of lean-burn combustioneffect in comparison with conventional stoichiometricof SI engines. These include charge stratification,engines. The advantages of lean-burn SI engineshydrogen-enhanced [3], HCCI [4, 5], pre-chamberare implemented mainly by reducing pump losseslean-burn [6], etc. For the use of charge stratification,associated with part-load operation and heat losses.in this way, a near stoichiometric air–fuel mixtureTherefore, it is always expected to run the air–fuelcan be maintained around the spark plug with acharge as leanly as possible. However, there areleaner mixture at a greater distance from the sparkcertain drawbacks associated with the lean-burnplug. This is a very effective way to extend thelean limit and improve the engine efficiency of SI

* Corresponding author: Department of Engineering and Design, engines and in the meantime to prevent knockingcombustion [7]. However, it is necessary to optimizeUniversity of Sussex, Brighton, UK. email: [email protected]

JAUTO376 © IMechE 2007 Proc. IMechE Vol. 221 Part D: J. Automobile Engineering



846 T Wang, Z Peng, S-L Liu, H-D Xiao, and H Zhao

the mixture stratification pattern for charge stratifi- the flame speed of optimized stratification com-cation. Otherwise, excessive stratification may cause bustion to be as fast as premixing combustion.increased NO

xemissions owing to possible local high Noticeable improvements were observed in fuel

combustion temperatures. consumption, NO emissions, and EGR tolerance.Stratified charge engines may be implemented in Oh et al. tested to inject fuel when the intake valve

two main ways. The first technology is realized was open and when the intake valve was closed andmainly by direct injection of fuel [7–11] and normally very different fuel distributions were obtained in theis referred to as gasoline direct injection (GDI) or cylinder [18]. When the fuel was injected when thespark ignition direct ignition (SIDI). Currently, GDI intake valve was open, the in-cylinder fuel distri-has been conducted with wall-guided, air-guided, bution was favourably stratified in both the axial andand spray-guided technologies. All concepts are radial directions. But with closed valve injection, abased on the formation of a stable, ignitable mixture slightly leaner mixture was observed around thecloud around the spark plug. Whilst GDI was mainly spark plug and richer fuel near the bottom of thecompleted by fuel injection inside the combustion cylinder. Yamato et al. [13] explored the effects ofchamber, significant effort is necessary to optimize different injection positions and optimized inletthe fuel injection system, combustion system, and ports (one swirl and one straight), and achievedtheir integrations in order to help the formation of different charge stratifications on a PFI gasolinemixture stratification. In some GDI engines, parti- engine. Their CFD and LIF measurement resultscularly, the fuel spray needs to be transported by the showed that different injection positions resulted inbulk flow to the vicinity of the spark plug at the right the charge ignitability around the spark plug beingtime. If the bulk flow is too strong, too weak, or if very different and this would change the total air–there are too many cycle-to-cycle variations, the fuel ratio (ARF) limit of lean-burn.initial flame kernel between the gaps of the spark For lean-burn combustion, it is necessary to controlplug may not be formed or may be moved about, the mixing method and combustion system designleading to a poor combustion process or large so as to achieve optimal combustion performancecycle-to-cycle combustion variation. Therefore, it is

and emissions. Appropriate fuel concentration nearnecessary to have precise control of those gas flows

the spark plug and stratification gradient acrossin the combustion chamber for GDI engines.

the combustion chamber should be accuratelyThe second approach for stratification combustion

calibrated to achieve an ideal flame speed andis to introduce the air–fuel mixture during the intake

the lowest emissions. As demonstrated by Aleiferisprocess with port fuel injection (PFI) [6, 12–16].

et al. [17, 19], most important in the stratificationConsidering that the PFI SI engine is still by far thecombustion mixture preparation is to produce amost widely used engine of passenger cars in thegood mixing and AFR around the spark plug soUSA [9] and still one of the main sectors in Europe,as to maintain a good combustion. Note that forthe possible approach of using PFI stratificationstratification lean burn, it is essential to sustain ancombustion to implement lean burn will produceinitial flame growth for the first 5 per cent of fuelsignificant benefits for reducing CO

2emissions and

burnt. Their results showed that the first 5 per centthe fuel consumption of passenger cars.mass fuel burnt (MFB) duration was significantlySome previous studies have demonstrated theaffected by the initial flame-kernel growth, whichpotential of PFI stratification combustion on CO

2 then controlled the level of cyclic variability andemission and fuel consumption improvements. Thelimited the operating range of lean-burn SI engines.accurate turning and integration of the intake flows

In addition to the fuel distribution around theand fuel injection are critical for forming the essentialspark plug, the AFR in the lean area and the totalcharge stratification for PFI lean burn. One widelyAFR are also restricted for reaching acceptableinvestigated is axial stratification, which consists ofcombustion performances [20, 21]. Lumsden et al.stratification of the cylinder charge along the vertical[20] showed that NO

xformation would increaseaxis of the combustion chamber. It is found that such

significantly if the total relative AFR exceeds 1.15.stratification can be achieved through a combinationThis is due to the NO

xformation mechanism beingof swirl and fuel injection timing [17]. The best

a balance between oxygen availability and tem-results were obtained with injection occurring late inperature. Although an increased AFR can reduce thethe intake stroke, realizing a richer mixture aroundcombustion temperature, it will increase the oxygenthe spark plug and a lean mixture towards the piston.concentration, which is one of the principal variablesMoriyoshi et al. [12] implemented axial stratification

by adjusting the injection timing of PFI and measured for controlling NOx

formation. From this point of

JAUTO376 © IMechE 2007Proc. IMechE Vol. 221 Part D: J. Automobile Engineering



847Optimization of stratification combustion in an SI engine

view, the AFR must be compromised between maxi- 2 EXPERIMENTAL APPARATUSmum combustion temperature and appropriate

2.1 Test engineoxygen concentration so as to achieve ideal NOx

emissions. The experiment was conducted on a three-cylinderIn the current study, new charge stratification SI production gasoline engine that is fitted with a PFI

combustion was explored with double-pulse fuel system and has 0.33 l swept volume per cylinderinjection on a PFI SI engine. With this method, it (76 mm bore, 73 mm stroke), 9.5 : 1 compression ratio.was attempted to examine the effects of injection The rated speed is 6000 r/min. The maximum torqueparameters on charge stratification and lean burn on and brake power are 76 Nm (0.96 MPa b.m.e.p.) anda modified production gasoline engine. To achieve 39 kW respectively. The relevant specifications of thedouble-pulse fuel injection and a variable fuel amount engine are listed in Table 1. In the table, the valveand timing, a purpose-built electronic control system timing unit CAD stands for crank angle. In orderfor fuel injection was developed. Then a double-pulse to reduce confusion for using the gas change TDCfuel injection (DFI) in the inlet port of SI engines was and compression or combustion TDC, 0 or 720 CADused to form the inhomogeneous mixture. The total is used as the compression or combustion TDC,amount of fuel needed for each cycle was divided while 360 CAD is used for the gas change TDC in allinto two parts and injected at different timings. following descriptions, whilst some negative valuesThe first injection was designed to deliver in con- of crank angle are used for timing those events thatventional timing to format a homogeneous mixture take place during the last cycle.in the whole cylinder. Then the second injection To meet the requirements of charge stratificationwas injected at an optimum moment to provide a combustion on in-cylinder flow, the intake portspherical charge stratification that formed a relatively shape was modified slightly and swirl and tumblerich mixture in the spark plug region. strengths were increased. To compensate for the

Compared with simply adjusting the timing of reduction of flow coefficient due to those portthe single fuel injection, DFI can form a better modifications in order to maintain a similar peakcharge stratification for maintaining a good initial torque and power, the cross-section area of the inletflame growth and a good combustion speed and port, the inner diameter of the valve seat, and the

inlet valve diameter were slightly increased. A seriestemperature in order to obtain reduced fuel con-of experiments was carried out at a steady state flowsumption and emissions. Experimental investigationsrig to evaluate the performance of the inlet port thatwere conducted and emphasis was placed on thewas optimized among five inlet port model versions.effects of fuel amounts and injection timings of twoThe flow performance including flow coefficient,injection pulses on fuel consumption and NO

x, HC,

swirl ratio, and tumble ratio was measured usingand CO emissions. Experimental results showed thatthe method suggested by Ricardo [22]. In Table 2,the proposed lean-burn method can extend the lean-specifications and flow performance of the originalburn limit of the AFR and can reduce the specificintake system and the modified intake systems arefuel consumption and exhaust emission noticeably.listed.To understand the mechanism of good charge

To intensify in-cylinder air motion is helpful forstratification formation with DFI, numerical analysisforming necessary charge stratification, while thewas undertaken. Using a phenomenological model,volumetric efficiency should not be reduced tothe mixture in the cylinder was assumed to be

distributed spherically around the spark plug and toTable 1 Specifications of the test engineconsist of a central region of stoichiometric air–fuel

mixture and a gradually leaner outside region. The Cylinder number 3volume of the central region and the AFR gradient Bore 76 mm

Stroke 73 mmof the outside region can be regulated in order toSwept volume per cylinder 0.33 l

analyse the influence of charge stratification level on Compression ratio 9.5 : 1Valve number per cylinder 2SI engine performance and NO emissions. ResultsIVO (intake valve open) 341 CAD

indicate that both reducing the central region IVC (intake valve closed) 591 CADEVO (exhaust valve open) 129 CADvolume and the outside region AFR gradient canEVC (exhaust valve closed) 379 CADreduce fuel consumption and NO emissions, but thatFuel injection Port fuel injection

there is a minimum central region volume and a Rated engine speed 6000 r/minPeak torque 90 Nmlowest outside region gradient for maintaining thePeak power 40 kW

benefits of charge stratification combustion.





Table 2 Comparison of the original and the modified inlet port

Original port Modified port

Exit diameter of inlet port (mm) 31 32Inner diameter of valve seat insert (mm) 35 36Diameter of inlet valve (mm) 36 37Port shape Plain TangentialFlow coefficient 0.405 0.430Swirl ratio 0.595 0.916Tumble ratio 0.492 1.123

maintain the maximum power and torque of the Recorded in-cylinder pressure data was used foranalysing heat release, i.m.e.p., etc. In this study, aengine. Hopefully, these modifications can help to

set up a good base for realizing lean burn through 10 per cent coefficient of variation of indicated meaneffective pressure (COV

i.m.e.p.) was used for definingorganizing an adequate air–fuel mixture distribution

or stratification. the lean-burn limit of the AFR.Initial experiments showed that the optimum

advanced ignition for the modified engine inlet port2.2 Test rigcould be shortened by 3 to 8 CAD at an engine speed

The test bed is shown in Fig. 1. It consists mainly ofof between 3500 and 5600 r/min, compared to the

the test engine, which has a modified intake systemoriginal inlet port. This was due to the intensified

and improved fuel injection system, dynamometer,in-cylinder air motion, which could lead to higher

exhaust gas analysers (for NOx, HC, and CO),

turbulence intensity, higher flame propagationin-cylinder pressure measurement, and an analysis

speed, and higher combustion speed. The increasedsystem. A UEGO sensor was fitted on the exhaust

volumetric efficiency resulted in a somewhat higherpipe for AFR ratio measurement.

peak power output and a somewhat lower fuelMost original parts of the fuel injection system

consumption.were used, except for the electrical control unit(ECU), which was replaced by a control PC. Withthese modifications, flexible fuel injection including

3 EXPERIMENTAL RESULTS AND DISCUSSIONmore than one injection pulse, variable amountand timing for each injection pulse was supplied.

3.1 Lean-burn experiment with single injectionIn-cylinder pressure was measured by a Kistler 7061B3.1.1 Effect of fuel injection timingspressure transducer and was recorded and real-time

analysed by a data acquisition system based onThe emphasis of this study is to explore the DFI of

National Instrument’s LabviewA system. NOx, HC,

PFI gasoline engines and its effects on lean burn. Inand CO emissions were measured with Signal gas

order to evaluate the charge stratification character-analysers. A NGK TL7111-W1 UEGO sensor, which has

istics and engine performance of DFI and to comparea 10.0–30.0 effective AFR range (or 0.68–2.05 effective

those with single injection, the first part of thelambda range), was used for AFR measurement.

experiment was to investigate the effects of singleinjection on lean-burn engine performance andemissions. Both this part of the experiment and thefollowing DFI test were carried out with the modifiedinlet port.

Figure 2 shows the effects of the injection timingson brake specific fuel consumption (BSFC) andexhaust emissions when the engine runs with singlefuel injection at lean-burn conditions. The measure-ment was fixed at 3000 r/min engine speed and0.3MPa brake mean effective pressure (b.m.e.p.). Thisoperating point was selected owing to the speed andtorque being among the most common operatingregions of the engine. AFR was set at 20 and the sparkignition timing for different fuel timing was alwaysFig. 1 Experimental set-up for stratification combustion

measurement calibrated at minimum ignition advance for best





Fig. 2 Effects of the injection timing of single injection on emissions and fuel consumption(3000 r/min, b.m.e.p. 0.3 MPa, AFR 20, spark ignition timing at MBT)

torque (MBT) timing. The valve timings are plotted the injection timing was continuously retarded. HCemission showed a similar trend. For CO emission,in the figure with four vertical dashed lines.the level was high for the injection timing just afterFrom Fig. 2, it can be seen that the injection timingintake valve opening. When the single injectionsuitable for low fuel consumption is just after thehappened close to IVO, CO was the highest comparedintake valve opening under lean-burn conditions. Itto the other injection timings, but it graduallyis assumed that swirl and tumble pushed the fueldecreased from injection timing at IVO until theto have an optimum distribution for stratificationlowest BSFC was achieved during the intake valvein the cylinder when the fuel was injected justopen period.after the intake valve was opened. Compared to the

injection happening as the intake valve is closing,3.1.2 Effect of AFRfuel injection happening after the inlet valve opens

can lead to a reduction of BSFC by 12 per cent. This Figure 3 shows the effects of AFR on the BSFC andis because those fuels injected during the closed exhaust emissions with 3000 r/min engine speedintake valve are easily pushed to the bottom area of and 0.3 MPa b.m.e.p. The injection timing is fixed atthe cylinder, thereby reducing the fuel amount TDC. From those results, it can be seen that the fuelaround the spark plug and restricting the initial consumption, HC, and CO emissions were all keptflame development. For single injection, the main reduced when the mixture changed from rich to lean.adjustable parameter is the injection timing, if the Around 16–18 AFR, they all had their lowest levels.evaluation is concentrated on a fixed engine speed But NO

xhad a reverse trend with AFR. On further

and torque. However, with a fixed AFR, reduced BSFC examination, it can be found that around 20 AFR, allwith optimized injection timing will need the throttle BSFC, NO

x, HC, and CO emissions had very good

to be closed a little. levels compared to the stoichiometric condition.The AFR used for the single injection experiment With lean burn, the benefit of BSFC comes mainly

is 20. For this lean-burn condition, the combustion from the following two criteria: reduced pumpingspeed is relatively low compared to conventional SI loss and higher thermal efficiency; the former owingcombustion. It is possible to form a favourable AFR to wider throttle open due to more air being needed;stratification by adjusting the injection timing, which the latter owing to the reduced combustion temper-would enrich the spark plug region locally when the ature reducing the heat loss from combusted chargefuel is injected during the induction process. This to coolant and hence more complete combustionwould lead to a stable combustion, a shorter com- with rich oxygen. However, as the AFR increasesbustion period, and a reduced BSFC. The variation continuously, the flame speed clearly slows and theof BSFC with a different single injection timing was combustion period lasts longer. The advance of thenot found in those experiments with a stoichiometric ignition timing has to be increased and the negativemixture (14.7 AFR). This suggests that injection power from compression therefore increased. Alltiming will be much more important for lean-burn the above considerations cause the BSFC to increase.than conventional SI engine combustion. In Fig. 4, the increased combustion duration with

For emissions, NOx

was still kept under an increased AFR is demonstrated; but from AFRacceptable range when the injection started just 17–19, the combustion flame speed has no obvious

reduction.after intake valve opening; but suddenly increased if





Fig. 3 Effects of AFR on emissions and fuel consumption (3000 r/min, b.m.e.p. 0.3 MPa, injectiontiming at 0 CAD, spark ignition timing at MBT)

Fig. 5 Load performance with optimized AFR by singleFig. 4 Combustion durations for different AFR injection (3000 r/min, near line number is AFR)

(3000 r/min, b.m.e.p. 0.3 MPa)

For NOx

emission, because it is formed with high 3.2 Charge stratification experiment with DFIcombustion temperatures and rich oxygen, too rich

The motivation for using DFI to optimize chargeor too lean a mixture is beneficial for its reduction.

stratification for lean burn is to produce a moreThis can be found from Fig. 3. Lower than 14 AFR

suitable fuel distribution in the cylinder and toreduces the oxygen concentration for forming NO

x improve the AFR gradient for improving emissions.and over 20 AFR produces a low combustion tem-

If some fuel is injected into the inlet port at TDC ofperature of lean burn which significantly reduces the

the compression or expansion strokes, there will beformation of NO

x. HC and CO emissions are high

enough time and heat for the fuel to atomize andwith a rich mixture (AFR less than 15). This is due to

vaporize to form a homogeneous mixture. Theninadequate oxygen causing incomplete combustion.

the second injection can be delivered during theWhen the mixture becomes lean, HC and CO have

induction stroke and it is possible to generatethe lowest emissions of around 16–17 AFR. Also, too

some range of AFR stratification with the help oflean a mixture reduces the combustion temperature,

in-cylinder swirl and tumble flow.resulting in high HC and CO emissions.

Figure 5 shows the results of the engine load per-3.2.1 Mixture stratification formed by DFI

formance at 3000 r/min and the reduced BSFCafter optimizing the single injection timings. The Figure 6 attempts to display AFR distributions for

different injection strategies. To better demonstrateAFRs for optimized conditions are based mainly onconsideration for the lowest BSFC. It can be seen that those characteristics created by conventional gasoline

engines, single injection for charge stratification, andat the optimized AFR condition the BSFC has beenconsiderably reduced compared to the stoichiometric DFI for charge stratification, only stratification in the

axial direction is shown. In Fig. 6(a) is a typical homo-mixture condition, though just with the singleinjection. On the test engine, the maximum BSFC geneous mixture formed by conventional gasoline port

fuel injection. In Fig. 6(b), the charge stratificationreduction is greater than 10 per cent.





the homogeneous mixture part can be increased.Therefore, with DFI, the AFR distribution can bebetter adjusted and optimized. In the followingsection are introduced those DFI test results.

3.2.2 Optimization test of DFI

Compared to only the one parameter being adjustedfor lean burn by the traditional single injection, thereFig. 6 AFR distributions formed by the first injection,are three parameters to be adjusted with DFI for leanby the second injection and by both: (a) ifburn SI engines, as shown in Fig. 7. The first test forjust the first injection; (b) if just the secondevaluating the engine performance of DFI was theinjection; (c) if simultaneously using both the

first and the second injections adjusting of the injection timing of the first injection.The result is shown in Fig. 8 in which the effectsof the timings of the first injection on the engineby single injection is plotted and the charge stratifi-performance and emissions are demonstrated. Thecation by DFI is shown in Fig. 6(c). Compared toadjusting range of the first injection timing variessingle injection, DFI would be possible to form abetween 145 CAD before the combustion TDC of thelower AFR gradient: the charge stratification by DFIlast cycle (here, a negative CAD is used for represent-is like the combination of homogeneous mixing withing the crank angle belonging to the last cycle) andsingle injection stratification.70 CAD of the current cycle. The second injectionFor a high AFR gradient created by singletiming and the fuel amount proportion were keptinjection, when the engine load is changed and theconstant for this part of the test. The engine was stillinjected fuel needs to be increased or decreased, itrun at 3000 r/min and 0.3 MPa b.m.e.p. From Fig. 8,is inevitable that overlean or over-rich regions tendit can be seen that there is a suitable first injectionto occur because there is only one chance for thetiming around 0–30 CAD where BSFC, NO

x, HC, andfuel injection. At the over-rich region, high com-

CO all lie at an acceptable level. This suggests thatbustion temperatures and soot are easily generated.the first injection timing in this area can provide theAt the overlean region it is easy to form HC emissions.best integration with the second injection at thisIt is anticipated that DFI can help to reduce bothengine operating condition. Compared to the resultsover-lean and over-rich regions.of single fuel injection shown in Fig. 3, it can beWith DFI, the charge stratification parameters fornoted that the best BSFC is reduced from 291 g/kWhlean burn can be optimized with more variables.to 274 g/kWh simply by adjusting the timing of theThis can assist the engine to work in more optimalfirst injection of DFI.conditions than with single injection. For a com-

In Fig. 9, the effects of the second injection timingparison, see Fig. 7. With DFI, when the engine loadon the fuel consumption and emissions are shown.needs to be decreased and the homogeneous part ofThe first injection timing was fixed at TDC of the lastthe mixture cannot be leaner, the fuel amount forcombustion cycle (0 CAD in Fig. 8). Two fuel pro-the stratified mixture part can be reduced. When theportions were tested. They are 30 per cent and 60engine load needs to be increased and the stratifiedper cent of the second injection respectively.mixture part cannot be richer, the fuel amount for

From Fig. 9, it can be seen that BSFC is clearlyimproved when the second injection is delivered atthe beginning of induction (360 CAD). Comparedto the second injection at 300 CAD when the inletvalve has not opened, the reduction of BSFC canbe up 10 per cent. For this operating condition, thefuel injection duration was about 50 CAD. If thesecond injection starts at around TDC (360 CAD),the injection will complete around 410 CAD whenthe inlet valve is completely opened. The fuel sprayfollows the intake airflow to form a rich mixture, thenthe mixture moves with the in-cylinder air motion toreach the upper part of the cylinder, resulting in afavourable stratification for the ignition and theFig. 7 Adjusting parameters and resulted AFR mixture

type by single injection and DFI lean burn.





Fig. 8 Effects of the first injection timing on fuel consumption and emissions. 0 CAD is TDC ofthe last combustion cycle. (3000 r/min, 0.3 MPa b.m.e.p., AFR 20, the first injection 60per cent fuel amount, the second injection timing at 410 CAD, ignition timing at MBT)

Fig. 9 Effects of the second injection timing on fuel consumption and emissions; 360 is TDC ofintake stroke. (3000 r/min, 0.3 MPa b.m.e.p., AFR 20, the first injection timing at TDCof the last cycle combustion stroke)

Although 360 CAD of the second injection timing fuel amount proportion is the most suitable for DFIlean burn under different operating conditions.is not good for NO

xemissions, the increase of NO

xat 360 CAD can be reduced if the fuel amount pro- In Fig. 10, the effects of the fuel amount proportion

of two injections on fuel consumption and emissionsportion is optimized. As there is a trade-off betweenNO

xemissions and BSFC, it is necessary to test what are shown. For fuel consumption, the variations

Fig. 10 Effects of the fuel amount proportion of two injections on fuel consumption andemissions. (3000 r/min, 0.3 MPa b.m.e.p., AFR 20, the first injection timing at TDC ofthe last cycle combustion stroke, the second injection timing at TDC of the earlyintake stroke)





with the second injection amount of AFR 17 and adjusted parameters include the total AFR, the firstinjection timing, the second injection timing, and theAFR 20 are very different. With AFR 17, it is clearlyfuel amount proportion of two injections. Figure 11beneficial if the second injection amount is low.shows the lean-burn limits at different engine loadHowever, for AFR 20, it is suggested that a highand different engine speeds. It can be seen that DFIsecond injection amount is delivered for the bestis clearly beneficial for lean burn and extends theBSFC. This is mainly because a high AFR needs morelean-burn limit by about 2–3 AFR units over quitecharge stratification to form a rich enough mixturewide load and speed ranges. During the test process,around the spark plug. For AFR 17, charge stratifi-it was also noticed that the potential of increasingcation is not very helpful for BSFC compared withthe lean-burn limit by DFI was most apparent withAFR 20. Considering different AFR, in the figure it canengine speed ranges from 3000 to 3500 r/min. Withbe seen that a 60 per cent second injection amountthe engine speed increased or decreased from thiscan always give an ideal fuel consumption. However,speed range, the extent of AFR for lean-burn limit bya low second injection amount is a favourable optionDFI was reduced.for low NO

xemissions but a high second injection

In Figs 12 and 13, the load performances and NOxamount is for low HC emissions. CO emission is

emissions at 3000 r/min and 4000 r/min are shownmuch more dependent on the AFR than on the fuelrespectively. The optimized values of AFR for leanamount proportionally of the two injections.burn are indicated along the curves. From Figs 12 and13, it can be seen that the fuel consumption can be3.2.3 Extension of lean-burn limit of AFR by DFIreduced significantly from stoichiometric conditions

With a series adjustment and optimization test, lean- to single injection charge stratification conditions. Ifburn limits of the test engine were investigated based using DFI, not only can fuel consumption be further

reduced, but also the NOx

emissions can be improvedon acceptable fuel consumption and emissions. The

Fig. 11 Lean-burn limit for different operating conditions

Fig. 12 Load performance and NOx

emissions at 3000 r/min





Fig. 13 Load performance and NOx

emissions at 4000 r/min

with a leaner mixture. This should be attributable to gas region can be modelled as multiple zones forimproved accuracy in predicting NO formation. In thissome extent to the greater AFR of lean-burn limits

with DFI. Therefore, DFI provides an easy solution study, only NO is calculated, because NOx

emissionsdue to NO

2are negligibly small for spark ignitionfor lean burn of PFI gasoline engines.

From the above results and discussions, it can engines. A more robust numerical method has beendeveloped to calculate the equilibrium burnt gasbe summarized that lean burn by single injection or

two injections can reduce fuel consumption signifi- composition using the Olikara and Borman method[24]. In addition, the model is capable of predictingcantly over quite a wide load range. The reasons

and principles using DFI for lean burn to reduce the occurrence of knocking combustion by simulatingthe chemical reactions in the end gas. The auto-fuel consumption and emissions, particularly NO

xemissions, will be further analysed in the next section ignition model is based on the extended Shell

model [25], which includes 15 generalized reactionby numerical simulation methods.steps for each component of a primary reference fuel(a mixture of iso-octane and n-heptane) and hence 30reactions are used in the simulation of gasoline fuel4 NUMERICAL SIMULATIONcomprising a mixture of n-heptane and iso-octane.

4.1 Model description4.2 Modelling results of spherical stratification

To simulate the effects of charge stratification onlean-burn performance and emissions, an engine The simulations were carried out based on a single-

cylinder test engine with a 0.49 l swept volume.simulation code that has a two-zone combustionmodel, with a burnt and unburnt zone, was used. The The specification of the engine is given in Table 3.

Engine speed, compression ratio, and cylinder wallmodel consists of a main thermodynamic model withwhich are linked a number of submodels describing temperature at intake valve closure (IVC) are set at

1600 r/min, 7.5, and 410 K respectively. The chargethe various engine phenomena, such as heat transfer,auto-ignition, combustion, and emissions. The com- temperature at IVC is assumed to be uniform at

362 K. Wide-open throttle operation is assumed forbustion processes are divided into four distinctphases: adiabatic compression, ignition and laminarburning, development and propagation of a turbulent Table 3 Input parameters describing engineflame front, and final burning of entrapped gas in conditionsthe burnt gas region. The flame propagation is

Bore (mm) 75described by the Keck eddy entrainment model [23], Stroke (mm) 111Swept volume (litre) 0.49which models flame propagation as a sphericalEngine speed (r/min) 1600expansion. Interaction with the cylinder walls andEngine load at reference point (b.m.e.p., MPa) 0.565

head is taken into account by a geometric submodel Charge pressure at IVC (bar) 1.1019Charge temperature at IVC (K) 362in a separate subroutine, thereby allowing the effectsCylinder wall temperature (K) 410of different cylinder head geometry to be included. Compression ratio 7.5

Heat transfer between in-cylinder charge and Combustible charge (vol %) 47.5Diluent (EGR or excess air) (vol %) 47.5coolant is modelled according to a derivative toBurnt gas residual (BGR) (vol %) 5

the Woschni heat transfer correlation. The burnt





all calculations. The engine simulation starts at IVC metric zone size decreased. This can assume thatthe fuel injection changes from single injection toand finishes at exhaust valve opening (EVO).

The simulation assumes that the charge stratifi- two injections and the second injection amount isgradually reduced. From the figure, both fuel con-cation is distributed spherically, which is shown in

Fig. 14. In the core, it is a stoichiometric mixture sumption and NO emissions are reduced with twofuel injections compared to only one. With two fuelwith a 14.7 AFR that is symmetrically distributed

around the spark plug. Lean mixture with higher injections, there is also an optimal lean zone sizefor the lowest fuel consumption and NO emission.AFR is also spherically distributed outside of the

stoichiometric mixture. The spark plug is assumed This suggests that the fuel amount proportion andinjection timings of two fuel injections must beto be located in the middle of the cylinder head. The

flame propagation is also assumed to be spherical, carefully optimized for ideal results.Figure 16 shows the effects of the lean zone sizeso the flame front meets the lean charge at all points

simultaneously. This provides a major simplification on i.m.e.p. and MBT spark timing. As the total fuelamount is fixed for different lean zone sizes (alsofor the modelling of a stratified charge engine cycle

of this type. different stoichiometric zone size), it can be seenthat the power output is increased with the optimizedIn this study, the total charge assumed is that

there are three zones (also named AFR zones): an fuel amount proportion of two injections. With twofuel injections, it is clear that the i.m.e.p. is higherinner stoichiometric mixture core region, and outer

annular air region without fuel, and an annular lean than with single injection (0 per cent lean zone size).The simulation also shows that the MBT spark timingregion between them with less than 14.7 AFR. Each

region is specified as a certain volumetric fraction of should be adjusted for different lean zone sizes.the overall cylinder charge. The total fuel is fixed,thereby increasing the stoichiometric zone size resultsin that the lean zone must be reduced. This means 5 CONCLUSIONSincreasing the second injection proportion. Also, ifthe lean zone size is zero, it means that there is only On a modified production engine for examining the

charge stratification combustion of a PFI gasolinethe second injection.Figure 15 shows the effect of the lean zone size engine, single-pulse fuel injection and double-pulse

fuel injection (DFI) were experimentally investigatedon fuel consumption and NO emissions. With theincrease of the lean zone size, it means the stoichio- for improving lean-burn performance. It was found

that both single injection and DFI can form chargestratification for PFI engines, but DFI clearly hadmore benefits for lean burn, compared to singleinjection.

For analysing the effects of two fuel injections,numerical simulation was also used in this study bycalculating engine performance, fuel consumption,and NO emission of a spherical charge stratificationcombustion. The charge was divided into three

Fig. 14 Schematics of spherical stratified charge zones: an inner stoichiometric mixture core region,

Fig. 15 Effects of lean zone size on fuel consumption Fig. 16 Effects of lean zone size on i.m.e.p. and MBTspark timingand NO emissions





cylinder emissions and exhaust after-treatmentand outer annular air region without fuel, and ansystem performance. SAE paper 2004-01-0043, 2004.annular lean region between them with less than

8 Kawahara, N., Tomita, E., Kasahara, D.,14.7 AFR.Nakayama, T., and Sumida, M. Fuel breakup

With these experiments and simulation results, the near nozzle exit of high-pressure swirl injectorfollowing conclusions are reached. for gasoline direct injection engine. SAE paper

2004-01-0542, 2004.1. Compared with conventional single injection, a9 Landenfeld, T., Kufferath, A., and Gerhardt, J.

leaner mixture can be used by DFI. With this Gasoline direct injection – SULEV emission concept.method, even more reduced fuel consumption SAE paper 2004-01-0536, 2004.was reached at a fairly wide engine load range. 10 Karl, G., Kemmler, R., Bargende, M., and Abthoff, J.

Analysis of a direct injected gasoline engine. SAEThe lowest value of BSFC can be lower thanpaper 970624, 1996.250 g/kWh at 3000 r/min, 0.63–0.76 MPa b.m.e.p.

11 Hatakeyama, S., Sekiya, Y., Murayama, T., and(50–60 Nm) at the current test engine.Tunemoto, H. A study of lean burn of a 4-stroke2. NO

xemissions can also be significantly reduced gasoline engine by the aid of low-pressure, air-

with DFI, compared to the single injection method assisted, in-cylinder injection-Part II. SAE paperused for lean burn. In this study, DFI can reduce 1999-01-3689, 1999.NO

xemissions by 35–50 per cent at a load range 12 Moriyoshi, Y., Morikawa, H., Kamimoto, T., and

Hatashi, T. Combustion enhancement of very leanof 0.25–0.76 MPa b.m.e.p. (20–60 Nm), 3000 r/min.premixture part in stratified charge conditions. SAE3. Using DFI for lean burn, the optimized values ofpaper 962087, 1996.AFR can be higher by 2–3 units than that of the

13 Yamato, T., Sekino, H., Hayashida, M., andsingle injection method. This directly resulted in Sugahara, K. Stratification of in-cylinder mixturereduced BSFC and improved exhaust emissions. distributions by tuned port injection in a 4-valve SI

gas engine. SAE paper 2001-01-0610, 2001.14 Ortmann, R., Arndt, S., Raimann, J., Grzeszik, R.,

and Wurfel, G. Methods and analysis of fuel injection,ACKNOWLEDGEMENTSmixture preparation and charge stratification indifferent direct injected SI engines. SAE paperThe financial supports from the Natural Science 2001-01-0970, 2001.

Foundation of China (NSFC 50576063) and the 15 Reynolds, C. C. O. B. and Evans, R. L. ImprovingRoyal Society (China-UK Network) are gratefully emissions and performance characteristics of leanacknowledged. burn natural gas engines through partial stratifi-

cation. Int. J. Eng Research, 2004, 5(1), 105–114.16 Arcoumanis, C., Hull, D. R., and Whitelaw, J. H.

Optimizing local charge stratification in a lean-burnREFERENCES spark ignition engine. Proc. Instn Mech. Engrs, Part D:

J. Automobile Engineering, 1997, 211(2), 145–154.1 Germane, G. J., Wood, C. G., and Hess, C. C. Lean 17 Aleiferis, P. G., Taylor, A. M. K. P., Ishii, K., and

combustion in spark-ignited internal combustion Urata, Y. The relative effects of fuel concentration,engines – A review. SAE paper 831694, 1983. residual-gas fraction, gas motion, spark energy

2 Das, A. and Watson, H. C. Development of a natural and heat losses to the electrodes on flame-kernelspark ignition engine for optimum performance. development in a lean-burn spark ignition engine.Proc. Instn Mech. Engrs, Part D: J. Automobile Proc. Instn Mech. Engrs, Part D: J. AutomobileEngineering, 1997, 211(5), 361–378. Engineering, 2004, 218(4), 411–425.

3 Goldwitz, J. A. and Heywood, J. B. Combustion 18 Oh, S., Lee, Y., Kang, K., Woo, Y., and Bae, C.optimization in a hydrogen-enhanced lean burn SI Stratification in a liquid-phase LPG injection engine.engine. SAE paper 2005-01-0251, 2005. SAE paper 2003-01-1777, 2003.

4 Thring, R. H. Homogenous charge compression- 19 Aleiferis, P. G., Taylor, A. M. K. P., Ishii, K., andignition (HCCI) engines. SAE paper 892068, 1989. Urata, Y. (2004) The nature of early flame develop-

5 Peng, Z., Zhao, H., Ma, T., and Ladommatos, N. ment in a lean-burn stratified-charge spark-ignitionCharacteristics of homogeneous charge compression engine. Combust. and Flame, 2004, 136(3), 283–302.ignition (HCCI) combustion and emissions of 20 Lumsden, G., Sykes, R., and Eddleston, D. Com-n-heptane. Combust. Sci. Technol., 2005, 177(11), paring lean burn and ERG. SAE paper 970505, 1997.2113–2150. 21 Einewall, P., Tunestal, P., and Johansson, B. Lean

6 Rajashekhar Swamy, K., Harne, V, Gunjegaonkar, burn natural gas operation versus stoichiometricD. S., and Gopalkrishnan, K. V. Study and develop- operation with EGR and a three way catalyst. SAEment of lean-burn systems on small 4-stroke gaso- paper 2005-01-0250, 2005.line engine. SAE paper 2001-01-1801, 2001. 22 Ricardo Consulting Engineers. Analysis of steady

7 D’Errico, G. and Onorati, A. An integrated simu- flow tests on inlet and exhaust ports, Dp. 93/0704,1993.lation model for the prediction of GDI engine





23 Beretta, G. P., Rashidi, M., and Keck, J. C. COVi.m.e.p.

coefficient of variation of indicatedTurbulent flame propagation in spark-ignition mean effective pressureengines. Combust. and Flame, 1983, 52, 217–245. CR compression ratio

24 Olikara, C. and Borman, G. A computer program DFI double-pulse fuel injectionfor calculating properties of equilibrium combustionECU electrical control unitproducts with some applications to I.C. engines. SAEEGR exhaust gas recirculationpaper 750468, 1975.EVC exhaust valve closure25 Cox, R. A. and Cole, J. A. Chemical aspects of the

autoignition of hydrocarbon–air mixtures. Combust. EVO exhaust valve openand Flame, 1985, 60, 109–123. GDI gasoline direct injection

HC hydrocarbonHCCI homogeneous charge compression

ignitionAPPENDIXIC internal combustioni.m.e.p. indicated mean effective pressureNotationinj injection

ABDC after bottom dead centre ISFC indicated specific fuel consumptionATDC after top dead centre IVC inlet valve closureAFR air/fuel ratio IVO inlet valve openBBDC before bottom dead centre LIF laser induced fluorescenceb.m.e.p. brake mean effective pressure MBT minimum ignition advance for bestBSFC brake specific fuel consumption torqueBTDC before the top dead centre MFB mass fraction burntCA crank angle NO nitric oxideCAD crank angle degree NO

xnitrogen oxides

CFD computational fluid dynamics PFI port fuel injectionCO carbon monoxide SI spark ignitionCO

2carbon dioxide SIDI spark ignition direct injection

TDC top dead centreCOV coefficient of variation




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