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SAE TECHNICALPAPER SERIES 2007-01-0199

Study of SI-HCCI-SI Transition on a Port FuelInjection Engine Equipped with 4VVAS

Yan Zhang, Hui Xie, Nenghui Zhou, Tao Chen and Hua ZhaoState Key Laboratory of Engines, Tianjin University

Reprinted From: Homogeneous Charge Compression Ignition Engines, 2007(SP-2100)

2007 World CongressDetroit, MichiganApril 16-19, 2007

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2007-01-0199

Study of SI-HCCI-SI Transition on a Port Fuel Injection EngineEquipped with 4VVAS

Yan Zhang, Hui Xie 1, Nenghui Zhou, Tao Chen and Hua Zhao 2State Key Laboratory of Engines, Tianjin University

Copyright 2007 SAE International

1 Corresponding author.2 Visiting Professor at SKLE and Professor at Brunel University, UK.

ABSTRACT

A strategy to actualize the dual-mode (SI mode andHCCI mode) operation of gasoline engine wasinvestigated. The 4VVAS (4 variable valve actuatingsystem), capable of independently controlling the intakeand exhaust valve lifts and timings, was incorporatedinto a specially designed cylinder head for a singlecylinder research engine and a 4VVAS-HCCI gasolineengine test bench was established. The experimentalresearch was carried out to study the dynamic controlstrategies for transitions between HCCI and SI modeson the HCCI operating boundaries.

Results show that equipped with the 4VVAS cylinderhead, the engine can be operated in HCCI or SI mode tomeet the demands of different operating conditions.4VVAS enables the rapid and effective control over thein-cylinder residual gas, and therefore dynamic

transitions between HCCI and SI can be stably achieved.It is easier to achieve transition from HCCI to SI thanreversely due to the influence of thermo-inertia. Twomajor approaches can be applied to controlling theHCCI-SI transitions, which are the dynamicmanagement of residual gas and the design of hybridheat release rate curve.

INTRODUCTION

Homogeneous Charge Compression Ignition (HCCI) hasbeen recognized as a potential technology for futureautomotive engines, to observe the increasing strictemission regulations and to meet the demand of highfuel consumption efficiency.

The modern HCCI combustion engines date back to thelate 1970s, when HCCI combustion was first realized inthe conventional ported two-stroke gasoline engines [1,2]for improved part-load engine performance and lowerexhaust emissions. Since then a lot of research efforthas been made to understand the HCCI combustion

characteristics and develop corresponding controltechniques, on which this new concept thrives.

Although it is a focus of recent research to extend itsoperating range in terms of speed and load with HCCIcombustion, HCCI is still found limited in that in-cylinderdeflagration, resulted from extremely high burning rate,might occur at insufficiently diluted high loads. Besides,even though it is possible to run the whole driving cyclesin HCCI mode with certain extension, higher outputs aredemanded to enhance the drivability of the HCCIvehicles in all driving conditions. Therefore, the researchon rapid transitions between HCCI and SI mode on theboundary of the HCCI regime becomes a key to availingthe application of product HCCI engines in the future. Itis even more challenging to achieve the required level ofcontrol during transient engine operation when thecharge temperature has to be correctly matched to theoperating condition during rapid transients with a highrepeatability as the speed and load are changing.Transient responses for various means of HCCI controlsuch as variable compression ratio and variable valvetiming must be carefully examined to achieve seamlesstransition [3].

The exhaust gas recirculation(EGR), obtained bytrapping residual gases inside the cylinder is supposedto have the potential to enable fast and smooth transitionfrom SI to HCCI to SI, and to provide, in the certainrange, control for the HCCI combustion [4].

Based on the understanding of the above problems, acylinder head [5], with intake and exhaust valves that are

continuously variable in lifts and timings, was developedfrom the BMW mass produced engines. As a result,transitions between HCCI and SI, as well as extensionsof HCCI operating range, were achieved through thedynamic management of in-cylinder fresh charge andtrapped residuals.



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1 Engine

The engine employed in this research was a singlecylinder, 4 stroke research engine equipped with 4VVAsystem. The photograph of the engine is shown inFigure 1. The 4VVAS, a special cylinder head newlydesigned by SKLE, was mounted to the original cylinderand base of a Ricardo Hydra single cylinder researchengine. The engine profile and specifications are shownin Figure 1 and Table 1 respectively.

Figure 1 Single-cylinder research engine with the4VVA system

Table 1 Engine specifications

Bore Stroke 86mm 86mmSwept volume 0.5L

Compression ratio 10.66Combustion chamber Pent roof / 4 valves

DOHCValve control 4VVA systemFuel injection Port fueledFuel Standar d gasolin e

(93 RON)Equivalence air/fuel ratio StoichiometricIntake temperature 25Inlet pressure Naturally aspiratedThrottle WOT

2. 4VVA System

In the original BMW cam systems, only three controlvariables are available, including intake valve phasing,intake valve lift and exhaust valve phasing. As a result, itis difficult to properly control the required amount of hotresidual gas through negative valve overlap without theassistance of the exhaust valve lift regulation. Therefore,a new 4VVAS system has been developed, the conceptof which is presented in literature [6] and the majorcomponents are shown in Figure 2. 4VVAS involves 2sets of valve systems and each is composed of a Vanossystem and a Valvetronics system. 4VVAS is coupled tothe specially designed cylinder head. It features specialoil galleries and mounting points for both the VVT(Vanos) and VVL (Valvetronics) systems. As is shown inFigure 3, continuous variation in lift profiles can beobtained from a minimum of 0.3mm to a maximum of9.5mm and positions of peaks are variable by the rangeof 60CA close to the original phase.

Figure 2 Mass produced VVA parts

-300 -200 -100 0 100 200 3000

2

4

6

8

10

Inlet

L i f t ( m m

)

Crank Angle (deg)

Exhaust

Figure 3 Valve lift profiles of the 4VVA system

EXPERIMENTAL SET-UP



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3 Experimental system

Figure 4 demonstrates the experimental system, with thecontrol sub-system and the data acquisition sub-systemdeveloped by SKLE. A layered-pattern technique isadopted for engine management. The control sub-system is composed of an operating unit an EFI ECUand a 4VVAS ECU. The EFI ECU is mainly responsiblefor the control of fuel injection, ignition and A/F ratio, andthe 4VVAS ECU is employed for the control of intakevalve timing, intake valve lifting, exhaust valve timingand exhaust valve lifting according the command of theoperating unit. Besides, there were two PCs, onecarrying out transient combustion analysis and the otherfulfilling the 4VVAS monitoring and calibration. In bothHCCI and SI modes, the throttle is kept wide-openduring the experiments.

The engine is coupled directly to a 30 kW AC electricdynamometer. An ETAS linear oxygen sensor with anaccuracy of 1.5% was mounted in the engine exhaust

pipe to measure the global lambda. In-cylinder pressureis measured with a Kistler 6125B piezoelectrictransducer and a Kistler 5011B charge amplifier.Cylinder pressure is calculated through averaging thecylinder pressures of 100 consecutive cycles. Theamount of airflow is measured by a vortex flow meterwith an accuracy of 1%.

During each experiment, the coolant and oiltemperatures are strictly maintained at 801C and551C to eliminate their effects on HCCI/SI combustionin all the experiments. The injector, which was

manufactured by Delphi Corporation, had four split holesto achieve better atomization. The fuel used is ordinary93# gasoline in the market. And it is introduced by Port-Fuel-Injection at a constant pressure differential by 2.9bars.

RESULTS AND DISCUSSION

TRANSITIONS BETWEEN HCCI MODE AND SI MODE

Quite in contrast to practical vehicle engines, currentHCCI engine is characterized by a narrow operatingrange. At the HCCI operating boundaries where eitherunstable combustion or deflagration occurs and also inthe regions where HCCI can not be achieved, it isnecessary to switch the operation into the conventionalSI mode for successive and steady power output. Sinceit is only through fulfilling all the required transition pointsthat an engine is able to run in all operating conditions,this paper attempts to investigate in details the transitionmethods at the representative points on the HCCI

operating boundaries.

3 operating points, as shown in Figure 5, are carefullyselected to investigate the inter-mode transitions:

1) n = 1000 rpm and IMEP net = 2.3 bar ( located onthe low-speed-low-load boundary )

2) n = 1500 rpm and IMEP net = 3.0 bar ( located onthe low/medium-speed-high-load boundary )

Figure 4 Schematic of the test setup



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3) n = 3000 rpm and IMEP net = 2.0 bar ( located onthe high-speed-high-load boundary )

1000 2000 3000 40001.0

1.5

2.0

2.5

3.0

3.5

SI

HCCI

Figure 5 Test points selected for mode transitions

HCCI SI MODE TRANSITION

Transition from HCCI to SI at test point 1 is presented inFigure 6. Data are obtained from 100 successive enginecycles and transition occurs between cycle 26 and cycle33. It is found that IMEP remains unchanged before andafter the transition takes place, with slight fluctuationsduring the transition process. With WOT maintained,ISFC still rises a little after the operation shifts into SImode, due to the increase in pumping loss which isrepresented by the Pumping Mean Effective Pressure(PMEP) reduction in the figure.

Variations in in-cylinder pressure from cycle 26 to cycle33 and the corresponding variations in the MassFraction Burned (MFB) of the combustion process arealso shown in Figure 6. It is noticed that, with 4VVAScontrolling the valve parameters, the amount of in-cylinder residuals gradually decreases (indicated by thereduced peak pressure around TDC in the gasexchange process) and step-by-step transition into SImode takes place (when the first derivative is reduced),no misfire within the entire process.

0 20 40 60 80 100-1

0

1

2

3

4

5

250

300

350

400

450

500

PMEP ISFC

Lambda

I S F C ( g / k w

h )

P M E P ( b a r ) ,

L a m

b d a ,

I M E P ( b a r )

Cycle

IMEP

HCCI SI

cycle 26~33

-100 0 100 200 300 4000

5

10

15

20

25

30

35

-40 -20 0 20 40 60

0

20

40

60

80

100

30

M F B ( % )

Crank Angle (deg ATDC)

2629

27

28

P r e s s u r e

( b a r )


Cycle 26~33

26

Figure 6 HCCI-SI mode transitions at 1000rpm IMEP2.3bar

HCCI-SI transitions at the second and the third testpoints, which show similar trends to that at test point 1,are shown in Figure 7 and Figure 8 respectively.However, at relatively high speeds, more cycles aredemanded for the transitions and increasing numbers ofdeteriorated cycles are observed, which can beexplained in the following two aspects:

(1) The initial conditions (e.g. residual gas temperature)and the boundary conditions (e.g. cylinder walltemperature) of the combustion vary as transitionproceeds. It appears that the transition process islonger at high speeds when time is measured inengine cycles. Since SI is comparatively insensitive

to initial and boundary conditions, suchphenomenon is not obviously observed. However,in the following section dealing with SI-HCCItransitions, it is quite notable that the transitionprocess occupies more engine cycles as enginespeed increases.

(2) HCCI combustion is different from SI combustion,especially in that it requires a RGF of approximately40%~80% to heat the fresh charge and realizeauto-ignition, while the RGF for conventional SI



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combustion should not be over 15%, or it will lead to misfire. Thus there is a RGF gap between the two

0 20 40 60 80 100-1

0

1

2

3

4

5

150

200

250

300

350

400

450

500

PMEPISFC

Lambda I S F C ( g / k w

h )

P M E P ( b a r ) ,

L a m

b d a , I

M E P ( b a r )

Cycle

IMEP

HCCISI

cycle 47~51

-100 0 100 200 300 4000

5

10

15

20

25

30

35

40

-40 -20 0 20 40 600

20

40

60

80

100

50 514847

HCCI M F B ( % )


49

SI

P r e s s u r e

( b a r

)


Cycle 47~51

49

Figure 10 SI-HCCI transitions at 3000rpm IMEP2.0bar

0 20 40 60 80 100-1

0

1

2

3

4

5

240

260

280

300

320

340

360

380

400

PMEP

ISFC

Lambda

I S F C ( g / k w

h )

P M E P ( b a r ) ,

L a m

b d a , I

M E P ( b a r )

Cycle

IMEP

HCCISI

cycle 16~21

-100 0 100 200 300 4000

5

10

15

20

25

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35

40

-20 0 20 40 600

20

40

60

80

100

HCCI M F B ( % )


21

SI

P r e s s u r e

( b a r )


Cycle 19~25

21

Figure 9 SI-HCCI transitions at 1500rpm IMEP3.0bar

Figure 7 HCCI-SI mode transitions at 1500rpmIMEP 3.0bar

0 20 40 60 80 100-1

0

1

2

3

4

5

240

260

280300

320

340

360

380

400

PMEPISFC

Lambda

I S F C ( g / k w

h )

P M E P ( b a r ) ,

L a m

b d a ,

I M E P ( b a r )

Cycle

IMEP

HCCI SI

cycle 19~24

-100 0 100 200 300 4000

5

1015

20

25

30

35

40

-40 -20 0 20 40 60

0

20

40

60

80

100

M F B ( % )


1920

22

23

24

21

P r e s s u r e

( b a r )


Cycle 19~24

19

0 20 40 60 80 100-1

0

1

2

3

4

5

250

300

350

400

450

500

PMEPISFC

Lambda

I S F C ( g / k w

h )

P M E P ( b a r ) ,

L a m

b d a ,

I M E P ( b a r )

Cycle

IMEP

HCCI SI

cycle 27~40

-100 0 100 200 300 4000

5

10

15

20

25

30

-20 0 20 40 600

20

40

60

80

100

M F B ( % )


2927

3331

P r e s s u r e

( b a r )


Cycle 27~40

27

Figure 8 HCCI-SI mode transitions at3000rpm IMEP 2.0bar



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combustion modes. Probably, the RGF falls into thegap as it is decreased to obtain transition fromHCCI to SI, resulting in deteriorated combustion oreven misfire in certain cycles. In order to achievesmoother transitions, attempts should be made toavoid the RGF gap; otherwise, the spark energyshould be increased in these cycles to offer betterignition and combustion assistance.

SI HCCI MODE TRANSITION

SI-HCCI transitions at test point 2 and 3 are shown inFigure 9 and 10 respectively, and it is found thatespecially at high engine speeds, it might take severalHCCI cycles, sometimes even tens of HCCI cycles, toachieve stable IMEP after the mode transition. Also athigh speeds, judging from the different MFB curves inFigure 10, advanced ignition and deflagration occur inthe first cycle of HCCI right after the transition from SI.The differences between the combustion characteristicsof SI and HCCI account for these phenomena. In SImode, the combustion of the current cycle is relativelyindependent of the conditions of the preceding cycle.However, in HCCI mode, combustion relies on thetrapped residuals as means to enable auto-ignition, andthus there is significant dependence of the current cyclebehavior on the preceding one. In the first HCCI cycleafter the transition, high temperature remains on thecylinder wall and in the combustion chamber, leading toearly auto-ignition. The HCCI ignition timing is furtheradvanced by the hot trapped residuals of the precedingSI cycle. In order to hold back the ignition timing, it isnecessary to precisely control the fuel-injection andspark timing and one potential method is to provideultra-lean mixture (e.g. to significantly reduce theamount of fuel injected [7]) in this cycle. And results of

this method will be presented in future papers.

HYBRID HEAT RELEASE CONTROL DURING MODETRANSITIONS

Generally, cycles number for transitions exist as long asthe mode transition is not done within a short period (i.e.one cycle), whether from SI to HCCI or reversely. Duringthese cycles, the MFB curve, shown in Figure 11, istypical of a smaller slope in the front part (from S to T)and a lager slope (from T to H) that follows, with theinflexion T, which is corresponding to the combustionprocess. In fact, each cycle during the mode transition iscomposed of both HCCI and SI combustions. At the verybeginning, SI combustion occurs with a low heat releaserate, resulting in the small slope between S and T. ThenHCCI combustion takes place when auto-ignitionconditions are achieved through the compressive effectof the expanding burn mixture, leading to the large slopebetween T and H. As SI combustion dwindles (line STshortens) and HCCI combustion increases (line THextends) in these cycles, the SI-HCCI transition, namelythe movement of T from S to H, is achieved. Similarly,the HCCI-SI transition can be regarded as the processwhen T moves from H to S in the MFB curve.

-20 0 20 40 60

0

20

40

60

80

100

HCCI combustion(auto-ignition at multi points)

Inflexion(dividing SI and HCCI combustion,representing the progress of the mode transition process)

SI combustion(flame propagation)

H

T

M F B ( % )


S

Figure 11 MFB in transition process

Thus it can be seen that it plays an important part in thecombustion mode transition processes to control theposition of T in the MFB curve, and here two relatedparameters are studied, which are spark timing and RGFrespectively.

1) EFFECTS OF SPARK TIMING ON T POSITON

As is shown in Figure 12, T position retards as sparktiming delays until the latter reaches 10CA BTDC, afterwhich T position coincides with that in compressionignition without the assistance of spark plug and theinfluence of spark timing can be neglected. Therefore, itcan be found out that spark ignition functions only beforeHCCI combustion occurs.

-15 -10 -5 0 5 10 15 20

0.0

0.2

0.4

0.6

0.8

1.0

M F B ( % )


Figure 12 Effects of Spark timing on T position

2) EFFECTS OF RGF ON T POSITON

MFB curves of several continuous cycles duringmode transition processes are shown in Figure 13.In Figure 13a, the influence of relatively low RGFon MFB is illustrated. In such cases, combustion isstill dominated by spark ignition and flamepropagation, and as RGF increases within this



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region, T moves downwards. However, when RGF is high enough to fall into the RGF gap,

-20 0 20 40 60

0.0

0.2

0.4

0.6

0.8

1.0

21%17%12%

8%

M F B ( % )

RGF = 4%

-20 0 20 40 60

0.0

0.2

0.4

0.6

0.8

1.0

40%

38%36%34%32%30%

M F B ( % )

RGF=26%

Figure 13 Effects of RGF on T position

0 1 2 3 4 5 6 7 8 9 100

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50

0

5

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-2

0

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4

-5051015202530

p r e s s u r e

( b a r )

H R R ( J / C A D )

Hybrid Mode

V a

l v e

L i f t ( m m )SIHCCI

I M E P ( b a r )

C A 5 0

Figure 14 HCCI to SI mode transition process

0 1 2 3 4 5 6 7 8

0

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0

5

10

0 1 2 3 4 5 6 7 80

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-2

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0481216202428

p r e s s u r e

( b a r )

H R R ( J / C A D )

Hybrid Mode

V a

l v e

l i f t ( m m

)SI HCCI

I M E P ( b a r )

C A 5 0

Figure 15 SI to HCCI mode transition process



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combustion extremely deteriorates with misfirecycles. If RGF further increases, as shown inFigure 13b, HCCI combustion gradually becomesdominant, and with higher and higher RGF, Tkeeps on descending until it reaches point S,when the SI-HCCI transition is completed andstable HCCI is achieved.

In general, two methods are available to control themode transition processes: dynamically manage theresidual gas and designing a hybrid heat release model.The model set-up of HCCI combustion is presented inliterature [8] and modeling of the mode transitionprocess is in progress. Dynamic control based on thisnew model is definitely a promising technique to achieveseamless mode transition.

Examples of successful HCCI to SI and SI to HCCItransitions are shown in Figure 14 and 15 respectively.IMEP and its fluctuation ( IMEP ) are employed toindicate the stability of the transition process. It is notedthat steady transition can be achieved when combustion

is in hybrid mode. Experiment results show that fromHCCI to SI mode transition, IMEP is less than 0.2bar,and, from SI to HCCI mode transition, IMEP is less than0.3bar.

CONCLUSIONS

Experiments were carried out on a 4VVAS-HCCI testbench to investigate the transitions between HCCI andSI in detail. Conclusions are drawn as follows:

1. With 4VVAS implemented on the engine cylinderhead, mode transitions between SI and HCCI

have been achieved at the HCCI operatingboundaries through the rapid and dynamicalmanagement of RGF.

2. A RGF gap exists, within which combustiondeteriorates and misfire occasionally occurs.Smooth transition can be obtained when this gapis avoided.

3. Transition from SI to HCCI is harder to achievethan that from HCCI to SI. This is because onlyRGF management is required in the lattersituation while in the former, the influence of

thermo-inertia should also be considered.

4. The control of mode transition processes hasbeen realized through either the dynamicmanagement of RGF or the design of hybrid heatrelease rate curve.

RECOMMENDATIONS FOR FUTURE WORK

At the time of writing, researches on in-cylindercombustion sensing method (e.g. the ion-currentsensing-based feedback closed loop control technology)

to judge transition area between SI and HCCI andadaptive algorithm in mode transition process areunderway in SKLE. According to the current resultsobtained from the research on gasoline HCCIcombustion in SKLE, series of problems have beensolved on the whole, including the control of ignition andcombustion phasing, the expansion of HCCI operatingrange and HCCI-SI mode transition. Therefore, atechnology of combustion feedback is demanded for

multi-cylinder HCCI vehicle engine. Once thistechnology is developed, the application of HCCI engineon vehicles can be expected soon.

ACKNOWLEDGMENTS

The study is a part of the State Key Project ofFundamental Research Plan (Grant Number:2001CB209204) supported by Ministry of Science andTechnology of China and National Science Fund Project(Grant Number: 50476064) supported by NationalScience Fund Committee of China.

REFERENCES1. Onishi S., Hong Jo S., Shoda K., et al, Active

Thermo-Atmosphere Combustion (ATAC)A NewCombustion Process for Internal CombustionEngines, SAE Paper 790501.

2. Noguchi M., Tanaka Y., Tanaka T., et al, A Studyon Gasoline Engine Combustion by Observation ofIntermediate Reactive Products during Combustion,SAE Paper 790840.

3. Zhao, F., Asmus, T.W., Assanis, D.N., Dec, J.E.,Eng, J.A., Najt, P.M., Homogeneous ChargeCompression Ignition (HCCI) Engines, Key

Research and Development Issues, SAEPublication PT-94, Soc. of Automotive Engineers,2003.

4. Koopmans L., Strm H., Lundgren S., Backlund O.et al, Demonstrating a SI-HCCI-SI Mode Change ona Volvo 5-Cylinder Electronic Valve Control Engine,SAE Paper 2003-01-0753.

5. Xie H., Hu S., Zhang Y., et al, A Highly EconomicalUltra-low-emissions-producing Hybrid-mode HCCIEngine Equipped with 4VVAS, Chinese Patent No:200610013415.8.

6. Xie H., Hou S., Qin J., Zhang Y., et al, ControlStrategies for Steady and Transient Operation of a4-Stroke Gasoline Engine with CAI Combustionusing a 4-Variable Valve Actuating System (4VVAS),SAE Paper 2006-01-1083.

7. Santoso H., Matthews J., Cheng W., ManagingSI/HCCI Dual-Mode Engine Operation, SAE Paper2005-01-0162.

8. Qin J., Xie H., Zhang Y., et al, A Combustion HeatRelease Correlation for CAI Combustion Simulationin 4-Stroke Gasoline Engines, SAE Paper 2005-01-0183.



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Dr. Hui XieState Key Laboratory of Engine,Tianjin University,P.R. ChinaEmail: [email protected]

ABBREVIATIONS

4VVAS 4 Variable Valve Actuating System

AC Alternating Current

ATDC After Top Dead Centre

BTDC Before Top Dead Centre

CA Crank Angle

DOHC Double Over Head Camshaft

EFI Electronic Fuel Injection

EGR Exhaust Gas Recirculation

EVP Exhaust Valve Phasing

HCCI Homogeneous Charge Compression Ignition

HRR Heat Release Rate

IMEP Indicated Mean Effective Pressure

ISFC Indicated Specific Fuel Consumption

IVP Inlet Valve Phasing

MFB Mass Fraction Burned

PMEP Pumping Mean Effective Pressure

RGF Residual Gas Fraction

SI Spark Ignition

TDC Top Dead Centre

VVT Variable Valve Timing

VVL Variable Valve Lift

WOT Wide Open Throttle

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2007-01-0199