A study on backfire control and performance improvement by changing the valve timings in a hydrogen fueled engine with external injection

A Study on Backfire Control and Performance Improvement by Changing the Valve Timings in a Hydrogen-Fueled Engine with External Injection Huynh Thanh Cong The Graduate School Sungkyunkw

A Study on Backfire Control and Performance Improvement by Changing the Valve Timings in a Hydrogen-Fueled Engine with External Injection

Huynh Thanh Cong

The Graduate School Sungkyunkwan University Department of Mechanical Engineering

A Study on Backfire Control and Performance

Improvement by Changing the Valve Timings in a Hydrogen-Fueled Engine with External Injection

Huynh Thanh Cong

A Dissertation Submitted to Department of Mechanical Engineering

and the Graduate School of Sungkyunkwan University

in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Mechanical Engineering)

January 2009

Approved by Professor Lee, Jong Tai Major Advisor

This certifies that the Dissertation for the degree of Doctor of Philosophy in Engineering of Huynh Thanh Cong is approved

December 2008

박사학위 청구논문

지도교수 이 종 태

흡기관분사식 수소기관에서 밸브 타이밍 변화에 의한 역화억제와 성능개선에 연구

성균관대학교 대학원

기계공학과 동력공학전공

현 탄 콩

박사학위 청구논문

지도교수 이 종 태

흡기관분사식 수소기관에서 밸브 타이밍 변화에 의한 역화억제와 성능개선에 연구

A Study on Backfire Control and Performance Improvement by Changing the Valve Timings in a Hydrogen-Fueled Engine with External Injection

성균관대학교 대학원

기계공학과 동력공학전공

현 탄 콩

박사학위 청구논문

지도교수 이 종 태

흡기관분사식 수소기관에서 밸브 타이밍 변화에 의한 역화억제와 성능개선에 연구

A Study on Backfire Control and Performance Improvement by Changing the Valve Timings in a Hydrogen-Fueled Engine with External Injection

이 논문을 공학박사학위 청구논문으로 제출합니다

2008 년 10 월 일

성균관대학교 대학원

기계공학과 동력공학전공

현 탄 콩

ACKNOWLEDGEMENTS

I express my gratitude to my advisor, Professor Jong Tai Lee, for his guidance, support, and encouragement during the period of this research at the Department of Mechanical Engineering, Sungkyunkwan University I am thankful to Professors Chul

Ju Kim (thesis committee chairman), Nak Won Sung, Hong Sun Ryou, and Kyu Hoon Choi for their advice and participation on the dissertation committee I would like also

to thank Dr Ki Chol Noh, Joon Kyoung Kang, and Kwang Ju Lee (Ph.D candidate) for their help in the various stages of the investigation I would like to express my heartfelt thanks to the technical staffs, whom are Mr Je Ha Lee and Mr In Seak Park, of Internal Combustion Engine Laboratory, Sungkyunkwan University, for full assistance with experimental apparatus setup and development of H2 research engine

I wish also to express my gratitude to my parents and my wife for their unconditional and unwavering support and encouragement during the course of this work

Huynh Thanh Cong Sungkyunkwan University

CONTENTS

List of Tables and Figures i

Nomenclature vi

Chapter 1 Introduction 1

1.1 Research background 1

1.1.1 Why backfire control in H2 engine? 1

1.1.2 Combustive properties of hydrogen related to backfire occurrence 3

1.2 Status of H2 researches on backfire control and enhancement of performance 6 1.2.1 Previous researches on backfire control in H2 engines 6

1.2.2 VVT studies for performance improvement and NOx reduction 9

1.3 Purpopes and objectives of research 10

1.4 Methods and contents of research 11

Chapter 2 Development of H2 research engine with MCVVT system 13

2.1 Introduction 13

2.2 Development of MCVVT system 13

2.2.1 Designing concept 13

2.2.2 Mechanical structure of MCVVT system 15

2.2.3 Characteristics of MCVVT system 18

2.3 H2 research engine with MCVVT system 19

2.4 Experimental setup 30

2.5 Experimental method 31

Chapter 3 Possibility of backfire control by changing valve overlap period 35

3.1 Introduction 35

3.2 Backfire occurrence with the change of valve overlap period 35

3.3 Backfire prevention due to the decrease of backflow period 43

3.4 Main prevention factor of backfire by the VOP 48

3.4.1 BFL equivalence ratios with change of the VOP center 48

3.4.2 IVO timing as the main prevention factor of backfire 54

Chapter 4 Method of obtaining both high power and high efficiency without backfire 59

4.1 Introduction 59

4.2 Engine performance with the change of valve overlap period 60

4.2.1 Valve timings for optimizing engine power and efficiency 60

4.2.2 Difference of engine performance with the change of valve timing for H2 and gasoline engines .72

4.3 Realization of high performance by using lean mixture and supercharging 77

4.3.1 Concept to obtain both high power and high efficiency 77

4.3.2 Optimization of engine performance in cases of supercharging and lean mixture 81

Chapter 5 Conclusions 86

Appendix A .89

List of publications 96

Bibliography 98

Abstract .104

List of Tables and Figures

Table 1-1 Comparative physical properties of H2, CNG, and gasoline 4 Table 2-1 Specifications of MCVVT H2 engine with external injection 25 Table 2-2 Specifications of base valve timings 25 Table 2-3 Test matrix #1 for change of the VOP 33 Table 2-3 Test matrix #2 for change of the VOP center 33 Table 2-5 Test matrix #3 for change of the valve timings 33 Table A-1 Specifications of fuel mass flow meters/controllers 89 Table A-2 Specifications of injection controller 91 Table A-3 Specifications of automotive emission gas analyzer 93

Fig 2-1 Designing concept of MCVVT system 14 Fig 2-2 Schematic diagram of MCVVT system 16 Fig 2-3 Main components of MCVVT system 17

Fig 2-5 Relation between the phasing angle of camshafts and displacement 20 Fig 2-6 Variation of engine power as a function of engine speed 20 Fig 2-7 Drawing of H2 research engine with MCVVT system 21 Fig 2-8 Photo of H2 research engine with MCVVT system 21 Fig 2-9 Drawing of developed cylinder block 22 Fig 2-10 Photo of developed cylinder block 22

Fig 2-11 Drawing of Ricardo type crankcase 23 Fig 2-12 Photo of Ricardo type crankcase 23 Fig 2-13 Schematic diagram of spark ignition system 26 Fig 2-14 Block diagram of engine oil and water supply system 26 Fig 2-15 Block diagram of water supply system 27 Fig 2-16 Block diagram of engine oil supply system 27

Fig 2-19 Drawing of exhaust intake system 29 Fig 2-20 Schematic diagram of experimental setup 30 Fig 2-21 Schematic of shifting phases of six valve overlap periods 34 Fig 2-22 Schematic of shifting phases of IVO and EVC timings 34 Fig 3-1 Curves of cylinder pressure and inlet pressure once backfire occurs 39 Fig 3-2 BFL equivalence ratio as a function of engine speed 39 Fig 3-3 BFL equivalence ratio as a function of valve overlap period 40 Fig 3-4 Brake torque as a function of valve overlap period 40 Fig 3-5 Brake thermal efficiency as a function of valve overlap period 41 Fig 3-6 Maximum gas temperature as a function of valve overlap period 41 Fig 3-7 Variation of air mass flow rate with change of valve overlap period 42 Fig 3-8 Variation of air mass flow rate for unequal supplied energy and

equal supplied energy with change of valve overlap period

45

Fig 3-9 BFL equivalence ratio as a function of VOP for the case where the

supplied energy is equal to the case of a VOP 30CA

46

Fig 3-10 Combustion duration as a function of VOP for supplied energy

equal to the VOP 30CA

46

Fig 3-11 Total heat release as a function of VOP for four equivalence ratios

for supplied energy equal to the VOP 30CA

47

Fig 3-12 Schematic diagram of valve overlap period center 47 Fig 3-13 Variation of BFL equivalence ratio with the VOP center 52 Fig 3-14 Variation of air mass flow rate with the VOP center 52 Fig 3-15 In-cylinder pressures versus crank angle for six VOP center 53 Fig 3-16 Change of COVimep with the VOP center 53 Fig 3-17 Combustion duration with change of the VOP center 54 Fig 3-18 BFL equivalence ratio as a function of IVO timings 57 Fig 3-19 BFL equivalence ratios as a function of EVC timings 58 Fig 3-20 Torque and efficiency as a function of the IVO timings at 0.5 and

1.0

58

Fig 4-1 In-cylinder pressures versus crank angle for each VOP 62 Fig 4-2 COV in IMEP with change of the VOP 63 Fig 4-3 Curves for COVimep as functions of fuel-air equivalence ratio 63 Fig 4-4 Torque and efficiency as a function of fuel-air equivalence ratio for

three IVO timings

67

Fig 4-5 Variation of NOx emissions with change of fuel-air equivalence

ratio for three IVO timings

67

Fig 4-6 Variation of NOx emissions and the rate of air mass flow rate with

the change of EVC timing

68

Fig 4-7 Influence of spark timings on NOx reduction for H2 and gasoline 68

Fig 4-8 Torque / max torque and efficiency / max efficiency as a function

of valve overlap period for various loads

71

Fig 4-9 Air mass flow rate and COVimep as a function of VOP 71 Fig 4-10 Rapid burning and combustion duration as a function of the VOP 72 Fig 4-11 Air mass flow rate with change of the VOP for H2 and gasoline

Fig 4-15 Brake torque with increase of fuel-air equivalence ratio for

naturally aspirated cases

79

Fig 4-16 Brake torque with increase of fuel-air equivalence ratio for equal

energy compensation to a VOP 30CA

80

Fig 4-17 Brake thermal efficiency as a function of fuel-air equivalence ratio

for naturally aspirated cases

80

Fig 4-18 Brake thermal efficiency as a function of fuel-air equivalence ratio

for equal energy compensation to a VOP 30CA

81

Fig 4-19 Air mass flow rate as a function of EVC timing for naturally

aspirated and supercharging cases

84

Fig 4-20 Brake torque as a function of EVC timing for naturally aspirated

and supercharging cases

84

Fig 4-21 Thermal efficiency as a function of EVC timing for naturally

aspirated and supercharging cases

85

Fig 4-22 NOx emissions as a function of EVC timing for naturally aspirated

and supercharging cases

Nomenclature

Alphabetic symbol

bTDC before Top Dead Center

aTDC after Top Dead Center

bBDC before Bottom Dead Center

aBDC after Bottom Dead Center

BMEP [bar] Brake mean effective pressure

BFL Backfire limit

CFR Cooperative fuel research

CI Compression ignition

CNG Compressed natural gas

COVimep [%] Coefficient of cycle variation of indicated mean effective pressure

DOHC Double over head cam

EGR Exhaust gas recirculation

EVO Exhaust valve opening

EVC Exhaust valve closing

ICE Internal combustion engine

IVO Intake valve opening

IVC Intake valve closing

NTP Normal temperature and pressure (300K and 1 atm)

PFI Port fuel injection

HC [ppm] Hydrocarbon

LHV [MJ/kg] Lower heating value

IMEP [bar] Indicated mean effective pressure

MBT [bTDC] Minimum spark advance for the best torque

MCVVT Mechanical continuous variable valve timing

NOx [ppm] Nitrogen oxides

rpm Engine revolution per minute

SA [bTDC] Spark advance

SI Spark ignition

TDC Top dead center

T Adiabatic flame temperature

VOP Valve overlap period

VVT Variable valve timing

WOT Wide-open throttle

Greek symbol

 Correlated tangent factor

 Fuel-air equivalence ratio

 1/ Air-fuel equivalence ratio

k Specific heat ratio

 Compression ratio

Chapter 1 Introduction

1.1 Research background

1.1.1 Why backfire control in H2 engine?

Hydrogen-fueled engines may be divided according to the fuel supply method [1-4] into: (1) external mixture formation (e.g using carburetion or port or manifold injection) and (2) internal mixture formation (e.g direct-cylinder fuel injection) The former has the advantages of a hydrogen engine with external mixture formation include potentially high thermal efficiency because of a more homogeneous inlet mixture, simplicity, and a lower-pressure fuel injection system However, the main problems of abnormal combustion that have limited in the development of operational hydrogen engine are backfire, pre-ignition, and knock The latter is not typically subjected to backfire occurrence and has the potential for the highest power Problems with a direct-cylinder fuel injection hydrogen engine, however, include a decrease of the thermal efficiency due to non-homogeneity of the inlet mixture and the development of the high-pressure fuel injection system with reliability and durability

Many attempts have been made to eliminate or minimize the backfire problem by means of the fuel delivery system Das and co-workers [5-6] introduced the timed manifold injection (TMI) that can overcome the problem of backfire in a hydrogen engine Swain and co-workers [7] demonstrated that inlet injection techniques can be used to suppress backfire problems

In addition, the combustion characteristics of hydrogen (such as wide flammability limits, fast burning velocities, and low ignition energy) enable stable engine operation, which results in high thermal efficiencies and low NOx emission levels As mentioned above, hydrogen engine with external injection has high thermal efficiency For a hydrogen engine with external injection, however, backfire under higher load conditions may be a serious problem If backfire can be eliminated, external mixture type hydrogen-fueled engine with high efficiency and high power will become to realize in a near-term possibility

The backfire phenomena of H2-air mixtures in the intake system are often a result of backflow during the valve overlap period (VOP) During this overlap period, the H2-air mixture may be pre-ignited due to an ignition source burning slowly in the combustion chamber The cause of this ignition (resulted by slow burning flame) is able to propagate backward into the intake system and causes the well-known backfire Many researchers [8-11] have tried to prevent this backfire by using a number of methods These methods have included: (1) a decrease of the ignition source¡s temperature, (2) a decrease of the burning velocity, (3) lean burn techniques, and (4) a reduction of crevice volume and elimination of abnormal discharges

In general, these methods have not been perfectly successful for preventing backfire Most of these previous works has described the difficulty of controlling the unknown ignition source and the rapid burning velocity

The causes of backfire may be divided into an unknown ignition source, fast combustion velocity and VOP When the VOP is short enough, backfire may not occur even under high load operating conditions due to the fact that the pre-ignited flame cannot flow backward into the inlet system Consequently, backfire may be controlled with a decrease of the VOP, but this has not been clearly demonstrated yet In addition, the trade-offs with respect to the engine performance are not known and need to be documented for reduced overlap periods

The maximum power of hydrogen-fueled engine is determined as a function of volumetric efficiency, fuel energy density, and backfire For most practical applications, the latter effect has been shown to be the limiting factor that determines maximum power output Premixed (or external mixture) type hydrogen-fueled engines inherently suffer from a loss in volumetric efficiency due to the displacement of intake air by the large volume of hydrogen in the intake mixture For example, a stoichiometric mixture of hydrogen and air consists of approximately 30% hydrogen by volume, whereas a stoichiometric mixture of fully vaporized gasoline and air consists of approximately 2% gasoline by volume The corresponding power density loss is partially offset by the higher energy density of hydrogen The stoichiometric heat of combustion per standard kg of air is 3.37MJ and 2.83MJ, for hydrogen and gasoline, respectively It follows that the maximum power density of a pre-mixed or port fuel injection

type hydrogen-fueled engine, relative to the power density of the identical engine operated on gasoline, is approximately 83% [12] For applications where peak power output is limited by backfire, the power densities of hydrogen-fueled engine with external mixture formation, relative to gasoline operation, can be significantly below 83% Furuhama et al [12] and Tang

et al [13] reported backfire-limited power densities of 72% and 50%, respectively, relative to operation with gasoline

Therefore, this thesis has motivated (1) to study the feasibility of preventing backfire occurrence and (2) to realize the method of enhancing both engine power and efficiency without backfire in a hydrogen-fueled engine with external injection

1.1.2 Combustive properties of hydrogen related to backfire occurrence

The general characteristics of hydrogen that is used as a fuel in the internal combustion engines have significant difference compared to the conventional fuels Table 1-1 shows a comparison of the most important properties of the various types of fuel

All temperature and pressure dependent data is given at normal conditions (NTP, 250C and

1 atm) Other chemical and physical properties have been found at [14-16] The different combustion characteristics of hydrogen are wide range of flammability, high auto-ignition temperature, and high flame velocity at stoichiometric ratios, high diffusivity, low ignition energy, small quenching distance, and very low density

Hydrogen has a wide range of flammability in comparison with other fuels Hydrogen engine, therefore, can be operated more effectively on excessively lean mixtures than gasoline engine As little as 4% hydrogen by volume with air produces a combustible mixture Generally, fuel economy is greater and the combustion reaction is more complete when an engine is run on

a lean mixture Additionally, the final combustion temperature is remarkably lowered in lean mixture, reducing the amount of pollutants, such as nitrogen oxides, emitted in the exhaust

However, there is a limit to how lean the engine can be run, as lean operation can significantly

reduce the power output due to a reduction in the volumetric heating value of the air-fuel mixture

Moreover, the auto-ignition temperature of hydrogen is relatively high and is over the values for CNG and gasoline This makes hydrogen more particularly suited for SI engine operation than that of CI engine (or Diesel configuration) operation The high auto-ignition temperature of hydrogen allows higher compression ratio in a hydrogen engine than in other hydrocarbon-fueled engine, allows increasing the engine thermal efficiency

A significant merit of hydrogen-fueled engine is fast flame velocity at stoichiometric ratios Under these conditions, the flame velocity of hydrogen is nearly 3 times faster than that of gasoline This means that hydrogen engines can more closely approach the thermodynamically ideal engine cycle (constant volume cycle) At leaner mixtures, however, the combustion duration of hydrogen-air mixture increases significantly with the decrease of the flame velocity Furthermore, hydrogen has ability of very high diffusivity that makes it easy to disperse in air This is considerably greater than gasoline and is advantageous for two main reasons: (1) it

Table 1-1 Comparative physical properties of H2, CNG, and Gasoline

Auto-ignition temperature in air (K) 858 813 ~500-750

Stoichiometric A-F ratio (mass) 34.3:1 17.2:1 14.7:1

Stoichiometric A-F ratio (vol.) 2.38:1 9.55:1 59.5:1

Mass diffusivity in air (cm2/s) 0.61 0.16c 0.05

facilitates the formation of a homogeneous mixture of fuel and air; (2) The unsafe conditions can either be avoided or minimized as a hydrogen leak develops, and the hydrogen disperses rapidly

In addition to the above significant advantages, however, hydrogen as a fuel also shows the important demerits that affect to the combustion and overall performance characteristics of hydrogen-fueled engine such as:

(1) As shown in Table 1-1, the amount of energy needed to ignite hydrogen is about one order of magnitude less than that required for gasoline This enables hydrogen engines

to ignite lean mixtures and ensures prompt ignition Unfortunately, the low ignition energy means that hot gases and hot spots on the cylinder can serve as sources of ignition, creating problems of premature ignition (pre-ignition) and backfire (flashback) Preventing this is one of the challenges associated with running an engine on hydrogen Beside, the wide flammability range of hydrogen means that almost any mixture can be ignited by a hot spot

(2) Hydrogen has a smaller quenching distance than gasoline Consequently, hydrogen flames travel closer to the cylinder wall than other fuels before they extinguish The smaller quenching distance can also increase the tendency for backfire since the flame from a hydrogen-air mixture (in crevice volumes) more readily passes a nearly closed intake valve, than a hydrocarbon-air flame

(3) Hydrogen has relative high auto-ignition temperature in air and high adiabatic flame temperature of hydrogen that may result in an increase of hot spots in the combustion chamber These can be the potential ignition sources for the occurrence of backfire (4) Beside the high auto-ignition temperature, the ignition energies of hydrogen-air mixtures are lower than that of hydrocarbon-air mixtures The low ignition energies of hydrogen-air mixtures mean that hydrogen-fueled engine with external mixture formation are predisposed towards the limiting effects of backfire and in general, results from surface ignition at engine hot spots, such as spark electrodes, valves or engine deposits

(5) The burning velocity of hydrogen is about 3 times higher in comparison with CNG and gasoline This may lead to the backflow of pre-ignited fast flame from the cylinder to intake manifold and causes the occurrence of backfire during extended valve overlap period

(6) Hydrogen has extremely low density that results in two problems: (a) a very large volume is necessary to store enough hydrogen to give a vehicle an adequate driving range, (b) the energy density of a hydrogen-air mixture, and hence the power output, is reduced

1.2 Status of H2 researches on backfire control and enhancement of performance

1.2.1 Previous researches on backfire control in H2 engines

The control of backfire occurrence in hydrogen-fueled engines has proven to be quite a challenge Many researchers have studied to take the counter-measures to avoid this abnormal combustion in order to have important implications for engine design, mixture formation and load control For SI engines, three common regimes of abnormal combustion include: (1) knock that is an auto-ignition of the end gas region, (2) pre-ignition which is known as an uncontrolled ignition induced by hot spots or slow combustion of lean or inhomogeneous mixture, (3) backfire, premature ignition during the intake stroke, which can be seen as an early form of pre-ignition Two first phenomena encouraged to increase the noise, the vibration or the damage of the engine in the worse case The last phenomenon created a sound bang in the inlet system with its occurrence and stopped the engine or the intake manifold can be destructed in the worse case

In practice, the knocking behavior of hydrogen engines is not benefit of combustion characteristics Knock in hydrogen-fueled SI engine is an acknowledged barrier to the further improvement of efficiency, increased power and the use of a wider range of fuel-air mixtures

Many experimental results and discussion of this phenomenon are reported generally Some papers fail to point out that the knock resistance is a property of the fuel-air mixture, stating octane numbers without the corresponding equivalence ratio Das and co-workers [17,18], Li and co-workers [19,20] claimed that the reason might be the octane number to be very low or very high Jing and co-workers [21] mentioned octane numbers as a function of the mixture richness Das and co-workers [18] have experimentally reported that show hydrogen to act as an anti-knock agent when added to unleaded iso-octane Moreover, his work showed that the causes of hydrogen engine knock can be different from gasoline knock, being caused by excessive flame speeds rather than an end-gas reaction Thus, reducing the rate of pressure rise might be more effective to control knock than limiting the combustion period

Reviewing the experimental literature on hydrogen SI engines, pre-ignition seems to be the limiting factor concerning compression ratios, spark timings and mixture equivalence ratios, rather than knock Berckmuller and co-workers [22] carried out measurements with a hydrogen-fueled engine at a compression ratio of 11:1 and a supercharging pressure of 0.85 bar (gauge) on stoichiometric mixtures Tang and co-workers [17] measured experimentally on lean mixtures using compression ratios of 14:1 and higher, all without any appearance of knock It thus seems safe to say that hydrogen has a higher effective octane number than regular gasoline, although it would be interesting to have quantitative data

Karim and co-workers [19,20,23] reported very wide knocking regions, where stoichiometric mixtures are claimed to knock even at compression ratios as low as 6:1 The results of his work stated that the avoidance of the incidence of knock is a most important consideration in H2 SI engine operation The compression ratio and intake temperature are the main parameters that affect the knock limited equivalence ratio while the effect of spark timing tends to be in comparison less

For backfire phenomenon, it has been a particularly tenacious obstacle to the development

of hydrogen engines Most, if not all, of the early literature mentions causes of backfire and countermeasures as it so frequently occurs in hydrogen engines with external mixture formation

(backfire can only occur when a combustible charge is in the intake port) Thus, the causes of backfire may be as follows:

(1) Hot spots in the combustion chamber: King [24], Das and co-workers [2,6,7], Kondo and co-workers [25,26] reported that deposits, particulates, spark plug, residual gas, exhaust valves, etc are the main factors of hot spots

(2) Residual energy in the ignition circuit: Kondo and workers [25,26], Meier and workers [27] found that the lower ion concentration of a hydrogen-air flame compared

co-to a hydrocarbon-air flame It is possible that the ignition energy is not completely deposited in the flame and remains in the ignition circuit until the cylinder conditions are such that a second, unwanted, ignition can occur, namely during the expansion or the intake stroke, when the pressure is low

(3) Combustion in the piston top land (piston crevice volumes): persisting up to inlet valve opening time and igniting the fresh charge Lee and co-workers [1,2] stated that this is caused by the smaller quenching gap of hydrogen mixtures compared to typical hydrocarbons, which enables a hydrogen flame to propagate into the top land

(4) Pre-ignition: Tang and co-workers [17], Koyanari and co-workers [28], Furuhama [29,30] concluded that pre-ignition is often encountered in hydrogen engines because of the low ignition energy and wide flammability limits of hydrogen As a premature ignition causes the mixture to burn mostly during the compression stroke, the temperature in the combustion chamber rises, which causes the hot spot that led to the pre-ignition to increase in temperature, resulting in another, earlier, pre-ignition in the next cycle This advancement of the pre-ignition continues until it occurs during the intake stroke and causes backfire MacCarley and co-workers [31] reported that the mechanism is termed a runaway pre-ignition and can result from a knocking cycle, increasing the chamber temperature and creating a hot spot

1.2.2 VVT studies for performance improvement and NOx reduction

One of the latest trends in ICEs to improve performance and to reduce exhaust emissions is

to develop a system that allows controlling the intake and exhausting valve timing and/or lifting for each engine speed and load condition These systems are generically referred as variable valve timing (VVT)

The literature on the effects of VVT on the performance improvement and reduction in NOxemission of H2 SI engine is quite limited: mainly simulating researches Sher and co-workers [32] demonstrated the valve timing strategies for maximizing engine torque and minimizing BSFC (brake specific fuel consumption) in terms of exhaust valve opening (EVO), intake valve opening (IVO), and intake valve closing (IVC) timings of a commercial SI engine through a MICE (modeling internal combustion engine) computer program By calculating the engine overall performance characteristics such as the cycle efficiency, engine power, and exhaust gas composition, his works concluded: (1) the optimal timing of each valve depends apparently linearly on the engine load, linearly (in a good approximation) on the engine speed, while the slope depends in a weak manner on the engine load; (2) when VVT is applied, the maximum torque at any engine load is shifted towards a lower engine speed The engine power and the engine BSFC are enhanced particularly at partial load and low engine speed, but the CO and

NOx emissions are not improved nor noticeably deteriorated; (3) the optimal IVO timing is practically insensitive to the engine speed and to a very limited degree of sensitivity to the engine load; (4) the optimal EVO timing and IVC timing for minimizing BSFC occurs in a good approximation when the engine torque maximizes in the entire range of operating conditions Hong and co-workers [33] reviewed the literature in the technology of intake and exhaust philosophies of VVT and their effects on the pressure - volume (PV) diagram of the engine The effects of different VVT philosophies simulated with a single-cylinder engine by using the GT-Power software are also analyzed and compared to those of the literature reviewed

Dresner and co-workers [34] or Yan and co-workers [35] provided a review of VVT strategies and mechanisms Basically, VVT mechanisms can be classified into three types based

on their principle: (1) variable phase timing; (2) variable cam profile; and (3) fully flexible mechanisms The first type is simple and has a little change in technique compared with fixed cam phase angle engine This kind of VVT mechanism has been initiated on the production application by some Nissan, Benz or Toyota

For the second type, cam profile may be changed on different engine operation (variable valve timing), so the valve lift, duration, and cam phase angle can be variable The engine performance is better than the first type The 3-stage VTEC [36] system of Honda, MIVEC (Mitsubishi Innovative Valve Timing Electronic Control) [37] system of Mitsubishi, VVTL-i (Variable Valve Timing and Lift with intelligent) system of Toyota are belonged to this type The fully flexible VVT mechanism employed to camless engine is one lately advanced new type, which allows changing intake and exhausting valve lift, timing, and duration based on varied engine working condition Therefore, it is used in the un-throttled load control and can realize the variable displacement For this type, all varied factors of valve can be optimized to enhance the engine performance It is an ideal mechanism for VVT technology This function can be realized by FEV electronic valve control system of FEV Motorentechnik [38] or ECV (Electro-hydraulic Camless Valve Train) system of Ford [39]

1.3 Purposes and objectives of research

The objectives of this thesis have been to contribute to the establishment of the basic technologies by using the change of valve overlap period:

- To control the occurrence of backfire at high load conditions

- To enhance the engine performances (such as power and efficiency) with supercharging concept at lean mixtures and to limit the NOx emissions

required in development of external injection type H2-fueled engines with both high thermal efficiency and high power without backfire for commercial H2-powered vehicles

1.4 Methods and contents of research

This work is to develop experimentally a method to prevent backfire by changing the valve timings and to realize method of enhancing engine power and efficiency without backfire in a hydrogen-fueled engine with external injection For this goal, a single-cylinder hydrogen-fueled research engine with a mechanical continuous variable valve timing system is developed The following chapters describe the experimental works performed during this Thesis includes:

Chapter 1: to present the research background and the status of H2 researches on backfire control and the enhancement of engine performance Also the purposes and objectives of research are shown The methods and contents and of research are covered

Chapter 2: to relate the development of a H2 MCVVT engine with external injection and MCVVT system The designing concept and operation of both engine and the MCVVT system have been covered In addition, the basic characteristics of the research engine have been compared to the same conventional engine without MCVVT system

Chapter 3: to show the possibility of backfire control by changing the valve overlap period Two most important factors affecting to the occurrence of backfire are examined including: (1) the reduction of backflow duration and (2) the change of intake valve opening timing For the former case, unavailable data is found in the literature and the present work proposed a new experimental data that opens the ways to prevent backfire occurrence efficiently The latter case describes the variation of backfire limit equivalence ratio as a function of intake valve opening timing and demonstrates that the intake valve opening timing is the main prevention factor for method of controlling backfire by the valve overlap period The proper selection of intake valve opening timing can allow preventing globally the occurrence of backfire in a hydrogen-fueled engine with external injection

Chapter 4: to realize how to optimize hydrogen-fueled engine with both high power and high efficiency and without backfire by employing the supercharging method at lean regions

and suitable intake valve opening timing The engine torque (power) can be enhanced at the lean mixture region, where the maximum efficiency occurred by increasing the air mass flow rate (or the positive intake pressure) at 0.5 and IVO timing at TDC The engine power, thermal efficiency, and NOx emissions are evaluated as the functions of the valve timing The obtained results may be referred to the commercial application of hydrogen-poared vehicles

Finally, a concluding chapter summarizes the main findings, the results of this work, and recommendation for further works

Chapter 2 Development of H2 research engine with MCVVT system

2.1 Introduction

In this Chapter, the development of a MCVVT system and a single-cylinder fueled engine using external mixture injection have been discussed as the first steps of the research Section 2.2 and 2.3 describe the design, construction, testing, and data obtained preliminary from the dedicated H2 engine with and without MCVVT system The H2 engine is based on a 2-liter DOHC SI commercial engine at a fixed compression ratio while the MCVVT system is properly designed to control continuously the intake and/or exhaust phasing (e.g., opening/closing timing) independently or simultaneously (e.g., dual cam control) It is thus very important to know the designing concept, the mechanical structure, and the characteristics of the system, as this is a main means for the current study After the discussion on the concept, the estimation for the mechanical loss of the system (due to the additional gears for idle, timing, and auto-tension functions) are discussed Several obtained results found in the preliminary experiments are then compared to each other and to the conventional timing belt system Additionally, Section 2.4 is devoted to the experimental setup which carries out for the purposes

hydrogen-of the next Chapters Lastly, the methods proposed for the experiments in this dissertation are also covered in Section 2.5

2.2 Development of MCVVT system

2.2.1 Designing concept

The designing concept proposed for the MCVVT system is shown in Fig 2-1 In accordance with the physical relations, the principal idea for the variable valve timing (VVT) is basically exposed by the fact that when the mechanically moving pulley has linearly moved a arbitrary distance X (e.g., from position 1 to position 1 in the figure) and created to a natural change of contacting angle  of the fixed pulley (e.g., from position 2 to position 2) This is resulted in a respond change in the valve timing (retarded/advanced) with a little difference in

the total length of the timing belt The auto-tentioners (also see Fig 2-2) are played an important role for overcoming this difference In the MCVVT system, moreover, the fixed length of timing belt and a pair of mechanically moving pulleys are used to make the above fact

Fig 2-2 shows the schematic diagram described for operation of the MCVVT system which includes: (a) the completely schematic diagram of the exhaust MCVVT system that is completely the same as the intake system and (b) the detail sketch of exhaust VVT system As stated above, valve timing is adjusted by the change of the angle of camshaft (α) as the VVT timing gear 1 moves from position 1 to position 2 in the figure The points A1 and B1 move to the new points A2 and B2 respectively On the other hand, the belt length between tangential points of belt and timing gear is not changed It is because when the timing gear 1 moves as above, the timing gear 2 which is installed on the opposite side moves from position 1 to

Fig 2-1 Designing concept of MCVVT system

position 2 Hence, the relation between the rotating angle of the camshaft and the displacement

of the VVT timing gear is as follows:

Here, X is the displacement of the timing gear Angle  is defined as the setting angle Angle  and  are the phasing angle of the camshaft gear and the phasing angle of the timing gear, respectively D1 and D2 are the given diameters of the camshaft gear and the timing gear, respectively Using the equations from (2-1) to (2-3), the displacement X and the phasing angle

of gear () has following relation:

2.2.2 Mechanical structure of MCVVT system

The principal targets of the MCVVT system design are the ability to change continuously valve timing at all of engine load or engine speed conditions and the ability to control independently the intake and exhaust valve timing (e.g., opening/closing timings) that varies as wide as possible Furthermore, its location must be suitably designated to mitigate the mechanical vibration of the additional gears and to avoid the overload of the bending moment for the camshaft Hence, it is installed at the front of the engine coupling with the intake and exhaust camshafts separately (see Fig 2-19 to Fig 2-22)

1B A B

1 3 1

3 3 1 1

1B A A A B X cos A B

)(

1 4 2

Fig 2-2 Schematic diagram of MCVVT system

Fig 2-3 Main components of MCVVT system

Fig 2-4 Photo of a MCVVT system

Fig 2-3 and Fig 2-4 show the main components and the photo of a MCVVT system, respectively The MCVVT system uses a driving shaft (referred to as (3) in the figure) that is manufactured with the right-hand thread and the left-hand thread in two separate position of the shaft Two timing gears (5) assembled in two gear supporters (4), ball bearings (6), and snap rings (7) All is then coupled in a way ready for the timing gears to move in the opposed directions along the driving shaft by the support of the LM (linear motion) guides below In addition, the MCVVT system includes one idle gear, an auto-tensioner and a traverse device (9)

to move the timing gear straight The auto-tensioner maintains the tension of the timing belt as the timing gear 1 and 2 (see in Fig 2-2) move simultaneously

2.2.3 Characteristics of MCVVT system

As noted previously, the MCVVT system is changed through the relation between the phasing angles of the intake or exhaust camshafts and the displacement X of the timing gears This relation is shown in Fig 2-5 It is clearly from the figure that the phasing angle of camshaft

is varied linearly with respect to the displacement as the timing gear moves The changing value

of phasing angle for intake camshaft and exhaust camshaft is almost similar As a rough estimation, the cam phasing angle varies a value of around 10CA for each about 4.5 mm of the displacement Rotating the handle will result in the change of the displacement X Therefore, the valve timing events can be controlled (e.g., advanced or retarded) during engine operation Fig 2-6 shows the power output of the motoring test engine installed the MCVVT system compared with that under conventional operation for a range of engine speed The lower power for the case with the MCVVT system is noted at all engine speeds and is a reflection of the mechanical loss associated with the additional timing and idle gears, and other items of the MCVVT system As engine speed increases, the mechanical loss increases linearly It is estimated an 11% decreasing value of power output at a constant engine speed of 2000 rpm for MCVVT system However, as the VOP changes, this loss may be changed due to the intake air flow rate It is able to demonstrate the feasibility of using changes of the VOP to control backfire (see Chapter 2)

2.3 H2 research engine with MCVVT system

The single-cylinder hydrogen-fueled research engine is based on a 2-liter DOHC (Double Over Head Cam) commercial engine and is modified to accommodate its cylinder head of four-valve pent-proof geometry Fig 2-7 and 2-8 illustrate the drawing and photo of the test engine used Two camshafts of the above engine are redesigned to connect the cam bearings in the front and the rotary encoders in the rear of the cylinder head The MCVVT system is installed in front of the engine (see Fig 2-8) for connecting two cam (intake and exhaust) gears A pressure transducer for measuring the pressure in cylinder is installed in the converted cylinder head that

is cut from the above 4-cylinder engine A cylinder liner assembled inside a developed cylinder block

2.3.1 Cylinder block

The photo and drawing of the developed cylinder block are shown in Fig 2-9 and Fig 2-10 Cylinder block is designed for coolant water to enter at the left side and to exit the right side Coolant flow is passed into the water jacket that is formed by the cylinder liner and cylinder block

2.3.2 Crankcase

The cylinder head and developed cylinder block are mounted on a Ricardo type crankcase The drawing and photo of this crankcase is presented in Fig 2-11 and 2-12 The overall dimension of crankcase is 416 x 170 x 465 The crank mechanism is specially modified from the parts of a 4-cylinder engine A piston 86 mm in diameter and 86 mm in stroke is used Also the intake system is properly designed to use a gases fuel injector The engine specifications are summarized in Table 2-1

Fig 2-5 Relation between the phasing angle of camshafts and displacement

Fig 2-6 Variation of engine power as a function of engine speed

Fig 2-8 Photo of H2 research engine with MCVVT system Fig 2-7 Drawing of H2 research engine with MCVVT system

Fig 2-10 Photo of developed cylinder block Fig 2-9 Drawing of developed cylinder block

Fig 2-12 Photo of Ricardo type crankcase Fig 2-11 Drawing of Ricardo type crankcase