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
Prof. Sung, Nak Won : Thesis Committee Member #2 Prof. Ryou, Hong Sun : Thesis Committee Member #3
Prof. Choi, Kyu Hoon : Thesis Committee Member #4 Prof. Lee, Jong Tai :

Thesis Supervisor
The Graduate School
Sungkyunkwan University
December 2008

박사학위 청구논문
지도교수 이 종 태 흡기관분사식 수소기관에서 밸브 타이밍 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


0
8

Huynh Thanh Cong
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 H
2
engine? 1
1.1.2 Combustive properties of hydrogen related to backfire occurrence 3
1.2 Status of H
2
researches on backfire control and enhancement of performance 6
1.2.1 Previous researches on backfire control in H
2
engines 6
1.2.2 VVT studies for performance improvement and NO
x
reduction 9
1.3 Purpopes and objectives of research 10
1.4 Methods and contents of research 11


4.2.2 Difference of engine performance with the change of valve timing for H
2

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

- i -
List of Tables and Figures Table 1-1 Comparative physical properties of H
2
, CNG, and gasoline 4
Table 2-1 Specifications of MCVVT H
2
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

Fig. 2-18 Photo of intake system 29
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

- iii -
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

the change of EVC timing
68
Fig. 4-7 Influence of spark timings on NO
x
reduction for H
2
and gasoline 68

- iv -
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 COV
imep
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 H
2
and gasoline
engines
75
Fig. 4-12 Brake torque with change of the VOP for H
2
and gasoline engines 75
Fig. 4-13 Brake torque with change of the VOP for H
2
and gasoline for
various loads
76
Fig. 4-14 Brake thermal efficiency with change of the VOP for H

and supercharging cases
85
Fig. A-1 Main components of mass flow meter/controller 90
Fig. A-2 MFC/MFM FM30V4 90
Fig. A-3 Electronic injection controller 91
Fig. A-4 Injector characteristic graph 92
Fig. A-5 Photo of supercharging system 94
Fig. A-6 Gasoline fuel supply system (carburetor) 95


[

]
Degree
d
q
Quenching distance
DOHC Double over head cam
EGR Exhaust gas recirculation
EVO Exhaust valve opening
EVC Exhaust valve closing
H
2
Hydrogen
FC Fuel cell
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

1/


Air-fuel equivalence ratio
k Specific heat ratio


Density


Compression ratio
- 1 -
Chapter 1 Introduction

1.1 Research background

1.1.1 Why backfire control in H
2
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

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

- 3 -
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

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 H
2
, CNG, and GasolineProperty Hydrogen CNG
*
Gasoline
Density (kg/m
3
) 0.0824 0.79 730
a
Flammability limits in air (vol. %) 4-75 5.3-15 1.0-7.6
Flammability limits ()
0.1-7.14 0.4-1.6 0.17-3.84
Minimum ignition energy (mJ)
b

Liquid at 0
o
C;
b
At stoichiometry;
c
Methane;
*
CNG (CH
4
86.8%, C
2
H
6
8.2%, C
3
H
8
3.9%, C
+
1.0%, N
2
0.1%)

- 5 -
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

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 H
2
researches on backfire control and enhancement of performance

1.2.1 Previous researches on backfire control in H
2
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.

- 7 -
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

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 co-workers [25,26], Meier and co-
workers [27] found that the lower ion concentration of a hydrogen-air flame compared
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.