WREC 1996
CONSTANT-VOLUME ADIABATIC COMBUSTION OF STOICHIOMETRIC
HYDROGEN-OXYGEN MIXTURES
T. Bohacik, S. De Maria, W. Y. Saman
Energy and Engines Research Croup,
School of Manufacturing and Mechanical Engineering
University of South Australia,
South Australia 5095
Australia
ABSTRACT
The paper evaluates and discusses the combustion characteristics of electrolytically produced stoichiometric
hydrogen-oxygen mixtures under constant volume adiabatic conditions. The results will be used in the
preliminary design of a combustion chamber of a hydrogen-oxygen fuelled engine. Maximum combustion
pressures are investigated and compared to theoretical predictions based on the first law of thermodynamics,
ideal gas laws and adiabatic flame temperature. High temperature dissociation is not taken into account.
Experimental results are within 10% of theoretical predictions. Ignition delay time and rate of pressure rise
are also investigated and discussed. The ignition delay time is a function of the initial mixture pressure and
decreases exponentially from a maximum value of 6.6 ms towards zero as the initial pressure of the mixture
is increased.
KEYWORDS
Alternative Fuels; Hydrogen; Electrolysis; Stoichiometric Hydrogen-Oxygen Mixtures; Combustion
INTRODUCTION
Due to the growing sensitivity to pollution of the air in municipal areas caused by motorised traffic, the
evaluation of alternative fuels and consequently, the quest for new automobile propulsion systems is well
underway. From a purely environmental point of view, hydrogen is the most likely candidate in the
foreseable future. The paper reports on the initial stages of a project undertaken at the University of South
Australia to develop and test a closed cycle (zero-emission) hydrogen-oxygen fuelled combustion engine
system specifically designed for city and urban passenger commuting. The engine system is an alternative to
the electrically powered vehicle, which, at present is considered to be zero polluting without regard to the
manner in which the electric motive power is generated. Vehicles powered by hydrogen join electric
vehicles as an option for effectively reducing the emission of greenhouse gases.
The use of hydrogen as a fuel for reciprocating piston engines is not a recent event, being utilised as early as
the nineteenth century. Hydrogen was burnt with oxygen, and the vacuum created upon cooling due to the
reduction of the number of moles of the products compared to the reactants, used to drive a piston. In
automotive vehicles it has been experimented as early as World War II in gazogene composition (50%
hydrogen) resulting from coal gasification. More recently it has been used in a large number of vehicles in
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different countries, primarily Germany, U.S.A. and Japan. Mercedes Benz, BMW and Mazda prototypes are
using gasoline designed engines that have heen modified for hydrogen, although not yet optimised for
performance. These engines use atmospheric air as a source of oxygen and hence similar levels of NO, are
emitted compared to gasoline fuelled engines. Work of Billings and Lynch (1973) showed that oxides of
nitrogen were produced in a hydrogen-fuelled engine with very low concentrations using lean mixtures, but
they sharply increased with equivalence ratio and reached a maximum at equivalence ratio of 0.8.
To eliminate NO, , pure oxygen must be used in conjunction with hydrogen. The disadvantages with this
scheme is the added complexity of carrying separate hydrogen and oxygen supplies leading to a heavy and
bulky energy storage system.
SYSTEM DESIGN/EXPERIMENTAL WORK
The proposed system overcomes the need to carry separate supplies of hydrogen and oxygen. Its principle of
operation results in a number of advantages over conventional methods of storing hydrogen in compressed
form on vehicles namely, hydrogen and oxygen are produced on-board by electrolysis of water, unlimited
generating pressures are possible without a compressor and the fact that hydrogen and oxygen are stored
together in one pressure vessel in stoichiometric proportion leads to a compact energy storage system.
Figure 1. shows the basic components of the proposed zero emission engine system.
Fig. 1. Schematic of zero emission engine system.
A prototype of the high pressure electrolysis system has been manufactured and tested up to generating
pressures of 24 MPa. The rate of pressure rise was linear with an average value of 60 kF’a per minute. The
electric power consumption of the high pressure electrolysis cell remained constant at 25 W up to the
highest generating pressure. This indicates that high pressure electrolysis cells are the most efficient form of
generating high pressure stoichiometric hydrogen-oxygen mixtures.
The combustion characteristics of stoichiometric hydrogen-oxygen mixtures produced by this system have
been evaluated utilising a 1.54 x lo” m3 ceramic combustion chamber fitted with a piezoelectric transducer
and utilising high-speed data logging software. The data logger, recording the instantaneous pressure inside
the chamber, was triggered at the instant of spark discharge at the spark plug. This enables the ignition delay
time to be measured. A spark plug with projected electrodes was used to ignite the mixture. The
experimental rig is depicted in fig. 2.
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Firing button
Charge Amplifier
Metering valve
0 0 0
Oscilloscope
*
To manometer
Three-way valve
Experimental rig and data collection.
Fig 2.
RESULTS AND DISCUSSION
Figures 3 and 4 describe the combustion characteristics obtained from the experimental test rig. Figure 3
shows the maximum combustion pressure reached versus the initial hydrogen-oxygen mixture pressure.
Comparison is made against theoretical predictions based on first law of thermodynamics, ideal gas laws
and adiabatic flame temperature. Experimental results are within a 10% range of theoretical predictions. The
deviations are due to high temperature dissociation not taken into account, heat leakage through the metal
jacket of the spark plug, valves and pipes and deviation from ideal gas behaviour.
2
1.6
1.6
1.4
1.2
1
0.6
0.6
0.4
0.2
0
20
40
60
60
Initial
mixture absolub3
100
120
146
pressure (IPa)
0
i0
40
60
60
100
120
14
Inltlalmbtture absolute pressure (kPa)
Fig. 3. Maximum combustion pressure versus initial
mixture pressure.
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Fig. 4. Ignition delay time versus initial mixture
pressure for stoichiometric hydrogenoxygen mixtures at 25” C.
WREC 1996
Figure 4 shows that the ignition delay time corresponding to the lowest pressure is considerable, but
decreases exponentially and approaches zero as the pressure of the mixture is increased. A reciprocating
hydrogen fuelled piston engine utilising a compression ratio of 10 and rotating at 3000 RPM would
experience an ignition delay time of less than 0.2 ms. This corresponds to approximately 4-5” crank angle
and indicates that hydrogen-oxygen fuelled engines approximate more closely to the ideal Otto cycle
because heat addition occurs closer to constant volume on the P-V diagram. Figure 5 shows the rate of
pressure rise as a function of the initial mixture pressure.
220
200
180
f
it
y
k
6
lb0
140
120
100
80
60
40
20
0
0
20
40
60
80
100
120
1 lo
Initialmlxturaabaoluta pressure (kPa)
Fig.5 Rate of pressure rise versus initial mixture pressure.
The high rate of pressure rise due to the high rate of combustion is evident. Figure 5 also suggests the
possibility that “knock” in a hydrogen-oxygen fuelled engine is not caused by the spontaneous auto-ignition
of the end gas as postulated for a hydrocarbon fuelled engine. The very high rate of combustion indicates
that knocking should not occur since the end gas has very little time to auto-ignite. The octane rating of
hydrogen is 106 which is higher than that for gasoline, yet knocking in a hydrogen fuelled engine is more
pronounced and occurs more frequently. Rather, it is caused by a high primary flame front which is
attributable to very high laminar and turbulent burning velocities and hence a very high rate of pressure rise.
This has also been validated by Swain et al (1988). If such is the case, then the combustion chamber of
hydrogen-fuelled engines needs to be of broad quiescent geometry, in which ignition by a spark plug occurs
as far as possible from the centre. Combustion chambers for hydrocarbon fuels can actually promote knock
for hydrogen.
In conclusion, the combustion chambers of hydrocarbon fuelled engines are not compatible with hydrogen
fuelled engines. A different design approach must be used employing clean, cool running chambers to
prevent pre-ignition of the admitted charge. The geometry should be disk-shaped to minimise knock.
REFERENCES
Billings R. E. and Lynch F. E. (1973). Performance and Nitric Oxide Control Parameters of the
Hydrogen Engine. -Research.Provo, UT.
.
.
Longman
Eastop T. D. and McConkey A. (1986). ).
Scientific and Technical, Essex, U.K.
. .
John
Somrtag R. E and Van Wylen G. J. (1991). S
Wiley and Sons, New York.
Swain M. R and Adt R. R. (1988). Considerations in the Design of an Inexpesive Hydrogen-Fuelled Engine.
SAE.
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