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SAE TECHNICAL
PAPER SERIES 2000-01-2206
Experimental Evaluation of SI Engine Operation
Supplemented by Hydrogen Rich Gas from a
Compact Plasma Boosted Reformer
J. B. Green, Jr., N. Domingo, J. M. E. Storey,
R. M. Wagner and J. S. Armfield
Oak Ridge National Lab.
L. Bromberg, D. R. Cohn, A. Rabinovich and N. Alexeev
MIT Plasma Science and Fusion Center
Government/Industry Meeting
Washington, D.C.
June 19-21, 2000
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1
2000-01-2206
Experimental Evaluation of SI Engine Operation
Supplemented by Hydrogen Rich Gas from
a Compact Plasma Boosted Reformer
J. B. Green, Jr., N. Domingo, J. M. E. Storey,
R. M. Wagner and J. S. Armfield
Oak Ridge National Lab.
L. Bromberg, D. R. Cohn, A. Rabinovich and N. Alexeev
MIT Plasma Science and Fusion Center
No copyright is asserted in the works of U.S. Government employees
ABSTRACT
It is well known that hydrogen addition to spark-ignited
(SI) engines can reduce exhaust emissions and increase
efficiency. Micro plasmatron fuel converters can be used
for onboard generation of hydrogen-rich gas by partial
oxidation of a wide range of fuels. These plasmaboosted
microreformers are compact, rugged, and provide
rapid response. With hydrogen supplement to the
main fuel, SI engines can run very lean resulting in a
large reduction in nitrogen oxides (NOx) emissions relative
to stoichiometric combustion without a catalytic converter.
This paper presents experimental results from a
microplasmatron fuel converter operating under variable
oxygen to carbon ratios. Tests have also been carried
out to evaluate the effect of the addition of a microplasmatron
fuel converter generated gas in a 1995 2.3-L fourcylinder
SI production engine. The tests were performed
with and without hydrogen-rich gas produced by the
plasma boosted fuel converter with gasoline. A one hundred
fold reduction in NOx due to very lean operation was
obtained under certain conditions. An advantage of
onboard plasma- boosted generation of hydrogen-rich
gas is that it is used only when required and can be
readily turned on and off. Substantial NOx reduction
should also be obtainable by heavy exhaust gas recirculation
(EGR) facilitated by use of hydrogen-rich gas with
stoichiometric operation.
INTRODUCTION
Decreasing emissions from automobiles and increasing
engine efficiency are necessary steps toward improving
air quality and decreasing greenhouse gases. Transportation
vehicles are the largest consumer of imported oil
and a major source of pollutants that affect urban areas.
A variety of potential improvements are currently being
investigated: spark-ignited direct-injection engines, new
catalyst formulations, close coupled catalysts, new types
of exhaust aftertreatment, electric and fuel-cell powered
vehicles, and alternative fuels.
Large reductions in emissions from SI engines are possible
by operation under lean conditions with the addition of
hydrogen. Hydrogen increases flame speed and extends
the lean limit of SI engine operation [1]. The combination
of enhanced flame speed and wider flammability limits of
hydrogen can thus stabilize combustion during lean operation.
Thus, a concept that could substantially reduce
emissions is onboard hydrogen generation using microplasmatron
fuel converters. Plasmatrons are electrical
gas heaters that make use of the conductivity of gases at
high temperature. Microplasmatron fuel converters are
compact, rugged, compatible with several fuels, and able
to respond rapidly.
Very lean fueling of SI engines could reduce NOx emissions
by a factor of one hundred relative to NOx emissions
at stoichiometric fueling [2,3,4]. Hydrogen addition
could also be used to reduce NOx emissions by facilitating
the use of increased exhaust gas recirculation (EGR)
[5]. Onboard production of hydrogen is also attractive for
reduction of cold start emissions, as well as for lean NOx
catalyst regeneration and post treatment.
Concepts for utilizing plasmatron generated hydrogenrich
gasoline in SI engines have been discussed in previous
papers [6-9]. Engine experiments have also been
performed using bottled synthesis gas [2, 3, 4, 10] and
conventional reformers operating on methane [11] or ethanol
[12,13]. To the knowledge of the authors, this paper
reports the first use of a compact plasma boosted
reformer to convert gasoline into hydrogen-rich gas for
the purpose of stabilizing lean combustion in a SI engine.
2
COMPACT PLASMA BOOSTED REFORMERS
Plasmatrons provide highly controllable, ohmic heating of
gases to elevated temperatures. At these temperatures,
the gas is partially ionized. The increased temperatures,
ionization levels, and mixing provided by plasmatron
heating accelerate reformation of hydrocarbon fuels into
hydrogen rich gas. The high temperatures can be used
for reforming a wide range of hydrocarbon fuels into
hydrogen-rich gas without using a catalyst. Thus, it is
possible to eliminate problems associated with catalyst
use, such as narrow operating temperature, sensitivity to
fuel composition, poisoning, and response time limitations.
By increasing the reaction rates, plasma heating could
significantly reduce size requirements for effective
reforming, increase speed of response and increase fuel
flexibility. A wide range of operation is possible, from partial
oxidation to steam reforming. The boosting of the
reaction rate would occur by creating a small high temperature
region (5000-10000 K) where radicals are produced
and by increasing the average temperature in an
extended region.
The additional heating provided by the plasma can
ensure a sufficiently high number of chemically reactive
species, ionization states, and elevated temperatures for
the partial oxidation reaction to occur with negligible soot
production and with a high conversion of hydrocarbon
fuel into hydrogen-rich gas. The effective conversion of
hydrocarbon fuel is aided by both the high peak temperature
in the plasma and the high turbulence created by the
plasma.
The rapidly variable plasmatron parameters (energy
input, flow rate, product gas composition, etc.) make this
technology able to respond to the dynamic demands of
vehicles. It should be practical to instantaneously produce
hydrogen-rich gas for use during cold startup.
Throughout the driving cycle, rapid changes in hydrogenrich
gas flow can be accommodated by variation of plasmatron
parameters. [8]
Figure 1 shows a diagram of a multi kilowatt plasmatron.
The device operates at atmospheric pressure, with air as
the plasma forming gas. The plasmatron operates in DC
mode. The plasmatron consists of a copper cathode with
a hafnium tip, and a copper tubular anode [14]. The electrodes
are separated by an electrical insulator made out
of fiberglass (G-10). The cathode and anode are water
cooled, and this cooling represents a sink of energy.
Measurements on the water temperature rise indicate
that the plasmatron is about 70–80% efficient. Hafnium
allows operation on air as the plasma forming gas. The
hafnium tip has a high electron emissivity and relatively
long lifetime using a current less than ~100 A.
The plasma arc ignites across the electrode gap. Air is
injected tangentially upstream from the electrodes to produce
a vortex that elongates the plasma inside the tubular
anode. The anode root of the arc is in constant
rotation in order to minimize electrode erosion. The
hydrocarbon fuel and additional air are injected downstream
from the electrodes. The mixture of hot air and
vaporized hydrocarbons enter the plasma reactor where
the reaction takes place.
Figure 1. Plasmatron fuel converter device. Reaction
extension cylinder and heat exchanger are
not shown.
Figure 2 shows a photograph of a microplasmatron without
a reaction extension cylinder, operating with air at
about 1.5 kW. The plasma jet is pointing vertically in the
figure. During reforming operation, fuel and additional air
are injected downstream from the stainless steel flange
shown in Figure 2.
Figure 3 shows a microplasmatron fuel converter that
includes a reaction extender and a heat exchanger. The
heat exchanger can be used to simultaneously cool down
the hydrogen-rich gas and to preheat the incoming air
and fuel. Preheating the air and fuel reduces the electrical
energy requirement to the plasmatron and increases
the hydrogen yield. Work is continuing in the development
of a high efficiency, high temperature heat recuperator.
Simple calculations show that using preheat can
reduce the electrical power requirements to the plasmatron
by half.
A typical microplasmatron fuel converter includes a steel
tube 4-cm in diameter and 15-cm long. It is thermally
insulated by fiberglass felt and steel screens. The
samples of hydrogen rich gas are cooled down and analyzed
using gas chromatography (GC). Table 1 shows
the typical plasmatron range of operating parameters for
a DC arc device. Materials that could be used for various
components of the plasma-boosted reformer are copper,
zirconium and molybdenum. Conversion efficiency, electrode
life, size, and weight are essential feasibility issues
and require detailed experimental investigation.
Anode
Cathode
Water inlet
Plasmatron
Air inlet
Water outlet
Fuel/air/water
3
Figure 2. DC arc microplasmatron operating in air
without fuel.
Figure 3. Microplasmatron with reaction extender
cylinder and two heat exchangers.
The microplasmatron used in the engine experiments
described in the following section is shown in Figure 3.
The plasmatron is followed by a reaction extension cylinder,
a simple heat exchanger (not cooled for the present
experiments), and a gas-to-water heat exchanger, used
to cool the reformate.
GASOLINE AND DIESEL REFORMING
Using a DC arc microplasmatron fuel converter, conventional
fuels were very efficiently converted to hydrogen
rich gas, with an electrical power input of ~10% of the
heating value of the fuel. However, with heat regeneration
and with improved reactor design, it is estimated that
the required electrical energy input to the microplasmatron
fuel converter will be on the order of 5% of the heating
value of the fuel. Furthermore, the reactor showed no
evidence of soot, even after extended operation. Innovations
to further decrease the energy consumption and to
further simplify the microplasma reformer are described
at the end of this discussion.
Figure 4 shows the specific energy consumption of the
hydrogen rich gas produced by the DC Arc microplasmatron
fuel converter, in units of MJ/kg H2 for diesel fuel.
The hydrogen and light hydrocarbon yield as a function of
oxygen to carbon ratio is shown in Figure 5. At the higher
oxygen to carbon ratios, the process becomes more exothermic.
For a given specific electrical power input, the
increase temperature increases the yield (as shown in
Figure 5) and decreases the specific energy consumption.
Figure 4. Energy consumption for plasma diesel
reforming as a function of the oxygen to
carbon ratio.
The experiments described above were conducted at
constant power. The power requirements and the reformate
composition were relatively insensitive to the flow
rates as long as the specific power input (power/unit
mass) and air/fuel ratios are kept constant. Under these
circumstances, although the residence time decreases
(because of the higher throughputs) increased efficiency
Table 1. Parameters of conventional DC Arc plasma
boosted reformer.
Power 1.5-10 kW.
Voltage 120-140 V DC
Current 15-75 A DC
Flow rates
Air 0.5-1.5 g/s
Fuel 0.3-0.5 g/s
Microplasmatron
Double wall
reaction
extender cylinder
(heat exchanger)
Heat
exchanger
Reaction
extender
cylinder
Secondary in
Secondary out
Thermal insulation
High temperature
insulation
Steel
Secondary in
Secondary out
Dependence of energy consumption on air/fuel ratio
820 kJ/mol diesel
no preheating
40
50
60
70
80
90
100
110
120
130
0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3
Oxygen to Carbon ratio
Energy Consumption,MJ/kg H2
related to electrical energy
referred to plasma energy
plasmatron heat losses
Referred to
electrical energy
Referred to
plasma energy
Plasmatron
heat losses
4
of the system makes up for the decreased residence
time. Figure 6 shows the composition of the reformate for
diesel as the fuel, as a function of the oxygen to carbon
ratio. The hydrogen concentration is relatively constant.
Microplasmatron fuel converters have substantial
dynamic range. The lower power is determined by the
maximum voltage capability of the power supply (the voltage
increases with decreasing current), while the highest
power is determined by erosion of the electrodes. It is
expected that a dynamic range of a factor of 10 is possible
without substantial modification to the plasmatron
device. This is sufficient to provide the required change
in throughput for conventional engines. For hybrid vehicles,
with engines operating at constant or near constant
conditions, the plasmatron fuel converter would operate
at near constant conditions, with air/fuel/power management
requirements that are much simpler than for conventional
drive.
Figure 5. Hydrogen and light hydrocarbon yield as a
function of the oxygen to carbon ratio.
Figure 6. Composition of reformate as a function of the
oxygen to carbon ratio, for diesel fuel.
Balance is N2.
LOW CURRENT MICROPLASMATRON DEVICE
A low current microplasmatron device that can operate at
power levels as low as 10 W was developed. The size of
the power supply was decreased and electrode wear was
reduced. Figure 7 shows a schematic of the low current
microplasmatron device used in the engine experiments
described in the following section. During the engine
experiments, the unit operated at 270 W.
The low current microplasmatron consists of a central
electrode, an insulator, and a tubular grounded electrode.
The air-fuel mixture is introduced into the gap between
the electrodes creating a plasma discharge with characteristics
similar to those of a glow discharge (500–1000 V,
20-100 mA). The reaction is created by intimate contact
of the air-fuel mixture with the discharge from the plasma
and propagates through the reaction extension cylinder.
ENGINE EXPERIMENTS
Microplasmatron reforming experiments were conducted
using a production in-line four-cylinder SI gasoline-fueled
engine (1995 General Motors Quad-4) at Oak Ridge
National Laboratory (ORNL). The Quad4 has a 2.3-L
displacement, 9.2-cm bore, 8.5-cm stroke, and compression
ratio of 9.5. This engine is port-fuel-injected and
does not utilize a turbocharger or exhaust gas recirculation.
The engine was coupled to a 130-kW (175-hp)
eddy-current (Power Dyne, Inc.) dynamometer for engine
speed and load control. Engine control management
was carried out with a TEC-II control system (Electromotive,
Inc.) The TEC-II provided access to all calibration
parameters (base fuel curve, enrichment, and spark
advance) for proper engine operation. The TEC-II control
allows the user to set a desired air/fuel ratio. It can also
adjust the fuel automatically to maintain stoichiometric
air/fuel ratio by monitoring the exhaust gas oxygen.
Figure 7. Schematic of the low current microplasmatron
fuel converter.
Dependence of diesel converion to light hydrocarbons on air/fuel ratio
820 kJ/mol diesel
no preheating
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30
Oxygen to Carbon ratio
Diesel to light hydrocarbons conversion
0
2
4
6
8
10
12
14
16
18
20
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
Oxygen to Carbon ratio
Volume fraction (%)
H2
CO
CO2
CH4
C2H4
C2H2
AIR -F UEL MIXTURE
GROUND ELECTRODE
ELECTRODE
HYDROGEN
RICH GAS
REACTION
EXTENSION
CYLINDER
INSULATOR
5
Along with engine operating parameters and in-cylinder
pressure, engine out regulated emissions (CO, HC and
NOx) and PM were measured at each operating point.
Total mass concentration and rate of PM was measured
with a Tapered Element Oscillating Microbalance (TEOM,
R and P Co. Model 1105). A scanning mobility particle
sizer (SMPS, TSI, Inc.) measured PM size and number.
CARB Phase II certification grade gasoline was used for
engine and plasmatron operation. The gasoline equivalent
air/fuel ratio was measured with a universal exhaust
gas oxygen (UEGO) sensor (Horiba MEXA 110) in the
exhaust stream. Therefore during reformate addition, the
reported equivalence ratio from the UEGO sensor is
higher (richer) than the actual equivalence ratio because
of the effect of burning both gasoline from the engine and
reformate from the microplasmatron on the UEGO sensor.
The microplasmatron was operated with a constant gasoline
throughput of 0.25 g/s. The reformate was cooled
down to room temperature by a low-pressure shell-intube
heat exchanger. The composition of the microplasmatron
output was continuously monitored using a conventional
tailpipe emissions monitor (Horiba MEXA 554).
Table 2 shows the measured parameters of the microplasmatron
during experiments conducted at ORNL and
MIT. The electrical power input to the microplasmatron
was about 2% of the heating value of the fuel processed.
This microplasmatron incorporated several design
improvements that will be discussed in a later publication.
Experiments were conducted at two engine operating
conditions: the first one at 2300 rpm and 4.2 bar brake
mean effective pressure (BMEP); and the second one at
1500 rpm and 2.6 bar BMEP. Maximum brake torque
(MBT) spark timing was defined for both operating conditions
at stoichiometric conditions with the engine in
closed-loop control mode. Once the MBT spark timing
was defined for each condition, spark timing remained
fixed as air/fuel ratio was increased with the engine in
open-loop control mode. BMEP was also kept constant
as air/fuel ratio was increased.
Reformate was introduced into the engine via the intake
manifold downstream of the throttle. The overall hydrogen
addition was relatively small, being about 4% of fuel
heating value at the 2300 rpm condition and about 9% at
the 1500 rpm condition. The maximum reformate flow
rate was determined by heat removal limitations of the
plasmatron reactor. Bench top tests are being conducted
in the laboratory to remove this limitation.
RESULTS AND DISCUSSION
Figures 8 and 9 show the Coefficient of Variation of the
gross Indicated Mean Effective Pressure (COV of IMEP)
as a function of equivalence ratio for the two operating
conditions. Both cases of baseline operation (without
reformate addition) and the case with reformate addition
are shown in the figures. The equivalence ratio in these
figures has been determined from the exhaust gas composition.
The presence of hydrogen in the engine substantially
reduces the COV of IMEP, even at the 2300 rpm
condition, when the reformate addition is a small fraction
of the total fuel.
Figure 8. COV of IMEP as a function of exhaust
equivalence ratio (1500 rpm, 2.6 bar BMEP).
Figure 9. COV of IMEP as a function of exhaust
equivalence ratio (2300 rpm, 4.2 bar BMEP).
Figures 10 and 11 show the NOx emissions as a function
of the COV of IMEP. The plots illustrate the reduction of
NOx emissions within acceptable levels of cycle-to-cycle
combustion variations (3 to 5% COV of IMEP). The NOx
concentration decreases with the reformate addition for a
given COV of IMEP, even with relatively small reformate
addition. At a COV of 5%, NOx is reduced by a factor of
Table 2. Operating conditions for the
microplasma reformer.
MIT ORNL
(GC) (Horiba)
Fuel Flow Rate, g/s NA 0.25
Composition of reformate
CO 20% 18-21%
CO2 3.5% 4%
CH4 0.5% NA
C2H4 + C2H6 0.2% NA
H2 16% NA
N2 60% NA
HC by FID (ppm C3) NA 260-410
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
1.10 1.00 0.90 0.80 0.70 0.60
Equivalence Ratio
COV IMEP (%)
Baseline Hydrogen Assist
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
1.10 1.00 0.90 0.80 0.70 0.60
Equivalence Ratio
COV IMEP (%)
Baseline Hydrogen Assist
6
about a hundred by the addition of plasma boosted
reformer generated hydrogen at 1500 rpm engine operation.
Higher reformate addition may be necessary, especially
for the higher load cases. The plasma-boosted
reformer is presently being modified to provide increased
hydrogen generation.
Figure 10. NOx emissions as a function of the COV of
IMEP (1500 rpm, 2.6 bar BMEP).
The corresponding concentration of hydrocarbons is
shown in Figures 12 and 13, for the 1500 and 2300 rpm
operating conditions, respectively. Decreases in HC
emissions of about 20-30% are possible. Larger effects
could be possible with the additional of increased
amounts of hydrogen rich gas.
Particulate mass emissions as measured by the TEOM
showed very low values for both the baseline and reformate
addition cases. Figure 14 shows the relative mass
emissions decrease from the baseline stoichiometric
case. Although the TEOM was approaching its lower
sensitivity limits, the trend is still clear; decreasing equivalence
ratio leads to lower PM mass emissions. In addition
to PM mass, PM size distribution was measured.
Figure 15 shows that there appears to be an increase in
particle number with decreasing equivalence ratio.
Although this seems to contradict the PM mass emissions
decrease, the proportion of larger diameter (and
heavier) particles decreases with decreasing equivalence
ratio.
Figure 11. NOx emissions as a function of the COV of
IMEP (2300 rpm, 4.2 bar BMEP).
Use of the microplasmatron caused a relatively small
decrease in thermal efficiency (see Figures 16 and 17).
Thermal efficiency was degraded because of the fuel that
is reformed and the microplasmatron electrical power
consumption. When the proportion of plasmatron output
is higher (e.g., the 1500 rpm engine condition), the efficiency
penalty is reduced at lower equivalence ratios due
to improved combustion stability.
Figure 12. Hydrocarbon emissions as a function of the
COV of IMEP (1500 rpm, 2.6 bar BMEP).
Figure 13. Hydrocarbon emissions as a function of the
COV of IMEP (2300 rpm, 4.2 bar BMEP).
Figure 14. Relative PM emissions as a function of
equivalence ratio (1500 rpm, 2.6 bar BMEP).
0.0
5.0
10.0
15.0
20.0
25.0
30.0
0.0 2.0 4.0 6.0 8.0 10.0
COV IMEP (%)
NOx (g/kW-hr)
Baseline Hydrogen Assist
0.0
5.0
10.0
15.0
20.0
25.0
30.0
0.0 2.0 4.0 6.0 8.0 10.0
COV IMEP (%)
NOx (g/kW-hr)
Baseline Hydrogen Assist
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0.0 2.0 4.0 6.0 8.0 10.0
COV IMEP (%)
HC (g/kW-hr)
Baseline Hydrogen Assist
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0.0 2.0 4.0 6.0 8.0 10.0
COV IMEP (%)
HC (g/kW-hr)
Baseline Hydrogen Assist
7
Figure 15. Particle size distribution for four different
equivalence ratios (designated by labels) at
1500 rpm, 2.6 bar BMEP. P designates
hydrogen assist. Note the overall numbers
are very low.
Figure 16. Engine thermal efficiency as a function of
equivalence ratio (1500 rpm, 2.6 bar BMEP).
Figure 17. Engine thermal efficiency as a function of
equivalence ratio (2300 rpm, 4.2 bar BMEP).
DISCUSSION
The addition of reformate stabilized engine operation
under lean fueling, resulting in lower NOx emissions.
Similar results should be obtainable with reformate addition
at high EGR levels. The exhaust gas has a larger
heat capacity than air (due to the higher concentration of
tri-atom molecules in the exhaust gas), and therefore
NOx reduction with EGR should be larger than with air
[1]. Tests are being planned to investigate the effects of
EGR with reformate addition.
In a similar fashion to other reformer concepts such as
those used for reforming ethanol [12,13], the plasmaboosted
reformer is also ideally suited for cold-start conditions,
due to the rapid response time of the microplasmatron.
The time of microplasmatron operation without
preheat and with a high fraction of fuel to the device is
limited to cold start and is thus very small. During cold
start, a high fraction of the fuel would be converted into
hydrogen-rich gas. As soon as the catalyst is warmed
up, the fraction of processed fuel would be decreased.
CONCLUSIONS
Onboard generation of hydrogen-rich gas using a
plasma-boosted microreformer could provide important
new opportunities for significantly reduced emissions. A
compact plasma boosted reformer was successfully integrated
with a gasoline engine on an engine test stand. SI
engine experiments were carried out to determine the
effect of reformate addition on emission and efficiency.
NOx emissions reduction of two orders of magnitude was
obtained. The plasma boosted microreformer operated
reliably for the relatively long duration of the experiments
(>6 hours per day), operating on gasoline. Additional
effort is required to decrease the electromagnetic noise,
as well as to better integrate the microplasmatron fuel
converter with the engine. The rapid response, as well
as the robustness to fuel characteristics and ambient
temperature, makes the plasma-boosted microreformer
suitable as a fuel converter for a variety of onboard applications.
ACKNOWLEDGMENTS
This work was supported by the Office of Heavy Vehicle
Technologies (OHVT), U.S. Department of Energy. Dr.
Sid Diamond and Mr. Richard Wares of DOE-OHVT provided
valuable guidance and support for this project.
Oak Ridge Natioinal Laboratory is managed by UT-Battelle,
LLC, under contract no. DE-AC05-00OR22725.
0
1500
3000
4500
6000
dN/dlogDp (#/cc)
5.42 11.9 26.4 58.2 128.
Dp (nm)
0.71 P
0.83 P
1.0 P
1.0
20
25
30
35
40
1.10 1.00 0.90 0.80 0.70 0.60
Equivalence Ratio
Thermal Efficiency (%)
Baseline Hydrogen Assist
20
25
30
35
40
1.10 1.00 0.90 0.80 0.70 0.60
Equivalence Ratio
Thermal Efficiency (%)
Ba seline Hydrogen Assist
8
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