Energy 263 (2023) 125896
Contents lists available at ScienceDirect
Energy
journal homepage: www.elsevier.com/locate/energy
Combined experimental-numerical analysis of hydrogen as a combustion
enhancer applied to wankel engine
Cheng Shi a, *, Sen Chai a, Liming Di a, Changwei Ji b, **, Yunshan Ge c, Huaiyu Wang c
a
School of Vehicle and Energy, Yanshan University, Qinhuangdao, 066004, China
Department of Automotive Engineering, Key Lab of Regional Air Pollution Control and Beijing Lab of New Energy Vehicles, Beijing University of Technology, Beijing,
100124, China
c
School of Mechanical Engineering, Beijing Institute of Technology, Beijing, 100081, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Hydrogen
Combustion
Wankel engine
Numerical simulation
Equivalent ratio
Hydrogen is regarded as one of the most potential alternatives for high-efficiency and eco-friendly combustion.
The influences of hydrogen fractions and equivalence ratios on chamber pressure, combustion phases, wall heat
loss, brake thermal efficiency, and cyclic fluctuation of Wankel engines were studied by testbench measurement.
The intrinsic mechanisms of these effects were analyzed through numerical investigation. Results indicated that
increasing hydrogen fraction increased the mass fraction of high-temperature regions, the timings taken for
formations of intermediate products were advanced, and the peak H2O2 and CH2O reduced while OH concen­
tration increased. These factors were responsible for increased chamber pressure and the shortened flame
development and propagation periods in experiments. A close inverse relationship can be drawn between the
wall heat loss and brake thermal efficiency with respect to hydrogen addition. A larger hydrogen fraction coupled
with a smaller equivalence ratio manifested lower wall heat loss and higher thermal efficiency. Compared with
hydrogen-free regimes, brake thermal efficiency of 6% hydrogen fraction was increased by 64.6%. There was a
positive proportional correspondence between the initial flame development and cycle-to-cycle fluctuation.
Increasing the hydrogen content caused the enhanced turbulent intensity during the initial combustion stage,
which contributed to the stability improvement of engine operations.
1. Introduction
curb weight and fuel consumption of the aircraft significantly.
As a clean gaseous fuel that can be used in engines, hydrogen (H2) is
an ideal alternative fuel and has been widely used in reciprocating
piston engines [4] and rotary piston engines [5]. Unlike other fossil
fuels, H2 is a zero-carbon secondary energy source, which contributes to
the goal of emission peak and carbon neutrality [6]. H2 has drawn much
attention due to its inherent advantages, such as a wide range of sources,
fast flame propagation, low ignition energy, and high diffusion coeffi­
cient [7]. Much of the published meaningful work has demonstrated that
adding a certain proportion of H2 to liquid fuels like gasoline, ethanol,
and butanol is one of the effective ways to enhance thermal efficiency
and emissions control of the engine, especially for Wankel rotary en­
gines [8]. Researchers have achieved significant achievements in H2
enrichment Wankel rotary engines [9]. The experiment and simulation
results showed that the thermal efficiency, cyclic variation, and fuel
consumption could be improved by H2 enrichment [10]. Besides,
The Wankel rotary engine has gained extensive attention in aviation
due to its simple structure, large power-to-weight ratio, favorable highspeed performance, low vibration and noise, and easy maintenance [1].
At present, rotary engines are primarily used in small-scale low-speed
aircraft, such as unmanned aerial vehicles, gliders, powered parafoils,
and helicopters [2]. The direct-injection stratified multi-fuel rotary en­
gine developed by Curtiss-Wright Corp. has successfully been placed into
small aircraft production. UAV Engines Ltd., an unmanned aerial vehicle
manufacturer in the United Kingdom, is well-known for its AR series
rotary engines as an alternative unmanned aerial vehicle power source
[3]. On the helicopter side, the rotary engine fueled with aviation gas­
oline of the unmanned helicopter S-100 Schiebel Corp. must be
mentioned. The power unit for the simple flight from Cessna Ltd. applied
rotary engines to the advanced airframe for the first time, reducing the
* Corresponding author.
** Corresponding author.
E-mail addresses: shicheng@ysu.edu.cn (C. Shi), chwji@bjut.edu.cn (C. Ji).
https://doi.org/10.1016/j.energy.2022.125896
Received 13 August 2022; Received in revised form 15 October 2022; Accepted 24 October 2022
Available online 27 October 2022
0360-5442/© 2022 Elsevier Ltd. All rights reserved.
C. Shi et al.
Energy 263 (2023) 125896
unburned hydrocarbon (HC) and carbon monoxide (CO) emissions
decreased, while nitrogen oxide (NOx) emissions increased [11]. This is
mainly due to the fact that the H2 is very diffusive, and its dispersion into
the medium presents faster than gasoline, which forms a more homo­
geneous mixture in the elongated combustion chamber and increases
turbulent flow intensity, thus making it easier to ignite the fuel-air
mixture [12]. The faster flame propagation speed of H2 enables the
fuel to burn more completely within the rotor chamber of rotary en­
gines, reducing cooling losses, exhaust losses, and the content of
incomplete combustion products in the exhaust port. H2 enrichment can
shorten the quenching distance of the mixture so that it can be burned in
the narrow space, the joint of the gap, and the end of the rotor,
decreasing the unburned HC and CO generations caused by the narrow
quenching gap [13]. Furthermore, the higher adiabatic flame tempera­
ture of H2 increases the average temperature in the rotor chamber,
which is conducive to promoting the combustion rate and flame spread
[14].
Extensive studies have pointed out the benefits of lean-burn oper­
ating conditions. Moreover, the H2-enriched fuel-air mixture aids in
improving the cyclic variation when the engine is performed in a diluted
mixture regime [15]. The reasons why the lean-burn condition enhances
the engine’s thermal efficiency by reducing these limitations of stoi­
chiometric combustion are summarized as follows: Firstly, the smaller
equivalence ratio enables more fuel-air mixture to be heated through
intake charge for minimizing pumping losses and lowering the tem­
perature in the combustion chamber [16]. Secondly, compared with
stoichiometric combustion, applying the lean operation leads to low
cooling losses and better thermodynamic performance of the combus­
tion products [17]. The latter means a small specific heat capacity,
thereby elevating the ratio of specific heat and work-extraction effi­
ciency of the expansion stroke [18]. Thirdly, adopting lean operation to
completely burn in the vicinity of TDC because of oxidation of active
radicals in the thermal H2-enriched mixture [19]. Fourthly, a leaner
mixture can reduce the pressure rise rate by smoothing the reaction rates
and thus has the benefit of extending the high-load operating range [20,
21]. The final attractive feature lies in the slow combustion process of
the lean-burn mixture, which suppresses excessive heat release, hence
reducing the exhaust temperature and NOx generation [22]. To sum up,
H2 addition, coupled with the lean mixture, could ensure basic re­
quirements for spark-ignition engines concerning power output, but
with the merit of enhancing combustion efficiency and decreasing
unwished pollutant formations.
Although previous investigations had illustrated significant infor­
mation on the intake H2-enriched lean-burn, these studies were only
related to reciprocating piston engines [23]. Moreover, the literature
survey implies that perhaps no effort was involved in H2 addition in
Wankel engines by a combined experimental-numerical approach.
Relevant studies on the correlation between H2 enrichment in
spark-ignition rotary engines and different lean-burn operations using
equivalence ratios are also unavailable. By adjusting the H2 fraction, it is
imperative to optimize the lean-burn regime because of the direct in­
fluence on combustion characteristics and general engine thermody­
namics [24]. Additionally, the experimental measurement is difficult in
analyzing flow field variation, initial flame development, combustion
proceeding under different H2 concentrations and equivalence ratios,
and the intrinsic mechanism of emission generations due to costs and
devices. In comparison with engine experiments, the CFD modeling re­
cords transient responses, and it is hard to analyze mixture preparation
and combustion behavior in detail [25]. The complete combustion
process of spark-ignition gasoline Wankel rotary engines by the coupling
function between H2 addition and equivalence ratio remains to be
investigated.
As stated above, the operation amelioration of hydrogen-enriched
rotary engines has special importance. In existing studies, there is no
combination investigation for rotary engines at different H2 fractions
with equivalence ratios. This is essential for the determination of the
operating parameters of rotary engines for UAV propulsions. Especially
for hybrid UAVs, in which the rotary engine as a power source can
output power continuously at a fixed speed [26]. In summary, based on a
rotary engine prototype, the H2 enrichment is varied from the original
gasoline rotary engine using a developed H2 injector in the intake port,
and operations are performed at different equivalence ratios. A
three-dimensional kinetic model of a Wankel engine coupled with the
reaction mechanism is developed and calibrated under varying experi­
mental conditions. Based on the combined experimental–numerical
investigation, the optimal match between H2 fraction and equivalence
ratio is explored to provide theoretical guidance for rotary engines
fueled with hydrogen enrichment in this study.
2. Materials and methods
2.1. Experimental setup
To meet the experimental requirements, some experimental in­
stallations were used in this work: (1) To monitor the flow of air, Tociel
20N060 thermal flowmeter was employed in the course of this work. (2)
To record the mean pressure within the cylinder, the Kistler 6117BFD17
piezoelectric pressure sensor was applied and embedded in the spark
plug. The Kibox combustion analyzer was adopted in the experiment
testbench to obtain the chamber pressure data. (3) To measure the
excess air ratio (λ), a wide-range oxygen sensor and Horiba MEXA-730λ
wide-range lambda analyzer were employed. (4) the Horiba MEXA-730λ
wide-range lambda analyzer was used under n1/P mode in order to
control the engine speed and throttle percentage of rotary engines. The
schematic overview and measurement uncertainty of the above in­
stallations are shown in Fig. 1 and Table 1, respectively.
As the main component of this experimental system, the engine is
selected as a single-cylinder single-spark plug rotary engine, the major
specifications of which have been listed in Table 2. Some modifications
were made to meet the experimental requirements: (1) A self-developed
optimized electronic control unit (ECU) is added to adjust the spark
timing and H2 fraction. The optimized ECU intercepts the original en­
gine’s signals, including the spark-ignition signal and the H2-injected
signal. Then the optimized ECU outputs the control signals according to
the parameters input to the corresponding control software. (2) A H2
supply system, as shown in Fig. 1, was employed to take the place of the
traditional gasoline supply system. The H2 purity and injection pressure
in this work is 99.99% and 5 bar. (3) Limited by the version of Kibox, the
original trigger wheel, which is unsymmetrical 36-2-2-2, can not be
recognized. Hence, a trigger wheel with 24–1 gears was added to the
center shaft. Besides, a photoelectron magnetic sensor communicating
with Kibox was also added to identify the engine speed signals.
To understand the role of H2 enrichment in combustion character­
istics of the gasoline Wankel rotary engine, a total of 21 operating points
for which intake charged mixture cases with varying H2 enrichment and
varying equivalence ratio (Φ) have been carried out in the present work.
Seven equivalence ratios of 0.77, 0.80, 0.83, 0.87, 0.91, 0.95, and 1.00
were used, the corresponding excess air ratios were varied from 1.3 to
1.0 in 0.05 paces, respectively. The H2 volumetric fractions of 0%, 3%,
and 6% were selected to contrast and clarify the H2 fraction effect. The
equivalence ratio and H2 fraction are severally defined as the following
equations. Where VH2 and Vair denote the volume of H2 and air sepa­
rately. ρH2 and ρair are the densities of H2 and air. AFst,H2 and AFst, gaso
represent the stoichiometric air-to-fuel ratios of H2 and gasoline,
respectively. mgaso is the mass flow rate of gasoline. The main operating
conditions for the engine test bench are summarized in Table 3.
Φ=
VH2 ρH2 AFst,H2 + mgaso AFst,gaso
Vair ρair
αH2 =
2
VH2
× 100%
VH2 + Vair
(1)
(2)
C. Shi et al.
Energy 263 (2023) 125896
Fig. 1. Schematic overview and actual display of the rotary engine testbench.
Table 1
Summary of measurement sensitivities.
Table 2
Summary of engine specifications.
Parameter
Instrument
Manufacturer
Uncertainty
Parameter
Value
Engine speed
Torque
Cylinder pressure
Gasoline mass flow rate
H2 volumetric flow rate
Air volumetric flow rate
Air-to-fuel ratio
CAC6
CAC6
6117BCD17
FC2210
D07-19BM
20N060
MEXA-730λ
Power link
Power link
Kistler
Power link
Seven star
Tociel
Horiba
≤±1 rpm
≤±0.4% F⋅S.
≤±0.3 bar
≤0.8% F⋅S
≤±0.02 L/min
≤±0.1 L/min
≤±0.007
Engine type
Fuel supplement
Generating radius
Eccentricity
Rotor width
Compression ratio
Displacement
Power output
Intake opening
Intake closure
Exhaust opening
Exhaust closure
Side-ported, air-cooled
Port fuel injection
69 mm
11 mm
40 mm
8:1
0.16 L
3.8 kW@4200 rpm
− 465◦ EA ATDC
− 209◦ EA ATDC
208◦ EA ATDC
250◦ EA ATDC
2.2. Computational models
The CFD simulation is a numerical analysis method. Considering the
computational accuracy and time cost, this numerical calculation used a
2 mm basic grid with adaptive encryption as the combustion chamber
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C. Shi et al.
Energy 263 (2023) 125896
Table 3
Summary of engine operating conditions.
Parameter
Value
Engine speed
MAP
Spark timing
Gasoline injection timing
Hydrogen injection timing
Gasoline injection pressure
Hydrogen injection pressure
Volumetric coefficient
4500 rpm
0.035 MPa
25◦ EA BTDC
195◦ EA BTDC
195◦ EA BTDC
0.3 MPa
0.3 MPa
0.78
grid size. At the moment of spark plug ignition, the total number of
meshes in the working chamber was about 160,000. The calculated
mesh model was post-processed using EnSight software to obtain the
overall mesh model of the rotary engine with different geometric pro­
files and the local mesh model of rotor chambers, as depicted in Fig. 2.
The mesh is automatically encrypted according to the temperature, ve­
locity field, etc. The computation in CONVERGE enhanced powerful
features of AMR function (i.e., adaptive mesh refinement), fixed
embedding, and acceleration algorithms, such as the Multi-zone source
term, which aids in ensuring the computational accuracy and saving the
simulation time [27]. Specifically, the Multi-zone model solves detailed
chemistry (SAGE) in zones to accelerate the solution of detailed chem­
ical kinetics based on work by Babajimopoulos et al. [28]. In the
Multi-zone model, at each discrete time t, every cell in CONVERGE is at
some thermodynamic state. The cells are grouped into zones based on
the thermodynamic state of the cells. More details of the procedure for
grouping into zones are elaborated in Ref. [29]. The summary of the
working routine of the current work is described in Fig. 3.
Appropriate turbulence models, spray models, ignition models,
combustion models, and emission generation models were selected ac­
cording to the engine operating conditions, and the engine was solved
numerically to obtain detailed not only microscopic characteristics of
the flow field in the working chamber but also effective macroscopic
information of the engine operating characteristics. In CONVERGE code,
the AMR is based on velocity and temperature used in intake, chamber,
and spark regions [30]. The selection of each model is shown in Table 4.
The appropriateness of boundary and initial conditions was directly
involved in model reliability and calculation precision. In order to
describe the specific engine modeling environment and meet the actual
operating conditions, this study adopted the full transient processing
method to set the boundary conditions and initial conditions of the
operating conditions. The boundary and initial conditions are shown in
Table 5. As a simplification in this work, the intake charge was assumed
perfectly homogeneous, and the fuel thoroughly evaporated during the
CFD calculation, and heat transfer with the environment was not taken
Fig. 3. Summary of the working routine for numerical simulation.
Table 4
Summary of computational models and chemical mechanisms.
Description
Models and mechanism
Turbulence
Wall heat transfer
Ignition
Combustion
Reaction kinetics
RNG k-ε model [31]
Han and Reitz model [32]
Spark-energy deposition model [33]
SAGE model [34]
PRF skeletal mechanism [35]
Table 5
Summary of boundary and initial conditions.
Region
Type
Value
Air inlet
Exhaust outlet
Rotor
Inlet port
Outlet port
Stator
Spark plug
Spark electrode
Inflow
Outflow
Moving wall
Fixed wall
Fixed wall
Fixed wall
Fixed wall
Fixed wall
0.035 MPa/293 K
0.1 MPa/700 K
550 K
293 K
293 K
550 K
750 K
850 K
into account as well [36].
2.3. Validation
In order to verify the accuracy of the CFD model, the hydrogenenriched rotary engine were simulated under different operating con­
ditions. Fig. 4 compares the simulated and measured chamber pressure
and heat release rate traces with various operating points, matching the
experimental data. Although there are some differences between simu­
lated measured traces, the accuracy of the CFD model meets the re­
quirements of engineering applications.
Fig. 2. Computational mesh of the operated rotary engine.
4
C. Shi et al.
Energy 263 (2023) 125896
Fig. 4. Model validation for chamber pressure and heat release rates versus eccentric-shaft position.
3. Results and discussion
Variation of the equivalence ratio does not affect the trend. For example,
Fig. 5(a) illustrates that when the equivalence ratio is 0.8, the peak
chamber pressure for αH2 = 0% is 0.89 MPa, while the peak chamber
pressure for αH2 = 6% is 1.29 MPa. Their corresponding eccentric-shaft
positions are 439.1◦ EA and 395.1◦ EA. At other equivalence ratio con­
ditions, as shown in Fig. 5(a) and (b), the variation tendency in the peak
pressure is similar. Besides, as the hydrogen addition is increased, dif­
ferences in peak chamber pressure increase, despite the peak chamber
pressure or corresponding eccentric-shaft position. Specifically, the
3.1. Analysis of combustion characteristics
Fig. 5 plots the peak chamber pressure and the corresponding
eccentric-shaft position under varying H2 fraction and equivalence ratio
conditions based on the test bench measurement. As can be observed in
Fig. 5, the peak chamber pressure increases, and the corresponding
eccentric-shaft position is advanced as the H2 is enriched larger.
Fig. 5. Peak chamber pressure and corresponding eccentric-shaft position under varying operating conditions.
5
C. Shi et al.
Energy 263 (2023) 125896
pressure difference in the αH2 = 6% condition, 1.01 MPa, is greater than
that of the H2-free condition (αH2 = 0%), 0.07 MPa.
Fig. 6 shows the flame development period (EA0-10) and flame
propagation period (EA10-90) versus the H2 fraction and equivalence
ratio for measured rotary engines. As depicted in Fig. 6, the flame
development period is slightly prolonged with a small equivalence ratio.
It can also be found that the flame development period is significantly
shortened with increasing H2 fractions. This phenomenon performs in
all operating points of rotary engines. In addition, the flame propagation
period reduces with the increment of H2 fractions at a specific equiva­
lence ratio. At a leaner point (0.77), the EA10-90 of (αH2)s = 3%, 6%
cases are shortened by 25.5◦ EA and 28.9◦ EA compared with H2-free
regimes, respectively. The flame propagation period and the flame
development period have a similar trend of being shortened with the
increase of the equivalence ratio. This demonstrates that the center of
heat release shifts earlier, as seen in Fig. 5(b), and contributes to a rapid
combustion process. Apparent is that the effect of the equivalent ratio on
the flame development period of H2-free regimes is greater than that of
hydrogen-enriched regimes.
To further understand the experimental data and clarify the under­
lying mechanism of hydrogen enrichment effect on combustion char­
acteristics in rotary engines under varying equivalence ratio conditions,
a three-dimensional comprehensive transient simulation is carried out
based on CONVERGE code. The cut in Fig. 7 runs through the median
plane of the rotary engine, which reveals the temperature distribution
across the rotor chamber for each tested case at TDC. The flame spreads
faster as the equivalence ratio is high. At TDC, when the equivalence
ratio is 1.00 (stoichiometric operation), the flame has spread towards
the leading part of the combustion chamber of the engine. Most of the
flames of the other equivalence ratios are still mainly distributed in the
vicinity of the spark plug. It is noteworthy to highlight that introducing
H2 brings about the burning acceleration and temperature elevation
obviously. Thus, a steep temperature difference is anticipated for the
main phase of combustion. Subsequently, the combustion temperature
in the expansion stroke is the crucial indicator in affecting the com­
bustion characteristics of this engine concept. Compared with the
maximum in-cylinder temperature, the maximum mass fraction of the
high-temperature domain above the fixed temperature (MMFT) can
better reflect the combustion temperature inside the rotary engine [33].
A fixed temperature of 2500 K is used as a criterion for the ongoing
research in Fig. 8. One can see that the significant MMFT enhancement
happens with the 3% increment of the αH2, which explains the large
influence on the chamber pressure and combustion rate. As can be seen
in Fig. 8, when the equivalence ratio varies from 0.77 to 1.00, the MMFT
exceeds 2500 K acts trend to the temperature distribution at TDC and
intensifies the level of thermal atmosphere in the rotor chamber of the
engine. Consequently, it is possible to shift the center of the heat release
earlier and reduce the period of the combustion process.
The underlying mechanism of these trends can also be clarified from
a species variance point of view. As crucial points to reflect the ignition
event and combustion intensity, several important combustion inter­
mediate products (H2O2, OH, and CH2O) are shown to reveal how H2
addition influences the combustion characteristics of the rotary engine
fueled with stoichiometric and lean mixtures as plotted in Fig. 9. From
H2-free to H2-enriched operations, the combustion velocity expedites
with the increase in temperature in the rotor chamber (Fig. 7), the
earlier eccentric-shaft position corresponding to the generation of some
crucial intermediates are available, and the peak mass fractions of H2O2
and CH2O reduce whereas the peak value of OH increases. Apparent is
the fact that with the increase in combustion temperature inside the
engine, less time is taken in the decomposition reaction of H2O2 (M) ⇔
OH + OH (M); hence a large amount of H2O2 is consumed and dissolved
in the high-temperature stage, thereafter decreasing the peak concen­
tration of H2O2. Then, at the higher temperature condition, OH is
accumulated at a larger level (reflected by the slope of the decline curves
of H2O2), and the eccentric-shaft position corresponding to the massive
generation of the OH active radical is somewhat advanced. Due to CH2O
is being mainly consumed via CH2O + OH ⇔ HCO + H2O, the eccentricshaft position corresponding to the consumption of CH2O shifts earlier at
a higher OH concentration, and the peak mass fraction of CH2O de­
creases. Additionally, when the H2 fraction is constant, the peak mass
fractions of H2O2, OH, and CH2O drastically reduce under a leaner
operating condition. The reason is that the quantity of reactants declines
and results in small-scale burning at a lower equivalence ratio tested
point, which is attributed to the equivalence ratio effect on the measured
data of combustion characteristics of the rotary engine in Figs. 5 and 6.
3.2. Analysis of combustion performance
Based on the experiment results of the rotary engine, Fig. 10 shows
the variations in the wall heat loss (Qw) and brake thermal efficiency
versus the equivalence ratio at different H2 fractions. It is generally
accepted that the Qw directly affects the thermal efficiency of the engine.
For the determination of the Qw, the heat transfer coefficient adapted for
the rotary engine is employed according to Ref. [37]. By adjusting the
engine operation to lean-burn conditions, it is bound to cause less Qw
than stoichiometric combustion. At a constant H2 content, decreasing
the equivalence ratio within the cylinder almost linearly reduces the Qw,
as shown in each subfigure of Fig. 10. As for brake thermal efficiency
histories with the equivalence ratio, a direct proportional correspon­
dence of the Qw and brake thermal efficiency trend is presented. The
difference in brake thermal efficiency at varying equivalence ratio
conditions is more distinct under the H2-free condition. When the
equivalence ratio changes from 0.77 to 1.00, brake thermal efficiency of
αH2 = 6% cases was severally increased by 6.5%, 6.2%, 6.2%, 5.7%,
2.9%, 1.6%, and 1.0% compared with hydrogen-free regimes. However,
as the H2 fraction increases, from Fig. 10(a)–(c), a sharp decrease of the
Qw is noticeable, whereas the increased tendency is available in brake
thermal efficiency. A close inverse correspondence can be obtained be­
tween the Qw and brake thermal efficiency indices provided with respect
to H2 addition.
It is reasonably considered that the Qw of the engine is generally
determined by the heat transfer area, in-cylinder temperature, and
duration of high-temperature combustion. Fig. 11 plots the duration of
the mass fraction of the high-temperature domain (DMFT) above 2500 K
under varying operating conditions by CFD modeling. Taken Fig. 8
together with Fig. 11, it can be confirmed that a higher MMFT and
longer DMFT are capable of increasing Qw of the rotary engine for a
Fig. 6. Flame development and flame propagation periods under varying
operating conditions.
6
C. Shi et al.
Energy 263 (2023) 125896
Fig. 7. Temperature distribution at TDC under varying operating conditions.
the three mentioned factors in the Qw effect, the dropped heat transfer
area outperforms, hence inflicting higher H2 fraction cases with lower
Qw .
The interpretation of brake thermal efficiency trends is possible by
means of analyzing the distributions of unburned fuels and combustion
products. Fig. 12 shows the spatial distribution of unburned iso-octane
(IC8H18) at EA90 in the investigated rotary engine. Looking at the
leading part of the rotor chamber at the end of the burning duration, it is
found that this area still distributes unburned IC8H18 under the H2-free
regime, which is responsible for worse combustion loss. The reason is
that as the equivalence ratio increases, the area of unburned reactants
becomes smaller. Particularly, the entire unburned IC8H18 in the leading
corner has dissolved completely in the case of stoichiometric operation.
This means a better combustion event is obtained if the burning pro­
ceeds to chemical equilibrium. As the combustion event continues, it is
intuitively reasonable that brake thermal efficiency is limited by the
level of complete combustion products immediately before the moment
of the exhaust opening (568◦ EA), typically corresponding to the spatial
distribution of CO2, as shown in Fig. 13. Enriching the air/fuel mixture
leads to the increase of CO2 formation, as well as the enlargement of the
burned area, which is partly because of flow motion. As shown in
Fig. 14, more squish flow moves towards the leading side of the rotor
chamber at a larger equivalence ratio, implying that compared with the
leaner operation, the burned area could reach the leading edge. With
regard to the H2 addition effect, increasing the H2 content in the com­
bustion chamber enlarges the spatial distribution of CO2, thus improving
the level of complete combustion, which confirms the analysis above.
Fig. 8. MMFT value (＞2500 K) under varying operating conditions.
specific H2 fraction level, which plays an important role in the com­
bustion performance of the rotary engine. But this effect is less pro­
nounced concerning the H2 adjustment. Although the DMFT is slightly
prolonged by H2 enrichment, unexpectedly the Qw at a high H2-enriched
level is reduced. The reason for this efficiency loss results from the
substantially shortened flame development period and flame propaga­
tion period after H2 blending, and fewer amounts of fuel-air mixtures are
combusted during the expansion stroke, which results in the dropped
heat transfer area [38]. It can be deduced that in the competition among
3.3. Analysis of combustion stability
On the basis of engine test-bench measurements, the COVPmax, COVn,
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C. Shi et al.
Energy 263 (2023) 125896
Fig. 9. Combustion intermediate products as a function of eccentric-shaft position.
Fig. 10. Wall heat loss and brake thermal efficiency under varying operating conditions.
and COVEA10-90 are imposed to assess the cycle-to-cycle fluctuation of
the rotary engine, which denotes the coefficients of cycle-to-cycle vari­
ation (COV) of the peak chamber pressure, engine speed, flame propa­
gation period, as plotted in Fig. 15. In general, one can see that the
combustion stability is more and more sensitive to engine operations as
the equivalence ratio decreases, and rough burning is obtained by
diluting the fuel-air mixture leaner. For the case that the equivalence
ratio is 0.77, the COVPmax and COVEA10-90 increase significantly
compared with a higher equivalence ratio. Of particular interest are the
results from the COVn profile for αH2 = 6% cases, and there is a negli­
gible difference in the COVn under varying operating points. When the
H2 fraction increases from 0% to 6%, the COVPmax and COVEA10-90
decrease sharply, hence enhancing the burning stability of the rotary
engine. In addition, introducing H2 has less influence on the COVPmax,
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C. Shi et al.
Energy 263 (2023) 125896
COVn, and COVEA10-90 at the equivalence ratio of 1.00, but it has an
evident effect under lean-burn operations.
There is no denying the fact that the early combustion stage has an
important effect on the combustion stability of the rotary engine [39].
However, the intrinsic mechanisms between early flame development
and combustion stability are still unclear. In order to acquire intuitive
information in detail, Fig. 16 gives the growth of the initial flame rep­
resented by the iso-contour of 2000 K (black line in Fig. 7). It is observed
that the flame front location for different operating points is similar in
the rotor chamber. The flame propagates chiefly in the same direction as
the rotor rotation, whereas the flame spread is retarded in the opposite
direction. This effect is most likely due to the existence of one-way
mainstream characteristics from the trailing to the leading edge of the
rotor chamber [40]. As shown in Fig. 16, the increase in the equivalence
ratio could lead to the faster formation of the flame kernels, which
contributes to a stable combustion initiation and shortened burning
duration [41]. In addition, when the H2 fraction is 6%, the flame front
has spread across the leading part of the combustion chamber after the
spark timing of 20◦ EA (355◦ EA). The majority of the burning sources of
remaining cases (0% and 3%) are mainly centralized in the vicinity of
Fig. 11. DMFT value (＞2500 K) under varying operating conditions.
Fig. 12. Unburned iso-octane distribution at EA90 under varying operating conditions.
9
C. Shi et al.
Energy 263 (2023) 125896
Fig. 13. CO2 distribution before exhaust opening moment under varying operating conditions.
the spark-plug location yet.
Although the coincidence of the initial flame growth and combustion
stability, an acquisition of the combustion rate almost fails to succeed
with observed initial kernels [42]. To evaluate the feature of early flame
development, a quantitative analysis is carried out by the burned vol­
ume rise rate (BVRR), which is calculated by the following formula:
BVRR = lim
ΔEA→0
VEA+ΔEA − VEA
ΔEA
power stroke. As anticipated, the BVRR increases quickly with H2
addition, as can be inferred from the fact that initial flame development
is accelerated, thus improving the combustion stability [43].
In addition to initial flame development, another probable cause is
the variation in the flow motion of turbulence, which affects the stability
of the rotary engine. For a better insight into the influences of the H2
fraction and equivalence ratio on the turbulent level inside the engine,
Fig. 18 shows a high-resolution median cross-section plane of the rotor
chamber using turbulent kinetic energy (TKE) and turbulent dissipation
(EPS). It is evidently observed from Fig. 18(a) that with a 3% increment
of the H2 fraction, the high region of TKE within the rotor chamber in­
creases to a certain degree. At spark timing, the turbulence level is
intensified around the spark-plug location with hydrogen additive. In
Fig. 18(b), when the H2 fraction augments from 3% to 6%, a higher EPS
is concentrated in burning areas at the leading part of the combustion
chamber, which contributes to an effective ignition and stable com­
bustion. Despite the minimal effect on TKE distribution in terms of the
(3)
where VEA is the volume of the burned area through flame propagation
at an EA, and △EA is the increment of the EA. Specifically, the volume
of the burned area is applied to quantify early flame development as the
local volume of the region enclosed by the isothermal of 2000 K. It can
be observed from Fig. 17 that fuel-burning speed decreases when the
flame propagates towards the lean mixture, which contributes to the
reduction of the BVRR. As the proceeding of combustion, the BVRR of
the rich-mixture scheme becomes increasingly higher in the stage of the
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Energy 263 (2023) 125896
4. Conclusions
This research investigates H2 as a combustion enhancer applied to
the gasoline rotary engine for UAV propulsion. The combined effect of
H2n enrichment and equivalence ratio on combustion characteristics,
thermal efficiency, and cyclic variations of the engine was measured
through an engine test bench, and the mechanisms behind experimental
data were revealed by CFD simulations. The present work gave a unique
insight into the combustion characteristics, performance, and stability of
the H2-enriched rotary engine under stoichiometric and lean conditions.
The contribution of the present work elaborately addressed the flow
phenomenon and combustion process in the rotor chamber. Also, they
were compared with existing investigations to dig out the potential of
improving combustion characteristics and thermal efficiency of rotary
engines for UAV propulsion. Based on the comprehensive analysis, the
conclusions available in the present work may be summarized as
follows:
Fig. 14. Velocity distribution before exhaust opening moment under varying
operating conditions.
(1) For a specific equivalence ratio, the mass fraction of hightemperature regions increased with the introduction of H2, the
timings taken for the formations of intermediate products were
notably advanced, and the peak mass fractions of H2O2 and CH2O
reduced while the OH concentration increased. These factors
contributed to increased chamber pressure and shortened com­
bustion phases in experiments. At a leaner point (0.77), the EA1090 of (αH2)s = 3%, 6% cases were shortened by 25.5◦ EA and
28.9◦ EA compared with H2-free regimes, respectively.
(2) A close inverse correlation can be deduced between the wall heat
loss and brake thermal efficiency indices provided in terms of H2
equivalence ratio effect, a remarkable difference in EPS can be observed
with the variation of the equivalence ratio. According to Fig. 18(b),
there is a lower EPS level at a leaner operation, and a high concentration
of EPS can be seen on both sides of the rotor chamber. This means that
the turbulent flow is not used for the combustion effect of the fuel but
has rather been consumed by the friction with the cylinder wall [44],
which inevitably aggravates the instability of the engine under the same
H2-enriched condition.
Fig. 15. COVPmax, COVn, and COVEA10-90 under varying operating conditions.
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C. Shi et al.
Energy 263 (2023) 125896
Fig. 16. Initial flame growth under varying operating conditions.
enrichment. Due to the synergistic effects of the heat transfer
area, combustion temperature, and the duration of hightemperature combustion, a larger H2 addition with leaner oper­
ation showed lower wall heat loss and higher brake thermal ef­
ficiency. When the equivalence ratio changes from 0.77 to 1.00,
brake thermal efficiency of αH2 = 6% cases was severally
increased by 6.5%, 6.2%, 6.2%, 5.7%, 2.9%, 1.6%, and 1.0%
compared with hydrogen-free regimes.
(3) There was a directly proportional correspondence between the
initial flame growth and cyclic fluctuations. The increase of the
equivalence ratio could lead to the faster formation of the flame
kernels, which contributed to a stable combustion initiation and
shortened burning duration.
(4) Increasing the hydrogen content caused a higher turbulent ki­
netic energy and turbulent dissipation in burning areas around
the spark-plug location during the initial combustion stage,
which was responsible for the stability improvement of the rotary
engine.
(5) There is a lower turbulence level at a leaner operation, and a high
concentration of turbulent dissipation can be seen on both sides
of the rotor chamber. This means that the turbulent flow is not
used for the combustion effect of the fuel but has rather been
consumed by the friction with the cylinder wall, which inevitably
Fig. 17. Burned volume rise rate as a function of eccentric-shaft position.
12
C. Shi et al.
Energy 263 (2023) 125896
Fig. 18. TKE and EPS distributions under varying operating conditions.
aggravates the instability of the engine at a specific H2-enriched
regime.
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Author contribution
Cheng Shi: Conceptualization, Data curation, Investigation, Meth­
odology, Software, Validation, Visualization, Writing – original draft,
Writing – review & editing, Sen Chai: Investigation, Writing – review &
editing, Liming Di: Formal analysis, Project administration, Changwei
Ji: Funding acquisition, Project administration, Software, Yunshan Ge:
Resources, Supervision, Huaiyu Wang: Investigation, Methodology.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
This work was financially supported by the Fundamental Research
Funds for the Central Universities, CHD (Grant No. 300102222512),
National Natural Science Foundation of China (Grant No. 51976003),
Natural Science Foundation of Hebei Province (Grant No.
E2020203127), and Cultivation Project for Basic Research and Innova­
tion of Yanshan University (Grant No. 2021LGQN011).
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