Journal of Thermal Science Vol.28, No.

4 (2019) 669681

https://doi.org/10.1007/s11630-019-1081-0 Article ID: 1003-2169(2019)04-0669-13

Numerical Investigation of the Effect of Hydrogen Enrichment on an

Opposed-Piston Compression Ignition Diesel Engine

ZHOU Jianhao*, SHENG Xueshuang, HE Longqiang

JiangSu Province Key Laboratory of Aerospace Power System, Nanjing University of Aeronautics and Astronautics,
Nanjing 210016, China

© Science Press, Institute of Engineering Thermophysics, CAS and Springer-Verlag GmbH Germany, part of Springer
Nature 2019

Abstract: High power-to-weight and fuel efficiency are bounded with opposed-piston compression ignition
(OPCI) engine, which makes it ideal in certain applications. In the present study, a dynamic three-dimensional
CFD model was established to numerically investigate the combustion process and emission formation of a model
OPCI engine with hydrogen enrichment. The simulation results indicated that a small amount of hydrogen was
efficient to improve the indicated power owing to the increased in-cylinder pressure. Hydrogen tended to increase
the ignition delay of diesel fuel due to both dilution and chemical effect. The burning rate of diesel fuel was
apparently accelerated when mixing with hydrogen and premixed combustion became dominated. NOx increased
sharply while soot was sufficiently suppressed due to the increase of in-cylinder temperature. Preliminary
modifications on diesel injection strategy including injection timing and injection pressure were conducted. It was
notable that excessive delayed injection timing could reduce NOx emission but deteriorate the indicated power
which was mainly attributed to the evident decline of hydrogen combustion efficiency. This side effect could be
mitigated by increasing the diesel injection pressure. Appropriate delay of injection coupled with high injection
pressure was suggested to deal with trade-offs among NOx, soot and engine power.

Keywords: two-stroke, opposed-piston compression ignition, hydrogen, combustion

1. Introduction auxiliary power unit or range extender for hybrid electric

vehicle (HEV) [5], etc.
In order to achieve ultra-high fuel efficiency and to With combination of the advantages of Junkers Jumo
meet the increasing stringent emission legislation, engine and boxer engine, opposed-piston opposed-
innovation and revolution of conventional internal cylinder (OPOC) engine was proposed and designed by
combustion engines is around the corner. Modern Hofbauer et al. [6] from FEV GmbH in 2005, including
opposed-piston compression ignition (OPCI) engine, the development of key components, such as controlled
gradually arises attentions and exhibits attractive power- boosting, uniflow scavenging, asymmetry port timing,
to-weight, power-to-size ratio and with a bonus of ~55% opposed injection, etc. Typically, OPOC engine employs
brake thermal efficiency (BTE) [1,2]. OPCI is widely at least one cylinder with axial-separated intake and ex
recognized as suitable for up to 485 kW ground vehicle haust port, mounted or assembled in the cylinder liner
[3], 5/10 kW unmanned aerial vehicle (UAV) [4], and uniflow scavenging is employed. A pair of pistons

Received: July 3, 2018 Corresponding author: ZHOU Jianhao E-mail: zhoujianhao@nuaa.edu.cn

www.springerlink.com
670 J. Therm. Sci., Vol.28, No.4, 2019

Nomenclature
AMR adaptive mesh refinement HRR heat release rate
ATDC after the top center IPC intake port close
BDC bottom dead center LTC low temperature combustion
BTDC before the top dead center OPCI opposed-piston compression ignition
BTE brake thermal efficiency OPOC opposed-piston opposed-cylinder
DIP diesel injection pressure PPC partially premixed combustion
DOE design of experiment SOI start of injection
EGR exhaust gas recirculation TDC top dead center
EPO exhaust port open TKE turbulent kinetic energy
FPEG free piston engine generator UAV unmanned aerial vehicle
HACA hydrogen abstraction and C2H2 addition ULSD ultra low sulfur diesel
HEV hybrid electric vehicle 4SDE four-stroke diesel engine

are disposed in opposition within the cylinder and the captured but was unobservable under reacting condition
combustion chamber is configured from two pistons’ due to the decreased penetration length and increased
crown at top dead center (TDC), hence the cylinder head vaporization rate during high temperature combustion.
is unnecessary. The opposed pistons, namely inner and Spray impingement level was affected by engine load
outer pistons, are driven by inner and outer conrod and speed owing to the variations of in-cylinder fuel
mounted on a single crankshaft, respectively. Two fuel quantity and temperature.
injectors are commonly mounted in the cylinder liner as It can be found that previous research mainly engaged
side injection configuration and are typically arranged in in the optimization of mixture formation via fuel spray
opposite direction. One may imagine that the fuel spray pattern (spray direction, nozzle geometry, etc). However,
and mixture formation through central injection are to the authors’ best knowledge, the fuel variety of OPCI
unique from that in conventional CI (compression engine was scarcely touched in current literature. In this
ignition) engines. regard, multi-fuel adaptability was widely studied and
Side injection of OPCI results in the differences validated in conventional 4SDE. Hydrogen is well-
during fuel-to-air mixing and combustion compared with established as a qualified candidate for 4SDE to enhance
conventional central injection of four-stroke diesel diesel combustion as well as reducing the tailpipe
engine (4SDE). The effect of spray cone angle, nozzle emissions due to its favorable inborn features [10-14].
size and number, injection timing on mixture formation For instance, high flame propagation speed and fast
was discussed by Hofbauer et al. [6] targeting to best burning rate of hydrogen accelerated the premixed
mixture distribution. General trends were concluded that combustion and accordingly shortened the diffusion
lower swirl and higher fuel flow rate contributed to better combustion of diesel fuel which increased the
mixture formation and three-hole injector showed higher homogeneity of diesel combustion and accordingly
air utilization and lower wall-wetting compared to improved BTE [10,11]. Wide flammability limits of
four-hole injector. Zhang et al. [7] presented a numerical hydrogen increased the exhaust gas recirculation (EGR)
study on the effect of split injection strategy on tolerance of a hydrogen-enriched diesel engine which
combustion process for OPOC engine using AVL Fire was capable to operate at advanced combustion mode,
software with ECFM-3Z model in which the pilot fuel such as low temperature combustion (LTC) [12] and
mass, split injection timing and interval were varied. It partially premixed combustion (PPC) [13]. Banerjee et al.
was observed that split injection with longer interval [14] concluded that the synergy of appropriate diesel fuel
performed better thermal atmosphere and space injection strategy (e.g. split injection) and EGR resulted
utilization compared to single injection. Appropriate split in the improvement of BTE and emissions performance.
injection strategy contributed to reduction of soot Besides, the superiority of hydrogen is also popular in
emission but resulted in penalty of NOx emission owing unconventional engines. Hydrogen was employed to
to the increase of in- cylinder pressure and temperature. enhance the limit burn capability of a gasoline fueled
Zhang et al. [8,9] implemented a modified droplet Wankel rotary engine [15]. The results indicated that
bouncing model in KIVA-3v code to study its application hydrogen addition improved the thermal efficiency
in the modeling spray impingement under realistic OPOC significantly and enhanced the cyclic stability. In addition,
engine operating conditions. Their results indicated that carbon based emissions were reduced due to the direct
the non-reacting diesel spray impingement could be substitution of liquid fuel. Unfortunately, the impact on
ZHOU Jianhao et al. Effect of Hydrogen Enrichment on an Opposed-Piston Compression Ignition Diesel Engine 671

NOx emission was not mentioned. Fan et al. [16] mechanism of hydrogen, hence can be adopted to
conducted numerical simulation on ported natural gas simulate the diesel-hydrogen dual-fuel combustion.
rotary engine with low pressure hydrogen injection. The Phenomenological soot model (Waseda model [26]) was
results indicated that early injection of hydrogen used to simulate soot formation and oxidation using
enhanced the mixture formation and resulted in higher detailed chemical kinetics. Extended Zel’dovich NOx
hydrogen concentration around the front of trailing spark model [27] was used to estimate NOx formation.
plug which is helpful for increasing the combustion rate. Fixed and uniform Cartesian grid and cut cell method
Moreover, Yuan et al. [17] used diesel as pilot fuel in a at the boundaries were implemented in CONVERGE.
spark-ignition free piston engine generator (FPEG) with Fixed embedding method was used to refine the grid at
hydrogen as secondary fuel. It was found that the free- specific location including inner/outer piston crown,
piston hydrogen engine has lower peak pressure and heat cylinder liner and dual-injectors consisting of each
release rate due to its faster piston motion around TDC nozzle. Adaptive mesh refinement (AMR) was employed
which resulted in lower NO emission. However, to automatically refine the grid based on the fluctuating
hydrogen combustion under OPCI engine operating and moving conditions of temperature and velocity with
condition is lack of study. specified threshold value, respectively, which is
With above considerations, it is of great interest to favorable to refine the grid surrounding the diesel spray
explore the impact of hydrogen in improving the mixture plume. With synergy of above techniques, the grid size
formation and combustion process of an OPCI diesel was regionally non-uniform and dynamically adjusted at
engine. To this end, a numerical investigation of the each time step as can be seen in Fig. 1. Although the
effect of hydrogen enrichment, with 10%, 20% and 30% basic grid size is 2.5 mm, it will be further refined after
energy share, on a two-stroke OPCI engine with dual- fuel injection and combustion commence, hence the grid
injector and side injection configuration is motivated size will be much smaller which is enough to capture the
which has not been reported in the literature. Particular instantaneous combustion phenomena.
attentions are paid to the impact of hydrogen energy
share ratio, injection timing/pressure of diesel fuel on
combustion process and tailpipe emissions.

2 Numerical scheme and model validation

2.1 Numerical model

In the present study, CONVERGE CFD software was
served as the computational framework for the OPCI
engine simulation. The injection of diesel fuel was
simulated using standard droplet discrete model. Spray
atomization and break-up were modeled using the
KH-RT hybrid model [18]. No time counter [19] and
wall-film models were adopted to simulate droplet
collision/coalescence and drop-wall interaction,
respectively. Turbulent flow was simulated with
implementation of RNG k- model [20] with wall
function which takes wall heat transfer into consideration.
Detailed chemical kinetics solver SAGE [21] coupled
with multi-zone model was employed to simulate the
combustion process. For diesel-hydrogen dual-fuel
engine, most of the study employed detailed or reduced
diesel mechanism where the detailed hydrogen
combustion mechanism was merged in different ways to
consider the influence of hydrogen [22-24]. A chemical
kinetic mechanism consisting of 141 species and 709
reactions was used which was reduced from the detailed
LLNL (Lawrence Livermore National Laboratory)
n-heptane mechanism using directed relation graph error Fig. 1 (a) Total cell number count (SOI: start of injection, EOI:
propagation and sensitivity analysis (DRGEPSA) method end of injection); (b) computational mesh in Y and Z
[25]. The mechanism contains detailed reaction cross sections at 5BTDC
672 J. Therm. Sci., Vol.28, No.4, 2019

An aviation OPOC engine (type M100) developed by

Hofbauer et al. [1] was used as the model OPCI engine in
the present study and its specification is listed in Table 1.
One may find the detailed working principle of an OPOC
engine in Refs. [7,9]. The motion profile of inner and
outer piston was user-defined as shown in Fig. 2. The
time-based simulation results were converted to equivalent
crank angle during data processing and used in the
following discussions. An imaginary surface representing
piston skirt was assembled with inner and outer piston
crowns to accomplish a sealing boundary such that both
the gas scavenging and combustion could be captured
[28]. The diesel fuel spray pattern highly influences the
combustion process, hence the primary principle of the
spray patter design it to avoid impingement of piston
crown and opposed fuel spray from other injector. An
optimal design (spray angle, etc.) suggested by Franke et
al. [29] with dual-injector and three nozzles in each was
adopted in the present study as shown in Fig. 3. The uni- Fig. 3 (a) Working principle of model OPCI engine [7]; (b)
flow scavenging port containing a dual-layer intake port Computational domain at BDC and TDC in
CONVERGE studio; (c) Injection direction of each
with upper straight inflow and lower swirl inflow and an
nozzle of two injectors
exhaust port are shown in Fig. 3(b). The scavenging
process is from exhaust port open (EPO) to intake port
close (IPC), while the in-cylinder combustion process is
Table 1 Specification of an opposed-piston opposed-cylinder from IPC to EPO. The swirl intake port was cut with a
(OPOC) engine tangential of 10° in order to generate low level swirl.
Items Specifications
2.2 Hydrogen supplementation method
Bore/mm 100
Inner, outer stroke/mm 90, 80
In the present study, hydrogen is supposed to be
injected through the intake manifold and naturally
Intake pressure/bar 2.55
aspirated into the cylinder. Since the intake charge is
Geometric compression ratio 18 partially replaced by hydrogen and the density and
Effective compression ratio 12.8 thermal capacity of hydrogen is quite different from air,
175 (inner conrod), the addition of hydrogen will majorly influence the
Connecting rod length/mm
440 (outer conrod)
charge properties at IPC and accordingly affect the initial
Nozzle diameter/mm 0.22
boundary conditions. The calculation from EPO to IPC
Number of nozzle 3 only serves to supply the initial swirl ratio, regional
Peak injection pressure/bar 650 temperature and pressure as boundary conditions. As for
Injection timing/°CA 16 BTDC~~5 ATDC real engine, the hydrogen fuel injector (injection timing,
injection pressure, injector position) should be well
designed and controlled to make sure the energy
substitution of hydrogen achieve target ratio with
particular attention to the intake temperature, pressure,
scavenging ratio, slip ratio of hydrogen, etc. However,
the scavenging performance and control of hydrogen slip
ratio did not fall into the scope of this study, hence were
not covered in the following sections. To this end, the
hydrogen quantity was assumed to reach to the cylinder
with desired energy substitution ratio at IPC and the slip
ratio of hydrogen was neglected.
Since the maximum in-cylinder pressure cannot
exceed 16 MPa, the addition amount of hydrogen is
limited. During the simulation, the total fuel energy,
Fig. 2 Motion profile of inner and outer piston including diesel fuel and hydrogen, maintained the same
ZHOU Jianhao et al. Effect of Hydrogen Enrichment on an Opposed-Piston Compression Ignition Diesel Engine 673

among different cases. The hydrogen energy share ratio combustion. The heat release rate (HRR) profile during
was 10%, 20% and 30%, namely H10, H20 and H30. The premixed combustion is commonly smooth and sharper
energy share ratio of hydrogen was calculated using Eq. than diffusion combustion as shown in the HRR profile
(1), which is widely employed in authors’ previous study of baseline (ULSD) in Fig. 4(a). However, when feeding
[10,30,31]. H10, the ignition delay is extremely extended and a clear
m H 2  LHVH 2 inflexion can be found at around 2°CA BTDC, thus the
X H2   100% (1) premixed combustion becomes different from that of
m diesel  LHVdiesel  m H 2  LHVH 2
baseline. It is well known that hydrogen cannot be
where, X H 2 refers to the energy share ratio of hydrogen; compressed to ignite; diesel is believed to ignite first and
m diesel and m H 2 refer to the mass flow rate of diesel then triggered the combustion of hydrogen. Since the
concentration of hydrogen around diesel plume is low
fuel and hydrogen; LHVdiesel and LHVH 2 refer to the (for H10 case), the combustion efficiency of hydrogen is
lower heating value of diesel fuel and hydrogen. Since quite low (see Section 3.2), hence depicted as premixed
hydrogen was aspirated into the cylinder, the mass flow diesel combustion followed by premixed combustion of
rate of hydrogen was converted from the mass diesel and hydrogen as well as diesel diffusion
concentration of hydrogen in the intake air (the air mass combustion. Therefore, a three-stage combustion process
is known). The enrichment of hydrogen shared part of the was stated When the concentration of hydrogen is high
total fuel energy contributed by diesel fuel and hydrogen, (H20 and H30), the premixed diesel combustion and
thus the injected diesel was reduced which affected its premixed diesel-hydrogen combustion merged together
original injection pressure. To maintain the peak diesel and cannot be divided into two stages. Due to the
injection pressure at 650 bar [6], the injection duration combustion-accelerating effect of hydrogen, diesel-
was shortened accordingly when feeding hydrogen but hydrogen premixed combustion is dominated and diesel
diesel injection timing, nozzle diameter and injection diffusion combustion is weakened in the H20 and H30
cases which results in shortened combustion duration.
profile were kept unchanged.
Although an increase of ignition delay is found with
2.3 Model validation hydrogen addition, the fast burning rate and flame
To validate the present model, the computed in-
cylinder pressure trace was validated with that obtained
from the prototype opposed-piston folded-cranktrain
engine developed by Beijing Institute of Technology
(BIT) at 50% load of 1500 and 2000 rev·min-1 [9]. It can
be found that the computational results are acceptable
which is close to the experimental data during
compression, combustion and expansion phases in both
studies.

3. Results and Discussion

3.1 Characteristics of hydrogen assisted diesel

combustion
The variations of in-cylinder pressure, heat release rate,
utilization efficiency of total fuel energy and turbulent
kinetic energy (TKE) are shown in Fig. 5. A remarkable
increase of peak in-cylinder pressure is observed along
with the increase of hydrogen energy share ratio. More
precisely, the maximum in-cylinder pressure increases
from 12.8 MPa (baseline) to 13.3, 14.7 and 15.3 MPa in
the H10, H20 and H30 cases, respectively. As a
consequence, the indicated power of the OPCI diesel
engine may improve accordingly which is consistent with
the previous study conducted in conventional 4SDE Fig. 4 Comparison of computational (present study) and
[11,32]. The diesel combustion in 4SDE commonly experimental in-cylinder pressure traces ( Zhang et al.
depicts as two-stage namely premixed and diffusion [9])
674 J. Therm. Sci., Vol.28, No.4, 2019

propagation speed of hydrogen contribute to the H30 cases, respectively. In this paper, the utilization
accelerated diesel-hydrogen combustion and results in efficiency of total fuel energy is represented by the ratio
reduction of combustion duration. of accumulated heat release to total fuel energy. The fuel
The TKE is majorly generated by the in-cylinder utilization efficiency of M100 is very poor since it is
charge motion (swirl, tumble, squish), inner and outer originally served as an aircraft engine which may need to
piston movement, fuel injection and combustion events. ensure no abrupt pressure rise and high mechanical
According to the TKE profile shown in Fig. 4(b), reliability, although it can be easily alleviated by
hydrogen enrichment has negligible impact of TKE advancing the fuel injection timing. However, it is not the
brought from diesel fuel injection although slight main concern in this study if the maximum in-cylinder
decrease of TKE is observed with hydrogen addition pressure is lower than 16 MPa [6]. Apparently, hydrogen
which is due to the reduction of injected diesel fuel. addition is helpful in improving the total energy
However, the intensified combustion of diesel-hydrogen utilization efficiency which increases from 85.8% in the
apparently contributes to the evident improvement of baseline case, to 91.1%, 98.4%, and 99.3% in the H10,
TKE, even in the H10 case. The maximum TKE H20 and H30 cases, respectively.
increases from 31 m2/s2 to 71 m2/s2 in the baseline and
3.2 Characteristic of hydrogen combustion
One of the primary intentions of this study is to
illustrate the hydrogen combustion process in an OPCI
diesel engine. Evolution of hydrogen mass distributions
at selected crank angle in the baseline, H10, H20 and
H30 cases are shown in Fig. 6. Evenly distributed
hydrogen dilutes the intake air and reduces the
surrounding available oxygen of diesel fuel, hence
retards its low temperature oxidation. In addition, the
enrichment of hydrogen consumes OH radical by
OH+H2=H+H2O and hydrogen atom abstraction rate of
n-heptane (n-C7H16+OH=C7H15+H2O) is suppressed.
Instead of generating OH, lower reactive HO2 and H2O2
radical is formed which also alleviates the low
temperature oxidation of diesel fuel and increases the
ignition delay as suggested by Guo et al. [33]. Therefore,
the retardation of diesel combustion phase as shown in
Fig. 5 is owing to both dilution and chemical effect of
enriched hydrogen. Due to the increase of ignition delay,
an extension of fuel-to-air mixing is offered which
enhances the premixed combustion and increase the
in-cylinder pressure and temperature.
The variation of total in-cylinder hydrogen mass
during the combustion process is presented in Fig. 7 in
the four cases. In the baseline case, hydrogen as
combustion by-product of diesel fuel formed at the
central area where the impingement of two opposed
diesel sprays occurs. This region is considered to be
under fuel-rich condition, thus hydrogen is easily formed
due to the incomplete combustion of diesel fuel [34].
With H10, H20 and H30 feeding, the consumption of
hydrogen initiates from the same region and gradually
spreads to the cylinder liner. In the meanwhile, formation
of hydrogen is also captured at the central area. It is
consistent with the results shown in Fig. 7 as presented
by a first increase and then decrease trend of total
hydrogen mass, especially in the H10 case. Gatts et al.
[35] adopted the variation of mass flow rate between
Fig. 5 Effect of hydrogen enrichment on in-cylinder pressure, emitted hydrogen and added hydrogen to evaluate the
heat release rate and turbulent kinetic energy profile combustion efficiency of hydrogen in a 4SDE. However,
ZHOU Jianhao et al. Effect of Hydrogen Enrichment on an Opposed-Piston Compression Ignition Diesel Engine 675

Fig. 6 In-cylinder hydrogen mass distribution at selected crank angles (1st row: baseline; 2nd row: H10; 3st row: H20; 4th row: H30)

at EPO among all cases are presented in the H30 case. It

may indicate that the net consumption rate (difference
between generation and consumption) of hydrogen is
highly associated with the concentration of added
hydrogen.

3.3 Characteristic of in-cylinder flow field

Some critical information is obtained during the
initiation of combustion which is expressed by the
velocity fields during 3~0.5°CA ATDC in the cut plane as
shown in Fig. 8. It can be seen that the impingement of
opposed diesel spray occurs during 3~2.5°CA BTDC.
Fig. 7 In-cylinder hydrogen mass variation during combustion The magnitude of velocity near the impingement region
process increases dramatically at 2°CA BTDC. Following closely,
a ring shape velocity field with high magnitude is formed
this method seems to be not feasible in an OPCI diesel near the cylinder liner at 1.5°CA BTDC which can be
engine when a slight amount of hydrogen is added (e.g. recognized as the flame propagation of hydrogen within
H10), since a competition between consumption (by the combustion chamber. Since the flame propagation
hydrogen combustion) and generation (by diesel speed of hydrogen is higher than that of diesel fuel, the
combustion) rate of hydrogen occurs and it’s hard to flame front of hydrogen reached to the cylinder liner
distinguish the source of hydrogen. In this regard, the faster. Meanwhile, the diesel injection is still in progress
total in-cylinder hydrogen mass is apparently higher than and the diesel fuel near the injector is easily to be
the added hydrogen mass during -5~55°CA ATDC and compressed to ignite when the shock wave of hydrogen
the maximum increasing rate even achieves 50% at reaches which is indicated by a sharp increase of velocity
12°CA ATDC in the H10 case. By contrast, direct near the right side injector as can be seen during
descent of hydrogen mass and the lowest hydrogen mass 1.5~1°CA BTDC. After that, the shock wave generated
676 J. Therm. Sci., Vol.28, No.4, 2019

Fig. 8 In-cylinder velocity fields at selected crank angle in the H30 case

from diesel-hydrogen combustion touches to the cylinder Therefore, the contours of instantaneous temperature,
liner and reflects back to the opposite direction which oxygen, NOx and soot concentration (in mass fraction)
collides to the shock wave at the other side (left side) within different cut planes with uniform range at selected
where the collision border between the two opposite crank angles are illustrated in Fig. 9. The results show
shock waves is exhibited at 0.5°CA BTDC. Owing to the good consistency. Since the variations of above
contribution of the second compression ignition of diesel parameters follow similar trend among the H10, H20 and
fuel from the right side injector, the pressure of the shock H30 cases, their differences between baseline and H30
wave at the right side is higher than that of the left side, are depicted as representative. The peak mean in-cylinder
hence the shock wave continues to propagate to the left gas temperature increases from 1330 K (baseline) to
side at TDC. Similarly, the fuel surround the left side 1709 K (H30) while the maximum in-cylinder
injector is compressed to ignite by the shock wave as can temperature keeps around 2800 K for all cases. Since the
be found at 0.5°CA ATDC. consumption of oxygen per unit mass of hydrogen is
The second compression ignition of tail diesel fuel more than two times higher than that of n-heptane
near the side-injector due to hydrogen combustion in (representing diesel fuel), higher oxygen consumption
OPCI diesel engine is not likely to occur in the 4SDE rate in the H30 case is exhibited by the contour of
[36], which is primarily owing to the different injection oxygen concentration. However, the reduction of oxygen
pattern, being side injection for the former and central leads to negligible impact on NOx and soot emission
injection for the latter. In addition, to ensure the since the energy share ratio of hydrogen is relatively low.
combustion commence at the central zone in order to In general, increase of mean in-cylinder temperature
reduce heat loss, swirl is limited in OPCI diesel engine. favors the formation of NOx and is also associated with
Nearly flat piston bowl results in negligible squish flow. the promotion of soot oxidation in the H30 case. More
Relative low bulk motion leads to limited guiding effect specifically, local temperature and oxygen concentration
of hydrogen flame propagation, hence the shock wave play an important role in forming thermal NOx, thus a
due to hydrogen combustion propagates radially to the major part of NOx concentrates at the outer contour of the
cylinder liner. However, for better utilization of air, high diesel plume where premixed combustion of diesel fuel
swirl and squish flow are generated in 4SDE which (baseline) and diesel-hydrogen (H30) occurs and a large
interferes the flame propagation of hydrogen and the amount of heat is released. The homogeneity of
combustion induced shock wave is easily dispersed. diesel-hydrogen combustion is higher than that of
baseline due to the extension of premixed combustion
3.4 Effect of hydrogen on emission formation phase which enlarges the volume of high temperature
NOx and soot formation is closely related to the region and accordingly increases NOx. With regard to
in-cylinder temperature and local oxygen concentration. soot, it is primarily formed and trapped inside the
ZHOU Jianhao et al. Effect of Hydrogen Enrichment on an Opposed-Piston Compression Ignition Diesel Engine 677

Fig. 9 The in-cylinder temperature, oxygen, NOx and soot concentration contours at selected crank angles in the H30 case (1st row:
baseline; 2nd row: H30)
678 J. Therm. Sci., Vol.28, No.4, 2019

impinged diesel plume, as the primary sooty zone, due to impact of SOI and DIP coupling on a hydrogen enriched
the lack of oxygen at TDC and 5°CA ADTC in the OPCI (H30) diesel engine. During the simulation, the
baseline and H30 case, respectively. Less soot is formed nozzle diameter and injection profile were unchanged
with H30 addition in comparison to that of baseline due and DIP was adjusted by the reduction of injection
to the reduction of diesel fuel. However, reverse duration.
evolution of soot concentration is observed in the Comparison of the impact of SOI/DIP on NOx
baseline and H30 case at 15°CA ATDC and 30°CA emission is shown in Fig. 10(a). It may conclude that
ATDC, representing as gradual increase (inception) and both lower DIP and delayed SOI are effective in reducing
decrease (oxidation) of soot, respectively. It may NOx emission. The decrease of in-cylinder temperature
conclude that the enrichment of hydrogen may alleviate becomes the main reason for NOx reduction. However,
soot inception and enhance soot oxidation. It is giving consideration to both enging power and emission,
reasonable since some researches [37,38] have proved the influence of proposed injection strategy on combus-
that hydrogen interferes the HACA (hydrogen abstraction tion process is further discussed. The in-cylinder pressure
and C2H2 addition) mechanism [39] which is shown to profile with various SOI/DIP and H30 enrichment is
reduce soot nucleation and soot surface growth rate, both shown in Fig. 10(b) and 10(d). For the convenience of
of which contribute to the suppression of soot inception. discussion, the influence of SOI on the combustion
Additionally, the increase of TKE is helpful for process is firstly discussed by employing original DIP of
fuel-to-air mixing and in-cylinder temperature, hence 650 bar. In Fig. 10(b), gradual distortion of in-cylinder
improves the oxidation of soot. pressure is exhibited with SOI changing from 10 to 0°CA
On the basis of above discussions, it is noteworthy BDTC, represented by delayed combustion phasing,
that the involvement of hydrogen in an OPCI diesel shortened combustion duration and decreased peak in-
engine effectively improves the indicated power and total cylinder pressure. In particular, the in-cylinder pressure
fuel utilization efficiency which is mainly attributed to shape is drastically distorted with 0°CA BDTC SOI.
the optimization of hydrogen on diesel combustion Meanwhile, total fuel energy utilization efficiency
process. Meanwhile, the combustion duration is dramatically decreases from 98.9% to 71.0% with 16 and
shortened and the profile of heat release rate is modified 0°CA BDTC SOI, respectively, which implies unacceptable
being dominated by premixed combustion, both of which incomplete combustion. The in-cylinder mass evolution
contribute to the excessive increase of in-cylinder of hydrogen in Fig. 10(c) shows a good consistency with
temperature and an accompanying NOx penalty. In the deteriorated total fuel energy utilization efficiency as
consequence, hydrogen enrichment seems to aggravate shown in Fig. 10(b). Although the competition of
the trade-off of NOx-soot. With above concerns, injection hydrogen consumption and generation still exists which
strategy of diesel fuel is preliminarily developed in order has been discussed before, the variation of hydrogen
to pursue the synergy effect of fuel injection strategy and mass during the competition period is relatively small,
hydrogen enrichment to achieve high efficiency and low hence the variation between emitted hydrogen at EPO
emissions for OPCI diesel engine. and added hydrogen can roughly represent the
combustion efficiency of hydrogen. Then it may find that
3.5 Effect of diesel injection strategy the combustion efficiency of hydrogen declines sharply
Diesel injection timing essentially controls the start of along with delayed SOI, being changing from 88.9% to
combustion [27]. Advancing start of injection (SOI) leads 31.3%, with 16 and 0°CA BDTC SOI, respectively. With
to longer fuel-to-air mixing evaporation time and excessive delay of SOI, the in-cylinder environment,
enriched hydrogen further prolongs the ignition delay as including the in-cylinder pressure, temperature, charge
mentioned before, thus the premixed combustion may density and TKE, is changed when diesel injection begins,
become more intensified and further promote NOx which may lead to negative impact on fuel-to-air mixing,
formation which violates the original intension. In fuel evaporation and accordingly increase the ignition
addition, bulk density becomes lower with earlier SOI delay [41,42]. It may conclude that hydrogen combustion
which leads to the increase of spray penetration length is sensitive to the SOI of diesel fuel, especially when
and results in the spray-wall impingement [1]. Therefore, compression ratio is low and hydrogen-air mixture is lean.
delayed SOI is employed in the following study. On the Due to highly delayed SOI of diesel fuel (e.g. SOI=
other hand, diesel injection pressure (DIP) plays an 0°CA BDTC), the decreased in-cylinder pressure and
important role for mixture formation which is helpful for temperature cannot support the flame propagation of
improving total fuel utilization efficiency [40]. Since the hydrogen which eventually results in very low
OPCI diesel engine is equipped with common rail combustion efficiency of hydrogen.
injection system, the DIP can be further increased. With In comparison to above-mentioned cases applying 650
above concerns, late SOI (10, 5, 0°CA BDTC) and high bar DIP, the drawback brought from late SOI on
DIP (1000 bar) were adopted to further investigate the hydrogen combustion can be mitigated when coupling
ZHOU Jianhao et al. Effect of Hydrogen Enrichment on an Opposed-Piston Compression Ignition Diesel Engine 679

Fig. 10 Effect of start of injection (SOI) and diesel injection pressure (DIP) on in-cylinder pressure, hydrogen combustion
efficiency and NOx emission with H30 addition (Baseline: DIP=650 bar, SOI=16° CA BDTC, without H30 addition)

with increased DIP (1000 bar). In Fig. 10(b) and 10(d), it -soot and NOx -power exist. Since the ignition of
can be seen that the peak in-cylinder pressure increases hydrogen is triggered by diesel compression ignition, the
from 12.8 MPa to 15.2 MPa and 15.3 MPa to 18.2 MPa SOI of diesel fuel is crucial to maintain high combustion
(exceed 16 MPa) in the baseline and H30 cases, efficiency of hydrogen, which cannnot be excessivly
respectively, compared to that with 650 bar DIP. Without delayed. In general, it is desirable to maintain the benefit
modification of SOI, the combustion phasing is advanced provided by delayed SOIS as well as increased DIP while
and the combustion duration is shortened owing to the mitigating their side effects which is harmful for
accelerated fuel-to-air mixing and increased inensity of hydrogen combustion and NOx emission. However, if one
turbulence by high DIP. Additionally, with comparison of wants to achieve high efficiency and low emission for
Fig. 10(c) and 10(e), the consumption rate of hydrogen OPOC engine, design of experiment (DOE) and
increased drastically with high DIP which implies faster multi-parameter optimization algorithm are necessary
burning rate of hydrogen and the combustion process is which cannot be covered in this paper.
dominated by premixed combustion, being more close to
constant volume combustion. However, it is noticed that, 4. Conclusions
as a possible side effect, the negative work during
compression stroke may increase due to early SOI which In the present study, a three-dimensional CFD model
influences the indicated power. With variation of SOI, the was established to investigate the transient combustion
tolerance of late SOI of an hydrogen enriched OPCI process of a model OPCI diesel engine with 10%, 20%
diesel engine is improved and no immoderate distortion and 30% energy share of hydrogen. Fixed embedding and
of in-cylinder pressure shape is not observed in all cases. AMR techniques are employed to generate dynamic
The total fuel energy utilization efficiency is higher than mesh. RNG k-ε model coupled with a reduced diesel-
90% and the combustion efficiency of hydrogen can hydrogen reaction mechanism was adopted to simulate
achieve at least 72.5% with 0°CA BDTC SOI. the combustion process and emissions formation.
According to the analysis of in-cylinder combustion Preliminary optimization of diesel injection strategy is
process with various SOI/DIP and the corresponding taken into consideration. Conclusions are obtained as
response of NOx emission, a preliminary combustion follows.
optimization strategy can be obtained. For a hydrogen- (1) Hydrogen is proved to be a performance enhancer
enriched OPCI diesel engine, trade-offs between NOx for OPCI diesel engine. The enrichment of hydrogen
680 J. Therm. Sci., Vol.28, No.4, 2019

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