energies

Article
Numerical Assessment on the Influence of Engine Calibration
Parameters on Innovative Piston Bowls Designed for
Light-Duty Diesel Engines
Federico Millo 1 , Andrea Piano 1, * , Salvatore Roggio 1 , Francesco C. Pesce 2 , Alberto Vassallo 2
and Andrea Bianco 3

1 Energy Department, Politecnico di Torino, 10129 Torino, Italy; federico.millo@polito.it (F.M.);

salvatore.roggio@polito.it (S.R.)
2 PUNCH Torino S.p.A., 10129 Torino, Italy; francesco_concetto.pesce@punchtorino.com (F.C.P.);
alberto_lorenzo.vassallo@punchtorino.com (A.V.)
3 POWERTECH Engineering S.r.l., 10127 Torino, Italy; a.bianco@pwt-eng.com
* Correspondence: andrea.piano@polito.it

Abstract: The optimization of the piston bowl design has been shown to have a great potential for air–
fuel mixing improvement, leading to significant fuel consumption and pollutant emissions reductions
for diesel engines. With this aim, a conventional re-entrant bowl for a 1.6 L light-duty diesel engine
was compared with two innovative piston designs: a stepped-lip bowl and a radial-bumps bowl. The
potential benefits of these innovative bowls were assessed through 3D-CFD simulations, featuring a
calibrated spray model and detailed chemistry. To analyse the impact of these innovative designs,
two different engine operating conditions were scrutinized, corresponding to the rated power and
a partial load, respectively. Under the rated power engine operating condition, a start of injection
Citation: Millo, F.; Piano, A.; Roggio,
sensitivity was then carried out to assess the optimal spray–wall interaction. Results highlighted
S.; Pesce, F.C.; Vassallo, A.; Bianco, A.
that, thanks to optimal injection phasing, faster mixing-controlled combustion could be reached with
Numerical Assessment on the
both the innovative designs. Moreover, the requirements in terms of swirl were also investigated,
Influence of Engine Calibration
Parameters on Innovative Piston
and a higher swirl ratio was found to be necessary to improve the mixing process, especially for the
Bowls Designed for Light-Duty radial-bumps design. Finally, at part-load operating conditions, different exhaust gas recirculation
Diesel Engines. Energies 2022, 15, (EGR) rates were analysed for two injection pressure levels. The stepped-lip and radial-bumps bowls
3799. https://doi.org/10.3390/ highlighted reduced indicated specific fuel consumption (ISFC) and soot emissions values over
en15103799 different rail pressure levels, guaranteeing NOx control thanks to the higher EGR tolerance compared
with the re-entrant bowl. The results suggested the great potential of the investigated innovative
Academic Editor: Constantine D.
Rakopoulos
bowls for improving efficiency and reducing emissions, thus paving the way for further possible
optimization through the combination of these designs.
Received: 4 April 2022
Accepted: 18 May 2022 Keywords: diesel engine; innovative piston bowl; computational fluid dynamics; additive-manufacturing-
Published: 21 May 2022
enabled design; stepped-lip bowl; radial-bumps bowl
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations. 1. Introduction
Nowadays, the design of a diesel engine combustion system requires a balance among
multiple and conflicting drivers. The compliance with even more stringent emission regu-
lations has to be reconciled with the need of improving fuel economy without deteriorating
Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
durability and reliability. In addition, the customer experience should be guaranteed,
This article is an open access article
achieving the requirements in terms of vehicle performance while keeping the noise and
distributed under the terms and
vibration under control. Lastly, the effort of balancing these multiple tradeoffs is made
conditions of the Creative Commons even more difficult by the always compelling need for cost reduction. In this context, the
Attribution (CC BY) license (https:// selection of the market segment target can affect the relative weight of the abovementioned
creativecommons.org/licenses/by/ drivers. There are, indeed, several critical differences between light- and heavy-duty appli-
4.0/). cations. On one side, the light-duty engines are mainly optimized in operating conditions

Energies 2022, 15, 3799. https://doi.org/10.3390/en15103799 https://www.mdpi.com/journal/energies

Energies 2022, 15, 3799 2 of 18

in which the spray is unable to deliver a proper turbulence level needed for efficient mixing,
requiring a supply by additional turbulence sources (e.g., swirl motion). On the other side,
heavy-duty engines usually work in high-load conditions, in which most of the energy
needed for the air–fuel mixing is provided by the spray. In recent years, several designs
have been developed for heavy-duty engines, increasing the interaction between the fuel
sprays and the piston bowl walls, thus enhancing the air–fuel mixing.
Among the different designs recently developed, the stepped-lip bowl has been con-
sidered the main alternative to the conventional re-entrant bowl [1–5]. This concept was
implemented by Mercedes Benz on the OM654 engine, highlighting great benefits in terms
of efficiency and soot reduction [6]. In addition, Cornwell and Smith, having a JCB off-
highway diesel engine as a case study, were able to reach the emissions legislation limits
even without any aftertreatment system [1,7]. Specifically, in this design, a chamfered lip is
added to split the fuel into two toroidal vortices, within the bowl and in the squish region,
thus increasing the air–fuel mixing [8]. The improved mixing enables higher exhaust gas
recirculation (EGR) to control the NOx emissions, and in combination with higher injection
pressure, allows significant soot mitigation to be achieved [9]. Considering a similar design,
Busch et al. have analysed the combustion process at different injection timings in [10].
They concluded that a faster mixing-controlled combustion phase (i.e., the 50–90% of mass
fraction burned) can be obtained. This enhanced combustion rate has shown a strong
correlation with the formation of intense toroidal vortices due to the step design, as experi-
mentally evaluated by means of the combustion image velocimetry (CIV) technique by Zha
et al. in [11]. Lastly, the stepped-lip bowl highlighted the great potential in terms of soot
reduction [3,12]. Indeed, the more uniform fuel distribution increases the soot oxidation
rate in the late portion of the combustion process.
Considering the low-swirl heavy-duty diesel engines, the flame to-flame interaction for
the typical open bowl shape [13] dramatically reduces the combustion rate and increases the
soot formation [14]. Therefore, to minimize this effect, Volvo proposed the wave bowl [15],
introducing radial bumps in the outer bowl rim where adjacent flames interact. Thanks
to this novel design, the late-cycle mixing can be improved, providing benefits both in
terms of efficiency and soot emissions [16]. The radial bumps drive the adjacent flames to
collide with a more favourable angle, significantly reducing the formation of rich zones
and enhancing the flame velocity of the so-called radial mixing zone (RMZ) toward the
cylinder axis. Therefore, the air entrainment onto the flame can be improved. Additionally,
after the end of injection, when the RMZ detaches from the wall, the trailing edge of the
flame leads to higher air entrainment, resulting in a faster burn rate [16]. Thanks to the
improved mixing process, a higher mixing-controlled combustion phase was highlighted
with respect to a conventional open bowl shape, gaining up to +1% thermal efficiency [17].
Moreover, the wave bowl has shown a remarkable improvement in the soot–NOx tradeoff
under different partial load operating conditions, providing up to 80% soot reduction [16].
Recently, a radial-bumps bowl was numerically assessed for a light-duty diesel engine [18].
In this case, the higher swirl ratio reduced the intensity of the RMZ propagation, while the
high bowl re-entrance resulted in enhanced flame recirculation toward the piston centre
as a tumbling vortex. In this study, the radial-bumps design led to a strong improvement
of the mixing rate with respect to a conventional re-entrant design. At part-load engine
operating conditions, this resulted in flat soot–NOx and brake-specific fuel consumption
(BSFC)–NOx tradeoffs for different EGR rates, leading to −50% soot and −5% BSFC with
respect to the re-entrant bowl [18].
Nowadays, the possibility to build up innovative piston designs thanks to steel-based
additive manufacturing (AM) procedures has enabled high-complexity and undercut
geometries for further geometrical optimization [19–21]. In this context, the potential syn-
ergies between the stepped-lip and the radial-bumps bowls were experimentally evaluated
by Belgiorno et al. in [21]. In this study, a re-entrant sharp-stepped bowl and a number of
radial bumps equal to the nozzle holes in the inner bowl rim were combined in a single-
piston concept, highlighting an impressive soot reduction without any fuel consumption
 piston concept, highlighting an impressive soot reduction without any fuel consumption
penalties. A similar concept was also investigated by means of both numerical and optical
Energies 2022, 15, 3799 techniques, showing enhanced air–fuel mixing [22]. 3 of 18
Considering all the above, the present study aims to investigate the potential of two
combustion systems, originally developed for the heavy-duty sector, in a light-duty diesel
engine: one
penalties. based on
A similar the stepped-lip
concept bowl and the
was also investigated by other
meansfeaturing radial bumps
of both numerical in the
and optical
outer bowl rim. Considering the different
techniques, showing enhanced air–fuel mixing [22]. engine applications, a detailed investigation has
beenConsidering
performed toall highlight
the above, the present study aims to investigate the potential In
the potential benefits of the proposed innovative designs. of
two combustion systems, analysis
this regard, a sensitivity originallyover differentfor
developed engine calibration sector,
the heavy-duty parameters has been
in a light-duty
performed
diesel providing
engine: additional
one based insights about
on the stepped-lip bowl theand
needstheof the proposed
other featuring designs in terms
radial bumps in
of engine
the calibration.
outer bowl With this aim,
rim. Considering 3D computational
the different fluid dynamics
engine applications, (3D-CFD)
a detailed simula-
investigation
tionsbeen
has of the combustion
performed process the
to highlight were carriedbenefits
potential out, following the already
of the proposed developed
innovative 1D-
designs.
/3D-CFD coupling methodology presented in [23]. The impact of different
In this regard, a sensitivity analysis over different engine calibration parameters has beencalibration pa-
rameters on the proposed piston bowl designs (re-entrant, stepped-lip, and
performed providing additional insights about the needs of the proposed designs in terms of radial-bumps)
was investigated
engine calibration.for
Withtwo engine
this operating
aim, 3D conditions.
computational fluidFirstly,
dynamicsat full load, an
(3D-CFD) injection
simulations
timing
of and swirl ratio
the combustion sensitivity
process analysis
were carried out,was carriedthe
following out. Then, developed
already at partial load, different
1D-/3D-CFD
EGR ratesmethodology
coupling at two rail pressure levels
presented wereThe
in [23]. considered,
impact of highlighting the potential
different calibration benefits
parameters
in terms
on of efficiency
the proposed andbowl
piston pollutant emissions.
designs (re-entrant, stepped-lip, and radial-bumps) was
investigated for two engine operating conditions. Firstly, at full load, an injection timing
2. Case
and swirlStudy
ratio sensitivity analysis was carried out. Then, at partial load, different EGR
rates at twoTest
2.1. Engine railCase
pressure levels were considered, highlighting the potential benefits in
termsThe
of efficiency
analysed andenginepollutant
is a 1.6emissions.
L light-duty diesel engine, whose main characteristics
areCase
2. highlighted
Study in Table 1.
2.1. Engine Test Case
Table 1. Test engine main features.
The analysed engine is a 1.6 L light-duty diesel engine, whose main characteristics are
Cylinders
highlighted in Table 1. 4
Displacement 1.6 L
Table 1. Test engine main
Bore features.
× Stroke 79.7 mm × 80.1 mm
Compression ratio 16:1
Cylinders 4
Displacement Single-Stage1.6 with
L Variable Geometry Tur-
Turbocharger
Bore × Stroke 79.7 mm × 80.1 bine
mm(VGT)
Compression ratio Common 16:1rail direct injection (CRDI)
Fuel injection system Single-Stage with Variable Geometry Turbine (VGT)
Turbocharger Max Rail Pressure 2000 bar
Fuel injection system Common rail direct injection (CRDI) Max Rail Pressure 2000 bar
Maximum power 100 kW @ 4000 rpm
Maximum power 100 kW @ 4000 rpm
Maximum
Maximum torque torque 320
320 Nm @ 2000 Nm rpm@ 2000 rpm

The baseline test engine features the conventional re-entrant piston bowl, as shown
The baseline test engine features the conventional re-entrant piston bowl, as shown in
in Figure 1—left. Then, two innovative piston bowl designs were investigated: a stepped-
Figure 1—left. Then, two innovative piston bowl designs were investigated: a stepped-lip
lip (Figure 1—middle) and a radial-bumps (Figure 1—right) bowls. The stepped-lip piston
(Figure 1—middle) and a radial-bumps (Figure 1—right) bowls. The stepped-lip piston
bowl was designed following the geometrical items reported in [1]. The radial-bumps
bowl was designed following the geometrical items reported in [1]. The radial-bumps bowl
bowl was developed by having the re-entrant bowl as a basis and adding a number of
was developed by having the re-entrant bowl as a basis and adding a number of radial
radial bumps in the outer bowl rim equal to the injector nozzle holes, as shown in [15].
bumps in the outer bowl rim equal to the injector nozzle holes, as shown in [15].

Figure 1. Isometric view of the piston bowl geometries: (left) re-entrant; (middle) stepped-lip;
Figure 1. Isometric view of the piston bowl geometries: (Left) re-entrant; (middle) stepped-lip;
(right)
(right) radial-bumps.
radial-bumps.

The two novel pistons have the same bore and squish height as the re-entrant bowl.
Then, the piston bowl curvature was adjusted to keep the compression ratio equal to the
nominal value (i.e., 16:1). The resulting bowl profile on the centre of the sector geometry
can be observed in Figure 2, while the dashed green line refers to the bump geometry
 Energies 2022, 15, x FOR PEER REVIEW 4 of 19

The two novel pistons have the same bore and squish height as the re-entrant bowl.
Energies 2022, 15, 3799
Then, the piston bowl curvature was adjusted to keep the compression ratio equal 4toofthe 18

nominal value (i.e., 16:1). The resulting bowl profile on the centre of the sector geometry
can be observed in Figure 2, while the dashed green line refers to the bump geometry in
in
thethe sector
sector periphery.
periphery. Moreover,
Moreover, the injector
the injector protrusion
protrusion was
was not not modified
modified for
for each each
investi-
investigated
gated design.design.

Figure2.2.Piston
Figure Pistonbowl
bowlprofiles.
profiles.

Theanalysis
The analysiswaswascarried
carriedout
outconsidering
consideringtwotwodifferent
differentengine
engineoperating
operatingconditions,
conditions,
oneatatpart-load
one part-loadand
andone
oneatatrated
ratedpower,
power,asaslisted
listedininTable
Table2.2.For
Forthese
theseengine
engineoperating
operating
conditions,extensive
conditions, extensivevalidation
validationof of
thethe numerical
numerical model
model for re-entrant
for the the re-entrant
bowlbowl was al-
was already
ready presented
presented in [23],inhighlighting
[23], highlighting
fairly fairly
good good agreement
agreement both both in terms
in terms of combustion
of the the combus-
tion process
process and emissions
and emissions prediction.
prediction.

Table2.2.Selected
Table Selectedengine
engineworking
workingpoints.
points.

Speed[rpm]
Speed [rpm] BMEP[bar]
BMEP [bar]
1500
1500 5.0
5.0
4000
4000 18.5
18.5

2.2.Simulation
2.2. SimulationSetupSetup
The1D-/3D-CFD
The 1D-/3D-CFD coupling
coupling methodology,
methodology, which which has hasbeen
beendeveloped
developedand andvalidated
validated
in[23],
in [23], was
was adopted
adopted for the numerical
numerical simulations.
simulations.The Themain
mainsteps
stepsof of
thethemethodology
methodol-
can can
ogy be summarized
be summarized as follows:
as follows:TheThe
1D-CFD
1D-CFD complete
complete engine model,
engine model,developed
developed in GT-in
SUITE andand
GT-SUITE validated in [24],
validated provides
in [24], the time-dependent
provides the time-dependent boundary
boundaryconditions (i.e., (i.e.,
conditions ther-
modynamic andand
thermodynamic species concentration)
species concentration) for the
for first 3D-CFD
the first 3D-CFDsimulation
simulationstep. step.
This latter,
This
developed
latter, in CONVERGE
developed in CONVERGE CFD,CFD,is a cold flowflow
is a cold simulation
simulationthatthat
waswascarried outout
carried for for
the
the analysis
analysis of the
of the gasgas exchange
exchange process.
process. Then,Then,
fromfrom the intake
the intake valvevalve closure
closure (IVC),(IVC), the
the com-
compression
pression stroke stroke
andand
thethe combustion
combustion were
were investigated
investigated considering
considering only
only a single
a single sector
sector of
of thefull-cylinder
the full-cylindergeometry,
geometry,which
whichwas wascentred
centred along
along aa single spray axis. For this this simula-
simula-
tion
tionstep,
step,the
theinjection
injectionrate
rateprofile
profilewas
wasprovided
providedby bythe
the1D-CFD
1D-CFDinjector
injectormodel
modeldeveloped
developed
in [25,26]. In the last step, the 3D-CFD results were post-processed by
in [25,26]. In the last step, the 3D-CFD results were post-processed by means of GT-SUITE means of GT-SUITE
to
toguarantee
guaranteethe thesame
samesolution
solutionmethodology
methodologyofofthe theinitial
initial1D-CFD
1D-CFDengineenginemodel.
model.
Regarding
Regardingthe the3D-CFD
3D-CFDsimulations,
simulations,the theReynolds-averaged
Reynolds-averagedNavier–Stokes
Navier–Stokes (RANS)-(RANS)-
based
basedrenormalization
renormalizationgroup group (RNG)
(RNG) k-εk-ε
model
model[27][27]
waswas adopted. For the
adopted. Formesh refinement,
the mesh refine-
the adaptive
ment, mesh refinement
the adaptive mesh refinement(AMR)(AMR)
technique, based based
technique, on theonvelocity and temperature
the velocity and temper-
sub-grid criterion,
ature sub-grid was setwas
criterion, [28].
setThe
[28].model settings
The model in terms
settings of mesh
in terms size, turbulence,
of mesh and
size, turbulence,
heat transfer models are listed in Table
and heat transfer models are listed in Table 3. 3.

Table 3. Mesh size, turbulence, and heat transfer models.

Table 3. Mesh size, turbulence, and heat transfer models.
Base grid 0.5 mm
Base grid 0.5 mm
Minimum grid 0.25 mm (fixed embedding and AMR)
Minimum grid
Turbulence model 0.25 mm (fixed
RNG embedding
k-ε model and AMR)
Heat transfer model O’Rourke and Amsden
Energies 2022, 15, 3799 5 of 18

For the spray model, the so-called “blob” injection method was considered and the
breakup of droplets was modelled by means of a calibrated Kelvin Helmholtz and Rayleigh
Taylor (KH-RT) model [29]. The spray sub-models are listed in Table 4.

Table 4. Spray sub-models.

Discharge coefficient model Cv correlation [28]

Breakup model Calibrated KH-RT
Turbulent dispersion O’Rourke model [30]
Collision model No Time Counter (NTC) collision [31]
Drop drag model Dynamic drop drag [32]
Evaporation model Frossling model [30]
Wall film model O’Rourke [33]

For the combustion simulation, the detailed chemistry kinetic solver (SAGE) was
adopted, considering the Skeletal Zeuch mechanism (121 species, 593 reactions) for the
n-heptane oxidation [34]. This reaction mechanism includes the NOx chemistry and the
polycyclic aromatic hydrocarbons (PAH) soot precursor chemistry, thus enabling the partic-
ulate mimic (PM) model for the in-cylinder soot prediction [35–37].

3. Results and Discussion

3.1. Full Load Engine Operating Condition—4000 RPM × 18.5 Bar BMEP
3.1.1. Start of Injection Sensitivity
The injection timing plays a crucial role to maximize the potential benefits of a bowl
design since the injection timing variation results in a different spray targeting which affect
the near-wall flame behaviour. Therefore, the injection timing was swept at the rated power
condition, keeping the injected fuel mass constant, considering the three different proposed
designs. Three different starts of injection (SOIs) were investigated: the nominal SOI for the
re-entrant bowl (baseline) and +5/+10 CAD with respect to the baseline SOI. The results
of the SOI sweep in terms of mass fraction burned data (CA 10, 50, 75, 90) are shown in
Figure 3a.
As expected, the early stages of the combustion process are not affected by the variation
of the bowl design, as shown by CA10 data among the SOI sweeps. The innovative bowl
designs start to influence the combustion process in the mixing-controlled combustion
phase, as highlighted by the CA50 which is slightly advanced for both the stepped-lip
and radial-bumps bowls. The CA75 shows even more evident differences: in fact, the
innovative bowls show advanced combustion with respect to the re-entrant bowl for each
SOI under investigation. During the late phase of the combustion process, the piston bowls
highlight a different behaviour depending on the injection timing. The re-entrant shows
the lowest sensitivity to the injection timing and the CA90 data are quite constant varying
the SOI. Instead, the stepped-lip bowl highlights a remarkable increment of the CA90 by
retarding the SOI. A similar result was experimentally assessed by Bush et al. in [10], in
which the stepped-lip bowl highlighted for retarded SOI a strong increment of CA90 in
comparison with a conventional re-entrant bowl. Indeed, in the late injection phase, the
spray–wall impingement occurs above the step, causing poor air utilization within the bowl,
as also reported in [18]. Regarding the radial-bumps bowl, a higher difference compared
with the re-entrant design is observed for the baseline SOI. Then, moving to retarded SOI,
the deviation from the CA90 of the re-entrant design is reduced. This suggests that the
adoption of radial protrusions in the outer bowl rim provides higher benefits when optimal
spray targeting is considered.
Energies2022,
Energies 2022,15,
15,3799
x FOR PEER REVIEW 66 of 19
of 18

Figure 3. SOI
Figure 3. SOI sweep
sweep results.
results. (a)
(a) Mass
Mass fraction
fraction burned
burned data;
data; (b)
(b) CA50-90;
CA50-90; (c)
(c) ISFC
ISFC normalized
normalized with
with
respect to
respect to baseline
baseline engine
engine configuration
configuration (bowl:
(bowl: re-entrant;
re-entrant; SOI:
SOI: base).
base). Engine
Engine operating
operating condition
condition
(baseline): 4000
(baseline): 4000 RPM
RPM ×× 18.5
18.5 bar
bar BMEP.
BMEP.

To
Asbetter understand
expected, the early thestages
injection timing
of the sensitivity
combustion for each
process are piston bowl under
not affected by theinves-
vari-
tigation, duration
ation of the bowl of the
design, aslast
shownphasebyofCA10
the mixing-controlled
data among the SOI combustion,
sweeps. Therepresented
innovative by
CA50-90, was further analysed as shown in Figure 3b. The radial-bumps
bowl designs start to influence the combustion process in the mixing-controlled combus- bowl shows lower
CA50-90
tion phase, with respect to the
as highlighted byre-entrant
the CA50 whichdesignisfor each investigated
slightly advanced forSOI. bothHowever,
the stepped- the
higher
lip anddeviation with respect
radial-bumps bowls. to The the re-entrant
CA75 showsdesign can beevident
even more observed for the nominal
differences: in fact,SOI,
the
suggesting
innovative once
bowls again
show that a propercombustion
advanced spray–wall interaction
with respect is to
required to enhance
the re-entrant bowl thefor
radial-
each
bumps
SOI underbenefits. When the stepped-lip
investigation. During thedesign is adopted,
late phase of theretarding
combustion the SOI resultsthe
process, in higher
piston
combustion duration in comparison with the other investigated designs
bowls highlight a different behaviour depending on the injection timing. The re-entrant due to the unbal-
anced
showsfuelthe splitting on the step.
lowest sensitivity to Lastly, the ISFC
the injection normalized
timing and thewithCA90 respect to baseline
data are engine
quite constant
configuration
varying the SOI. Instead, the stepped-lip bowl highlights a remarkable increment ofwith
was investigated and it is depicted in Figure 3c. The radial-bumps bowl the
the
CA90nominal SOI highlights
by retarding the SOI.the lowest result
A similar ISFC, reaching a −3% reduction
was experimentally in comparison
assessed by Bush et al. within
the
[10],baseline
in whichre-entrant bowl. The
the stepped-lip highlighted
bowl highlighted potential improvement
for retarded is comparable
SOI a strong increment withof
the results obtained for a similar bowl design in a heavy-duty
CA90 in comparison with a conventional re-entrant bowl. Indeed, in the late injectiondiesel engine application,
as assessed
phase, by Zhang et
the spray–wall al. in [17]. In
impingement this work,
occurs abovehigher thermal
the step, causingefficiency
poor air(up to +1%)
utilization
was experimentally assessed over different high-load engine operating
within the bowl, as also reported in [18]. Regarding the radial-bumps bowl, a higher conditions. Similar
dif-
behaviour has been found considering the stepped-lip design at nominal
ference compared with the re-entrant design is observed for the baseline SOI. Then, mov- SOI, with an ISFC
reduction lowerSOI,
ing to retarded thanthe1%.deviation
Nevertheless,
from by theretarding
CA90 of the the re-entrant
SOI the stepped-lip bowl leads
design is reduced. to
This
Energies 2022, 15, 3799 7 of 18

a worsening of ISFC with respect to the re-entrant bowl, confirming again the importance
of a proper fuel split on the lip for an efficient combustion process.
To further analyse the injection timing impact on the combustion development among
the piston bowls under investigation, the heat release rate (HRR) was scrutinized for two
different SOIs (nominal and +10 CAD), as shown in Figure 4. For each investigated SOI,
the premixed combustion stage is not significantly affected by the piston bowl design,
confirming the results previously presented in Figure 3 (i.e., CA10). However, moving
ahead in the combustion process, during the mixing-controlled phase, the re-entrant bowl
shows a reduced HRR compared to the stepped-lip and radial-bumps designs. Indeed,
considering the SOIbase (Figure 4—left), from −5 to +5 CAD aTDC, both the stepped-lip and
the radial-bumps bowls lead to higher HRR. In this phase, indeed, the re-entrant bowl has
shown the strongest jet-to-jet interaction and a reduced air–fuel mixing rate with respect to
the other bowls [18]. From +5 CAD aTDC to the end of injection (EOI), the HRR for the
stepped-lip bowl drops below the one obtained with the radial-bumps bowl. At this stage,
the unbalanced fuel split on the step reduces the air utilization within the bowl slowing
down the combustion process [18]. Conversely, the radial-bumps bowl shows the highest
HRR at this stage, due to the improved air–fuel mixing rate thanks to the adoption of radial
bumps [18]. Retarding the SOI (i.e., SOIbase +10 shown in Figure 4—right), as expected,
the main injection premixed combustion is significantly attenuated. From +5 to +15 CAD
aTDC, both the stepped-lip and the radial-bumps bowls show a higher combustion rate.
However, moving ahead in the combustion the higher fuel in the squish region results
Energies 2022, 15, x FOR PEER REVIEW 8 ofin
19
lower air utilization within the bowl, especially for the stepped-lip bowl, and the HRR
reduces its intensity, even lower than the one obtained adopting the re-entrant bowl.

Figure4.4. Heat
Figure release rate
Heat release rate and
andinjection
injectionrate
rateprofile:
profile:(Left)
(left)SOI
SOIbase ; (right)
; (right)
base SOISOI +10. +10.
base base Engine
Engine oper-
operating condition
ating condition (baseline):
(baseline): 40004000
RPM RPM × 18.5
× 18.5 bar BMEP.
bar BMEP.

The
The combustion
combustion system behaviour for
system behaviour for the
thetwotwoSOIs
SOIsininFigure
Figure4 was
4 was then
then further
further in-
investigated by looking at the in-cylinder flame evolution. With this aim, the stoichiometric
vestigated by looking at the in-cylinder flame evolution. With this aim, the stoichiometric
iso-surface
iso-surfacecontoured
contouredby bytemperature
temperaturewaswasconsidered
consideredrepresentative
representativeofofthetheflame
flamefront,
front,as
as
shown
shownininFigures
Figure55andand6 Figure
for the 6SOI theand
forbase SOISOI and+10,
base base SOI respectively. The three different
base +10, respectively. The three
crank angles,
different θi , highlighted
crank in Figure 4, in
angles, θi, highlighted areFigure
representative of the same degree
4, are representative of theinterval after
same degree
the start of injection to properly compare the different SOIs.
interval after the start of injection to properly compare the different SOIs.
• At θ1 = +18 CAD aSOI
At this stage, the flame/wall interaction plays the main role in the overall combustion
rate. Considering the SOI base, the re-entrant bowl highlights an HRR lower than the
other bowls (see Figure 4) due to the interaction of adjacent flames, as shown in Figure
5a. Different mechanisms can be instead highlighted for the other designs under in-
vestigation. The stepped-lip bowl shows higher flame propagation above the step,
increasing the air utilization in the squish area, while the radial-bumps bowl avoids
the interaction of adjacent flames, increasing the combustion rate. Retarding the in-
jection (SOI base +10, Figure 6a), all the bowls under investigation show similar results
with respect to the baseline SOI due to the same piston position (−/+5 CAD aTDC) and
thus similar spray–wall interaction. Therefore, even reducing the ignition delay, the
re-entrant bowl shows the less intense HRR among the proposed designs, as already
depicted in Figure 4.
 +10, Figure 6c), all the piston bowl designs lead to a remarkable redistribution of the
flame in the squish region. This effect is detrimental, especially for the radial-bumps
bowl, where the reduced flame recirculation within the bowl leads to a lower impact
Energies 2022, 15, 3799 of the bumps, providing a lower difference among the analysed SOIs than with8 the of 18
re-entrant design in terms of CA 50-90 (see Figure 3).

Figure 5. SOI base: stoichiometric iso-surface contoured by temperature. (a) 18 CAD aSOI; (b) 23 CAD
Figure 5. SOI base : stoichiometric iso-surface contoured by temperature. (a) 18 CAD aSOI; (b) 23 CAD
aSOI; (c) 38 CAD aSOI. (Left) Re-entrant; (middle) stepped-lip; (right) radial-bumps. Engine oper-
aSOI; (c) 38 CAD aSOI. (left) Re-entrant; (middle) stepped-lip; (right) radial-bumps. Engine operating
ating condition (baseline): 4000 RPM × 18.5 bar BMEP.
condition (baseline): 4000 RPM × 18.5 bar BMEP.

• At θ1 = +18 CAD aSOI

At this stage, the flame/wall interaction plays the main role in the overall combustion
rate. Considering the SOI base , the re-entrant bowl highlights an HRR lower than
the other bowls (see Figure 4) due to the interaction of adjacent flames, as shown
in Figure 5a. Different mechanisms can be instead highlighted for the other designs
under investigation. The stepped-lip bowl shows higher flame propagation above the
step, increasing the air utilization in the squish area, while the radial-bumps bowl
avoids the interaction of adjacent flames, increasing the combustion rate. Retarding
the injection (SOI base +10, Figure 6a), all the bowls under investigation show similar
results with respect to the baseline SOI due to the same piston position (−/+5 CAD
aTDC) and thus similar spray–wall interaction. Therefore, even reducing the ignition
delay, the re-entrant bowl shows the less intense HRR among the proposed designs, as
already depicted in Figure 4.
• At θ2 = +23 CAD aSOI
Moving ahead in the engine cycle, the re-entrant bowl highlights a strong interac-
tion between the adjacent flames for each SOI under investigation. This results in
lower HRR compared with the other bowls (see Figure 4). Considering the SOI base
(Figure 5b), the fuel split on the stepped-lip bowl allows a more even distribution of
the flame downward within the bowl and upward in the squish region, improving
Energies 2022, 15, 3799 9 of 18

the air–fuel mixing [18]. Differently, in the radial-bumps bowl, the flames collision is
significantly attenuated and the bumps coupled with the swirling flow enable a flow
recirculation that improves the air–fuel mixing near the tip of the bump [18]. Retarding
the SOI (SOI base +10, Figure 6b), similar results can be observed. Nevertheless, the
more advanced piston position leads to higher flame recirculation in the squish area.
This results in a reduced impact of the bump without completely jeopardizing its
beneficial effect as a flames separator.
• At θ3 = +38 CAD aSOI
Near the EOI of the main injection event, the flame evolution is strongly dependent on
the injection phasing. As shown in Figure 5c, at SOI base , the stepped-lip bowl shows an
intense flame redistribution above the step and the unbalanced split results in a lower
combustion rate, as already highlighted in Figure 4. Retarding the SOI (SOI base +10,
Figure 6c), all the piston bowl designs lead to a remarkable redistribution of the flame
in the squish region. This effect is detrimental, especially for the radial-bumps bowl,
Energies 2022, 15, x FOR PEER REVIEW where the reduced flame recirculation within the bowl leads to a lower impact 10 of
of the
19
bumps, providing a lower difference among the analysed SOIs than with the re-entrant
design in terms of CA 50-90 (see Figure 3).

Figure 6. SOI base +10: stoichiometric iso-surface contoured by temperature. (a) 18 CAD aSOI; (b) 23
Figure 6. SOI base +10: stoichiometric iso-surface contoured by temperature. (a) 18 CAD aSOI;
CAD aSOI; (c) 38 CAD aSOI. (Left) Re-entrant; (middle) stepped-lip; (right) radial-bumps. Engine
(b) 23 CAD
operating aSOI; (c)
condition 38 CAD 4000
(baseline): aSOI.RPM
(left) Re-entrant;
× 18.5 (middle) stepped-lip; (right) radial-bumps.
bar BMEP.
Engine operating condition (baseline): 4000 RPM × 18.5 bar BMEP.
3.1.2. Swirl Ratio Sensitivity
The swirl ratio impact on the proposed combustion chamber has been already inves-
tigated under nonreacting conditions considering the full-cylinder geometry [18]. It is
worth recalling that the stepped-lip bowl highlighted a lower swirl amplification than the
re-entrant bowl due to the reduced squish flow intensity. Differently, the radial-bumps
bowl showed an intense swirl collapse due to the bumps that can break the swirling flow,
Energies 2022, 15, 3799 10 of 18

3.1.2. Swirl Ratio Sensitivity

The swirl ratio impact on the proposed combustion chamber has been already inves-
tigated under nonreacting conditions considering the full-cylinder geometry [18]. It is
worth recalling that the stepped-lip bowl highlighted a lower swirl amplification than the
re-entrant bowl due to the reduced squish flow intensity. Differently, the radial-bumps bowl
showed an intense swirl collapse due to the bumps that can break the swirling flow, thus
resulting in higher turbulent kinetic energy within the bowl [18]. To further understand the
swirl impact on the combustion process, a swirl ratio at the IVC equal to zero (hereafter
“zeroed”) was imposed, zeroing the velocity components perpendicular to the cylinder
Energies 2022, 15, x FOR PEER REVIEW 11 of 19
axis. The HRR and the cumulative heat release (HR) for each investigated swirl ratio (i.e.,
nominal and zero) and combustion system are reported in Figure 7.

Figure
Figure7.7. Swirl
Swirlratio
ratioresults.
results.(Top)
(top) Heat
Heat release
release rate
rate and
and injection
injectionrate;
rate;(bottom)
(bottom)cumulative
cumulativeheat
heat
release.
release.(Left)
(left) Re-entrant;
Re-entrant; (middle)
(middle) stepped-lip;
stepped-lip; (right)
(right) radial-bumps.
radial-bumps. Engine
Engineoperating
operatingcondition
condition
(baseline):
(baseline):4000
4000RPM
RPM××18.5
18.5bar
barBMEP.
BMEP.

For
Foreach
eachcombustion
combustionsystem systemunder under investigation,
investigation, thethezeroed
zeroedswirlswirlratioratio
leads to re-
leads to
duced
reduced air–fuel mixing
air–fuel mixing during
during thethe pilot
pilotinjection
injectionevent,
event,thus
thusminimizing
minimizingthe theover-leaning
over-leaning
of
ofthe
thefuel
fueljetjetand
andresulting
resultingin in more
more intense
intense pilot
pilot combustion.
combustion.Due Dueto tothat,
that,the
thepremixed
premixed
combustion
combustionintensity
intensityofofthethemain
main injection
injection is reduced.
is reduced. Among Among the the
investigated
investigated piston ge-
piston
ometries, the stepped-lip bowl is less affected by the swirl ratio variation
geometries, the stepped-lip bowl is less affected by the swirl ratio variation due to its wider due to its wider
geometry,
geometry,whosewhosemain mainflow
flowstructures
structuresare arethethetoroidal
toroidalvortices
vorticesininthe
thecylinder
cylinderaxisaxisplane.
plane.
Instead,
Instead,the thecumulative
cumulative HRHRwithwith thethere-entrant
re-entrant andandthe the
radial-bumps
radial-bumps designs appears
designs sig-
appears
nificantly
significantlyaffected by by
affected thetheswirl
swirl ratio. In In
ratio. particular,
particular, thethere-entrant
re-entrantdesign
designhas hascomparable
comparable
HR
HRwith
withand andwithout
without swirling
swirling flow flowup touptheto EOI of theofmain
the EOI event.event.
the main After that,Afterthethat,
zeroedthe
swirl
zeroedratio reduces
swirl the air–fuel
ratio reduces mixing of
the air–fuel the residual
mixing fuel in the
of the residual fuellate
in thecycle,
lateand conse-
cycle, and
consequently,
quently, the cumulative
the cumulative HR drops HR down.
drops down.
The HRR The forHRR for the radial-bumps
the radial-bumps bowl is
bowl is signifi-
significantly
cantly reduced reduced
duringduring the injection
the injection event in event in the
the case ofcase
a nullof swirl
a nullratio.
swirlTherefore,
ratio. Therefore,
since
since
the the radial
radial bumpsbumps
tend totend
breakto the
break the organized
organized swirlingswirling
motionmotion
[18], the[18], the radial-bumps
radial-bumps bowl
bowl requires
requires a highera higher swirlto
swirl ratio ratio to increase
increase the air–fuel
the air–fuel mixing mixing rateachieve
rate and and achieve
a more a more
effi-
efficient
cient combustion
combustion process.
process.
Theequivalence
The equivalenceratio
ratiodistribution
distributionwas wasinvestigated
investigatedto tofurther
furtherunderstand
understandthe theflow
flow
structures induced by the different swirl ratios for each combustion
structures induced by the different swirl ratios for each combustion system under inves- system under inves-
tigation.Two
tigation. Twocutting
cuttingplanes
planeswere wereselected
selectedto toshow
showthe thenumerical
numericalresults:
results:the thespray
sprayaxisaxis
plane and the cylinder axis plane. The equivalence ratio contour plot for the selected
planes and the isoline at the constant temperature equal to 1500 K (black line), which was
selected as representative of the flame front, are shown in Figure 8 for the nominal swirl
ratio and in Figure 9 for the zeroed swirl ratio.
Energies 2022, 15, 3799 11 of 18

plane and the cylinder axis plane. The equivalence ratio contour plot for the selected planes
and the isoline at the constant temperature equal to 1500 K (black line), which was selected
as representative of the flame front, are shown in Figure 8 for the nominal swirl ratio and
in Figure 9 for the zeroed swirl ratio.
Regarding the nominal swirl ratio, at TDC (Figure 8a), the re-entrant bowl shows an
intense flame-to-flame interaction, while both the stepped-lip and radial-bumps bowls
reduce this counterproductive effect, as already depicted in Figure 5. The stepped-lip
bowl highlights higher flame propagation upward above the step, reducing the flame
propagation in the tangential direction and thereby the collision of two adjacent flames.
Instead, the radial-bumps bowl provides not only a reduced flame-to-flame interaction
but also a different tangential flame propagation along the bowl surface. In particular, the
swirling flow induces an asymmetrical spray–wall interaction and the fuel is driven by the
bump into the consecutive sector, where the high air content enhances the air–fuel mixing.
Considering the absence of swirl motion (Figure 9a), a symmetric spray–wall interaction is
observed for all the investigated geometries, creating a more pronounced radial-mixing
zone (RMZ) in the collision area of two adjacent flames, for the re-entrant and radial-bumps
bowls. Nevertheless, for the radial-bumps bowl, the behaviour of the RMZ is significantly
affected by the geometry of the bump, as also shown in [16]. The fuel plumes are driven by
the bumps tip toward the cylinder centre where the available oxygen improves the air–fuel
mixing onto the flame front. However, the intensity of this recirculation is high only in the
upper-bump region, while in the bottom-bump region the tumbling vortex is the main flow
structure due to the high bowl re-entrance [18]. Therefore, the radial-bumps bowl provides
Energies 2022, 15, x FOR PEER REVIEW 12 of 19
a more efficient air–fuel mixing with the nominal swirl ratio, since the bumps coupled with
higher swirling flow enable beneficial flow structures.

Figure 8. Nominal swirl ratio: equivalence ratio contour plot on the spray axis and cylinder axis
Figure 8. Nominal swirl ratio: equivalence ratio contour plot on the spray axis and cylinder
planes. Black line: the constant temperature at 1500 K. (a) 0 CAD aTDC; (b) 20 CAD aTDC. (Left)
axis planes. Black line: the constant temperature at 1500 K. (a) 0 CAD aTDC; (b) 20 CAD aTDC.
Re-entrant; (middle) stepped-lip; (right) radial-bumps. Engine operating condition (baseline): 4000
RPM ×Re-entrant;
(left) (middle) stepped-lip; (right) radial-bumps. Engine operating condition (baseline):
18.5 bar BMEP.
4000 RPM × 18.5 bar BMEP.
 Figure 8. Nominal swirl ratio: equivalence ratio contour plot on the spray axis and cylinder axis
planes. Black line: the constant temperature at 1500 K. (a) 0 CAD aTDC; (b) 20 CAD aTDC. (Left)
Energies 2022, 15, 3799 124000
Re-entrant; (middle) stepped-lip; (right) radial-bumps. Engine operating condition (baseline): of 18
RPM × 18.5 bar BMEP.

Figure 9. Zeroed swirl ratio: equivalence ratio contour plot on the spray axis and cylinder axis
Figure 9. Zeroed swirl ratio: equivalence ratio contour plot on the spray axis and cylinder axis
planes. Black line: the constant temperature at 1500 K. (a) 0 CAD aTDC; (b) 20 CAD aTDC. (Left)
planes. Black
Re-entrant; line: stepped-lip;
(middle) the constant(right)
temperature at 1500Engine
radial-bumps. K. (a)operating
0 CAD aTDC; (b) (baseline):
condition 20 CAD aTDC.4000
(left) Re-entrant; (middle)
RPM × 18.5 bar BMEP. stepped-lip; (right) radial-bumps. Engine operating condition (baseline):
4000 RPM × 18.5 bar BMEP.

After the EOI of the main event (+20 CAD aTDC), the overall air–fuel mixing of the
re-entrant bowl is significantly affected by the swirl ratio. At this stage with the nominal
swirl ratio (Figure 8b), an almost homogeneous equivalence ratio toward the stoichiometric
value can be observed, while removing the swirling flow (Figure 9b) the fuel-rich zones are
still significant. This residual fuel is slowly oxidized, resulting in lower HRR in the late
cycle, as shown in Figure 7. For the stepped-lip bowl, the equivalence ratio distribution
within the bowl is comparable among the two tested swirl ratios. Indeed, the air–fuel
mixing is mainly driven above the step which is slightly affected by the swirling flow. As a
consequence, small differences can be observed in the late-cycle HRR considering different
swirl ratios. Regarding the radial-bumps bowl, the bumps coupled with a higher swirl ratio
continue to play a fundamental role on air–fuel mixing, leading to a more homogeneous
mixture in the late-cycle with respect to the zeroed swirl ratio condition.
To globally characterize the effect of different swirl ratios on the combustion process at
+20 CAD aTDC, the whole cylinder mass was binned by equivalence ratio into ten intervals,
for both the nominal and the zeroed swirl ratio, as highlighted in Figure 10. For each piston
bowl, the nominal swirl ratio shows the mode of the distribution closer to the stoichiometric
range, while a lower cylinder mass fraction is shown in the tails of the distribution, suggest-
ing a faster and more efficient combustion process. Nevertheless, only the radial-bumps
bowl shows a strong sensitivity to the swirl ratio. Indeed, with the nominal swirl ratio,
a higher cylinder mass fraction can be observed in the 0.8–1.2 equivalence ratio range,
increasing the mixture homogeneity towards the stoichiometric value.
 tervals, for both the nominal and the zeroed swirl ratio, as highlighted in Figure 10. For
each piston bowl, the nominal swirl ratio shows the mode of the distribution closer to the
stoichiometric range, while a lower cylinder mass fraction is shown in the tails of the dis-
tribution, suggesting a faster and more efficient combustion process. Nevertheless, only
the radial-bumps bowl shows a strong sensitivity to the swirl ratio. Indeed, with the nom-
Energies 2022, 15, 3799 13 of 18
inal swirl ratio, a higher cylinder mass fraction can be observed in the 0.8–1.2 equivalence
ratio range, increasing the mixture homogeneity towards the stoichiometric value.

Figure 10. Equivalence ratio bins distribution for nominal and null swirl ratio at +20 CAD aTDC.
(left) Re-entrant; (middle) stepped-lip; (right) radial-bumps. Engine operating condition (baseline):
4000 RPM × 18.5 bar BMEP.

3.2. Partial Load Engine Operating Condition—1500 RPM × 5.0 bar BMEP
EGR and Rail Pressure Sensitivity
At the partial load engine working point, different EGR rates and rail pressure levels
were considered to assess the combustion system sensitivity in terms of fuel consumption
and emissions. Three EGR rates were analysed: the baseline EGR for the re-entrant bowl
and +/−5% with respect to the baseline EGR. The EGR rate was modified by varying the
gas species concentration, evaluating only the different dilution and thermal effects thus
keeping the same volumetric efficiency. Then, for each EGR rate, two rail pressure levels
were considered (i.e., baseline and 50% higher than the baseline). The injection profile
referred to the higher rail pressure was obtained by means of the injector model described
in [25,26]. For each investigated calibration, the energizing time of the main injection was
varied to keep the engine load equal to the one obtained for the baseline configuration (i.e.,
re-entrant bowl–nominal EGR–nominal rail pressure).
Figure 11 shows the EGR and rail pressure sweep analysis results in terms of ISFC,
indicated specific NOx (ISNOx ) and soot, each of them normalized with respect to the
value of the baseline engine configuration. Starting from the ISFC under the nominal
rail pressure (solid lines of Figure 11a), both the stepped-lip and the radial-bumps bowls
highlight a reduced ISFC with respect to the re-entrant bowl (−1% and −4% at baseline
EGR, respectively). Interestingly, going towards higher EGR rates, a further reduction in
ISFC compared with the re-entrant bowl can be appreciated for both the innovative bowls,
suggesting higher EGR tolerance due to the improved mixing process. As already assessed
in [18], the radial-bumps design highlights a faster combustion process and the resulting
higher temperature leads to higher NOx production, as also confirmed in Figure 11b.
Nevertheless, a further increase in the EGR rate can be adopted for NOx mitigation due
to the improved EGR tolerance. This is possible thanks to the impressive soot reduction
highlighted by the stepped-lip and radial-bumps bowls with respect to the re-entrant
design (−40% and −60% at baseline EGR), as shown in Figure 11. Additionally, it is worth
noting that the radial-bumps bowl shows a flat trend over the EGR sweep, confirming that
a higher EGR rate can be used for NOx mitigation even without soot penalties. This result
is in line with the flatness of the soot–NOx tradeoff observed for a heavy-duty diesel engine
application that features a similar bowl design [16], confirming the great potential of radial
bumps on soot reduction.
Energies 2022, 15,
Energies 2022, 15, 3799
x FOR PEER REVIEW 1514of
of 19
18

Figure 11.
Figure EGR and
11. EGR and rail
rail pressure
pressure sweep. (a) ISFC;
sweep. (a) ISFC; (b)
(b) ISNO
ISNOxx;; (c)
(c) soot
soot normalized
normalized with
with respect
respect to
to
baseline engine configuration. Engine operating condition: 1500 RPM ××5.0 5.0bar
barBMEP.
BMEP.

Once we assessed the combustion systems sensitivity over different EGR rates, the
4. Conclusions
rail pressure was increased with respect to the nominal condition (+50%) and the results
This work aims to assess through numerical simulations the influence of different
for the higher rail pressure are reported in Figure 11 with dashed lines. As expected,
engine calibration parameters on the combustion process considering three piston bowl
increasing the rail pressure, a further improvement of the ISFC can be observed for all the
designs for ageometries
investigated light-duty duediesel
to engine.
the moreThe baseline
intense re-entrant
premixed design was
combustion compared
phase. Nevertheless,with
two innovative designs, originally developed for heavy-duty applications,
the radial-bumps bowl with the baseline rail pressure still shows lower ISFC than the other named
stepped-lip and radial-bumps
bowls with higher rail pressure.bowl. Two the
Moreover, operating
enhanced conditions
premixedwere considered
combustion phase at leads
high
and low engine
to higher flame load. Firstly, under
temperature, the rated power
thus increasing the NO condition, the optimal injection tim-
x formation. However, the radial-
ing and swirl ratio were identified. Then, at the partial load
bumps bowl highlights the lowest NOx increment with respect to the baseline operating point, an
railEGR sen-
pressure.
sitivity at two different rail pressure levels was carried out to assess
As far as soot emissions are concerned, the higher rail pressure results in a remarkablethe impact on engine
efficiency
reduction andthanks emissions. The main
to the enhanced outcomes
spray of this that
atomization work can be
limits thesummarized
soot formation as follows.
for each
First of all, both innovative designs showed high potential in
investigated design. Interestingly, at baseline EGR, the re-entrant bowl with higherimproving the combus- rail
tion process
pressure showshighlighting
comparable significant
soot thanbenefits
with theinradial-bumps
terms of indicated
bowl atspecific fuel
baseline railconsump-
pressure.
tion
Then,(ISFC) reduction
the reduction in in comparison
soot achievablewith the conventional
by increasing re-entrant
the rail pressure geometry.
in the This re-
conventional re-
sult is more evident for the radial-bumps design, thanks to which −3%
entrant design could be reached in the radial-bumps bowl thanks to the improved mixing, and −4% ISFC re-
ductions
thus avoidingwereany achieved
worseningat full load andlosses
in parasitic at partial
due toload, respectively.
the increased power Instead,
demandfor of the
the
stepped-lip bowl,
high-pressure fuel pump.a reduced improvement of the ISFC (−1%) was observed for two inves-
tigated operating conditions. In addition, the adoption of the stepped-lip or the radial-
bumps designs led to a noticeable air–fuel mixing enhancement, thus halving the soot
emissions in almost all the analysed operating conditions.
Energies 2022, 15, 3799 15 of 18

4. Conclusions
This work aims to assess through numerical simulations the influence of different
engine calibration parameters on the combustion process considering three piston bowl
designs for a light-duty diesel engine. The baseline re-entrant design was compared with
two innovative designs, originally developed for heavy-duty applications, named stepped-
lip and radial-bumps bowl. Two operating conditions were considered at high and low
engine load. Firstly, under the rated power condition, the optimal injection timing and
swirl ratio were identified. Then, at the partial load operating point, an EGR sensitivity at
two different rail pressure levels was carried out to assess the impact on engine efficiency
and emissions. The main outcomes of this work can be summarized as follows.
First of all, both innovative designs showed high potential in improving the combus-
tion process highlighting significant benefits in terms of indicated specific fuel consumption
(ISFC) reduction in comparison with the conventional re-entrant geometry. This result is
more evident for the radial-bumps design, thanks to which −3% and −4% ISFC reductions
were achieved at full load and at partial load, respectively. Instead, for the stepped-lip bowl,
a reduced improvement of the ISFC (−1%) was observed for two investigated operating
conditions. In addition, the adoption of the stepped-lip or the radial-bumps designs led to
a noticeable air–fuel mixing enhancement, thus halving the soot emissions in almost all the
analysed operating conditions.
Several sensitivity analyses were carried out to clearly highlight the potential of the
proposed combustion systems.
• As far as the start of injection (SOI) is concerned, the geometry modifications to
enhance the mixing led to a higher impact on the injection timing due to the strong
spray targeting sensitivity. This effect has been clearly highlighted for the stepped-lip
design, in which the strong unbalanced fuel split toward the squish region, resulting
in a less efficient combustion process.
• The analysis of the swirl ratio has clarified significant insights about the need for
the proposed combustion systems in a light-duty engine. In fact, on one side, the
stepped-lip bowl showed a slight impact of the swirl on the combustion rate, since
the mixing process is mainly driven by the two toroidal vortices due to the fuel split
on the step. On the other side, differently from the heavy-duty requirements, the
radial-bumps design in combination with the swirling motion enables an efficient
recirculation of the fuel, remarkably improving the air–fuel mixing rate.
• The analysis at different EGR rates has demonstrated the effect of the improved air–fuel
mixing. Higher EGR tolerance was achieved, significantly reducing the soot emission
and almost flattening the NOx–soot tradeoff, without any detrimental impact on the
engine efficiency. In particular, at baseline EGR, −40% and −60% soot reductions were
obtained by the stepped-lip and radial-bumps bowls, respectively.
Future analysis will be focused on further geometrical optimization with the aim to
combine the potential benefits of these innovative profiles in terms of combustion and
pollutant emissions.

Author Contributions: Conceptualization, F.M., A.P., F.C.P. and A.V.; methodology, A.P., S.R. and
A.B.; validation, A.P. and S.R.; formal analysis, A.P., S.R. and A.B.; investigation, A.P., S.R. and A.B.;
writing—original draft preparation, A.P. and S.R.; writing—review and editing, F.M., F.C.P., A.V. and
A.B.; supervision, F.M., F.C.P. and A.V.; project administration, F.M. and F.C.P.; funding acquisition,
F.M. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Acknowledgments: Computational resources were provided by HPC@POLITO, a project of Aca-
demic Computing within the Department of Control and Computer Engineering at the Politecnico di
Torino (http://www.hpc.polito.it). The authors would like to acknowledge General Motors (GM)
and PUNCH Torino for their invaluable support to the research activity.
Conflicts of Interest: The authors declare no conflict of interest.
Energies 2022, 15, 3799 16 of 18

Abbreviations

AM additive manufacturing
AMR adaptive mesh refinement
BMEP brake mean effective pressure
BSFC brake-specific fuel consumption
CA10 crank angle at 10% of burned mass fraction
CA50 crank angle at 50% of burned mass fraction
CA50-90 duration for 50-90% of burned mass fraction
CA75 crank angle at 75% of burned mass fraction
CA90 crank angle at 90% of burned mass fraction
CAD aSOI crank angle degrees after start of injection
CAD aTDC crank angle degrees after top dead centre
CFD computational fluid dynamics
CIV combustion image velocimetry
CRDI common rail direct injection
EGR exhaust gas recirculation
EOI end of injection
HR heat release
HRR heat release rate
ISFC indicated specific fuel consumption
ISNOx indicated specific NOx
IVC intake valve closure
KH-RT Kelvin Helmholtz and Rayleigh Taylor
NTC no time counter
PAH polycyclic aromatic hydrocarbons
PM particulate mimic
RANS Reynolds-averaged Navier–Stokes
RMZ radial mixing zone
RNG renormalization group
SOI start of injection
TDC top dead centre
VGT variable geometry turbine

References
1. Cornwell, R.; Conicella, F. Direct Injection Diesel Engines, Ricardo UK Limited, West Sussex (GB). U.S. Patent 8770168 B2, 8 July 2014.
2. Styron, J.; Baldwin, B.; Fulton, B.; Ives, D.; Ramanathan, S. Ford 2011 6.7L Power Stroke® Diesel Engine Combustion System
Development. In Proceedings of the SAE 2011 World Congress & Exhibition, Detroit, MI, USA, 12–14 April 2011; SAE Technical
Paper 2011-01-0415. [CrossRef]
3. Yoo, D.; Kim, D.; Jung, W.; Kim, N.; Lee, D. Optimization of Diesel Combustion System for Reducing PM to Meet Tier4-Final
Emission Regulation without Diesel Particulate Filter. In Proceedings of the SAE/KSAE 2013 International Powertrains, Fuels &
Lubricants Meeting, Seoul, Korea, 21–23 October 2013; SAE Technical Paper 2013-01-2538. [CrossRef]
4. Kogo, T.; Hamamura, Y.; Nakatani, K.; Toda, T.; Kawaguchi, A.; Shoji, A. High Efficiency Diesel Engine with Low Heat Loss
Combustion Concept-Toyota’s Inline 4-Cylinder 2.8-Liter ESTEC 1GD-FTV Engine. In Proceedings of the SAE 2016 World
Congress and Exhibition, Detroit, MI, USA, 12–14 April 2016; SAE Technical Paper 2016-01-0658. [CrossRef]
5. Lückert, P.; Arndt, S.; Duvinage, F.; Kemmner, M.; Binz, R.; Storz, O.; Reusch, M.; Braun, T.; Ellwanger, S. The New Mercedes-Benz
4-Cylinder Diesel Engine OM654–The Innovative Base Engine of the New Diesel Generation. In Proceedings of the 24th Aachen
Colloquium Automobile and Engine Technology, Aachen, Germany, 5–7 October 2015; pp. 867–892.
6. Eder, T.; Kemmner, M.; Lückert, P.; Sass, H. OM 654–Launch of a New Engine Family by Mercedes-Benz. MTZ Worldw. 2016, 77,
60–67. [CrossRef]
7. Smith, A. Ricardo Low Emissions Combustion Technology Helps JCB Create the off-Highway Industry’s Cleanest Engine.
Ricardo Press Release. 2010. Available online: https://ricardo.com/news-and-media/news-and-press/ricardo-low-emissions-
combustion-technology-helps (accessed on 20 May 2022).
8. Busch, S.; Zha, K.; Perini, F.; Reitz, R.; Kurtz, E.; Warey, A.; Peterson, R. Bowl Geometry Effects on Turbulent Flow Structure
in a Direct Injection Diesel Engine. In Proceedings of the International Powertrains, Fuels & Lubricants Meeting, Heidelberg,
Germany, 17–19 September 2018; SAE Technical Paper 2018-01-1794. [CrossRef]
Energies 2022, 15, 3799 17 of 18

9. Millo, F.; Piano, A.; Paradisi, B.P.; Boccardo, G.; Mirzaeian, M.; Arnone, L.; Manelli, S. The Effect of Post Injection Coupled with
Extremely High Injection Pressure on Combustion Process and Emission Formation in an Off-Road Diesel Engine: A Numerical
and Experimental Investigation. In Proceedings of the 14th International Conference on Engines & Vehicles, Naples, Italy, 15–19
September 2019; SAE Technical Paper 2019-24-0092. [CrossRef]
10. Busch, S.; Zha, K.; Kurtz, E.; Warey, A.; Peterson, R. Experimental and Numerical Studies of Bowl Geometry Impacts on Thermal
Efficiency in a Light-Duty Diesel Engine. In Proceedings of the WCX World Congress Experience, Detroit, MI, USA, 10–12 April
2018; SAE Technical Paper 2018-01-0228. [CrossRef]
11. Zha, K.; Busch, S.; Warey, A.; Peterson, R.C.; Kurtz, E. A Study of Piston Geometry Effects on Late-Stage Combustion in a
Light-Duty Optical Diesel Engine Using Combustion Image Velocimetry. SAE Int. J. Engines 2018, 11, 783–804. [CrossRef]
12. Perini, F.; Busch, S.; Zha, K.; Reitz, R.; Kurtz, E. Piston Bowl Geometry Effects on Combustion Development in a High-Speed
Light-Duty Diesel Engine. In Proceedings of the 14th International Conference on Engines & Vehicles, Naples, Italy, 15–19
September 2019; SAE Technical Paper 2019-24-0167. [CrossRef]
13. Andersson, Ö.; Miles, P.C. Diesel and Diesel LTC Combustion. Encycl. Automot. Eng. 2014, 1, 1–36. [CrossRef]
14. Eismark, J.; Balthasar, M.; Karlsson, A.; Benham, T.; Christensen, M.; Denbratt, I. Role of Late Soot Oxidation for Low Emission
Combustion in a Diffusion-controlled, High-EGR, Heavy Duty Diesel Engine. In Proceedings of the SAE 2009 Powertrains Fuels
and Lubricants Meeting, San Antonio, TX, USA, 2–4 November 2009; SAE Technical Paper 2009-01-2813. [CrossRef]
15. Eismark, J.; Balthasar, M. Device for Reducing Emissions in a Vehicle Combustion Engine, Volvo Lastvagnar AB, Göteborg (SE).
U.S. Patent 8499735B2, 6 August 2013.
16. Eismark, J.; Andersson, M.; Christensen, M.; Karlsson, A.; Denbratt, I. Role of Piston Bowl Shape to Enhance Late-Cycle Soot
Oxidation in Low-Swirl Diesel Combustion. SAE Int. J. Engines 2019, 12, 233–249. [CrossRef]
17. Zhang, T.; Eismark, J.; Munch, K.; Denbratt, I. Effects of a wave-shaped piston bowl geometry on the performance of heavy duty
Diesel engines fueled with alcohols and biodiesel blends. Renew. Energy 2019, 148, 512–522. [CrossRef]
18. Millo, F.; Piano, A.; Roggio, S.; Bianco, A.; Pesce, F.C. Numerical Investigation on Mixture Formation and Combustion Process
of Innovative Piston Bowl Geometries in a Swirl-Supported Light-Duty Diesel Engine. SAE Int. J. Engines 2021, 14, 247–262.
[CrossRef]
19. Dolan, R.; Budde, R.; Schramm, C.; Rezaei, R. 3D Printed Piston for Heavy-Duty Diesel Engines. In Proceedings of the 2018 Ndia
Ground Vehicle Systems Engineering and Technology Symposium Power & Mobility (P&M) Technical Session, Novi, MI, USA,
7–9 August 2018. Available online: https://events.esd.org/wp-content/uploads/2018/07/3D-Printed-Piston-for-Heavy-Duty-
Diesel-Engines.pdf (accessed on 20 May 2022).
20. Millo, F.; Piano, A.; Roggio, S.; Bianco, A.; Pesce, F.C.; Vassallo, A.L. Numerical Assessment of Additive Manufacturing-Enabled
Innovative Piston Bowl Design for a Light-Duty Diesel Engine Achieving Ultra-Low Engine-Out Soot Emissions. SAE Int. J.
Engines 2021, 15, 427–443. [CrossRef]
21. Belgiorno, G.; Boscolo, A.; Dileo, G.; Numidi, F.; Pesce, F.C.; Vassallo, A.; Ianniello, R.; Beatrice, C.; Di Blasio, G. Experimental
Study of Additive-Manufacturing-Enabled Innovative Diesel Combustion Bowl Features for Achieving Ultra-Low Emissions and
High Efficiency. SAE Int. J. Adv. Curr. Pract. Mobil. 2021, 3, 672–684. [CrossRef]
22. Millo, F.; Piano, A.; Roggio, S.; Pastor, J.; Micó, C.; Lewiski, F.; Pesce, F.; Vassallo, A.; Bianco, A. Mixture formation and combustion
process analysis of an innovative diesel piston bowl design through the synergetic application of numerical and optical techniques.
Fuel 2021, 309, 122144. [CrossRef]
23. Millo, F.; Piano, A.; Peiretti Paradisi, B.; Marzano, M.R.; Bianco, A.; Pesce, F.C. Development and Assessment of an Integrated
1D-3D CFD Codes Coupling Methodology for Diesel Engine Combustion Simulation and Optimization. Energies 2020, 13, 1612.
[CrossRef]
24. Piano, A.; Millo, F.; Boccardo, G.; Rafigh, M.; Gallone, A.; Rimondi, M. Assessment of the Predictive Capabilities of a Combustion
Model for a Modern Common Rail Automotive Diesel Engine. In Proceedings of the SAE 2016 World Congress and Exhibition,
Detroit, MI, USA, 12–14 April 2016; SAE Technical Paper 2016-01-0547. [CrossRef]
25. Piano, A.; Millo, F.; Postrioti, L.; Biscontini, G.; Cavicchi, A.; Pesce, F.C. Numerical and Experimental Assessment of a Solenoid
Common-Rail Injector Operation with Advanced Injection Strategies. SAE Int. J. Engines 2016, 9, 565–575. [CrossRef]
26. Piano, A.; Boccardo, G.; Millo, F.; Cavicchi, A.; Postrioti, L.; Pesce, F.C. Experimental and Numerical Assessment of Multi-Event
Injection Strategies in a Solenoid Common-Rail Injector. SAE Int. J. Engines 2017, 10, 2129–2140. [CrossRef]
27. Orszag, S.A.; Yakhot, V.; Flannery, W.S.; Boysan, F. Renormalization Group Modeling and Turbulence Simulations. Near-Wall
Turbul. Flows 1993, 13, 1031–1046.
28. Ichards, K.J.; Senecal, P.K.; Pomraning, E. Converge 2.3 Manual; Convergent Science Inc.: Madison, WI, USA, 2016.
29. Reitz, R.D.; Bracco, F.V. Mechanisms of Breakup of Round Liquid Jets. Encycl. Fluid Mech. 1986, 3, 233–249.
30. Amsden, A.A.; O’Rourke, P.J.; Butler, T.D. KIVA-II: A Computer Program for Chemically Reactive Flows with Sprays; Los Alamos
National Laboratory Technical Report LA-11560-MS; Los Alamos National Lab: Los Alamos, NM, USA, 1989.
31. Schmidt, D.P.; Rutland, C. A New Droplet Collision Algorithm. J. Comput. Phys. 2000, 164, 62–80. [CrossRef]
32. O’Rourke, P.; Amsden, A. The Tab Method for Numerical Calculation of Spray Droplet Breakup. In Proceedings of the 1987
SAE International Fall Fuels and Lubricants Meeting and Exhibition, Toronto, ON, Canada, 2–5 November 1987; SAE Technical
Paper 872089. [CrossRef]
Energies 2022, 15, 3799 18 of 18

33. O’Rourke, P.; Amsden, A. A Spray/Wall Interaction Submodel for the KIVA-3 Wall Film Model. In Proceedings of the SAE 2000
World Congress, Detroit, MI, USA, 6–9 March 2000; SAE Technical Paper 2000-01-0271. [CrossRef]
34. Zeuch, T.; Moréac, G.; Ahmed, S.S.; Mauss, F. A comprehensive skeletal mechanism for the oxidation of n-heptane generated by
chemistry-guided reduction. Combust. Flame 2008, 155, 651–674. [CrossRef]
35. Frenklach, M.; Wang, H. Detailed modeling of soot particle nucleation and growth. Symp. Int. Combust. 1991, 23, 1559–1566.
[CrossRef]
36. Kazakov, A.; Wang, H.; Frenklach, M. Detailed modeling of soot formation in laminar premixed ethylene flames at a pressure of
10 bar. Combust. Flame 1995, 100, 111–120. [CrossRef]
37. Kazakov, A.; Frenklach, M. Dynamic Modeling of Soot Particle Coagulation and Aggregation: Implementation with the Method
of Moments and Application to High-Pressure Laminar Premixed Flames. Combust. Flame 1998, 114, 484–501. [CrossRef]