TEOiNICAL NOTE 1
Computational analysis of the lean-burn direct-
injection
jet
ignition hydrogen engine
A Borelli 1., H Watson " and A Tempia 2
'School of
Science
and
Enginesing, University of Ballarat, Ballarat, Vidoria, Auatralia
2Robert
Bosch
(Australia), Pty
ltd,
Clayton,
Vidoria,
Australia
The
manus::ript was
recEived
on 28 May 2009 and was
accepted
after revison for publication on 6
August
2009.
001: 10.1243/09544070J6.UT01278
Abstract: This paper presents a new in-cylinder mixture preparation
and
ignition
system
for
various gaseous fuels including hydrogen. The
system
consists
of
a centrally located direct-
injection
(01) injector and a jet ignition (J) device
for
combustion
of
the main chamber (MC)
mixture. The fuel
is
injected in the MC with a new-generation fast-actuating high-pressure
high-flowrate
01
injector capable
of
injection shaping
and
multiple
events.
This injector
produces a
bulk
lean stratified mixture. The J
9j'SIem
uses
a second
01
injector to inject a small
amount
of
fuel in a small pre-chamber
(PC).
A spark plug then ignites a slightly rich mixture.
The
MC mixture
is
then bulk ignited through
multiple
jets
of
hot reacting
gases.
Bulk ignition
and
combustion
of
the lean jet-controlled stratified
MC
mixture resulting from coupling
01
with J makes
it
possible to burn MC mixtures with fuel-to-air equivalence ratios reducing
almost to zero
for
a throttle less control
of
load diesel-likeand high efficiencies over almost the
full range
of
loads. Computations are performed with hydrogen
as
the
PC
and
MC fuel.
Keywords:
gas
engines, direct injection, jet ignition, lean-burn stratified combustion,
bulk
ignition, combustion
1 I
NTROOU
CTI
ON
lid standard temperature and pressure, hydrogen is a
colourless odourless non-metallic
tasteless,
highly
flammable
diatomic gas H
2
. With
an
atomic weight
of
1.00794, hydrogen
is
the lightest element. Hydro-
gen
is al9) the most abundant element in the
universe, making up
75
per cent
of
normal matter
by
mass
and over
90
per cent by number
of
atoms.
Under ordinary conditions on Earth,
elemental
hydrogen exists
as
the diatomic gas H
2
. However,
hydrogen
gas
is
very rare in the Earth's atmosphere
because
of
its
light weight which enables
it
to
escape
from Earth's gravity more easily than heavier
gases.
Hydrogen in chemically combined form
is
the third
most abundant
element on the Earth's surface. Most
of
the Earth's hydrogen
is
in the form
of
chemical
compounds
such
as
hydrocarbons
and
water.
*Correponding author:
S:hool
d S:iSlce
and
Enginwing,
Unive-sity
d Ballarat,
PO
Box
663,
Ballarat, Vidoria
3353,
Auatralia.
e-nail:
a_boratiCWahoo.
aJIIl
Jl\UT01278
Oespite the fact that hydrogen
can
be prepared in
several
different
ways,
the economically most
im-
portant processes involve removal
of
hydrogen from
hydrocarbons.
Commercial
bulk
hydrogen
is
usually
produced by the
steam
re-forming
of
natural
gas.
Hydrocarbons other than methane
can
be
used
to
produce synthesis
gas
with various product ratios.
Other important methods for
H2
production include
partial
oxidation
of
hydrocarbons. Hydrogen may
al9) be produced from water by electrolysis
at
substantially greater cost than production from
natural
gas.
Hydrogen
gas
is highly flammable and will burn in
air at a very wide range
of
concentrations between
4vol/%
and 75vol/%. Hydrogen-<lxygen mixtures
are
explosive across a wide range
of
proportions. Its
autoignition temperature, the temperature
at
which
it
ignites spontaneously in air,
is
858K.
H2
reacts
with every oxidizing element.
A hydrogen
internal combustion engine
(H
2ICE)
is
a hydrogen-fuelled internal combustion engine
providing efficiencies in
excess
of
today's ga9)line
engines
and
operating relatively cleanly with nitro-
Proc. IMechE Vol.
224
Part
D:.l
Automobile Engineering
2 A Boretti, H Watson, and A Tempia
gen
oxides
(NO.)
being the
only
emission pollutant
[1-3]. Table
1 (from references [1] tor 5]) presents
Slme properties
of
hydrogen to outline the unique
combustion properties in
internal combustion en-
gi
ne applications.
These
properties are beneficial at
certain engine operating conditions
and
pose
tech-
nical challenges
at
other engine operating condi-
tions. The presented values are not all well accepted
and
may be slightly different in other references. The
definition
of
the
research
octane number
(RON)
in
particular is open to dis::ussion. Rigorously,
RON
determination
of
H2
with the conventional ASrM
method is not possible. The value proposed in
Table 1 (from references [1], [3],
and
[5]) simply
indicates the decrease in knock tendency
of
hydro-
gen
(when 9Jrface ignition
and
residual
gas
ignition
are eliminated) with reference to
gaSlline fuel
experimentally demonstrated in the
limited number
of
engine tests performed Sl far.
Favourable properties
of
H2
are the wide flamm-
ability range for ultra-lean operation, the high
laminar flame
speed
for good stability,
and
the high
octane number
for
high compression ratios with
improved thermal efficiency.
Unfavourable proper-
ties
of
H2
are the high percentage stoichiometric
volume fraction
of
the vapour with the consequent
air
displacement effects, the low minimum ignition
energy with consequent propensity to pre-ignite, the
small quenching distance for thin thermal boundary
layers,
and
the low density that make
it
difficult to
provide
large injected
mass
flowrates.
Temperatures
below
200K
are collectively known
as
cryogenic temperatures,
and
liquids
at
these
temperatures are known ascryogenic liquids. Boiling
isthe transition from liquid to gas. Hydrogen has the
second-lowest boiling point
of
all
9Jbstances,
sec-
ond
only
to helium. The boiling
point
of
a pure
Table 1
Fuel
properties at
25£
(except when otha--
wise noted),
1b
ar, and stoichiometry (when
appli
cable)
(from
refa-ences
[1] to[ 5])
Minimum iglition ena-gy
(MJ)
Flame velocity
(m/5)
Ad
iabaticfiane
temperature
(K)
Minimum quenching distance (mm)
Fuel-to-air
mass
ratio
Vapour volume fraction
(%)
Heat
d combustion
(M"
(kg
air))
Flammability
limit (volume)
(%)
Flammability limit w
Minimum autoignilion temperature (K)
Research
octane numba-
Vapour density
at
20
LC(kglm
3
)
Uquid density
at
normal boiling point (kg/m
3
)
Lower
heating value (MJ'
kg)
Higher heating value
(M"
kg)
0.02
1.85
2480
0.64
0.029
29.53
3.48
4-75
0.1-7.1
858
.
120
0.0838
70.8
119.93
141.86
Proc. IMechE Vol.
224
Part
D:
J.
Automobile Engineering
9Jbstance increases with applied presSJre up to a
point. Unfortunately, hydrogen's
boiling
pOint
can
only
be increased to a maximum
of
33.145K
through
the
application
of
a presSJre
of
12.964b
ar,
beyond
which additional
presSJre has no beneficial effect.
The
fuel properties
playa
key role in development
of
the direct-injection (01) mixture preparation
system.
Figure 1 presents the fluid properties
of
methane, propane,
and
hydrogen along isothermal
lines
[6]. This picture clearly
states
the problems
and
opportunities
of
gas
injection with variable presSJre
levels.
Late
01
overcomes the air displacement
effects
of
port fuel injection (PFI)
of
gaseous fuets.
However, development
of
a direct injector providing
adequate flowrates
is
difficult. Propane
(C3He)
has a
critical temperature
Tc
5
369.81<,
critical
presSJe
PeS
42.5bar,
and
critical density
rc
5
22O.Okglm
3
,
while the normal boiling
point
is 231.1K. Methane
(CH.) has a critical temperature Tc5 190.6K, critical
presSJre
Pc
5
46.Ob
ar,
and
critical density
rc
5
162.7kglm 3
while the normal boiling
point
is
111.71<.
Hydrogen
(H
2
) has a critical temperature
Tc
5 33.1K, critical
presSJre
Pc
5
13.Ob
ar, and criti-
cal
density r c 5 31.3kglm 3 while the normal boiling
point
is
20.4K.
IV.
a temperature
T5
3OOK,
propane
is a vapour
for
presSJres below
10.Ob
ar,
and
liquid
above. Conversely, methane
is
avapour for presSJres
below 48.4b ar,
and
9Jpercritical above. Hydrogen
is
Fig. 1
100
80
40
-methane
300 K
- propane 300 K
-hydrogen
300
K
.
-hydrogen
70 K
!
-hydrogen
50 K
i -
ro
n30K
0~~-4----~----~---+----~
o
40
80
120
160 200
plbarj
lsotha-mal density data
of
propane, methane,
and
hydrogen [6]
JAUT01278
Lean-burn direct-injection
jet
ignition hydrogen engine
3
a vapour for
presBJres
below
13.5b
ar,
and SJper-
critical above. Therefore, while a dedicated liquefied
petroleum
gas
(LPG)
engine may inject fuel in the
liquid
phase,
a flexi-fuel
LPG-<:ompressed
natural
gas (CNG) engine would have injection in the
vapour-phase
if
at a low pressJre, and
in
the
liquid
or SJpercriticai
phase
if
at
a high pressJre. If
hilt!
flowrates are possible with
LPG,
CNG
is
certainly
much more demanding,
even
if
not
SJ
challenging
as
hydrogen. Apart from
fast
actuation, pressJre build-
up
in
the injection
line
seems
to be the key factor to
deliver a high flowrate within short periods
of
time.
2
PRC>PC>S:O
ADVANCEO
H
2
1CES
Oifferent design options
and
engine management
strategies are
available
for
advanced H
2
1CEs
with
high power
densty
to satisfy SJper-ultra-low emis-
son
vehicle (SJLEV) emissions while providing
hilt!
efficiencies
and
regular, smooth, and stable opera-
tion over the
full range
of
engi
ne
speed
and
loads [1,
2]. The recent European Union (EU) HylCE project
'Optimization
of
a hydrogen powered internal
combustion engine'
(7)
has shown cryogenic
PFI
and
01
to be the
best
options currently available to
develop
H
2
ICEs.
HylCE developed
and
tested
two
concepts
of
mixture formation for specific hydrogen
engines,
01
at
10-200b ar and cryogenic
PFI
at about
22001£:. In both methods the performance
was
doubled while conSJmption
was
reduced with
reference to
prior
state-of-the-art hydrogen combus-
tion engines designed
for
both gaSJline and hydro-
gen
usage.
01
is
alSJ
being
used
in
the ongoing
EU
NICE project 'New integrated combustion
system
for
future
passenger
car engines'
(8)
as
the
best
option
for
gas engines Hydrogen-assisted
jet
ignition (HAJ)
devices have
been
designed, built, and
tested
at
the
University
of
Melbourne over more than a decade for
enhanced combustion
of
homogeneous lean mix-
tures in sngle- and multiple-cylinder
research
engines [5, 9-17]. SJme
of
the options
for
advanced
H
2
1CEs
are now presented
and
reviewed .
SJLEV operation
mayor
may not require after
treatment depending on the fuel-to-air equivalence
ratio w Operation
at
stoichiometry (wS
1)
requires
after-treatment with fuel-rich reduction catalyst.
Lean
operation (w<
0.7)
requires a lean-NO. trap
(LNT). Operation in the
ultra-lean region
(w.
0.
45)
and
below does not require after-treatment.
Hydrogen engines
can
be run stoichiometric to the
ultra-lean (w.
0.45)
region with very high rates
of
combustion. Backfiring (or back flashing)
and
pre-
ignition are major problems
especially at stoichio-
JAUT01278
metry,
and
knock may
limit
spark advances up
to
ultra-lean operation and beyond . Backfiring, pre-
ignition,
and
knock
alSJ
depend on additional
factors, including the combustion chamber
SJrface
materials, SJrface temperature, compression ratio,
pressJre boosting, charge cooling, and recirculation
of
exhaust
gases.
The power densty, rate
of
com-
bustion,
and
stability decrease by reducing the fuel-
to-air equivalence ratio, while the efficiency
at
first
increases
and
then decreases going towards leaner
operation.
The combustion rate deteriorates moving from
ultra-lean combustion
(w.
0.45)
to further ultra-lean
combustion (w<
0.2).
Moreover, ultra-lean combus-
tion requires stratified charge with
01
or
jet
ignition
(J) to
enSJre
stability. High compression ratiosand
pressJre boosting may help
to
keep
the rate
of
com-
bustion
hilt!.
The load
can
be reduced
by
lowering the fuel-to-
air equivalence ratio rather than by throttling for
better thermal efficiencies through the reduction in
pumping
10SIES.
However, the rate
of
combustion
decreases
and
stability worsens when the fuel-to-air
equivalence ratio
is
reduced and, after certain values
offuel-to-air equivalence ratios, the efficiency drops
sgnificantly.
Conventional
PFI
engines with gaseous fuel are
the
most SJs:eptible to pre-Ignition and knock. This
'warm'
PFI
alSJ
has the disadvantage that the large
displacement
of
air (stoichiometric volume fraction,
29.53
per cent)
limits
the air-based volumetric
efficiency
and
therefore the power output. Cryogenic
PFI
with the embedded charge cooling sgnificantly
reduces displacement effects and senstivity to
knock and pre-ignition and
dramatically improves
the volumetric efficiency.
o I
of
gas-phase
H2
is
alSJ
less SJsceptible to pre-
ignition, produces reduced
or
even
no displacement
effects,
and
eliminates back flash . Oisplacement
effects with early 0
I are not too far from those
of
PFI,
while late
01
may almost cancel these effects.
01
enables stratified operation with the engine able
to
run in a more ultra-lean
way
for
lower No,. emission
and higher efficiency.
01
alSJ
considerably improves
the fuel economy at
part-load, mainly because
of
reduced pumping
10SIES.
Late
01
has the penalty
of
needingto deliver
H2
at
a pressJre
of
100b
ar
or
more
with consequent fuel
presSJrization work, which is
lessfor cryogenic H
2
.
The power density may
be
increased by introdu-
Cing
presSJre boosting, SJpercharging
or
turbochar-
ging. However, with pressJre boosting, the tempera-
ture increases. the heat transfer increases, the knock
Proc
. IMechE
Vol
.
224
Part
0:
J.
Automobile Engineering
4 A Boretli, H WatSJn, and A Tempia
tendency increases,
and
the maximum allowable
compression ratio is reduced. With the addition
of
an
intercooler, the temperature rise across the
compressor
can
be sgnificantly offset.
The rate
of
combustion (and therefore
its
stability)
and
the lean
limit
may be improved by replacing the
standard
l:Park
ignition
(51)
in the main chamber
(MC) with ignition in a pre-chamber
(PC).
In the
HAJ
system,
additional fuel
is
injected in a
PC
connected to the MC via calibrated orifices, creating
fuel-rich conditions in the pre chamber.
Following
ignition by a conventional l:Park plug in the
PC,
the
MC lean mixture is then ignited by the jets
of
hot
products
and
radicals from the
PC,
enabling faster
combustion
of
the lean MC mixture to occur. Flame
l:Peed
enhancement
of
up
to
sx
times has
been
mea9.Jred
.
Exhaust
gas recirculation
(EGR)
allows lean equi-
valent conditions with stoichiometric
inflow
.
EGR
allows after-treatment with a reduction catalyst,
and
the
excess
air
is
replaced by
EGR
dilution.
EGR
also
mitigates pre-ignition effects in the
case
of
warm
PFI.
Cool
EGR
controls the charge temperature
within the cylinder
and
may
also
be beneficial
for
lean operation.
3 THE LEAN-BURN
OIRECT-IN.ECTION .ET
IGNITION H
2
1CE
MC
01
of
fuel with fast-actuating high-flowrate high-
pres9.Jre
injectors capable
of
injection shaping
and
multiple
events,
and
MC
J,
with ignition by
l:Park
or
autoignition in a small-volume
PC
providing
mini-
mal
complication
of
cylinder
head
deSgn with
PC
mixture preparation by
PC
01
[18, 19] has never
been
consdered before
for
both stationary
and
tranl:Port applications.
In
large-volume
PC
ignition
systems
for large
gas
engines. the
PC
fuel
is
not
negligible, the
cylinder
head
deSgn is strongly
affected,
and
PC
combustion
is
important
also
in
itself
and
not just in initiating MC combustion
whereas,
in standard
01
injectors, the
01
injector is
a traditional low-cost slow-actuating solenoid
low-
pres9.Jre
low-flowrate injector; finally, with standard
MC
51
coupled to
01,
combustion
is
wall initiated
with a relatively
small energy
9.Jpply
in just one
location.
The new-generation fast-actuating
high-pres9.Jre
high-flowrate
01
injector capable
of
injection shap-
ing
and
multiple events produces a bulk lean jet-
controlled
stratified mixture. Late
01
overcomes the
air
dil:Placement effects
of
PFI
of
gaseous fuels.
PrOC.
IMechE Vol.
224
Part
0: J Automobile Engineering
High-energy bulk ignition isthen achieved by
usng
PCJ
.
The proposed ignition
PC
is
very small in sze,just
afew per cent
ofthe
combustion chamber volumeat
top
dead
centre
(TO
C)
and
about
1c
m
3;
it
is
de-
sgned to be fitted within the traditional l:Park plug
thread
of
diameter
14m
m. The ignition device there-
fore
only
marginally increases the level
of
complexity
of
deSgning a cylinder
head
with a standard
l:Park
plug.
The jets
of
reacting
gases
from the ignition
PC
enhance the rate
of
combustion
of
the MC mixture
and
allow bulk ignition
and
combustion
for
reduced
heat
losses
and
faster heat
release.
The coupling
of
J
and
01
technologies allows
development
of
an
engine permitting operation with
overall fuel-to-air equivalence
ratiOS
reduced to
almost zero,
because
combustion
is
always started
in the
J
PC
provided that there
is
a very small
amount
of
fuel,
and
the jets
of
hot reacting gases
from the J
PC
may extend combustion to globally
very lean MC mixtures provided that
only
a
mini-
mum amount
of
fuel
is
behind the J nozzle, thus
replicating
diesel-like light-load operation.
The lean-burn
01
J engine
uses
a fuel injection
and
mixture ignition
system
conssting
of
the follow-
ing:
(a)
one MC
01
fuel injector per engine cylinder;
(b) one
J device per engine cylinder, the latter
being made
of
one
PC
connected to the MC
through one
or
more calibrated orifices, one
PC
01
fuel injector,
and
one
PC
(51
version) or one
PC
(autoignition version);
(c)
all the ancillaries required to 9.Jpply the fuel at
the
deS
red
pres9.Jres
by the
01
injectors
and
to
operate the
01
injectors
and
the
51
or
the auto-
ignition
PC.
The fuel injection and mixture ignition
system
operation is
as
follows.
1.
One
fuel is injected directly within the cylinder by
an
MC direct injector operating one sngle
injection
or
multiple injections to produce a lean
stratified mixture.
This
non-homogeneous mix-
ture
is
mildly
lean in
an
inner region 9.Jrrounded
byair
and
some reSduaisfrom the previouscycle.
The extenson
of
the inner region may be reduced
in
sze
to achieve
mean
chamber average mix-
tures ranging from slightly to extremely lean.
2. This mixture
is
then ignited by
oneor
more
jets
of
reacting
gases
that
iS9.Je
from a
PC
connected to
the MC via
calibrated orifices, sourced from the
same
or
an
alternative fuel that
was
injected into
.Io\UT01278
Lean-burn direct-injection jet ignition hydrogen engine 5
it
by a direct injector and then ignited by a spark
plug dis::harge (spark plug version).
3.
Combustion which started slightly fuel rich in the
very-small-volum e
PC
moves to the MC through
one
or
more J nozzles, with one
or
more jets
of
reacting
gases
bulk igniting the MC mixture. The
jets
of
reacting
gases
enhance combustion
of
lean
stratified mixtures within the MC through a
combination
thermal energy and the presence
of
active radical
species.
With reference
to
homogeneous
01
or
PFI
and
MC
spark ignition, non-homogeneous
01
and
J offer the
following advantages
(a)
much faster, more complete, much leaner com-
bustion;
(b)
less
sensitivity to mixture
state
and composition;
(c)
reduced heat
losses
to the MC walls.
This is
because
of
better fuel
for
same
chamber-
averaged
lean conditions, combustion in the bulk
of
the in-cylinder
gases,
heat transfer cushion
of
air
between
hot reacting
gases
and walls, very high
ignition energy
at
multiple simultaneous ignition
sites igniting the
bulk
of
the in-cylinder
gases,
aided
by
large concentrations
of
partially oxidized com-
bustion products initiated in the
PC
that accelerate
the oxidation
of
fresh reactants.
The advantages
of
the
system
are
as
follows:
(a)
higher brake efficiency (ratio
of
the engine
brake power to the total fuel energy)
and
therefore reduced brake specific fuel conSJmp-
tion
(~C)
(ratio
of
the engine fuel flowrate to
the brake power)
for
improved full load opera-
tion
of
stationary
and
transport engines;
(b) efficient combustion
of
variable-quality fuel
mixtures from
globally near stoichiometry to
globally extremely lean
for
load control mostly
throttleless
by the quantity
of
fuel injected for
improved
part-load operation
of
non-stationary
engines.
(c) opportunity in the
ultra-lean mode to produce
near-zero
NO •.
4 COMPUTATIONAL
PROOF
OF
CONCEPT
The concept has
been
applied to a
1.51
SI
four-
cylinder galDline engine with double overhead
camshafts
and
four valves per cylinder. This engine
is
a V-Four
1.51
engine, with a bore
of
78m
m, a
stroke
of
78m
m,
an
intake valve seat insert inside
diameter
of
32m
m,
an
exhaust valve seat insert
JAUT01278
inside diameter
of
26m
m, a connecting-rod length
of
109m
m,
and
a pent
roof
combustion chamber.
While much
less
than
O.08m
glcycle has to be
introduced by the
PC
injector, and therefore a
galDline
01
injector
can
be
used
as
the
PC
injector
for prototype
applications where durability and
dry
run capability are not
an
iSSJe,
a specific hydrogen
injector must be developed
for
the short injection
time, high temperature, high injection
preSSJres,
high durability,
and
dry
run capability
of
the MC
injector having to introduce up to
14.8m
glcycle
and
to produce the charge stratification.
Lean
stratified
mixtures
would be possible by adopting charge
motion
controlled by
jet
and shaped piston
SJr-
face-wall
or
fully
jet
controlled configurations
depending on the injector performance.
Figure 2 presents a view
of
the in-cylinder plus
PC
volumes, while Fig. 3 presents a sectional view with
a
plane passing through the
PC
axis.
The
01
injector
and
the J device are placed at the centre
of
the
cylinder head. A
preSSJre
senlDr
for
combustion
studies
is
allD located in the centre. The J device
isa
six-nozzle type. The J device
is
designed to
fit
an
standard spark plug thread
of
diameter
14m
m.
It
accommodates one racing spark plug
of
diameter
Fig.
2
View
of
the
in-cylinder
and
PC
IIOlumes
Fig.
3
Aane
cut
of
the
i
n-cyti
nder
and
PC
IIOlumes
Proc. IMechE Vol.
224
Part
D:
J.
Automobile Engineering