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The Alfa Romeo 156 JTS (“Jet Thrust
Stoichiometric”) engine uses Bosch
direct injection. This gives the
performance of a 2.3-litre unit, yet
consumes 10% less fuel. The 2-litre
engine has the same capacity as the
Twin Spark engine it replaces, yet
power is up from 114kW to 121kW
and torque rises from 187Nm to
206Nm. [Alfa Romeo]
By Julian Edgar
Direct Petrol Injection
Complex electronic control and direct fuel
injection can provide fuel savings of up to
40%. The injectors work at much higher
fuel pressures and must inject the fuel in
extremely short times, within each cylinder’s
intake or compression stroke.
C
ONVENTIONAL ELECTRONIC
fuel-injected engines use injectors
to squirt fuel into the intake ports. In
a sequentially injected engine, each
injector opens a short time before
its associated intake valves, while in
simultaneous-injected engines the
injectors all open at the same time.
In either case, the cloud of small fuel
droplets is drawn into the engine only
22 Silicon Chip
when the intake valves open. But a
new breed of fuel-injected engines is
now appearing.
As the name suggests, Direct Petrol
Injection uses injectors that add fuel
directly to the combustion chamber.
Like diesel engines, the air/fuel mixing occurs inside the combustion
chamber, rather than in the inlet ports.
Taking this approach gives far greater
control over the combustion process,
allowing for a variety of combustion
operating modes, including those
having ultra-lean air/fuel ratios. However, the degree of electronic control
required to make smooth transitions
from one combustion operating mode
to another is complex and engine operating processes need to be monitored
far more closely than is the case with
conventional port fuel injection.
Manufacturers are deciding that
the added complexity is worth it –
Mitsubishi has produced more than
700,000 direct-injected petrol engines
(although none has been sold outside
Japan) and BMW and Alfa Romeo are
now selling cars with direct injected
engines in Australia.
Direct injected engines have even
won major motor sports events, with
siliconchip.com.au
HIGH PRESSURE
PUMP
CARBON CANISTER
FUEL RAIL
CANISTER
PURGE VALVE
AIR-MASS METER WITH
TEMPERATURE SENSOR
ACCELERATOR
PEDAL MODULE
INJECTOR
THROTTLE
DEVICE
(EGAS)
IGNITION COIL
PRESSURE CONTROL
VALVE
INTAKE
MANIFOLD
SENSOR
PHASE SENSOR
OXYGEN
SENSOR
(LSU)
FUEL PRESSURE
SENSOR
KNOCK
SENSOR
PRE-CATALYST
EGR VALVE
EXHAUST
GAS TEMP.
SENSOR
ELECTRONIC
CONTROL UNIT
NOX
CATALYST
DIAGNOSIS INTERFACE
MALFUNCTION INDICATOR LAMP
DELIVERY MODULE, INCLUDING
LOW PRESSURE PUMP
IMMOBILISER
BOSCH COMPONENTS SPECIFICALLY FOR DE
CAN
BOSCH COMPONENTS
OXYGEN
SENSOR
(LSF)
Fig.1: the Bosch MED Motronic engine management system, which uses direct fuel injection, looks similar to
conventional engine management. However, it uses a new Electronic Control Unit with many different functions, a
second high pressure fuel pump, new design injectors, a fuel rail pressure sensor, and a second NOx catalyst. Some
of the sensors are also used in unconventional ways. [Bosch]
Audi’s direct-injected 455kW twin
turbo 3.6-litre V8 triumphing at Le
Mans.
FUEL INJECTOR CROSS-SECTION
1
System mechanicals
Fig.1 shows the layout of the Bosch
direct injection system. Its mechanical
elements differ from conventional port
injection in several ways.
First, the fuel supply system uses
two fuel pumps – a conventional
electrical fuel pressure pump (in the
past dubbed a high-pressure pump
but now referred to in this system as
a low-pressure pump) and a mechanically driven high-pressure pump. The
low-pressure pump works at pressures
of 0.3-0.5MPa while the high-pressure
pump boosts this very substantially
to 5-12MPa.
The high-pressure fuel is stored in
the fuel rail that feeds the injectors.
The fuel rail is made sufficiently large
that pressure fluctuations within it
are minimised as each injector opens.
The pressure of the fuel in the injector supply rail is controlled by an
siliconchip.com.au
2
3
The fuel injectors used for direct injection
are subjected to enormous pressures.
Minimum injector opening time is just
5ms and droplets are on average smaller
than 20µm – just one-fifth the size of
traditional injectors and one-third the
diameter of a human hair. [Bosch]
Fig.2 (right): cross-section of a direct petrol
injection injector. (1) fuel inlet with strainer,
(2) electrical connections, (3) spring, (4)
solenoid, (5) injector housing, (6) nozzle
needle with solenoid armature, (7) valve seat,
(8) injector outlet passage. [Bosch]
4
5
6
7
8
August 2005 23
TORQUE (M)
THROTTLE-VALVE POSITION/
EXCESS AIR FACTOR (l)
Stratified Mode
STRATIFIED MODE
HOMOGENEOUS MODE
Fig.3: although at least six different modes of combustion can occur in a direct
injection engine, this diagram shows the two main modes. In Stratified Mode,
the injector adds the fuel during the compression stroke, just before the spark
plug fires. In the period between the injection finishing and the spark plug
firing, the airflow movement within the combustion chamber transports the
air/fuel mixture towards the spark plug. This results in a portion of relatively
rich air/fuel mixture surrounding the spark plug electrode while the rest of the
combustion chamber is relatively lean. In Homogenous Mode, injection starts
on the intake stroke, so there is sufficient time for the air/fuel mixture to be
distributed throughout the combustion chamber. Stratified Mode can result in
air/fuel ratios as lean as an incredible 55:1. [Bosch]
electronically-controlled bypass valve
that can divert fuel from the highpressure pump outlet back to its inlet.
The fuel bypass valve is varied in flow
by being pulse-width modulated by
the Electronic Control Unit (ECU). A
fuel pressure sensor is used to monitor
fuel rail pressure.
a
1
0
Ivm
b
Ih
tvm
c
0
ton
INJECTED
FUEL
QUANTITY
NEEDLE LIFT
CURRENT
Imax
0
toff
d
INJECTION DURATION
Fig.4: direct injection injectors use
a peak/hold system of operating;
(a) is the triggering signal from
the ECU, (b) is the actual injector
current, (c) injector needle lift, (d)
injected fuel quantity. A booster
capacitor is used to provide the
high opening current. [Bosch]
24 Silicon Chip
Second, compared with a conventional port fuel injection system,
the fuel injectors must be capable of
working with huge fuel pressures and
also injecting large amounts of fuel
in very short periods. Fig.2 shows a
cross-sectional view of an injector. The
reason for the much reduced time in
which the injection can be completed
is due to the fact that all the injection
must at times occur within a portion
of the induction stroke.
Port fuel injectors have two complete rotations of the crankshaft in
which to inject the fuel charge – at an
engine speed of 6000 RPM, this corresponds to 20ms. However, in some
modes, direct fuel injectors have only
5ms in which to inject the full-load
fuel. The fuel requirements at idle can
drop the opening time to just 0.4ms.
Direct injection fuel droplets are on
average smaller than 20µm – one-fifth
the droplet size of traditional injectors and one-third the diameter of a
human hair.
Finally, the very lean air/fuel ratios
at which direct injection systems can
operate result in the production of
large quantities of oxides of nitrogen
(NOx). As a result, direct-injected cars
require a primary catalytic converter
fitted close to the engine and also a
main catalytic converter – incorpo-
Homogeneous Mode
a
b
l=1
0
ACCELERATOR PEDAL POSITION
Fig.5: during Stratified Charge
Mode, the throttle is held wideopen, irrespective of the driver’s
accelerator pedal input. In this
mode, the air/fuel ratio (Bosch
refer to an increased ‘excess
air ratio’) is very lean when the
torque request is low, with the
air/fuel ratio gradually becoming
richer as more torque is required.
At a certain point, the engine
changes to Homogenous Mode.
With the change in modes, the
throttle valve opening becomes
related to the driver’s torque
request and the air/fuel ratio
holds a constant stoichiometric
ratio (ie, 14.7:1 or Lambda = 1)
across the rest of the engine load
range. [Bosch]
rating an NOx accumulator – fitted
further downstream.
Combustion modes
The really radical nature of direct
siliconchip.com.au
OPERATING MODE COORDINATION
Priority
OPERATING MODE SWITCHING
Function
1 Monitoring concept
2 Component protection
3 Limp-home functions
4 Adjustability of torque
and oxygen content limits
5 Catalytic converter heating
6 NOx accumulator
regeneration
7 SOx accumulator
regeneration
8 Starting/warming up
9 Diagnostic system manager
10 Operating mode map
Required
operating mode
Changeover
Function
Control
Functions
•
•
•
•
Exhaust
recirculation
Fuel tank venting
Swirl flap
EGAS
Fig.6: the direct injection system incorporates an operating-mode co-ordinator
which uses a 10-stage priority system. Before the selected combustion mode
starts to occur, control functions for exhaust-gas recirculation, fuel tank
ventilation, charge-movement flap and electronic throttle settings are initiated
as required. The system waits for acknowledgement that these actions have
been carried out before altering fuel injection and ignition timing to provide the
appropriate combustion mode. [Bosch]
fuel injection can be seen when the
different combustion modes are examined. There are at least six different
ways in which combustion can take
place.
• Stratified Charge Mode: at low
torque output up to about 3000 RPM,
the engine is operated in Stratified
Charge Mode. Here, the injector adds
the fuel during the compression stroke,
just before the spark plug fires.
In the period between the injection
finishing and the spark plug firing,
the air-flow movement within the
combustion chamber transports the
air/fuel mixture into the vicinity of
the spark plug. This results in a portion of relatively rich air/fuel mixture
surrounding the spark plug electrode,
while the rest of the combustion chamber is relatively lean.
The gas filling the rest of the chamber
Current
operating mode
For example:
• Homogeneous mixture
• Homogeneous lean mixture
• Stratified mixture
• Double injection
often comprises recirculated exhaust
gases, resulting in a reduced combustion temperature and decreased NOx
emissions. In Bosch direct injection
systems, the air/fuel ratio within the
whole combustion chamber can be as
lean as 22:1 - 44:1.
Mitsubishi states that total combustion chamber air/fuel ratios of
35:1 - 55:1 can be used. Compare this
with a conventional port fuel injected
engine that seldom uses an air/fuel
ratio leaner than 14.7:1.
• Homogenous Mode: Homogenous
Mode is used at high torque outputs
and at high engine speeds. Injection
Left & above: Audi has won Le Mans using direct-injected engines. The twinturbo 3.6-litre V8s developed 455kW, despite having to breathe through two
32.4mm restrictors and having the boost of the twin turbos limited to just under
10 psi. The adoption of direct petrol injection allowed the compression ratio
to be lifted a point and gave a fuel reduction of 8-10%, a torque increase of 9%
over a wide RPM range and quicker starting and better driveability. [Audi]
siliconchip.com.au
August 2005 25
CONTROL UNIT
HIGH PRESSURE PUMP
PRESSURE CONTROL VALVE
PRESSURE SENSOR
FUEL DISTRIBUTOR
INJECTORS
Fig.7: the high-pressure pump is driven directly from the engine and develops fuel pressures as high as 12MPa. This high
pressure is regulated by a fuel pump bypass valve which is pulse width modulated by the Electronic Control Unit. The
injectors are opened with a burst of high current from a capacitor that delivers up to 90V. A dedicated sensor monitors
fuel rail pressure. [Bosch]
The BMW 760 uses a direct injection 6-litre V12 developing 327kW and
600Nm. It complies with the EU4 emission standard in Europe and the LEV
(Low Emission Vehicle) standard in the USA. Fuel consumption in the EU test
cycle is 13.4 litres/100 km. The car accelerates from 0-100km/h in 5.5 seconds
and continues on to 200km/h in 17 seconds. [BMW]
26 Silicon Chip
starts on the intake stroke so there is
sufficient time for the air/fuel mixture
to be distributed throughout the combustion chamber. In this mode, Bosch
systems use an air/fuel ratio of 14.7:1
(the same as with port fuel injection at
light loads), while Mitsubishi use air/
fuel ratios from 13:1 – 24:1.
• Homogenous Lean-Burn Mode:
during the transition between Stratified and Homogenous Modes, the engine can be run with a homogeneously
lean air/fuel ratio.
• Homogenous Stratified Charge
Mode: initially, this mode appears
nonsensical – how can the combustion process be both homogenous and
stratified? However, what occurs is not
one but two injection cycles.
The initial injection occurs during
the intake stroke, giving plenty of
time for the fuel to mix with the air
throughout the combustion chamber.
Then, during the compression stroke,
a second amount of fuel is injected.
This leads to the creation of a rich zone
around the spark plug. The rich zone
easily ignites, which in turn ignites the
leaner air/fuel ratio within the remainder of the combustion chamber.
Of the total fuel addition, approxisiliconchip.com.au
mately 75% occurs during the first
injection and 25% during the second.
The Homogenous Stratified Charge
Mode is used during the transition
from Stratified Charge to Homogenous
Modes.
In addition there are at least two
more modes: Homogenous Anti-Knock
and Stratified Charge Cat-Heating. The
first is used at full throttle and the
second to rapidly heat the catalytic
converter to operating temperature.
A final mode – mentioned in only
some of the literature – is Rich Homogenous Mode, which is used to
regenerate the NOx accumulator in
the main catalytic converter. (The
NOx accumulator deposits oxides of
nitrogen in the form of NHO3 nitrates.
When the accumulator is regenerated,
the nitrates – together with carbon
monoxide – are reduced in the exhaust
to nitrogen and oxygen.)
Fig.3 shows the two primary combustion modes.
Electronic control systems
As indicated earlier, the injectors
must be opened against very high fuel
pressures. This is done using a peak/
hold technique, whereby the opening
current is very high and the “hold”
current much reduced. A dedicated
triggering module is used to control
the injectors, with a booster capacitor
providing 50-90V to initially open the
injector. Fig.4 shows this process.
The sensing of the mass of cylinder
charge is more complex on a directinjected engine than a conventional
port-injected engine. This is because at
times recirculated exhaust gas forms a
major component of the total cylinder
charge. As a result, two cylinder charge
sensors are used. These comprise a
conventional hot-film mass air-flow
sensor (similar to a hot-wire air-flow
meter) and a manifold pressure sensor
(MAP sensor).
The flow through the air-flow meter
is used as an input into the calculation of the pressure within the intake
manifold and this is then compared
with the actual intake manifold pressure measured by the MAP sensor. The
difference between the two indicates
the mass flow of the recirculated exhaust gas.
As with many conventional engine
management systems, direct injection
requires the use of an electronically
controlled throttle. In conventional
systems, the actual throttle opening
siliconchip.com.au
Fig.8: fuel is introduced directly into the combustion chamber by the highpressure injectors. Depending on the operating mode, the fuel can be added
during the intake stroke, the compression stroke or during both the intake
and compression strokes. [Bosch]
Fig.9: there are two main approaches to aiming the fuel spray. In Wall Guided
(left), the air movement within the combustion chamber guides the area of rich
mixture in the direction of the spark plug. In Spray Guided (right), the fuel is
injected directly into the vicinity of the spark plug. The latter approach gives
improved emissions and fuel economy but thermally stresses the spark plug.
[Bosch]
more or less follows the driver’s accelerator pedal torque request. By
contrast, in direct injected engines,
the throttle is fully open for much of
the time and varying the fuel delivery
regulates engine torque output.
Fig.5 shows how this occurs. During
Stratified Charge Mode, the throttle
is held wide-open, irrespective of the
driver’s accelerator pedal input. When
the torque request is low, the air/fuel
ratio is very lean (Bosch refer to this
as an increased ‘excess air ratio’), with
the air/fuel ratio gradually becoming
richer as more torque is required.
At a certain point, which corresponds on an engine-specific basis
to engine speed and the amount of
torque required, the engine changes
to Homogenous Mode. (For simplicity,
the transitional Homogenous LeanBurn Mode is ignored in this diagram.)
With the change in modes, the throttle
valve opening becomes related to the
driver’s torque request and the air/
fuel ratio holds a constant stoichioAugust 2005 27
losses – the engine is no longer trying
to breathe through the restriction of
the nearly-closed throttle. However,
the downside of this is that the partial
vacuum that’s normally available for
the brake booster will be lacking. To
overcome this problem, a vacuum
switch or pressure sensor monitors
brake booster vacuum and if it is
necessary, the combustion mode is
altered so that vacuum again becomes
available.
Increased efficiencies
This photo shows the BMW V12 injectors and fuel rail. Fuel rail pressure varies
from 30-100 Bar. The injection pumps are fitted above the outlet camshafts and
are driven by an additional cam. [BMW]
metric air/fuel ratio (that is, 14.7:1 or
Lambda = 1) across the rest of the
engine load range.
The system incorporates an operating-mode co-ordinator which maps
operating mode against engine speed
and torque request. Fig.6 shows a
schematic diagram of the functioning
of this controller.
As can be seen, a 10-stage priority
allocation is used when determining
the required operating mode. Before
the selected combustion mode starts
INJECTION
to occur, control functions for exhaustgas recirculation, fuel tank ventilation,
charge-movement flap (ie, port tumble
valves or variable length intake manifold) and electronic throttle settings
are initiated as required. The system
waits for acknowledgement that these
actions have been carried out before
altering the fuel injection and ignition
timing.
The advantage of having the electronic throttle valve fully open at low
loads is a huge reduction in pumping
IGNITION
COMBUSTION &
ENGINE START
As well as the huge reduction in
pumping losses at low loads (due to
the throttle being wide open), thermodynamic efficiencies are also increased
during Stratified Charge Mode. This is
because the rich cloud of combustible
air/fuel mixture around the spark plug
is thermally insulated by the layer of
air and recirculated exhaust gas that
surrounds it.
Together with the much leaner
air/fuel ratios than can be used in a
conventional port injected engine,
the result is a fuel efficiency improvement that can be up to 40% at idle.
Mitsubishi state that at 35km/h, their
direct injected engines use 35% less
fuel than a comparably-sized conventional engine and that in the Japanese
10-15 Urban Driving Cycle (albeit a
slow speed cycle), the direct-injected
engine uses less fuel than even a comparable diesel engine.
During homogenous mode operation, both the use of an air/fuel ratio
that is never richer than 14.7:1 and the
higher compression ratios normally associated with direct injection engines
result in a fuel saving of about 5%.
Conclusion
Fig.10: another advantage of direct injection is that it is possible to start a
hot engine without using the starter motor. This approach reduces startup hydrocarbon emissions, making it more attractive to use engines that
automatically switch off if the car has been stationary for a period. [Bosch]
28 Silicon Chip
Mitsubishi has been building direct
injection petrol engines since 1996
but they have not been able to achieve
worldwide success with their designs
due, it is said, to the engines’ reliance
on high-quality fuel. Now Bosch has
developed technology which is allowing direct injected engines to be sold
around the world. Combine direct
injection technology with the hybrid
cars being widely developed – plus
perhaps downsized and turbocharged
engines – and we’re certainly in for
some interesting times ahead.
But without the sophisticated electronics working behind the scenes,
none of these engines would be posSC
sible.
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