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Fuel injection in
economy cars
While most electronic engine management
systems in today’s cars are based around multipoint fuel injection, the Bosch Mono-Jetronic is
based on just one injector and no airflow meter
or MAP sensor. It is used in the Mazda 121 and
some other small economy cars.
By JULIAN EDGAR
The majority of today's EFI systems
use one injector for each of the engine’s
cylinders. These so-called multi-point
systems have the advantage of allowing the fuel to be added just before the
inlet valves, giving benefits in mixture
accuracy and overcoming manifold
wall wetting.
However, the cost of such a system
is higher than that of a single-point
system which normally uses only one
or two injectors.
10 Silicon Chip
In the cost-sensitive small, economy
car sector, every extra dollar saved is
crucial. If the injector count can be
more than halved and at the same time
the airflow meter or MAP sensor done
away with, the cost of the system can
be made very low.
Unfortunately, the technical compromises implicit in a single point
system require complex engineering
solutions, if the car is to perform at
a level near to that which would be
achieved by a more expensive system. This article looks at how Bosch
engineers developed a simple, cheap
EFI system using just one injector
and only four major input sensors.
Their approach is also used when aftermarket programmable EFI systems
are fitted to very “hot” piston engines
and peripheral ported rotary engines.
In these cases, a manifold pressure
signal is not a reliable indicator of
engine load and airflow meters are
only rarely used.
System layout
On paper, the Bosch Mono-Jetronic
appears similar to any of the more
common EFI systems. Fuel is pressurised by an electric pump, fed through
a fuel filter and then fixed at a level
above the manifold air pressure by a
fuel pressure regulator, before being
fed to an electronically-controlled
injector. Induction air passes through
Fig.1: in the Mono-Jetronic
system, many normally-discrete
components are integrated into
one unit:
(1) fuel injector
(2) intake air temperature
sensor
(3) throttle butterfly
(4) fuel pressure regulator
(5) fuel return
(6) fuel inlet
(7) throttle position sensor
(hidden)
(8) idle air bypass motor
(Bosch)
a filter, is monitored by an intake air
temperature sensor and then it passes through the throttle body into the
engine.
However, as Fig.1 shows, the physical layout of the system is quite unusual. The fuel injector, air temperature
sensor, fuel pressure regulator, throttle
valve, idle speed control actuator and
throttle position sensor are all integrated into one unit. Combining the
various components into one package
in this way obviously reduces manufacturing and installation costs. The
assembly is positioned in a similar
location to that used by a carburettor
in an old car – on top of a multi-branch
intake manifold.
Collecting engine data
The two major inputs determining
the injector pulse width are engine
speed and throttle position. Engine
speed is easily derived by monitoring the ignition signal but accurate
sensing of throttle position is more
difficult. When load sensing is derived
by monitoring the throttle angle, the
relationship between the throttle valve
opening and the flow area within the
throttle body must be maintained to
within very close tolerances on all
Fig.2: the single injector is located directly above the throttle butterfly,
with the fuel pressure regulator incorporated into the same housing. The
rest of the fuel supply system is similar to any other EFI system: (1) fuel
tank, (2) electric fuel pump, (3) fuel filter, (4) fuel pressure regulator, (5)
fuel injector, (6) throttle butterfly. (Bosch)
production units. This is because
small throttle movements can make
huge changes to the engine load.
The first step in developing the
system is to subject the engine to
accurate dynamometer testing. This
is so that the air charge for one intake
cycle at various engine speeds and
throttle openings can be measured.
Fig.3 shows an example of these “air
charge” amounts.
Several interesting aspects can be
noted about Fig.3. First, the amount of
air breathed per intake stroke is at its
maximum at peak torque, as is shown
by the air charge line indicative of full
throttle (the butterfly open by 90°). As
can be seen, the greatest ingestion per
intake stroke occurs on this engine at
about 3000 rpm.
However, of more importance when
attempting to measure the correct
amount of fuel to be added are the
differences in air charge amount which
occur at small throttle openings. At
idle and low-load, a change of ±1.5° in
throttle opening causes an air-charge
difference of ±17%! On the other hand,
the same amount of throttle movement
at high loads can cause a change of
only ±1%.
From this, it follows that small
throttle openings must be measured
with extreme accuracy.
In the Mono-Jetronic system this is
carried out by an unusual throttle position sensor (TPS). All other EFI sysJuly 1996 11
Fig.3: an ‘air charge’ map is developed on an engine dynamometer to show the amount of air
ingested during one cycle at different rpm and throttle openings. Note that at idle and low
loads, a change of ±1.5° in the throttle opening causes an air charge difference of ±17%, while
the same amount of throttle movement at high loads causes a change of only ±1%. This means
that very accurate throttle position sensing is required. (Bosch)
Fig.4: schematic diagram of the Mono-Jetronic ECU. (Bosch)
12 Silicon Chip
A single point injection system can
have major problems with manifold
wall-wetting, even with a very finely
atomised fuel spray. Mono-Jetronic uses
sophisticated techniques to overcome
these potential problems.
tems also use a TPS but it is often just a
two-position switch, with contacts for
idle and full throttle. The Mono-Jetronic system uses two potentio-meters in
its TPS. Each wiper arm carries four
wipers, each of which contacts one of
the potentiometer tracks. Track 1 covers the angular range from 0-24°, while
Track 2 covers the range from 18-90°.
The angle signals from each track are
each converted by dedicated analog/
Fig.5: this Lambda Map shows the
injection duration which gives a
14.7:1 air/fuel ratio at all loads and
engine speeds. This is the actual base
map, with the injector pulse widths
then modified on the basis of the
inputs of the other sensors. (Bosch)
digital converter circuits. The ECU
also evaluates the voltage ratios,
using this data to compensate for
wear and temperature fluctuations
at the pot.
Because the engine load cannot
be assessed in this way as accurately as with MAP sensing or airflow
metering, the system requires the
feedback of an exhaust gas oxygen
(EGO) sensor, if it is to comply
with emissions legislation. The
EGO sensor is the normal type,
where the output is a small voltage
which changes rapidly either side
of the stoichiometric (14.7:1) air/
fuel ratio.
Other sensor inputs include
coolant, intake air temperature
and control signals from the air
conditioning and/or automatic
transmission. The latter two inputs
are used as part of the idle speed
control.
Processing of input data
Fig.4 shows a schematic diagram
of the system’s ECU. The inputs
from the TPS, EGO, engine temperature and intake air temperature sensors
are converted by the analog to digital
converter and transmitted to the
microprocessor by the data bus. The
microprocessor is connected through
the data and address bus with the
EPROM and RAM. The read memory
contains the program code and data
for defining the operating parameters.
In particular, the RAM stores the
adaptation values developed during
Fig.6: this graph shows the intake air
temperature correction to the injector
pulse width. Note that the system
is calibrated to work over a 100°C
range! (Bosch)
self-learning, which occurs on the
basis of the EGO sensor input. This
memory module remains permanently
connected to the vehicle’s battery to
maintain the adaptation data whenever the ignition is switched off. A 6MHz
quartz oscillator provides the stable
basic clock rate needed for arithmetic
operations.
A number of different output stages
are used to generate the control signals
for the fuel injector, the idle speed control actuator, the carbon canister purge
valve (which allows the burning of
stored petrol tank vapour) and the fuel
pump relay. The fault lamp warns the
driver of sensor or actuator problems
and also acts as a diagnostics interface.
Mixture control
The starting point for the calculation of the fuel injector pulse width
is a stored 3-dimensional map derived
from dyno test data. This “Lambda
Map” (Fig.5) contains the optimum
pulse widths to deliver a stoichiometric air/fuel ratio under all operating
conditions.
It consists of 225 control co-ordinates, made up of 15 reference co-ordinates for throttle position and 15 for
engine rpm. Because of the extremely
non-linear shape of the air-charge
curves, the data points are situated
very closely together at the low-load
end of the map. The ECU interpolates
between the discrete points within
the map.
If the ECU detects deviations from
stoichiometric air/fuel ratios and as
Fig.7: because only a single injector
is used, manifold wetting can cause
major problems during transients.
Acceleration enrichment (1) and
deceleration lean-off (2) is used, with
both based on the speed of throttle
movement. (Bosch)
July 1996 13
Fig.8: the mixture signal from the exhaust gas oxygen sensor is used as
a correction factor. Note that the greater the length of time for which the
mixture is rich (or lean), the greater the amount of correction which is
applied. (Bosch)
a result is forced to correct the basic
injection duration for an extended
time, it generates mixture correction
values and stores them as part of the
adaptation process. In this way it can
compensate for engine-to-engine variations and engine wear.
However, because the Lambda map
is designed only for the engine’s normal operating and temperature range,
it becomes necessary at times to correct
the base injector pulse widths. The first
of these is when starting.
Because the Mono-Jetronic system
uses just one injector, manifold wall
wetting through condensation is a
much bigger problem than in multi-point systems. As in all EFI systems,
injector pulse width is increased
when the engine is cold but because
Very “hot” engines using radical cam specifications sometimes make use
of just engine speed and throttle position inputs to calculate the required
fuel addition. Doing so overcomes the problem of the poor vacuum
signal at low loads which can occur with high valve overlap. The system
described here takes the same approach but for reasons of economy.
14 Silicon Chip
condensation of the fuel also depends on the air velocity,
the starting injector duration is reduced as engine speed
increases. To counteract the possibility of flooding, the
longer the engine cranks, the less fuel is injected; it is
reduced by as much as 80% after six seconds of cranking.
Once the engine has started, the injector opening duration is based on values stored within the Lambda map,
suitably modified on both a time and temperature basis
by the engine coolant temperature input.
As the temperature of the intake air increases, its density
is reduced, meaning that at a constant throttle position
the cylinder charge reduces with increasing temperature.
Fig.7 shows the relative enrichment at different intake
air temperatures.
Transition compensation
While all EFI systems use the equivalent of a carburettor
accelerator pump during rapid throttle movements, the
single injector of the Mono-Jetronic system makes this a
critical aspect. During sudden changes in throttle position,
three factors need to be taken into consideration.
First, fuel vapour in the central injector unit and intake
manifold is transported very quickly, at the same speed
as the intake air.
Second, fuel droplets are generally transported at the
same speed as the intake air but are occasionally flung
against the intake manifold walls, where they form a film
which then evaporates. Third, liquid fuel is transmitted
as a film on the intake manifold walls, reaching the combustion chambers after a time lag.
At idle and low loads, the air pressure within the
manifold is low (there is a high vacuum) and the fuel is
almost entirely vapour with no wall wetting. When the
throttle valve is opened, the intake manifold pressure rises
and so does the proportion of fuel on the manifold walls.
This means that, when the throttle is opened, some
form of compensation is necessary to prevent the mixture
becoming lean due to the increase in the amount of fuel
deposited on the walls. When the throttle is closed, the
wall film reduces and without some form of leaning-compensation the mixture would become rich.
Rather than basing the transitional compensation on
throttle position alone, the system uses the speed with
which the throttle is opened or closed as the determining
factor. Fig.8 shows this compensation, with the maximum
correction occurring when the throttle is opened at more
than 260° per second. Also incorporated in these dynamic
mixture corrections are inputs from the engine and intake
air temperature sensors.
If you are seeing a blank page here, it
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misunderstandings. Please feel free to
visit the advertiser’s website:
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Mixture adaptation
The mixture adaptation system uses the EGO sensor input. The system must compensate for air-density changes
when driving at high altitudes, for vacuum leaks after the
throttle butterfly and individual differences in injector
response times.
Fig.8 shows the variation in the Lambda correction
factor with different EGO sensor output voltages. Updates
occur at between 100 milliseconds and one second, depending on engine load and speed.
Acknowledgment: thanks to Robert Bosch (Australia) Pty Ltd
for providing much of the information used in this article. SC
July 1996 15
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