Page 21
K-Jetronic
Barrel with metering slits
1 Intake air, 2 Control pressure, 3 Fuel inlet,
4 Metered quantity of fuel, 5 Control plunger,
6 Barrel with metering slits, 7 Fuel distributor.
7
2
5
4
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6
4
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3
1
UMK1496Y
The air-flow sensor is located upstream
of the throttle valve so that it measures all
the air which enters the engine cylinders.
It comprises an air funnel in which the
sensor plate (suspended body) is free to
pivot. The air flowing through the funnel
deflects the sensor plate by a given
amount out of its zero position, and this
movement is transmitted by a lever system to a control plunger which determines the basic injection quantity required for the basic functions. Considerable pressure shocks can occur in the
intake system if backfiring takes place in
the intake manifold. For this reason, the
air-flow sensor is so designed that the
sensor plate can swing back in the
opposite direction in the event of misfire,
and past its zero position to open a relief
cross-section in the funnel. A rubber
buffer limits the downward stroke (the
upwards stroke on the downdraft air-flow
sensor). A counterweight compensates
for the weight of the sensor plate and
lever system (this is carried out by an
extension spring on the downdraft airflow sensor). A leaf spring ensures the
correct zero position in the switched-off
phase.
Fig. 13
Fuel distributor
Depending upon the position of the plate
in the air-flow sensor, the fuel distributor
meters the basic injection quantity to the
individual engine cylinders. The position
of the sensor plate is a measure of the
amount of air drawn in by the engine. The
position of the plate is transmitted to the
control plunger by a lever.
Fig. 14
Barrel with metering slits and control plunger
a Zero (inoperated position), b Part load, c Full load.
1 Control pressure, 2 Control plunger, 3 Metering slit in the barrel, 4 Control edge, 5 Fuel inlet,
6 Barrel with metering slits.
a
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1
2
b
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c
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3
4
5
UMK1497Y
6
19
Page 22
20
Depending upon its position in the barrel
with metering slits, the control plunger
opens or closes the slits to a greater or
lesser extent. The fuel flows through the
open section of the slits to the differential
pressure valves and then to the fuel
injection valves. If sensor-plate travel is
only small, then the control plunger is
lifted only slightly and, as a result, only a
small section of the slit is opened for the
passage of fuel. With larger plunger
travel, the plunger opens a larger section
of the slits and more fuel can flow. There
is a linear relationship between sensorplate travel and the slit section in the
barrel which is opened for fuel flow.
A hydraulic force generated by the socalled control pressure is applied to the
control plunger. It opposes the movement
resulting from sensor-plate deflection.
One of its functions is to ensure that the
control plunger follows the sensor-plate
movement immediately and does not, for
instance, stick in the upper end position
when the sensor plate moves down again.
Further functions of the control pressure
are discussed in the sections “Warm-up
enrichment” and “Full-load enrichment”.
Control pressure
The control pressure is tapped from the
primary pressure through a restriction
bore (Figure 16). This restriction bore
serves to decouple the control-pressure
circuit and the primary-pressure circuit
from one another. A connection line joins
the fuel distributor and the warm-up
regulator (control-pressure regulator).
When starting the cold engine, the
control pressure is about 0.5 bar. As the
engine warms up, the warm-up regulator
increases the control pressure to about
3.7 bar (Figure 26).
The control pressure acts through a
damping restriction on the control
plunger and thereby develops the force
which opposes the force of the air in the
air-flow sensor. In doing so, the restriction dampens a possible oscillation of the
sensor plate which could result due to
pulsating air-intake flow.
The control pressure influences the fuel
distribution. If the control pressure is low,
Barrel with metering slits
The slits are shown enlarged (the actual slit is
about 0.2 mm wide).
UMK0044Y
Gasolineinjection
systems
Fig. 15
the air drawn in by the engine can deflect
the sensor plate further. This results in
the control plunger opening the metering
slits further and the engine being allocated more fuel. On the other hand, if the
control pressure is high, the air drawn in
by the engine cannot deflect the sensor
plate so far and, as a result, the engine
receives less fuel. In order to fully seal off
the control-pressure circuit with absolute
certainty when the engine has been
switched off, and at the same time to
maintain the pressure in the fuel circuit,
the return line of the warm-up regulator is
fitted with a check valve. This (push-up)
valve is attached to the primary-pressure
regulator and is held open during operation by the pressure-regulator plunger.
When the engine is switched off and the
plunger of the primary-pressure regulator
returns to its zero position, the check
valve is closed by a spring (Figure 17).
Differential-pressure valves
The differential-pressure valves in the
fuel distributor result in a specific pressure drop at the metering slits.
The air-flow sensor has a linear characteristic. This means that if double the
quantity of air is drawn in, the sensor-
Page 23
2
1
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6
a In zero (inoperated)
position,
b In operating position.
1 Primary pressure
intake,
2 Return (to fuel tank),
3 Plunger of the
primary-pressure
regulator,
4 Push-up valve,
5 Control-pressure
intake (from warmup regulator).
4
5
Fig. 16
Fig. 17
Primary-pressure
regulator with pushup valve in the
control-pressure
circuit
K-Jetronic
UMK1498Y
1 Control-pressure
effect (hydraulic
force),
2 Damping restriction,
3 Line to warm-up regulator,
4 Decoupling restriction bore,
5 Primary pressure
(delivery pressure),
6 Effect of air pressure.
3
a
b
1
,,,,,
,,,,,
,,,,,
,,,,,
,,,,,
5
2
plate travel is also doubled. If this travel is
to result in a change of delivered fuel in
the same relationship, in this case double
the travel equals double the quantity,
then a constant drop in pressure must
be guaranteed at the metering slits
(Figure 14), regardless of the amount of
fuel flowing through them.
3
4
UMK1499Y
Primary pressure
and control pressure
The differential-pressure valves maintain the differential pressure between the
upper and lower chamber constant regardless of fuel throughflow. The differential pressure is 0.1 bar.
The differential-pressure valves achieve
a high metering accuracy and are of the
flat-seat type. They are fitted in the fuel
21
Page 24
Gasolineinjection
systems
Differential-pressure valve
a Diaphragm
position with a
low injected
fuel quantity
,,,
,,,
,
,,,,,,,,
,,
,,,,,,,,
,,,,,,,,
,
,,,,,,,,
,,,,,,,,
UMK1656Y
b Diaphragm
position with a
large injected
fuel quantity
,,,
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,,
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,,,,,,,,
,
,,,,,,,,
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Fig. 18
22
distributor and one such valve is allocated to each metering slit. A diaphragm
separates the upper and lower chambers
of the valve (Figures 18 and 19). The
lower chambers of all the valves are connected with one another by a ring main
and are subjected to the primary pressure (delivery pressure). The valve seat
is located in the upper chamber. Each
upper chamber is connected to a
metering slit and its corresponding connection to the fuel-injection line. The
upper chambers are completely sealed
off from each other. The diaphragms are
spring-loaded and it is this helical spring
that produces the pressure differential.
Page 25
K-Jetronic
Fuel distributor with differential-pressure valves
2
4
5
6
,,,,
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8
UMK1602Y
1
3
7
Fig. 19
Fig. 20
If a large basic fuel quantity flows into the
upper chamber through the metering slit,
the diaphragm is bent downwards and
enlarges the valve cross-section at the
outlet leading to the injection valve until
the set differential pressure once again
prevails.
If the fuel quantity drops, the valve crosssection is reduced owing to the equilibrium of forces at the diaphragm until the
differential pressure of 0.1 bar is again
present.
This causes an equilibrium of forces to
prevail at the diaphragm which can be
maintained for every basic fuel quantity
by controlling the valve cross-section.
Mixture formation
The formation of the air-fuel mixture
takes place in the intake ports and
cylinders of the engine.
The continually injected fuel coming from
the injection valves is “stored” in front of
the intake valves. When the intake valve
is opened, the air drawn in by the engine
carries the waiting “cloud” of fuel with it
into the cylinder. An ignitable air-fuel
mixture is formed during the induction
stroke due to the swirl effect.
Mixture formation with air-shrouded fuelinjection valve
1 Fuel-injection valve, 2 Air-supply line,
3 Intake manifold, 4 Throttle valve.
1
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2
3
4
UMK0068Y
1 Fuel intake
(primary
pressure),
2 Upper chamber of
the differentialpressure valve,
3 Line to the fuelinjection valve
(injection
pressure),
4 Control plunger,
5 Control edge and
metering slit,
6 Valve spring,
7 Valve diaphragm,
8 Lower chamber of
the differentialpressure valve.
Air-shrouded fuel-injection valves favor
mixture formation since they atomize
the fuel very well at the outlet point
(Figures 10, 20).
23
Page 26
In addition to the basic functions described up to now, the mixture has to be
adapted during particular operating
conditions. These adaptations (corrections) are necessary in order to optimize
the power delivered, to improve the
exhaust-gas composition and to improve
the starting behavior and driveability.
24
Basic mixture adaptation
The basic adaptation of the air-fuel mixture to the operating modes of idle, part
load and full load is by appropriately
shaping the air funnel in the air-flow
sensor (Figures 21 and 22).
If the funnel had a purely conical shape,
the result would be a mixture with a constant air-fuel ratio throughout the whole
of the sensor plate range of travel (metering range). However, it is necessary to
meter to the engine an air-fuel mixture
which is optimal for particular operating
modes such as idle, part load and full
load. In practice, this means a richer
mixture at idle and full load, and a leaner
mixture in the part-load range. This
adaptation is achieved by designing the
air funnel so that it becomes wider in
stages.
If the cone shape of the funnel is flatter
than the basic cone shape (which was
specified for a particular mixture, e.g. for
λ = 1), this results in a leaner mixture. If
the funnel walls are steeper than in the
basic model, the sensor plate is lifted
further for the same air throughput, more
fuel is therefore metered by the control
plunger and the mixture is richer. Consequently, this means that the air funnel can
be shaped so that it is poss-ible to meter
mixtures to the engine which have different air-fuel ratios depending upon the
sensor-plate position in the funnel (which
in turn corresponds to the particular
engine operating mode i.e. idle, part load
and full load). This results in a richer
mixture for idle and full load (idle and fullload enrichment) and, by contrast, a
leaner mixture for part load.
Influence of funnel-wall angle upon
the sensor-plate deflection for identical air
throughput
a The basic funnel
shape results
in stroke “h”,
b Steep funnel
walls result in
increased
stroke “h” for
identical air
throughput,
c Flatter funnel
shape results
in reduced
deflection “h”
for identical air
throughput.
A Annular area
opened by the
sensor plate
(identical in
a, b and c).
A
h
a
A
h
b
A
UMK0071Y
Adaptation to operating
conditions
h
c
Fig. 21
Fig. 22
Adaptation of the air-funnel shape
1 For maximum power, 2 For part load,
3 For idle.
1
2
3
UMK0155Y
Gasolineinjection
systems
Cold-start enrichment
Depending upon the engine temperature,
the cold-start valve injects extra fuel into
the intake manifold for a limited period
during the starting process.
In order to compensate for the condensation losses due to condensation on the
cold cylinder walls, and in order to facilitate starting the cold engine during cold
starting, extra fuel must be injected at the
instant of start-up. This extra fuel is injected by the cold-start valve into the
intake manifold. The injection period of
the cold-start valve is limited by a
thermo-time switch depending upon the
engine temperature.
This process is known as cold-start enrichment and results in a “richer” air-fuel
Page 27
Cold-start valve in operated state
Thermo-time switch
1 Electrical connection, 2 Fuel supply with
strainer, 3 Valve (electromagnet armature),
4 Solenoid winding, 5 Swirl nozzle, 6 Valve seat.
2
1 Electrical connection, 2 Housing, 3 Bimetal,
4 Heating filament, 5 Electrical contact.
1
K-Jetronic
1
2
3
4
4
6
3
UMK0118Y
5
UMK0125-1Y
5
Fig. 23
Fig. 24
mixture, i.e. the excess-air factor λ is
temporarily less than 1.
The thermo-time switch (Figure 24)
consists of an electrically heated bimetal
strip which, depending upon its temperature opens or closes a contact. It is
brought into operation by the ignition/
starter switch, and is mounted at a
position which is representative of engine
temperature. During a cold start, it limits
the “on” period of the cold-start valve. In
case of repeated start attempts, or when
starting takes too long, the cold-start
valve ceases to inject.
Its “on” period is determined by the
thermo-time switch which is heated by
engine heat as well as by its own built-in
heater. Both these heating effects are
necessary in order to ensure that the
“on” period of the cold-start valve is
limited under all conditions, and engine
flooding prevented. During an actual cold
start, the heat generated by the built-in
heater is mainly responsible for the
“on” period (switch off, for instance,
at –20 °C after 7.5 seconds). With a
warm engine, the thermo-time switch has
already been heated up so far by engine
heat that it remains open and prevents
the cold-start valve from going into
action.
Cold-start valve
The cold-start valve (Figure 23) is a
solenoid-operated valve. An electromagnetic winding is fitted inside the
valve. When unoperated, the movable
electromagnet armature is forced against
a seal by means of a spring and thus
closes the valve. When the electromagnet is energized, the armature which
consequently has lifted from the valve
seat opens the passage for the flow of
fuel through the valve. From here, the fuel
enters a special nozzle at a tangent and
is caused to rotate or swirl.
The result is that the fuel is atomized very
finely and enriches the mixture in the
manifold downstream of the throttle
valve. The cold-start valve is so positioned in the intake manifold that good
distribution of the mixture to all cylinders
is ensured.
Thermo-time switch
The thermo-time switch limits the duration of cold-start valve operation, depending upon temperature.
25
Page 28
Gasolineinjection
systems
Warm-up regulator
a With the engine
cold,
b With the engine at
operating
temperature.
1 Valve diaphragm,
2 Return,
3 Control pressure
(from the mixturecontrol unit),
4 Valve spring,
5 Bimetal spring,
6 Electrical heating.
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
1
a
6
3
4
,,,,,,
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,,,,,,
,,,,,,
UMK1567Y
b
5
2
Fig. 25
26
Warm-up enrichment
Warm-up enrichment is controlled by
the warm-up regulator. When the engine
is cold, the warm-up regulator reduces
the control pressure to a degree dependent upon engine temperature and thus
causes the metering slits to open further
(Figure 25).
At the beginning of the warm-up period
which directly follows the cold start, some
of the injected fuel still condenses on the
cylinder walls and in the intake ports.
This can cause combustion misses to
occur. For this reason, the air-fuel mixture must be enriched during the warmup (λ < 1.0). This enrichment must be
continuously reduced along with the rise
in engine temperature in order to prevent
the mixture being over-rich when higher
engine temperatures have been reached.
The warm-up regulator (control-pressure
regulator) is the component which carries
out this type of mixture control for the
warm-up period by changing the control
pressure.
Warm-up regulator
The change of the control pressure is
effected by the warm-up regulator which
is fitted to the engine in such a way that it
ultimately adopts the engine temperature. An additional electrical heating system enables the regulator to be matched
precisely to the engine characteristic.
Page 29
The warm-up regulator comprises a
spring-controlled flat seat (diaphragmtype) valve and an electrically heated
bimetal spring (Figure 25).
In cold condition, the bimetal spring
exerts an opposing force to that of the
valve spring and, as a result, reduces the
effective pressure applied to the underside of the valve diaphragm. This means
that the valve outlet cross-section is
slightly increased at this point and more
fuel is diverted out of the control-pressure circuit in order to achieve a low
control pressure. Both the electrical
heating system and the engine heat the
bimetal spring as soon as the engine is
cranked. The spring bends, and in doing
so reduces the force opposing the valve
spring which, as a result, pushes up the
diaphragm of the flat-seat valve. The
valve outlet cross-section is reduced and
the pressure in the control-pressure
circuit rises.
Warm-up enrichment is completed when
the bimetal spring has lifted fully from the
valve spring. The control pressure is now
solely controlled by the valve spring and
maintained at its normal level. The control pressure is about 0.5 bar at cold start
and about 3.7 bar with the engine at
operating temperature (Figure 26).
Idle stabilization
In order to overcome the increased
friction in cold condition and to guarantee
smooth idling, the engine receives more
air-fuel mixture during the warm-up
phase due to the action of the auxiliary
air device.
When the engine is cold, the frictional
resistances are higher than when it is at
operating temperature and this friction
must be overcome by the engine during
idling. For this reason, the engine is
allowed to draw in more air by means of
the auxiliary-air device which bypasses
the throttle valve. Due to the fact that this
auxiliary air is measured by the air-flow
sensor and taken into account for fuel
metering, the engine is provided with
more air-fuel mixture. This results in idle
stabilization when the engine is cold.
K-Jetronic
Auxiliary-air device
In the auxiliary-air device, a perforated
plate is pivoted by means of a bimetal
spring and changes the open crosssection of a bypass line. This perforated
plate thus opens a correspondingly large
cross-section of the bypass line, as a
function of the temperature, and this
cross-section is reduced with increasing
engine temperature and is ultimately
closed. The bimetal spring also has an
electrical heating system which permits
the opening time to be restricted dependent upon the engine type. The in-
Fig. 26
Warm-up regulator characteristics at various operating temperatures
C
C
0°C
+2
C
0°
2
1
0
30
60
90
120
Time after starting
150
s
0
30
60
90
120
Time after starting
150
s
UMK1658E
1.0
0
0°
0°
1.5
−2
2.0
0°
C
3
0°
C
2.5
+2
4
bar
Control pressure
3.0
−2
Enrichment factor
Enrichment factor 1.0 corresponds to fuel metering with the engine at operating temperature.
27
Page 30
1
2
4
Fig. 29
Fig. 28
Dependence of the control pressure
on engine load
Control pressure
Idle and part load
UMK0019E
Full load
Engine load
Fig. 29
Acceleration response
Behavior of the K-Jetronic when the throttle valve
is suddenly opened.
Open
Closed
0.1
0.2
0.3
Time t
0.4
s
UMK1659E
0
28
3
UMK0127Y
1 Electrical connection, 2 Electrical heating,
3 Bimetal spring, 4 Perforated plate.
Throttle-valve
opening
Full-load enrichment
Engines operated in the part-load range
with a very lean mixture require an enrichment during full-load operation, in
addition to the mixture adaptation resulting from the shape of the air funnel.
This extra enrichment is carried out by a
specially designed warm-up regulator.
This regulates the control pressure depending upon the manifold pressure
(Figures 28 and 30).
This model of the warm-up regulator
uses two valve springs instead of one.
The outer of the two springs is supported
on the housing as in the case with the
normal-model warm-up regulator. The
inner spring however is supported on a
diaphragm which divides the regulator
into an upper and a lower chamber. The
manifold pressure which is tapped via a
hose connection from the intake manifold
downstream of the throttle valve acts in
the upper chamber. Depending upon the
model, the lower chamber is subjected to
atmospheric pressure either directly or
by means of a second hose leading to the
air filter.
Due to the low manifold pressure in the
idle and part-load ranges, which is also
present in the upper chamber, the diaphragm lifts to its upper stop. The inner
spring is then at maximum pretension.
The pretension of both springs, as a
result, determines the particular control
pressure for these two ranges. When the
throttle valve is opened further at full
load, the pressure in the intake manifold
increases, the diaphragm leaves the
upper stops and is pressed against the
lower stops.
The inner spring is relieved of tension
and the control pressure reduced by the
specified amount as a result. This results
in mixture enrichment.
Auxiliary-air device
Sensor-plate travel
stallation location of the auxiliary-air device is selected such that it assumes the
engine temperature. This guarantees
that the auxiliary-air device only functions
when the engine is cold (Figure 27).
Engine speed
Gasolineinjection
systems