Note: Descriptions are shown in the official language in which they were submitted.
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Description
DRIVER CIRCUIT FOR SOLENOID OPERATED FUEL INJECTORS
Technical Field
The present invention relates generally to
fuel controls for internal combustion engines, and
more particularly to a driver circuit for operating
fuel injectors.
Background Art
Compression type internal combustion engines
require the use of fuel injectors which deliver fuel
under pressure to one or more cylinders. Such fuel
injectors may be of the solenoid operated type which
are operated by an engine control to deliver
accurately measured quantities of fuel to the cylin-
ders at precise instants in time based upon the
positions of the pistons in the cylinders. The timing
of fuel injection and the quantity of fuel injected
during each injection operation affect the efficiency
of the engine and the emissions therefrom. Thus, it
is important to precisely control the timing and
quantity of fuel delivered to the cylinders by means
of a solenoid driver circuit which accurately controls
the fuel injectors.
A prior solenoid driver circuit which is
capable of precise timing and fuel quantity control is
disclosed in Pflederer U.S. Patent No. 4,604,675,
entitled "Fuel Injection Solenoid Driver Circuit",
assigned to the assignee of the present application.
Figure 1 is a greatly simplified drawing of the
Pflederer driver circuit wherein certain elements are
identified by the same reference numerals
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as used in such patent. Each of a series of six fuel
injector solenoid coils 168a-168f is coupled through a
modulation switch 164 to a voltage source 10.
Cylinder select switches 184a-184f are coupled between
S the solenoid coils 168a-168f and a series combination
of an inductor 186 and a current sensing resistor 188.
Flyback diodes 260a-260f include anode terminals which
are coupled to the junctions between the coils
168a-168f and the switches 184a-184f. Cathode termi-
nals of the diodes 260a-260f are coupled together to
the voltage source 10. During operation of this
circuit, an engine control 12 develops command signals
which are coupled to cylinder select and current
sense/control circuits that in turn operate the
lS switches 184a-184f and a modulation switch 164. When
a particular solenoid coil is to be actuated, for
example the solenoid coil 168a, the switch 184a is
closed by the cylinder select circuit 14. In
addition, the current sense/control circuit 16
operates the switch 164 in a pulse width modulated
(PWM) mode of operation to control the current
delivered to the solenoid coil 168a according to a
predetermined control strategy such that power
dissipation is kept at a low level.
When the coil 168a has been energized for a
sufficient time to insure that the proper quantity of
fuel will be delivered to the associated engine
cylinder, the switches 184a and 164 are opened, in
turn causing flyback currents to flow from ground
potential through the parallel combination of a
resistor 252 and an inductor 254, a diode 256, the
coil 168a and the diode 260a to the voltage source 10.
This places a reverse potential across the coil 168a
to quickly deenergize same.
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While the driver circuit illustrated in the
Pflederer patent is effective to control solenoid
operated fuel injectors in an efficient manner, it has
been found that the driver circuit can be totally
disabled under certain circumstances, in turn leading
to a complete shutdown of the engine. Specifically,
if either terminal of any of the coils 168a-168f
should be shorted to ground potential, there is no way
to continue to energize the remaining coils in a
controlled manner. Thus, under such a fault condition,
there is no way to provide fuel to the cylinders and
hence limp-home capability cannot be realized.
Disclosure of the Invention
lS In accordance with the present invention, a
driver circuit for energizing at least first and
second coils permits one of the coils to be energized
in a controlled manner even when the other coil has
been shorted to ground.
More specifically, a driver circuit for
first and second coils includes first and second
selector switches coupled in series between first
terminals of the first and second coils, respectively,
and a first common junction. First and second diodes
2S are coupled in series between second terminals of the
first and second coils, respectively, and a second
common junction. A source of first potential is cou-
pled to the first common junction and a modulation
switch is coupled between the second common junction
and a source of second potential. Means are coupled
to the selector switches for selectively closing the
switches at desired points in time and means are
provided for operating the modulation switch while at
least one of the selector switches is closed such that
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currents of controlled magnitude flow through the
coils.
In a preferred embodiment of the invention,
the driver circuit is particularly adapted for use in
S controlling solenoid operated fuel injectors which
control the flow of fuel into associated cylinders of
an internal combustion engine. Significantly,
shorting to ground of a terminal of one of the
solenoid coils does not totally disable the engine,
inasmuch as the diodes isolate the coils and prevent
the flow of shorting currents to at least some of the
remaining coils. Thus, at least a portion of the
remaining coils can continue to be controlled to
provide fuel to one or more of the engine cylinders.
This provides a limp-home capability which is not
realized by the prior art.
Brief Description of the-Drawings
Fig. 1 is a simplified combined schematic
and block diagram of the prior art solenoid driver
circuit disclosed in the above-identified Pflederer
patent;
Fig. 2 is a simplified combined diagrammatic
and block diagram of an internal combustion engine
together with associated control and driver circuit
according to the present invention;
Fig. 3 is a simplified combined schematic
and block diagram of the driver circuit according to
the present invention; and
Fig. 4 is a pair of waveform diagrams
illustrating the current and voltage delivered to the
solenoid coils illustrated in Fig. 3.
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Description of the Preferred Embodiment
Referring now to Fig. 2, an internal
combustion engine 20 of the compression or diesel type
includes N cylinders 22 which are provided fuel by N
S solenoid operated fuel injectors 24. In the
illustrated embodiment, N = 6, and hence there are six
cylinders 22a-22f and six fuel injectors 24a-24f
associated therewith, respectively. The fuel
injectors 24a-24f include solenoid coils, described in
greater detail hereinafter in connection with Fig. 3,
which are energized by a solenoid driver circuit 26
according to the present invention. The driver
circuit 26 in turn receives signals developed by an
engine control 28.
lS It should be noted that the engine control
28 forms no part of the present invention, and hence
will not be described in greater detail herein.
Illustrated in Fig. 3 is a simplified
diagram of the driver circuit 26. Solenoid coils
30a-30f of the fuel injectors 24a-24f, respectively,
include first terminals 32a-32f and second terminals
34a-34f. A plurality of N selector switches 36a-36f
are coupled in series between the first terminals
32a-32f of the coils 30a-30f, respectively, and a
first common junction 38. The selector switches
36a-36f may comprise, for example, bipolar
transistors, although this need not be the case. The
switches 36a-36f are controlled by a cylinder select
circuit 40 which is in turn responsive to the command
signals developed by the engine control 28.
Second terminals of pairs of associated
coils 30a-30f are connected together to form N/2 coil
junctions 42-1 through 42-3. More specifically, the
second terminals 34a and 34b of associated coils 30a
3S and 30b are connected together to form the coil
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junction 42-1. In like fashion, the second terminals
34c, 34d of associated coils 30c, 30d are connected
together to form the coil junction 42-2 whereas the
second terminals 34e and 34f of associated coils 30e,
S 30f are connected together to form the coil junction
42-3.
A plurality of N/2 isolation diodes 44-1
through 44-3 include anode terminals coupled to the
coil junctions 42-1 through 42-3, respectively.
Cathode terminals of the isolation diodes 44-1 through
44-3 are connected together at a second common
junction 44.
A source of first potential in the form of a
voltage source 46 is coupled via a current sensing
circuit 48 to the first common junction 38. A turn
off flyback diode 50 is coupled between the second
common junction and the voltage source 46. As noted
in greater detail hereinafter, the isolation diodes
44-1 through 44-3 and the diode 50, in conjunction
with diodes 52a-52f coupled between the first
terminals 32a-32f and chassis ground potential,
respectively, provide a path for flyback currents to
quickly deenergize the coils 30a-30f.
A modulation switch 56 is coupled between
the second common junction and a source of second
potential, illustrated by chassis ground symbol 58.
The modulation switch 56 is operated by a current
control logic circuit 59 which is in turn responsive
to the current detected by the current sensor 48 and
the signals developed by the engine control 28.
In operation of the circuit shown in Fig. 3,
the engine control 28 operates the cylinder select
circuit 40 and the current control logic 59 to
successively close different ones of the switches
36a-36f in synchronism with the position of pistons 23
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(only three of which are 23b, 23d and 23f are shown in
Fig. 2). Upon closure of each switch 36a-36f, the
current control logic 59 operates the modulation
switch 56 in accordance with the waveform illustrated
in the bottom waveform diagram of Fig. 4. As seen in
such waveform diagram, the switch 56 is operated in a
PWM mode of operation wherein the duration of time the
switch 56 is closed is dependent upon the current
provided by the voltage source 46. During a first
period of time during which the coil is energized to
move an associated actuator (not shown) from a closed
position to a fully opened position (hereinafter the
"pull-in period"), the current delivered to the coil
is controlled between first and second limits. More
lS specifically, when the current from the voltage source
reaches a first predetermined upper limit, as detected
by the current sensor 48, the current control logic
circuit 59 opens the switch 56, in turn causing an
exponential decay of current supplied by the voltage
source 46. When the current magnitude drops to a
second predetermined lower limit, the switch 56 is
again closed, causing the current supplied by the
voltage source to rise.
At the end of the pull-in period, the
2S current control logic 59 substitutes third and fourth
current limits which are less than the first and
second limits in effect during the pull-in period.
Thus, the average current flowing through the coil
during a subsequent period of time (hereinafter the
"hold-in period") is less than the average current
during the pull-in period.
At the end of the hold-in period, the
selector switch 36a-36f which was closed is now
opened, as is the switch 56. Inasmuch as the current
through the associated coil 30a-30f cannot decay to
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zero instantaneously, current is drawn from chassis
ground, through the associated diode 52a-52f, the coil
30a-30f, one of the diodes 44-1 through 44-3 and the
diode 50 to the voltage source 46. This flyback
S current flow causes the potential across the respec-
tive coil 30a-30f to reverse polarity, in turn causing
a rapid decay in the current flowing through the coil.
This flyback operation causes the fuel injector to
shut off rapidly, thereby permitting precise control
over the quantity of fuel delivered to each engine
cylinder.
Of particular significance in the circuit of
Fig. 3, if a first terminal or a second terminal of a
coil 30a-30f is shorted to chassis ground, only that
coil and the coil connected to the same coil junction
42-1 through 42-3 will be adversely effected. This is
due to the isolation provided by the diodes 44-1
through 44-3, which prevent current flow in a
direction which would cause the short to propagate to
the remaining coils. Thus, continued operation of the
engine during such a fault is possible, albeit under
reduced power so that the vehicle can be driven to a
repair facility. This limp-home capability is a
significant advantage realized by the present
invention.
A further advantage of the present invention
resides in the fact that the modulation switch 56 is
connected between the coils 30 and chassis ground. If
any of the coils should be shorted to ground, the
current sensor 48 and the modulation switch 56 are not
subjected to high current levels when supplying
current to the non-shorted coils, and hence the switch
56 continues to modulate the currents through the
non-shorted coils 30 in a controlled fashion. On the
other hand, if the switch 56 were instead coupled
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between the voltage source 46 and the coils 30, a
short to ground of a coil terminal could cause high
magnitude currents to flow through the current sensor
48, even when attempting to supply current to the
non-shorted coils, in turn causing the control logic
59 to open the switch 56 and thus prevent the delivery
of controlled currents to such coils. It can be seen
that connecting the switch 56 between the coils 30 and
ground further enhances the limp-home capability noted
above.
It should be noted that additional isolation
may be provided to further limit the adverse effects
of chassis ground shorts. This may be achieved by
coupling each second terminal of each coil through an
associated diode to the second common junction 44.
Thus, a short to chassis ground at either terminal of
a coil 30a-30f will be limited to such coil alone and
the remaining coils will continue to operate in normal
fashion. This further isolation is obtained through
the use of N isolation diodes rather than N/2 isola-
tion diodes, as is the case in the above-described
embodiment, however.
It can be seen that the driver circuit of
the present invention is simple in design and provides
2S the desired protection against complete engine shut
down in the event of a ground short.
