Control circuit for predominantly inductive loads in particular electroinjectors

A control circuit for supplying a load with current having a high-amplitude ortion with a rapid leading edge, and a lower-amplitude portion. The circuit is input-connected to a low-voltage supply source, and comprises a number of actuator circuits parallel-connected between the input terminals and each including a capacitor and a load. Each actuator circuit also comprises a first controlled switch between the respective load and a reference line, for enabling energy supply and storage by the respective load. A second controlled switch is provided between the capacitor line and the load line, for rapidly discharging the capacitors into the load selected by the first switch and recirculating the load current, or for charging the capacitors with the recirculated load current.

BACKGROUND OF THE INVENTION 
The present invention relates to a control circuit for predominantly 
inductive loads, in particular, electroinjectors forming part of an 
internal combustion engine supply system. 
For controlling internal combustion engine injectors, the supply current to 
the injectors must present a pattern comprising, in general, a rapidly 
increasing portion, a portion increasing more slowly, a portion 
oscillating about a mean value, and a rapidly decreasing portion. The 
circuits currently employed for achieving such a pattern substantially 
comprise a low-voltage supply source and a reactive circuit consisting of 
an inductor and capacitor for storing the energy required for producing a 
rapid current pulse in the load. For this purpose, the inductor is charged 
to a given current and then connected to the capacitor, so as to form a 
resonant circuit and transfer energy from the inductor to the capacitor, 
which is thus charged for subsequently supplying the load (injector 
actuator) with the required current pulse. 
A major drawback of the above known circuit is that, for achieving the high 
currents required, large-size components such as cup-shaped or toroidal 
cores are used as inductors on the reactive circuit, thus increasing the 
size and cost of the overall circuit. 
The above problem is further compounded by the fact that, for protecting 
the control elements of the actuators, each actuator presents a so-called 
"snubber" circuit comprising a capacitor and resistor connected parallel 
to the actuator, and which provide for absorbing and dissipating the 
energy of the recirculating current of the actuator. Such capacitors 
further increase the overall size of the circuit. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a more compact control 
circuit as compared with known types. 
According to the present invention, there is provided a control circuit for 
predominantly inductive loads, in particular electroinjectors, for 
supplying the load with current having a high-amplitude portion with a 
rapid leading edge, and a lower-amplitude portion; said circuit comprising 
a first and second input terminal connectable to a low-voltage supply 
source; an energy storage circuit connected between said input terminals 
and including at least a capacitive element and an inductive element; a 
first controlled switch element located between said inductive element and 
a reference line, for enabling selective charging of said inductive 
element; a second controlled switch element for enabling rapid discharge 
of said capacitive element into said load; and a control unit for 
generating control signals for said first and second switch elements; 
characterized by the fact that said inductive element consists of said 
load.

DETAILED DESCRIPTION OF THE INVENTION 
Number 30 in FIG. 1 indicates a supply system for an internal combustion 
engine 32, more specifically, a supercharged diesel engine. In FIG. 1, the 
continuous lines indicate the fuel conduits, and the dotted lines the 
electric lines relative to measured quantity signals, controls and supply. 
More specifically, system 30 comprises: 
an electric supply pump 1 for ensuring a given head (1-3 bar) in fuel 
supply conduit 31; 
a fuel filter 2 on conduit 31, downstream from pump 1; 
a high-pressure pump 3 downstream from filter 2, for generating as high an 
injection pressure as required (up to 1500 bar); 
a high-pressure supply line 5 from pump 3; 
a pressure regulator 4 on high-pressure supply line 5 and consisting of an 
electronically controlled two-way valve; 
a high-pressure fuel manifold or "rail" 6 connected to supply line 5 and 
having one or more connecting pipes to a number of injectors 7, one for 
each cylinder of engine 32; 
a low-pressure fuel return line 8 having a number of branches: branch 8a 
connected to pressure regulator 4, branch 8b connected to manifold 6, and 
branch 8c connected to injectors 7; 
a radiator 9 on return line 8, for cooling the feedback fuel; 
a fuel tank 10 from which fuel is withdrawn by supply conduit 31 and into 
which fuel is drained by return line 8; 
a system supply battery 11; 
a control and power unit (central control unit) 12 supplied by battery 11 
via lines 33, and by which the unit is controlled on the basis of signals 
from various sensors; 
spark plugs or starters 13, one for each cylinder of engine 32, for heating 
the cylinder when the engine is started, and which are controlled by unit 
12 via output line 34; 
an overpressure valve 21 inside manifold 6 and connected to branch 8b of 
return line 8; 
a combustion product exhaust conduit 45 connected to the exhaust manifold 
(not shown) of engine 32; 
a turbine 22 of variable geometry on exhaust conduit 45 and controlled by 
unit 12 via output line 46; 
an exhaust gas recirculating valve 23 on exhaust conduit 45, downstream 
from turbine 22, and connected to an output of unit 12 over line 47; 
a compressor 48 connected to output shaft 49 of turbine 22, supplied with 
ambient air by air supply conduit 50, and supplying intake manifold 36 via 
pressurized air supply conduit 51; 
a first pressure sensor 14 on manifold 6 and connected to an input of unit 
12 over line 35; 
a second pressure sensor 15 on intake manifold 36 of engine 32, for 
detecting the air pressure in the intake manifold and accordingly 
supplying an electric signal to unit 12 over line 37; 
a first temperature sensor 16 on the cylinder head of engine 32, for 
detecting its temperature and connected to an input of unit 12 over line 
38; 
an engine speed and stroke sensor 17 on output shaft 40 of the engine and 
connected to an input of unit 12 over line 41; 
a third pressure sensor 18 and second outside (ambient) air temperature 
sensor 19 on air supply conduit 50, and connected to respective inputs of 
unit 12 over respective lines 53 and 54; 
an accelerator pedal position sensor 20 connected to an input of unit 12 
over line 55. 
Central control unit 12 is connected to a control circuit 100 for the 
injectors 7 over a number of supply lines 56, one for each injector 7, for 
controlling the injection phases and to pressure regulator 4 over line 57. 
Unit 12 and control circuit 100 are also connected over line 58 from unit 
12 and line 59 from circuit 100, as explained in more detail later on. 
With reference to FIG. 2, control circuit 100 comprises two input terminals 
102 and 103 connectable to a supply source B consisting of a low-voltage 
battery. More specifically, terminal 102 is connected to the anode of a 
diode D2, the cathode of which is connected to a first common line 104 
(e.g., actuator line); and terminal 103 is connected directly to a second 
common line 105 (ground). 
Circuit 100 also comprises a number of actuator circuits 106 parallel 
connected between lines 104 and 105, and each comprising an actuator Li, a 
storage capacitor Ci, a coupling diode Di, and a controlled electronic 
switch SWi. More specifically, each actuator Li, consisting of a coil 
wound about a core and defining the predominantly inductive load, presents 
one terminal connected to line 104, and an opposite terminal, defining a 
node 107, connected to the anode of diode Di for connecting actuator Li to 
a third common line 112 (capacitance line). The cathode of each diode Di 
is connected to a second node 113 that is in turn connected to the 
capacitance line 112 and to the a first terminal of respective capacitor 
Ci, which provides for storing energy at a higher voltage than battery B, 
and the other terminal of which is connected to the ground line 105. Each 
switch SWi, which provides for connecting actuator Li to battery B and for 
transferring energy from actuator Li to the circuit consisting of the 
parallel connection of storage capacitors Ci, is located between node 107 
and ground 105, and presents a control input 108 connected to unit 12 via 
control line 56, over which unit 12 supplies a signal s.sub.i for 
selecting the actuator to be enabled, as described in more detail later 
on. 
Circuit 100 also comprises the series connection of an electronic switch 
SWR and a diode D1, which provide for connecting capacitance line 112 to 
actuator line 104 and for recirculating the current in load Li. More 
specifically, switch SWR presents a first terminal connected to 
capacitance line 112; a second terminal connected to the anode of diode 
D1, the cathode of which is connected to actuator line 104; and a control 
terminal 114 connected to unit 12 via control line 58 over which unit 12 
supplies a signal s.sub.1 for controlling switch SWR. Finally, line 112 is 
connected to unit 12 via line 59 for enabling unit 12 to monitor the 
voltage on line 112. 
Circuit 100 charges storage capacitors Ci to an appropriate voltage, and 
supplies actuators Li with current Ii, the pattern of which presents a 
high-amplitude portion with a rapid leading edge, followed by a 
lower-amplitude portion terminating with a rapid trailing edge, as 
described below with reference to FIGS. 3 to 5. 
With reference to FIG. 3, let us assume, to begin with, that switches SWR 
and SWi are open (low logic level of signals s.sub.1 and s.sub.i); and 
storage capacitors Ci are charged to a given high voltage (voltage V.sub.C 
of value V.sub.1), so that the voltage drop between capacitance line 112 
and actuator line 104 is such as to reverse-bias diodes Di, and current Ii 
in the actuators is zero. 
At instant t.sub.0, switch SWR is closed, so as to switch actuator line 104 
to the voltage level of capacitance line 112. 
At instant t.sub.1, unit 12 selects the required actuator Li by switching 
respective signal s.sub.i to high and so closing respective switch SWi, so 
that the selected actuator Li is connected between capacitance line 112 
and ground 105, parallel to capacitors Ci with which it forms a resonant 
circuit. In the selected actuator, a current pulse is therefore formed 
consisting of a high-frequency sinusoid portion (the value of which is 
determined by the inductance of actuator Li and the capacitance of 
capacitors Ci) and produced by rapid discharge of the energy stored in 
capacitors Ci, thus resulting in a simultaneous rapid reduction in voltage 
V.sub.C of capacitors Ci. The capacitors continue discharging up to 
instant t.sub.2, at which point voltage V.sub.C in line 112 is 
approximately equal to the voltage of battery B, so that diode D2 is 
biased directly and connects battery B to actuator line 104. As of instant 
t.sub.2, the selected actuator Li is supplied by low-voltage battery B, 
and its current Ii increases slowly with a time constant of L/R, where L 
is the inductance of actuator Li, and R the resistance of the actuator 
coil, battery B, components D2 and SWi, and the connecting line. In this 
phase, the selected actuator diode Di remains reverse-biased. 
The above phase continues up to instant t.sub.3, at which point switch SWi 
is opened (signal s.sub.i switched to low), so that the selected actuator 
diode Di is biased directly and operates as a "free-wheeling" diode, thus 
enabling discharge of the previously charged actuator Li and recirculation 
of current Li via capacitance line 112 and switch SWR. In this phase, 
current Ii therefore decreases with a time constant of L/R, where R is the 
resistance of the actuator coil and components Di, SWR and D1. 
At instant t.sub.4, switch SWi is again closed, the selected actuator Li is 
again charged by battery B, and respective diode Di opens to disconnect 
capacitance line 112. In this phase, current Ii in the actuator again 
increases with a time constant of L/R, where R is the resistance of the 
actuator coil, components B, D2 and SWi, and the connecting line, despite 
the L value differing as compared with phase t.sub.2 -t.sub.3, due to the 
different current level. When switch SWi is opened at instant t.sub.5, 
actuator Li is again discharged, so that, by appropriately opening and 
closing switch SWi, the current in actuator Li may be maintained in such a 
manner as to oscillate about a predetermined medium-low value. 
For rapidly discharging actuator Li, switches SWR and SWi are opened 
successively. In the FIG. 3 case, in particular, switch SWR is opened at 
instant t.sub.6 with switch SWi open. In this phase, diode Di is biased 
directly, so as to connect actuator Li to capacitance line 112 and again 
form a resonant circuit; actuator Li therefore discharges rapidly into 
capacitors Ci; current Ii decreases in the form of a high-frequency 
sinusoid portion; and the energy previously stored by actuator Li is 
transferred to capacitors Ci, the voltage of which thus increases rapidly. 
The above phase continues until the current in actuator Li is zeroed, 
which corresponds to a first charge of capacitors Ci to voltage V.sub.2, 
at which point diode Di is disabled for preventing the sign of the current 
in the inductor from being inverted (instant t.sub.7). Subsequently, 
capacitors Ci remain charged to voltage V.sub.2, by virtue of being 
isolated from the rest of the circuit. 
As shown in FIG. 3, at instant t.sub.8, unit 12 again closes one or more of 
switches SWi, so as to again close the circuit including battery B and the 
actuator Li relative to each closed switch SWi, so that each actuator Li 
is supplied with current increasing with a time constant of L/R. In this 
phase, capacitors Ci remain isolated. At instant t.sub.9, switch SWi (or 
all the switches closed previously) is again opened, so that, as in 
interval t.sub.6 -t.sub.7, energy is transferred from the actuator to 
capacitors Ci, current Ii in actuator Li is zeroed (instant t.sub.10), and 
the voltage in capacitance line 112 increases. By repeating the above two 
phases and appropriately selecting the closing times of switch/es SWi, it 
is possible to charge the capacitors gradually to the required level 
V.sub.1, by first charging actuators Li to such a value as to avoid 
activating them, and then discharging the actuators into the capacitors. 
The FIG. 2 circuit also provides for a second operating mode, as shown in 
FIG. 4. In this case, as in the FIG. 3 mode, capacitors Ci are initially 
charged to level V.sub.1 ; switches SWR and SWi are open; actuator line 
104 is switched to level V.sub.1 when switch SWR is closed (instant 
t.sub.0); closure of a given switch SWi (instant t.sub.1) provides for 
selecting a given actuator Li, generating a current pulse in the actuator, 
and rapidly charging the actuator at the expense of capacitors Ci, which 
discharge to approximately the value of battery B (instant t.sub.2); and 
the selected actuator Li is subsequently supplied by battery B, until the 
relative switch SWi is opened (instant t.sub.3). The fact that, in the 
second operating mode, switch SWR is opened in the interval t.sub.2 
-t.sub.3 in no way affects operation of the circuit as described above. 
Unlike the FIG. 3 mode, however, when switch SWi is opened (instant 
t.sub.3), actuator Li is prevented from discharging through the circuit 
including switch SWR, so that energy can only be transferred from actuator 
Li to capacitors Ci, thus resulting in a first charge of capacitors Ci in 
interval t.sub.3 -t.sub.4, as shown in FIG. 4. When switch SWi is closed 
(instant t.sub.4), actuator Li is again connected to the circuit including 
battery B, and so begins charging via diode D2, while the relative diode 
Di is disabled for disconnecting actuator Li from capacitance line 112, 
which is thus maintained at the previous voltage level. At instant 
t.sub.5, switch SWi is again opened, so that the energy stored by actuator 
Li in the foregoing interval t.sub.4 -t.sub.5 is transferred to capacitors 
Ci, which are thus charged directly by the selected actuator during the 
low-current operating phase, using the recirculating current of the 
actuator itself. 
The current in the actuator is zeroed by keeping the relative switch SWi 
open subsequent to instant t.sub.7, as shown in FIG. 4. 
In the FIG. 4 operating mode, the voltage of capacitors Ci may be limited 
to a predetermined value by appropriately delaying the opening of switch 
SWR subsequent to instant t.sub.3, so that the initial opening phases of 
switches SWi provide for recirculating the actuator current through switch 
SWR, without charging capacitors Ci, which are only charged after a given 
number of opening and closing cycles of switches SWi. 
In other words, according to the present invention, the energy stored in 
actuators Li, instead of being dissipated, as in known circuits, during 
the recirculating phase, is employed for charging capacitors Ci, which in 
turn provide for rapidly supplying the selected actuators. As such, energy 
is transferred continually in alternate phases between the actuators and 
capacitors, thus reducing the number of components and dissipation of the 
circuit, as well as increasing the rapidity with which the various phases 
are performed. Moreover, connection of actuator circuits 106 to the same 
line 104 provides for transferring energy from one circuit 106 to the next 
according to the injection phases provided for by unit 12. 
The resulting high-speed response of the circuit also provides for 
achieving a pilot injection phase prior to actual injection. Proposals 
have been made, in fact, for preceding actual injection with a shorter 
pilot injection phase, for initiating combustion with a limited amount of 
fuel and so reducing the rate of heat release, noise level, and the 
formation of nitric oxide. Despite the proved effectiveness of a pilot 
injection phase, particularly at low speed and/or under partial load 
conditions, the delays introduced by the control circuit components and 
injectors and the operating frequency involved currently prevent two 
distinct injection phases from being achieved in rapid succession. In 
actual practice, in fact, the two phases merge, with one continuous 
opening operation of the injector ranging from the start of the pilot 
phase to the end of the actual injection phase. 
By virtue of transferring energy from the actuators to the capacitors 
during the discharge phase, however, the present invention provides for 
achieving a pilot phase temporally distinct from the actual injection 
phase. 
One embodiment of such a pilot injection phase will be described with 
reference to FIG. 5 showing time graphs of quantities s.sub.1, s.sub.i, 
V.sub.C and Ii. Initially, signals s.sub.1 and s.sub.i are low, capacitors 
Ci are charged to voltage V.sub.C of value V.sub.1, and the actuators are 
discharged. As in FIGS. 3 and 4, at instant t.sub.0, switch SWR is closed 
(by switching signal s.sub.1) and, at instant t.sub.1, switch SWi of the 
selected actuator is closed, thus generating a current pulse Ii in the 
actuator due to rapid discharge of capacitors Ci. At instant t.sub.2, the 
voltage in capacitance line 112 equals that of battery B, which therefore 
takes over supply of the actuator from capacitors Ci, thus enabling a 
further, slower, increase in current Ii of actuator Li (pilot injection 
phase). At instant t.sub.3, switch SWR is again opened; and, at instant 
t.sub.4, switch SWi is also opened, so that the current in actuator Li 
falls rapidly to zero at instant t.sub.5, and, at the same time, the 
voltage in capacitors Ci increases rapidly to value V.sub.3 by virtue of 
the energy in actuator Li being transferred to capacitors Ci. At instant 
t.sub.6, switch SWR is again closed; and, at instant t.sub.7, switch SWi 
of the actuator previously selected for the pilot phase is again closed, 
followed by the actual, longer, injection phase according to either one of 
the operating modes in FIGS. 3 and 4. In the FIG. 5 example, the actual 
injection phase is performed as shown in FIG. 3 and therefore requires no 
further description. 
By virtue of employing the actuators for charging capacitors Ci, the 
circuit according to the present invention provides for achieving the 
required current patterns with no need for auxiliary inductors or 
capacitors. Moreover, by virtue of the recirculating current of actuators 
Li being absorbed by and charging capacitors Ci, no "snubbing" capacitors 
are required, as on known circuits, for protecting switches SWi, thus 
greatly reducing the size and cost of the circuit according to the present 
invention. 
To those skilled in the art it will be clear that changed may be made to 
the circuit as described and illustrated herein without, however, 
departing from the scope of the present invention. For example, the number 
of circuits 106 depends on the number of actuators Li, and may vary as 
required.