U.S. patent number 3,889,648 [Application Number 05/347,729] was granted by the patent office on 1975-06-17 for fuel systems for engines.
This patent grant is currently assigned to C.A.V. Limited. Invention is credited to Geoffrey Albert Kenyon Brunt, Christopher Robin Jones, Malcolm Williams.
United States Patent |
3,889,648 |
Williams , et al. |
June 17, 1975 |
Fuel systems for engines
Abstract
A fuel system for an engine has an electronic governor
controlling a pump, the governor receiving a number of signals
including one signal from a transducer meansuring engine speed.
This transducer includes a pump circuit, and is characterised in
that the capacitor of the pump circuit is connected across the
input and output of an operational amplifier.
Inventors: |
Williams; Malcolm (Solihull,
EN), Brunt; Geoffrey Albert Kenyon (Glastonbury,
EN), Jones; Christopher Robin (Alcester,
EN) |
Assignee: |
C.A.V. Limited (London,
EN)
|
Family
ID: |
26251223 |
Appl.
No.: |
05/347,729 |
Filed: |
April 4, 1973 |
Foreign Application Priority Data
|
|
|
|
|
Apr 4, 1972 [GB] |
|
|
15341/72 |
Apr 4, 1972 [GB] |
|
|
15339/72 |
|
Current U.S.
Class: |
123/357; 123/497;
327/185; 327/552 |
Current CPC
Class: |
F02D
41/38 (20130101) |
Current International
Class: |
F02D
41/38 (20060101); F02m 039/00 () |
Field of
Search: |
;123/139E,32EA,32AE,102
;60/39.28 ;73/398 ;307/247,266,268 ;328/165,158 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Myhre; Charles J.
Assistant Examiner: Cox; Ronald B.
Attorney, Agent or Firm: Holman & Stern
Claims
We claim:
1. A fuel system for an engine, comprising in combination a pump
supply fuel to the engine, an actuator controlling the pump, and an
electronic governor for controlling the actuator, said governor
receiving electrical signals representing engine speed and at least
one further engine parameter, the speed signal being obtained using
a transducer producing an a.c. output at a frequency proportional
to engine speed, a pump circuit for converting said signal to a
d.c. signal, first, second and third supply lines, an operational
amplifier having an inverting input terminal and a non-inverting
input terminal, said amplifier being powered by the first and
second supply lines and having its non-inverting input terminal
connected to the third supply line, and said pump circuit including
a capacitor across which is developed a voltage proportional to the
frequency of the a.c. output, said capacitor being connected across
the inverting input and output terminals of said operational
amplifier, a resistor across the capacitor, a second capacitor and
a diode in series between an input terminal and the inverting input
terminal, and a second diode coupling the junction of the second
capacitor and first diode to the third supply line.
2. A circuit as claimed in claim 1 including a resistor coupling
the inverting input terminal of the amplifier to the second
line.
3. A system as claimed in claim 1 including in addition a further
pump circuit providing an input to the operational amplifier or
directly to the governor whereby said d.c. signal increases from
zero to a maximum level at a minimum engine speed and then
decreases substantially linearly to zero at a maximum engine
speed.
4. A system as claimed in claim 1 in which the operational
amplifier is biased so that it produces a maximum output at low
engine speeds, falling to zero near a maximum engine speed.
5. A system as claimed in any one of claim 1 including a shaping
circuit between the transducer and the first mentioned pump
circuit, the shaping circuit converting the output from the
transducer to substantially square wave form.
6. A system as claimed in claim 5, including a pair of supply lines
providing power to the governor, said shaping circuit producing a
square wave output having an amplitude proportional to the voltage
between said supply lines.
7. A system as claimed in claim 6 including means modifying said
amplitude to compensate for temperature dependance of components in
the pump circuit.
8. A system as claimed in any one of claim 6 in which the shaping
circuit includes first and second transistors connected as a long
tailed pair with a constant current source in the tail, the first
transistor having its base connected to the transducer, and the
second transistor having a collector-base cross coupling with a
third transistor providing the output from the pulse shaping
circuit.
9. A system as claimed in any one of claim 5 in which the shaping
circuit includes switching components producing a second square
wave output out of phase with the first output, and the capacitor
across the operational amplifier is common to two pump circuits
driven by the two outputs from the shaping circuit.
10. An electronic pump circuit in which a voltage dependent on the
frequency of an a.c. input is developed across a capacitor
connected across the input and output terminals of an operational
amplifier, including first, second and third supply lines, the
amplifier being powered by the first and second lines and having
its non-inverting input terminal connected to the third line, and
the pump circuit including, in addition to the capacitor between
the output terminal and the inverting input terminal of the
amplifier, a resistor across the capacitor, a second capacitor and
a diode in series between an input terminal and the inverting input
terminal, and a second diode coupling the junction of the second
capacitor and first diode to the third line.
11. A circuit as claimed in claim 10 including a resistor coupling
the inverting input terminal of the amplifier to the second
line.
12. A circuit as claimed in claim 10 in combination with a
conventional pump circuit providing a further input to the
amplifier, whereby with increasing frequency the output of the
amplifier rises from zero to a maximum value and then decreases
with further increase in frequency towards zero.
13. A fuel system for a compression-ignition engine, comprising in
combination a pump for supplying fuel to the engine, an
electro-mechanical actuator controlling the pump, and an electronic
governor for controlling the actuator, said governor receiving
electrical signals from three transducers representing demand, pump
output and engine speed, the governor including three power supply
lines, the third line being kept at a specified proportion of the
potential between the first and second lines, the speed signal
being obtained using a transducer producing an a.c. output at a
frequency proportional to engine speed, a signal-shaping circuit to
which said a.c. output is applied, said signal-shaping circuit
producing a square wave output whose amplitude is substantially
proportional to the potential between the first two supply lines,
and a pump circuit for converting said square wave output to a d.c.
signal which at least over the working speed range decreases in
magnitude relative to the third supply line with increasing speed,
said d.c. signal being applied to the governor, said pump circuit
including a capacitor across which is developed a voltage
proportional to the frequency of the said a.c. output, said
capacitor being connected across the input and output terminals of
an operational amplifier.
Description
This invention relates to fuel systems for engines, particularly,
but not exlcusively, compression-ignition engines. The invention
further resides in electronic pump circuits for use in such systems
and for other purposes.
In one aspect, the invention resides in a fuel system for an
engine, comprising in combination a pump supplying fuel to the
engine, an actuator controlling the pump, and an electronic
governor for controlling the actuator, said governor receiving
electrical signals representing engine speed and at least one
further engine parameter, the speed signal being obtained using a
transducer producing an a.c. output at a frequency proportional to
engine speed, an a pump circuit for converting said signal to a
d.c. signal, said pump circuit including a capacitor across which
is developed a voltage proportional to the frequency of the a.c.
output, said capacitor being connected across the input and output
terminals of an operational amplifier.
In another aspect, the invention resides in an electronic pump
circuit in which a voltage dependent on the frequency of an a.c.
input is developed across a capacitor, characterised in that the
capacitor is connected across the input and output terminals of an
operational amplifier.
In the accompanying drawings,
FIG. 1 is a circuit diagram, partly in block form, illustrating one
form of fuel system with which the invention can be used,
FIGS. 2 to 4 are graphs illustrating the outputs of three
transducers used in FIG. 1,
FIG. 5 represents a fuel-speed characteristic for an engine to be
controlled by the arrangement of FIG. 1,
FIG. 6 is a view similar to FIG. 1 of a second form of fuel
system,
FIG. 7 is a view similar to FIG. 5 but showing the characteristic
obtained by FIG. 6,
FIG. 8 is a circuit diagram illustrating one example of the pulse
shaping circuit used in FIG. 1 or FIG. 6,
FIG. 9 is a circuit diagram illustrating a modification of the
arrangement shown in FIG. 8,
FIG. 10 is a circuit diagram illustrating one form of pump
circuit,
FIG. 11 is a circuit diagram illustrating another form of pump
circuit,
FIG. 12 is a circuit diagram of a third form of pump circuit,
and
FIGS. 13 to 15 are wave forms illustrating the operation of FIG.
12.
The examples described relate to a fuel injection system for a
diesel engine driving a road vehicle, so that demand is set by an
accelerator pedal. However, the arrangements shown can be used with
other engines, and the engine employed need not drive a road
vehicle, in which case the demand is of course set in some other
way.
Referring first to FIG. 1, a fuel pump 11 supplies fuel to the
cylinders of an engine 12 in turn, the fuel pump being driven in a
conventional manner, with the timing of injection controlled in the
usual way. The driving of the fuel pump forms no part of the
present invention and is not therefore described. Moreover, the
type of pump used is not critical, but in the example shown the
pump is a conventional in-line pump having a control rod 14 the
axial position of which determines the rate of supply of fuel to
the engine 12 by the pump 11. The axial position of the control rod
14 is controlled by an electro-mechanical actuator 13 to determine
the pump output.
The system further includes three transducers 15a, 16 and 17. The
transducer 15a produces an output having a frequency proportional
to engine speed, this output being fed by way of a pulse shaping
network 15b to a pump circuit 15c which produces an output in the
form of a voltage shown in FIG. 2, the magnitude of the voltage
being dependent on the rotational speed of the engine. The
transducer 16 produces an output voltage shown in FIG. 3 the
voltage being dependent on the rate of supply of fuel to the
engine, (i.e. the pump output). For this purpose the transducer 16
conveniently senses the axial position of the control rod 14 as
indicated by the dotted line. The transducer 17 produces a voltage
representing demand. Typically, the transducer 17 is controlled by
the accelerator pedal of vehicle which is driven by the engine, and
in the particular example being described, the engine is controlled
by an all-speed governor, so that the output from the transducer 17
is a voltage representing demanded engine speed. The form of this
voltage is shown in FIG. 4, and it should be noted that the slope
of this output is opposite to the slopes of the outputs from the
transducers 15. 16.
The outputs from the transducers 15, 16 and 17 are all applied, by
way of resistors 15d, 16a, 17a converting the signals to current
signals, to the inverting terminal of an operational amplifier 18
connected as a summing amplifier, whilst the output from the
transducer 16 is also connected through a resistor 16b to the
inverting terminal of an operational amplifier 19 connected as a
summing amplifier. The amplifier 18 and 19 are powered by positive
and negative supply lines 21, 22 and have their non-inverting
terminals connected to a line 23 which is kept at a reference
potential mid-way between the potentials of the lines 21, 22. The
origin of FIGS. 2 to 4 is the potential of the line 23, and the
supply for the power lines is derived from the vehicle battery.
The output from the amplifier 18 is fed through a diode 24 to a
drive circuit 25 which incorporates a power amplifier and which
serves to control the electro-mechanical actuator 13. Similarly,
the output termnal of the amplifier 19 is connected to the drive
circuit 25 through a diode 26. The diodes 24 and 26 together
constitute a discriminator, which ensures that only the amplifier
18, 19 producing the more positive output is coupled to the drive
circuit 25 at any given instant. Thus, if the amplifier 18 is
producing the more positive output, then the diode 26 is reverse
biased, and if the amplifier 19 is producing the more positive
output, the diode 24 is reverse biased. FIG. 1 also shows the
feedback resistors 27, 28 associated with the amplifiers 18, 19
respectively, and it will be noted that the feedback circuit for
each amplifier is taken from the input terminal of the drive
circuit 25. By virtue of this arrangement, the effective forward
voltage drop across the diodes 24 and 26 is reduced by a factor
dependent upon the amplifier open-loop gain, and so the temperature
characteristics of the diodes become negligible when considering
the temperature characteristics of the system. Also, there is a
very sharp changeover from control by one amplifier to contol by
the other amplifier.
The basic operation is as follows. The amplifier 18 compares the
input currents it receives and modifies the pump output until the
sum of the input currents is zero.
The amplifier 19 receives a signal by way of the resistor 16b
representing pump output and also receives a reference current from
a reference source 20. If the demanded pump output set by the
amplifier 18 exceeds a value set by the source 20, then the
amplifier 19 produces an output which is more positive than the
output of the amplifier 18, so that the diode 24 ceases to conduct
as previously explained and the amplifier 19 produces an output to
the drive circuit 25. It should be noted that an increasing output
from an amplifier 18 or 19 is in fact a demand for a decrease in
fuel, that is to say there is an inverting stage between the
amplifiers 18, 19 and the pump. When the amplifier 19 is producing
an output, the system operates in the same way as when the
amplifier 18 is producing an output to reduce the output of the
amplifier 19 to a value such that the output from the drive circuit
25 keeps the control rod 14 in the position it has assumed. The
system will stay in this condition until the amplifier 18 demands
less fuel than the maximum set by the amplifier 19. When the
amplifier 18 demands less fuel, it produces a greater positive
output than the amplifier 19, and so takes over the operation.
Referring now to FIG. 5, the way in which the governor is designed
and operates can be seen from the graph of pump output against
speed. This graph also shows the effect of a number of controls not
yet mentioned in relation to FIG. 1. The line 40 is set by the
amplifier 18 by virtue of the way in which the comparison of actual
and demanded speeds is modified in accordance with the input from
the transducer 16. The line 40 in the drawings represents 50%
demand, and is one of a family of lines stretching from 0% demand
to 100% demand. The extremes of this family, that is to say no
demand and full demand, are indicated at 38 and 43. The line 38 is
set by a current source 31 providing an input to the inverting
terminal of the amplifier 18, to ensure that the engine speed
varies with pump output in the manner indicated by the curve 38
even when the demand is zero. The maximum speed is set by a control
29 shown in FIG. 1 and which acts by limiting the maximum demand
from the transducer 17. The line 35 is the maximum fuel line which
is set by the amplifier 19 as previously explained.
The boundary line 39 is a function of the engine, not the governor,
and represents the no-load fuel requirements of the engine under
different demands, so that the points 41 and 42 are the no-load
engine speeds at zero and full demand, (i.e.) with the pedal
released and fully depressed respectively.
FIG. 5 explains how the engine will behave in any circumstances.
Suppose that the pedal has been set to demand 50%, corresponding to
the line 40 shown in FIG. 5. The exact position on the line 40 at
any given instant will depend upon the load on the engine, and so
for this given setting of the pedal, the engine speed can vary
within the limits set by the lines 35 and 40. The slope of the line
40 is, as previously explained, a result of the input to the
amplifier 18 from the transducer 16. Assuming that the engine is
operating at a particular point on the line 40, then if the vehicle
starts to go up an incline, the load will increase, and so for a
given position of the pedal the operating point will move up the
line 40, so that the speed is reduced. If the load becomes
sufficiently great, the line 35 will be reached, and no further
increase in pump output will be permitted. At this point, the speed
falls rapidly. If the load decreases, then the operating point
moves down the line 40 with the corresponding increase in speed. If
the load decreases to zero, the line 39 is reached.
If the demand is changed, then assuming for the sake of argument
that it changes from 50% demand to 100% demand, the pump output
will increase as rapidly as the pump and governor will allow until
the line 35 is reached, and the engine will then move along the
line 35 onto the maximum demand line 43, and will assume a position
on the line 43 which is dependent upon the load.
If the demand is reduced, then assuming the demand is reduced from
50 to 0%, the operating point will move downward until the fuel
supply is zero. The speed then decreases until the line 38 is
reached, after which the operating point moves up the line 38,
finishing at a point on the line 38 determined by the load on the
engine.
Turning now to FIG. 6, there is shown a second example in which the
governor is a two-speed governor, that is to say a governor in
which the demand signal is a fuel signal which is compared with the
actual fuel, the pump output then being modified to provide the
desired fuel output. In FIG. 6, the amplifier 18 receives a signal
from the transducer 16 by way of the resistor 16a, this signal
representing actual fuel. A signal representing demanded fuel is
fed by way of the resistor 17a to the amplifier 18, but it will be
noted that there is no speed term fed to the amplifier 18 by way of
the resistor 15d. The characteristics of the system are shown in
FIG. 7. The line 40a is one of a family of horizontally extending
lines which are set by the governor, and can be taken to represent
the 50% demand line. When the pedal sets a demand of 50%, the
amplifier 18 sets the required fuel level. The operating point on
the line 40a will of course then depend on the load on the
engine.
The amplifier 19 overrides the amplifier 18 in FIG. 6 in a similar
manner to the arrangement in FIG. 1, except that the amplifier 19
now receives a signal by way of the resistor 15d representing
speed, and also a reference current from a source 20a indicating
the maximum engine speed. The amplifier 19 sets the maximum speed
of the engine, which is indicated by the line 43 in FIG. 7. It will
be noted that the line 43 has a slope, that is to say the maximum
permitted speed varies with pump output. This slope is obtained by
feeding to the amplifier 19 a signal representing pump output, this
signal being fed by way of the resistor 16b.
The maximum pump output, that is to say the line 35 in FIG. 7, is
set by a control 29a which limits the maximum demand, in much the
same way as the control 29 limits the maximum speed in FIG. 1. The
minimum engine speed, indicated by the line 38, is set by a current
source 31a, which is similar to the current source 31 except that
because the current source 31a acts on the amplifier 18, which does
not receive a speed term, the current source 31a must receive a
speed term as indicated by its connection to the pump circuit
15c.
FIG. 8 shows one form of the pulse shaping circuit 15b. Referring
to FIG. 8, the circuit is powered by the supply lines 21, 22, 23.
The input to the circuit is taken from the transducer 15a and is
fed between an input terminal 45 and the line 23, and the output
from the circuit is taken between a terminal 46 and the line
23.
The terminal 45 is connected to the line 23 through a resistor 47
and a capacitor 48 in series, the junction of the resistor 47 and
capacitor 48 being connected to the line 23 through a resistor 49
and a capacitor 51 in series, and the junction of the resistor 49
and capacitor 51 being connected to the base of an n-p-n transistor
52. The transistor 52 has its collector connected to the line 21,
and its emitter connected to the emitter of a further n-p-n
transistor 53, and to the collector of an n-p-n transistor 54. The
emitter of the transistor 54 is connected through a resistor 55 to
the line 22, and the collector of the transistor 53 is connected
through a resistor 56 to the line 21. The base of the transistor 53
is connected to the line 23 through a resistor 57. The lines 23 and
22 are further interconnected through a resistor 58 and a pair of
diodes 59, 61 in series, and the junction of the resistor 58 and
diode 59 is connected to the base of the transistor 54.
The collector of the transistor 53 is connected to the base of a
p-n-p transistor 62 having its emitter connected to the line 21 and
its collector connected to the output terminal 46, to the collector
of the transistor 53 through a capacitor 63, and through a pair of
resistors 64 and 65 in series to the line 22. The junction of the
resistors 64 and 65 is connected to the base of the transistor 53.
Moreover, the terminal 46 is connected to the collector of an n-p-n
transistor 66, the emitter of which is connected through a resistor
67 to the line 22 and the base of which is connected through a
diode 68 to the line 23.
As will be seen from FIG. 1, the output between the terminal 46 and
the line 22 is applied to the pump circuit 15c, which can take a
number of forms, but usually includes a pair of diodes. The purpose
of the transistor 66, the resistor 67 and the diode 68 is merely to
provide two diodes which in operation will compensate for the
voltage drop across the diodes in the pump circuit, so that the
circuit is not affected by changes in temperature.
The a.c. signal from the transducer 15a is fed between the terminal
45 and the line 23, and is filtered by the resistor-capacitor
network 47, 48, 49, 51. The transistors 52, 53, 54 and 62 with
their associated components then produce a square wave between the
terminal 46 and line 22 in the following manner.
For ease of explanation, assume that the circuit is in one of its
stable states with the transistors 53, 54 and 62 conducting, the
terminal 46 approximately at the potential of the line 21. The
potential at the base of the transistor 53 at this stage is
determined by the resistors 57, 64 and 65. This is the state the
circuit assumes with the voltage at the terminal 45 relatively low.
As the voltage at the terminal 45 increases, the transistor 52
starts to conduct, and since the transistor 54 acts as a constant
current source, current flow through the transistor 53 is reduced.
Since the base current for the transistor 62 flows through the
transistor 53, conduction of the transistor 62 is alos reduced, so
that the base potential of the transistor 53 is varied, and the
transistors 53 and 62 switch off rapidly by regenerative action. It
will be appreciated that the rate at which the transistor 53
switches off is extremely rapid, and is not dependent on the rate
at which the voltage is rising at the terminal 45 once the
transistor 52 has started to conduct. Once the transistor 62 is
off, then the potential at the terminal 46 is equal to the
potential of the line 23, less the voltage drop across two diodes,
namely the diode 68 and the base-emitter diode of the transistor
66, these diodes compensating for the diodes in the pump circuit as
previously explained.
The resistors 64, 65 and 57 are selected so that the base potential
of the transistor 53 when the transistor 62 is conducting is at a
predetermined voltage above the line 23, and is at the same
predetermined voltage below the voltage of the line 23 when the
transistor 62 is off. As the input to the terminal 45 now falls, a
stage is reached at which the transistor 52 conducts less, and the
transistor 53 starts to turn on. As soon as the transistor 53
starts to turn on, it provides base current to the transistor 62,
and by regenerative action the circuit again switches rapidly,
independently of the rate of fall of voltage at the terminal 45, to
the opposite state in which the transistors 53 and 62 are on and
the terminal 46 is at the potential of the line 21. Thus, the
circuit produces a square wave output which in this example has a
mark-space ratio of approximately unity. The use of the transistor
54 as a constant current source permits operation over a wide range
of supply voltage between the lines 21, 22. The minimum amplitude
of the square wave is set by the resistors 57, 64, 65.
Turning now to FIG. 9, there is shown a modification in which both
halves of each cycles of the input at terminal 45 are used. In FIG.
9, the circuit connections are not shown in detail, but are exactly
the same as in FIG. 8, except for some additional components
associated with the transistor 52. Thus, the transistor 52 now has
its collector connected to the line 21 by way of a resistor 81, and
its collector also connected to the base of a p-n-p transistor 83
with its emitter connected to the line 21 through a resistor 84 and
its collector connected to a terminal 46a and further connected to
the collector of an n-p-n transistor 85, the base of which is
connected through a diode 86 to the line 23 and the emitter of
which is connected through a resistor 87 to the line 23. It will be
seen that the transistors 83 and 85 and their associated components
are equivalent to the transistors 62 and 66 and their associated
components in FIg. 8, and that the switching of these transistors
is effected in accordance with the voltage across the resistor 81,
in the same way as the switching of the transistors 62 and 66 is
effected by the voltage across the resistor 56 in FIG. 8. The
effect is that the wave form at the terminal 46a is complementary
to the wave form at the terminal 46. Obviously by taking an output
from the terminals 46 and 46a the circuit can be made to have a
more rapid response, or alternatively can have the same response
rate for a lower frequency input at terminal 45.
In some examples, it is preferred that the diode 68 in FIG. 8, and
its equivalent diode 86 in FIG. 9, should have it anode connected
to the junction of a pair of resistors connected between the lines
23, 22.
FIG. 10 illustrates one form of pump circuit which is intended for
use with the arrangement of FIG. 8. Referring to FIG. 10, the
terminal 46 seen in FIG. 8 is connected to the line 23 through a
series circuit including a resistor 91, a capacitor 92 and the
cathode-anode part of a diode 93. The junction of the capacitor 92
and diode 93 is connected through the anode-cathode path of a diode
94 to the inverting input terminal of an operational amplifier 95
having its non-inverting input terminal connected to the line 23.
The output terminal of the amplifier 95 is connected to the
resistor 15d, which provides the required input to the circuit as
described with reference to FIG. 1 or FIG. 6. The feedback between
the output terminal of the amplifier 95 and its inverting input
terminal includes a resistor 97 and a capacitor 96 in parallel and
moreover the inverting input terminal of the amplifier 95 is
connected to the line 22 through a resistor 98.
If the capacitor 96 and its associated conventional discharge
resistor 97 were to be connected in parallel between the inverting
input terminal of the amplifier 95 and the line 23, then the
arrangement would constitute a conventional diode pump circuit
followed by an amplifier. With such an arrangement, the capacitor
96 would acquire a charge dependent upon the frequency of the
signal at the terminal 46, but the charge would not necessarily be
directly proportional to the frequency at the terminal 46. However,
by using the capacitor 96 in the position shown, the voltage
developed across the capacitor 96 is proportional to the frequency
of the input signal at the terminal 46.
Without the resistor 98, the arrangement of FIG. 10 would produce
an output which for zero frequency would be at the potential of the
line 23, and then would decrease towards the potential of the line
22 as the frequency increased. As will be seen with reference to
FIG. 2, this is not the characteristic required, and the addition
of a bias by way of the resistor 98 lifts the curve to the correct
position, as shown in FIG. 2.
The arrangement of FIG. 11 is similar to that shown in FIG. 10, but
is designed for use with the circuit of FIG. 9. The components 91,
92, 93 and 94 are duplicated in FIG. 11, and are indicated with the
same reference numerals and the suffix A. The operation is
indentical to that of FIG. 10, except tht the input frequency to
the amplifier 95 is doubled.
It is to be understood that the particular pulse shaping circuit
described has advantages even when used with a conventional diode
pump circuit. Moreover, the particular pump circuit described has
advantages with or without the particular form of pulse shaping
circuit. Additionally, the pump circuit described can be used in
applications other than fuel systems.
It is a matter of some importance that the engine speed should not
exceed its maximum value, becuase if it does serious damage could
result. It will be noted that in the event of a failure in the pump
circuit of FIG. 10 or FIG. 11 resulting in a low output voltage at
the resistor 15d, then this fault will be interpreted as a high
engine speed, so that the circuit will reduce the engine speed, and
no damage will result. However, a fault resulting in a high output
voltage at the resistor 15d will be interpreted as a low engine
speed, and this is a potentially dangerous situation. This
difficulty can be overcome by monitoring the output of the
amplifier 95, so that if the amplifier fails action is taken to
prevent damage. However, one particular convenient way of achieving
the desired effect is shown in FIG. 12. The arrangement of FIG. 12
is shown for convenience as applied to a pump circuit for use with
the arrangement of FIG. 8.
Referring to FIG. 12, the arrangement is similar to that shown in
FIG. 10 with the omission of the resistor 98. In addition, however,
the junction of the resistor 91 and capacitor 92 is connected to
the line 23 through a capacitor 101 and the anode-cathode part of a
diode 102 in series. The junction of the capacitor 101 and diode
102 is connected to the line 23 through the cathode-anode part of a
diode 103 and a capacitor 104 in series, and the junction of the
diode 103 and capacitor 104 is connected through a resistor 105 to
the inverting input terminal of the amplifier 95.
The operation of the arrangement shown in FIG. 12 is best explained
with reference to the wave forms in FIGS. 13, 14 and 15. Without
the input from the resistor 105, the amplifier 95 would have an
output of the form shown in FIG. 13 (remembering that the resistor
98 is not present). The capacitors 101, 104 and their diodes 102,
103 form a conventional diode pump circuit producing an input to
the amplifier 95 which is out of phase with the input through the
diode 94. The form of the resultant output of the amplifier 95 is
shown in FIG. 14, and it will be seen that it rises exponentially
to a maximum value. The actual output of the amplifier 95 is the
sum of the outputs shown in FIGS. 13 and 14, and is shown in FIG.
15. The output rises to a maximum voltage at the point 105 and
falls to zero at the point 106. The portion of the curve between
the points 105, 106 is substantially linear, and the curve shown in
FIG. 15 replaces the curve shown in FIG. 2. The points 106 and 105
are then close to the maximum and minimum engine speeds
respectively, and in normal operation the output from the amplifier
95 will be between the points 105, 106. It will be seen that the
form of the curve produced ensures that both possible fault
conditions of the amplifier 95 and the preceding circuits result in
a low voltage output, so that the circuit fails safe. It will of
course be understood that the arrangement of FIG. 12 can be used
anywhere where it is desirable for a maximum output to be obtained
at a particular frequency, this frequency being represented by the
point 105 in FIG. 15.
It is not necessary for the resistor 105 to provide an input to the
amplifier 95. Where the circuit is being used with the arrangement
of FIG. 1, then if the diodes 102, 103 are appropriately connected
the resistor 105 can instead provide an input to the inverting
input terminal of the amplifier 18. Similarly, where the
arrangement is used with FIG. 6, the resistor 105 can be used to
provide an input to the inverting input terminal of the amplifier
19. Because the amplifier 95 and the amplifier 18 (or the amplifier
19 in FIG. 6) successively sum their input signals, it will be
appreciated that the overall effect of providing the input from the
second pump circuit in FIG. 12 to one of the amplifiers 18 or 19
does not alter the operation of the circuit.
* * * * *