Glow plug heating control apparatus for a diesel engine

A control apparatus for diesel engines controls the energization of glow plugs each of which is disposed in a cylinder of a diesel engine and heated by an energizing current flowing therethrough. When fuel supply to the engine is resumed after the engine temperature has fallen due to the interruption of fuel supply during a decelerating operation of the engine and further when fuel supply is enriched during an accelerating operation of the engine while it is cold, the control apparatus controls the energization of the glow plugs, thereby promoting the combustion of the engine and improving the combustion efficiency of the engine.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a control apparatus for diesel engines 
equipped with glow plugs which are energized to generate heat, and more 
particularly to a control apparatus for a diesel engine which control the 
heat generation of the glow plugs in association with the control of the 
combustion stroke of the engine. 
2. Description of the Prior Art 
In the past, glow plugs have been used in a diesel engine mainly for the 
purpose of preheating before the start of the engine and the after glow 
operation after the start of the engine. Japanese Utility Model 
Application Laid-Open (Kokai) No. 56-39868 may for example be cited as an 
example of such prior art glow plugs. 
However, conventional control apparatuses have been unable to meet the 
requirements for various operating conditions of a diesel engine. In other 
words, the operating condition of a diesel engine changes from time to 
time in accordance with environmental conditions and demands of a driver, 
so that, under these various operating conditions, e.g., during a 
decelerating operation of a vehicle mounted with the diesel engine, where 
cut-off of fuel supply to the engine occasionally causes a rapid drop in 
the engine temperature, conventional control apparatuses have been unable 
to meet effectively the requirements for such an operating condition of 
the engine. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a control apparatus for 
diesel engines which can solve the foregoing problem in the prior art. 
To accomplish the above object, on the basis of the results of experiments 
which showed that the temperature of a diesel engine falls rapidly in 
connection with particular fuel supply conditions, it is a feature of this 
invention to provide a control apparatus which is connected to an 
energization circuit for glow plugs so that the glow plugs are energized 
in response to the respective conditions in which fuel is supplied to the 
engine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described with reference to an embodiment 
thereof shown in the accompanying drawings. FIG. 1 shows the overall 
construction of a control apparatus for diesel engines embodying the 
present invention. Numeral 10 designates a digital computer constituted by 
a microcomputer. In the computer 10, a central processing unit 11 performs 
a series of computing operations in accordance with the control program 
and control functions which are stored in a program memory (ROM) 12. The 
input data required for the operations performed by the CPU 11 are taken 
in from external input units through an input buffer 13 having a 
multiplexer and an A-D converter circuit 14 in the execution of 
instructions determined by the control program in the course of execution 
of the control program. In addition, the CPU 11 supplies control output 
signals to external apparatuses through an output buffer 15 under 
specified conditions in the execution of instructions determined by the 
control program. 
A fuel injection device 16 is shown as an operating element for a diesel 
engine which element is controlled by the computer 10. The fuel injection 
device 16 is of a known construction including a first operating unit 17 
responsive to an electric signal for adjusting the quantity of fuel 
injection and a second operating unit 18 responsive to another electric 
signal for adjusting the timing of fuel injection. 
Further, glow plugs 19 are provided as a unit for preheating the diesel 
engine which unit is also controlled by the computer 10. There are 
provided the same number of glow plugs as the engine cylinders, and each 
of the glow plugs is arranged in respective one of the engine cylinders. 
The glow plugs 19 are connected electrically in parallel with one another. 
They have a positive resistance-temperature coefficient, and, for example, 
they have a characteristic such that the value of their resistances 
increases linealy with an increase in the temperature thereof. 
An energizing circuit for the glow plugs 19 is formed to supply an electric 
current to the glow plugs 19 from a dc battery 20 via a current adjusting 
circuit 21 and a current detecting resistor 22 having a small resistance 
value of a preselected resistance-temperature coefficient. The current 
adjusting circuit 21 comprises a first switch 23, a series circuit of a 
second switch 24 and a current limiting resistor 25, whereby an electric 
current is supplied to the glow plugs 19 directly from the battery 20 when 
the first switch 23 is closed and an electric current is supplied to the 
glow plugs 19 from the battery 20 through the current limiting resistor 25 
when only the second switch 24 is closed. When both switches 23 and 24 are 
open, no current is supplied to the glow plugs 19. Thus, the current 
adjusting circuit 21 switches the value of the energization current 
between two levels by opening and closing the first and second switches 23 
and 24. The resistance values of the glow plugs 19 and the current 
limiting resistor 25 are selected so that, upon closing the first switch 
23, the glow plug temperature increases rapidly due to a relatively large 
energization current thereby to exceed an upper limit of a desired 
temperature range, while, upon closing only the second switch 24, a 
relatively small current flows thereby to decrease gradually the glow plug 
temperature and stabilize it at a value which is around a lower limit of 
the desired temperature range or below. 
The energization of the glow plugs 19 is controlled by the computer 10 in 
connection with the control of the fuel injection device 16 so that the 
quantity of their heat generation is thereby controlled. Although the 
description of this embodiment does not refer thereto in detail, the 
energization of the glow plugs 19 is controlled by the computer 10 for the 
purpose of the heating before the start of the engine and the after glow 
operation after the start of the engine in the same way as conventional 
glow plugs do. As means for energizing the glow plugs 19, in accordance 
with this invention the glow plugs 19 can be used in combination with any 
temperature adjusting means provided that a suitable quantity of heat 
generation is ensured. In this embodiment, the temperature of the glow 
plugs 19 due to their own heat generation as well as the heat generated by 
the engine operation is measured, and the measured value is compared with 
a preset desired value, thereby controlling the current adjusting circuit 
21 to bring the glow plug temperature within the desired temperature 
range. 
This embodiment provides a method for determining the resistance value of 
the glow plugs 19 as one of the methods for measuring the glow plug 
temperature. For this purpose, a voltage drop across the current detecting 
resistor 22 and the potential at the junction point of the current 
detecting resistor 22 and the glow plugs 19 are utilized. In this case, 
the voltage drop across the current detecting resistor 22 is amplified by 
a differential amplifier 26, and, under the instruction of the CPU 11, the 
output voltage of the differential amplifier 26 and the junction point 
potential are converted to digital values through the input buffer 13 and 
the A-D converter circuit 14 and the digital values are supplied to the 
CPU 11. In the first place, the CPU 11 stores the digital values in an 
internal temporary memory RAM therein (not shown), and then it computes 
the glow plug temperature on the basis of the digital values in accordance 
with the control program. 
Electromagnetic actuators 27 and 28 are provided to effect the opening and 
closing of the first switch 23 and the second switch 24 of the current 
adjusting circuit 21, respectively, in response to output signals of the 
computer 10. 
The computer 10 is connected to necessary external signal generating means 
through the input buffer 13 in order to control the fuel injection device 
16 and the glow plugs 19. In this embodiment, the computer 10 is connected 
to a temperature sensor 29 for detecting the cooling water temperature of 
the engine, an intake air temperature sensor 30 for detecting the intake 
air temperature of the engine, a throttle valve opening sensor 31 for 
detecting an engine load and an engine rotation sensor 32 for detecting an 
engine speed. These sensors 29 to 32 are publicly known in the art. 
FIG. 2 shows schematically the control program executed by the CPU 11, that 
is, a series of processing steps which are executed by the computer 10 in 
accordance with the control program preset in the ROM 12. In the like 
manner as a general control program, the control program comprises a 
combination of inputting, processing and outputting steps, and the steps 
of the operational processings executed in accordance with the control 
program of this invention will now be described with reference to the 
accompanying drawings. 
When the engine key switch (not shown) is closed, the computer 10 is fed 
from a vehicle-mounted battery through a suitable voltage regulator 
circuit, and the execution of the control program is started at a start 
step where the power supply is turned on. Then, at an instruction step 
designated by a step 100, the internal temporary memory (RAM), registers, 
input/output ports, etc. are set into a predetermined initial state 
(initialization). 
At steps 101 through 104, the computer 10 receives through its input 
circuit necessary data for determining the fuel injection quantity and 
fuel injection timing. Firstly, at the step 101 the temperature data from 
the engine cooling water temperature sensor 29 is converted to a digital 
value through the input buffer 13 and the A-D converter circuit 14 and 
then stored in the RAM of the CPU 11. This digital value is hereinafter 
referred to as a digital value Tw. At the step 102, the intake air 
temperature data from the intake air temperature sensor 30 is similarly 
received as a digital value Ta. At the step 103, the throttle opening data 
from the throttle valve opening sensor 31 is received as a digital value 
.theta.a. At the step 104, the data based on the pulse train signal from 
the engine rotation sensor 32 is received as a digital value N. Only, as 
regards the engine speed data N, the input buffer 13 reshapes the pulse 
train signal from the sensor 32 to make the period of the pulse train 
signal clear and then transfers the data to the CPU 11 without passing 
through the A-D converter 14. The CPU 11 computes the period of the pulse 
train signal in accordance with a known time measuring program and stores 
the result of the computation as the digital value N in the temporary 
memory. 
A step 105 generally shows a procedure for the control of the fuel 
injection device 16 by the computer 10. At this combustion stroke control 
step 105, the computer 10 determines optimum values of the fuel injection 
quantity and fuel injection timing in accordance with the digital values 
Tw, Ta, .theta.a and N received from the input circuit. A known method for 
determining basic values from the throttle valve opening .theta.a and the 
engine speed N may be adopted in a program for determining the fuel 
injection quantity and fuel injection timing. 
In addition, the combustion stroke control step 105 includes several known 
correcting operations performed according to control conditions. For 
example, the step 105 may perform correcting operations for correcting the 
basic values in accordance with the cooling water temperature Tw and the 
intake air temperature Ta to obtain the final fuel injection quantity and 
fuel injection timing. Further, the step 105 includes a fuel supply 
interruption processing for the decelerating operation of the engine and a 
fuel supply enrichment operation for the cold engine acceleration, both of 
which are also known in the art. 
In this case, the fuel supply interruption processing compares the data 
.theta.a stored in the temporary memory with preset reference data to 
decide a condition that the throttle valve opening .theta.a is smaller 
than a preset opening value (corresponding to a substantially fully closed 
state). When this condition is detected, the CPU 11 fixes the fuel 
injection quantity to a value of zero or nearly zero independently of the 
result of the computation of the basic values. 
On the other hand, the fuel supply enrichment operation is performed on the 
basis of both decisions that the engine is cold and that the opening rate 
of the throttle valve opening .theta.a is great. The decision as to 
whether the engine is cold or not is made, for example, by comparing the 
cooling water temperature data Tw stored in the temporary memory with 
preset reference data and/or comparing the intake air temperature data Ta 
with preset reference data, thereby determining whether the former is 
lower than the latter. While, the decision on the throttle valve opening 
rate is made by comparing a quantity of a change .DELTA..theta.a in the 
throttle valve opening data .theta.a taken at preset time intervals (the 
quantity of the changes is computed sequentially) with preset reference 
data, thereby determining whether the opening rate of the throttle valve 
is great or not. When these conditions are satisfied, during a 
predetermined time or a suitable time the CPU 11 increases the fuel 
injection quantity by adding to the basic fuel injection quantity a preset 
increment and/or, when required, an increment determined in accordance 
with the cooling water temperature Tw and the opening rate of the throttle 
valve opening .theta.a. 
At a step 106, the CPU 11 decides whether the first glow plug energization 
control step (I) at a next step 107 should be executed. The decision is 
made on the following three items. The first item is to decide whether it 
is proper timing for performing the processing of the step 107 in 
accordance with a count of a timer counter which is not shown. Here, it is 
arranged that the processing of the step 107 is performed at intervals of 
50 msec, for example. The second item is to decide whether the fuel supply 
interruption processing has terminated (has been reset) after the 
processing has once been performed. The third item is to decide whether 
one sequence of processing at the step 107 has been completed after the 
initiation of the processing at the step 107. 
When the result of the decision on the three items at the step 106 is YES, 
upon resetting of the fuel supply interruption processing, the computer 10 
executes at the step 107 the first energization control step (I) at 
predetermined time periods until a sequence of processings is completed. 
When the result of the decision at the step 106 is NO, the processing 
jumps to a step 108. The details of the first energization control step 
(I) at the step 107 will be described later with reference to FIG. 3. 
At the step 108, the CPU 11 decides whether a second glow plug energization 
control step (II) at a next step 109 should be executed. The decision is 
made on the following three items. The first item is to decide whether the 
second energization control step (II) at the step 109 is executed, for 
example, at intervals of 50 ms. The second item is to determine whether 
the processing of the fuel supply enrichment operation has once been 
initiated. The third item is to determine whether a sequence of 
processings of the step 109 has been completed. Upon initiation of the 
fuel supply enrichment operation for cold engine acdeleration, the 
computer 10 executes at the step 109 the second energization control step 
(II) at predetermined time periods until a sequence of processings is 
completed. The details of the second energization control step (II) at the 
step 109 will also be described hereunder with reference to FIG. 3. 
FIG. 3 shows the details of the processing steps of the first and second 
energization control steps (I) and (II) at the steps 107 and 109, 
respectively, shown in FIG. 2. It should be noted that FIG. 3 shows a 
common example for explaining the processings of the steps 107 and 109, 
but each of the steps 107 and 109 is formed as a separate program. 
However, a temperature computing step 111, for example, may be formed as a 
commonly available subroutine program. 
When, for example, a condition is established for executing the first 
energization control step 107, the computer 10 starts the processing at a 
point A.sub.1 and executes a sequence of processings until it terminates 
at a point A.sub.2. If a point B.sub.2 is reached in the course of 
execution, the processing of the step 107 in this time ends. However, 
since the fact of reaching the point B.sub.2 is retained in the temporary 
memory, the next processing of the step 107 starts from a point B.sub.1. 
Further, if a point C.sub.2 is reached in the course of execution, the 
next processing of the step 107 starts from a point C.sub.1. The above 
fact applies in the same way to the processing of the second energization 
control step 109. 
In FIG. 3, the computer 10 starts the energization control from a step 110. 
In accordance with the instruction stored in the step 110 of the control 
program, the CPU 11 energizes the electromagnetic actuator 27 through the 
output buffer 15 to close the first switch 23 of the current adjusting 
circuit 21. In this case, the electromagnetic actuator 28 may also be 
energized to close the second switch 24 at the same time. The current 
adjusting circuit 21 supplies an electric current to the glow plugs 19 
from the battery 20 through the current detecting resistor 22. Since the 
resistance value of the current detecting resistor 22 is small and the 
rated voltage of the glow plugs 19 is selected to be lower than the normal 
voltage of the battery 20, each of the glow plugs 19 generates heat 
immediately and rises rapidly to a high temperature, thus reaching the 
upper limit of the desired temperature range in a few seconds. 
As soon as the computer 10 has energized the electromagnetic actuator 27, 
the computer 10 performs the computation of the temperature T of the glow 
plugs 19 at the step 111. This computation is performed in accordance with 
the following procedure: 
(1) The signals indicative of the voltage drop V across the current 
detecting resistor 22 and the potential E at the junction between the 
current detecting resistor 22 and the glow plugs 19 are successively 
supplied through the input buffer 13 and the A-D converter circuit 14 to 
the CPU 11 and stored in the temporary memory. 
(2) In accordance with the preliminarily known resistance value (e.g., 10 
m.OMEGA.) of the current detecting resistor 22 and the input data of the 
voltage drop (V), the value (I) of an electric current flowing through the 
glow plugs 19 is computed by using the following equation: 
EQU I(A)=100.multidot.V(V) 
(3) Where the number of the glow plugs 19 is 4, the resistance value 
R.sub.T of a glow plug 19 is computed by using the following equation and 
in accordance with the current value I and the input data of the potential 
E: 
##EQU1## 
(4) In accordance with the resistance-temperature coefficient of the glow 
plugs 19 represented by the formula R.sub.T =K.multidot.T+C (where K and C 
are constants), the glow plug temperature T is computed by using the 
following formula: 
EQU T=(R.sub.T -C)/K 
Since the constants K and C are known preliminarily, the foregoing 
computation procedure may be shown in a simpler form. 
The computer 10 compares the temperature T of the glow plugs 19 computed at 
the step 111 with a reference value T.sub.1 indicative of the upper limit 
of a preset desired temperature range. If the glow plug temperature has 
not yet reached the upper limit, the processing of the energization 
control step is stopped for a time at the point B.sub.2. Then, upon 
expiration of 50 msec, the processing of the temperature computing step 
111 is resumed from the point B.sub.1 to decide whether the actual glow 
plug temperature has reached the upper limit. The processing of the steps 
111 and 112 are repeated periodically until the glow plug temperature 
reaches the upper limit. 
Further, in order to provide against a case where the processing continues 
passing through the point B.sub.2 even after the lapse of a predetermined 
time from the generation of an instruction to energize the electromagnetic 
actuator 27 at the step 110 (namely, after the start of the processing of 
the energization control step), it is possible to add a fail-safe program 
for overriding the decision of the step 112 to de-energize the 
electromagnetic actuator 27. 
When the temperature of the glow plugs 19 rises rapidly and it is decided 
that the desired upper limit temperature is reached, the processing of the 
CPU 11 advances from the step 112 to the step 113. At the step 113, the 
computer 10 produces an output control signal through the output buffer 15 
to cause the electromagnetic actuator 27 to be de-energized and only the 
electromagnetic actuator 28 to be energized. Accordingly, in the current 
adjusting circuit 21 only the second switch 24 is closed, so that an 
electric current is supplied to the glow plugs 19 from the battery 20 
through the current limiting resistor 25 and the current detecting 
resistor 22. As a result, the glow plug temperature falls gradualy toward 
a value near the lower limit of the desired temperature range and it 
remains stabilized there. Steps 114 through 117 act to determine the 
stabilized preheating time. 
At the step 114, the CPU 11 computes the stabilized preheating time. Here, 
the CPU 11 may simply set a predetermined time. If necessary, the 
stabilized preheating time may be varied in accordance with the operating 
conditions of the engine. For instance, the stabilized preheating time may 
be increased as the cooling water temperature Tw or the intake air 
temperature Ta decreases. At the step 115, the CPU 11 starts the counting 
of an internal timer counter, and at the step 116 the CPU 11 checks 
whether the count value of the timer counter has reached a value 
corresponding to the time determined at the step 114. Also in this case, a 
similar procedure of causing the processing of the program to exit at the 
point C.sub.2 and to resume at the point C.sub.1 is repeated until the 
stabilized preheating time elapses. When the stabilized preheating time 
has elapsed, at the step 117 the CPU 11 produces an instruction to 
de-energize the electromagnetic actuator 28 through the output buffer 15, 
and thereby the glow plug energization through the second switch 24 
ceases. Thus, the sequence of the glow plug energization control steps is 
completed. 
Further, in the first energization control step 107 for the fuel supply 
interruption processing and the second energization control step 109 for 
the fuel supply enrichment operation, it is possible to change the set 
value for the upper limit value T.sub.1 at the step 112 and/or the set 
value of the stabilized preheating time at the step 114, as occasion 
demands. 
As described hereinabove, it is possible to improve the combustion 
efficiency of the engine by supplying an electric current to the glow 
plugs 19 through the energization of the first switch 23 and second switch 
24 when fuel supply is resumed after the interruption thereof and during 
the cold engine acceleration, respectively, in connection with the engine 
operating conditions, particularly the fuel injection quantity. 
Now summarizing the above-described embodiment, when fuel supply is 
enriched during the cold engine acceleration or the like, an endothermic 
action due to the fuel vaporization increases. However, by energizing the 
glow plugs to be heated thereby, it is possible to promote the combustion 
in an engine and to improve its acceleration performance. On the other 
hand, during the deceleration of an engine-mounted vehicle, especially 
when the vehicle goes down on a long descent with its accelerator pedal 
released, fuel supply is generally cut off and the cylinder temperature 
falls gradually. However, upon resumption of fuel supply after the 
interruption thereof, the energization of the glow plugs to heat them by 
the apparatus of this invention can promote the combustion efficiency of 
the engine. 
Further, the present invention is not limited to the foregoing embodiment. 
For instance, in the construction of the above-described embodiment, it is 
possible to supply an electric current to the glow plugs 19 by using only 
one or the other of the first switch 23 and second switch 24 and to 
determine the energization time simply to be a suitable constant time 
instead of making a decision as to whether the desired temperature has 
been reached or not, as mentioned previously. Further, the present 
invention is also applicable to a case where the switching means in the 
glow plug energizing circuit comprises a single switch instead of a 
plurality of switches, namely, a first switch and a second switch. 
Further, the present invention is also applicable to cases where the 
energizing circuit comprises one or more semiconductor switches to effect 
chopper control of the energizing current of the glow plugs or where the 
conductivity of the semiconductor switches is controlled. Further, in the 
present invention, instead of using a microcomputer, an engine control 
computer and a separately constructed glow plug energization control 
circuit may be used, whereby a fuel supply interruption signal and/or a 
fuel supply enrichment signal are supplied from the engine control 
computer and in accordance with these signals the glow plugs are energized 
for a predetermined time or for a time period determined according to the 
engine cooling water temperature. From the foregoing, it will be seen that 
the present invention has a great advantage such that the combustion 
efficiency of a diesel engine can be improved by energizing the glow plugs 
arranged therein in accordance with the operating conditions of the 
engine.