Boost pressure control system for an engine

A boost pressure control system for a vehicle engine equipped with a supercharger and a torque converter having a lock-up clutch, in which the boost pressure is controlled by adjusting the degree of the opening of an air bypass valve and the ON/OFF position of the supercharger in such manner that the different control characteristics of the boost pressure are selected in accordance with whether the lock-up clutch of the torque converter is turned ON or OFF, i.e., when the lock-up clutch is turned ON, the supercharger is started at a lower engine load, and the degree of the opening of the air bypass valve is set smaller, thereby enabling the lock-up operation of the torque converter under running conditions in which a higher driving torque is required and, thereby widening the operating range of the lock-up operation of the torque converter and improving the fuel economy of the vehicle.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to a boost pressure control system for a 
vehicle engine equipped with a torque converter having a lock-up clutch. 
Description of the Related Art 
A torque converter having a lock-up clutch is commonly used for a vehicle 
engine. 
The lock-up clutch is used for mechanically coupling the input shaft and 
output shaft of the torque converter when enhancement of the driving 
torque is not required, thereby avoiding a power loss and an increasing in 
the fuel consumption as a result of a "slippage loss" of the torque 
converter (i.e., transmission loss caused by a difference between the 
speeds of the input shaft and the output shaft of the torque converter). 
Usually, the lock-up operation of the torque converter is carried out 
(i.e., the lock-up clutch is turned "ON") under predetermined running 
conditions defined by engine load parameters such as the degree of 
throttle opening of the engine and the running speed of the vehicle. From 
the view point of fuel economy, it is preferable to set the region of the 
lock-up operation of the torque converter as wide as possible. 
Also, in the vehicle engine equipped with a super charger driven from an 
engine output shaft, it is known to provide a power transmission clutch 
between the engine output shaft and the supercharger, and an air bypass 
passage bypassing the supercharger for controlling the boost pressure of 
the engine. 
In such an engine, the boost pressure is controlled by connecting or 
disconnecting (i.e., ON or OFF) the power transmission clutch ( 
hereinafter, the power transmission clutch is referred to as the 
supercharger clutch ) and by adjusting the degree of opening of an air 
bypass valve disposed in the air bypass passage (see Japanese Unexamined 
Patent Publication No. 62-276220) 
Namely, when the engine is operated under lower load conditions in which 
supercharging is not required, the supercharger clutch is turned OFF 
(i.e., disconnected) to stop the operation of the supercharger, thereby 
reducing power loss incurred by driving the supercharger, and improving 
the fuel consumption of the engine. When the engine is operated under 
higher load conditions, in which supercharging is required, the 
supercharger clutch is turned ON to start the operation of the 
supercharger and, also the degree of opening of the air bypass valve is 
decreased as the engine load increases, so that the boost pressure becomes 
higher in the higher engine load region thereby increasing the engine 
boost pressure. 
Japanese Unexamined Patent Publication No. 61-101622 discloses a boost 
pressure control system for an engine equipped with a supercharger that is 
used in combination with a torque converter having a lock-up clutch. 
Since the lock-up clutch is used to terminate the torque enhancing effect 
of the torque converter, the driving torque of the vehicle decreases when 
the lock-up clutch is turned ON. On the other hand, when the supercharger 
is operated, the engine output torque increases due to an increase in the 
inlet air pressure. Therefore, if the lock-up clutch and the supercharger 
begin operating successively over a short period of time, a sudden 
decrease in the engine speed owing to the operation of the lock-up clutch 
and a sudden increase in engine speed owing to the operation of the 
supercharger occur over a short period of time, which deteriorates the 
driveability of the vehicle significantly. 
The boost pressure control system disclosed in Japanese Unexamined Patent 
Publication No. 61-101622 prevents this deterioration in the driveability 
by initiating the ON / OFF operation of the lock-up clutch and start / 
stop of the supercharger at the time when the ON / OFF operation of the 
lock-up clutch is required by a shift down operation of an automatic 
transmission and if the engine load condition enters the operating region 
of the supercharger due to said shift down operation of the automatic 
transmission. 
In prior arts, when an engine equipped with a supercharger is used in 
combination with a torque converter having a lock-up clutch, the load 
conditions thereby initiating an ON / OFF operation of the lock-up clutch 
and the load conditions thereby starting/stopping the supercharger are 
determined independently from each other. In other words, the supercharger 
in the prior arts starts or stops at the same load conditions regardless 
of whether the lock-up clutch is turned ON or OFF. Though an ON / OFF 
operation of the lock-up clutch and start / stop operations of the 
supercharger are initiated simultaneously in the boost pressure control 
system of Japanese Unexamined Patent Publication No. 61-101622, the load 
conditions themselves initiating such operations are determined 
independently from each other. 
However, even during the operating conditions in which the engine load is 
too low for the lock-up operation of the torque converter without 
operating the supercharger, the lock-up operation of the torque converter 
may become possible if the engine torque is increased by starting the 
operation of the supercharger. 
Further, usually slippage loss incurred by a non-lock-up operation of the 
torque converter (i.e., an operation in which the lock-up clutch is turned 
OFF) increases the fuel consumption of the engine by an amount larger than 
the power loss incurred by operating the supercharger. 
In the prior art, there are cases in which the torque converter is operated 
under non-lock-up conditions, even though the lock-up operation of the 
torque converter can be carried out if the supercharger is started. 
Accordingly, in such cases, fuel economy is impaired compared to the case 
in which the lock-up operation of the torque converter is carried out by 
starting the supercharger. 
To prevent this, the load conditions of the start/stop operation of the 
supercharger can be set so that the supercharger is started at a lower 
engine load thereby enabling the lock-up operation of the torque 
converter. 
However, if the operation range of the supercharger is extended to a lower 
engine load range irrespective of other operating conditions, the fuel 
economy may be impaired because of the power loss resulting from 
unnecessary operation of the supercharger. Also, if the load conditions 
starting or stopping the supercharger are determined regardless of the 
ON/OFF state of the lock-up clutch, an ON/OFF operation of the lock-up 
clutch is initiated under operating conditions in which the difference in 
driving torque caused by an ON/OFF operation of the lock-up clutch is very 
large. If the ON/OFF operation of the lock-up clutch is initiated under 
such conditions, a large torque shock occurs and driveability 
deteriorates. 
The above problems are depicted by FIG. 2A, which shows an example of the 
boost pressure control in the prior arts. 
In FIG. 2A, the horizontal axis represents the degree of opening of the 
throttle valve of the engine (i.e., the load of the engine), and the 
vertical axis represents the driving torque of the vehicle (i.e., the 
torque at the output shaft of the torque converter). Curves I and II in 
the figure represent the driving torque of the vehicle at a constant 
vehicle speed, where curve shows the driving torque during the lock-up 
operation of the torque converter, and curve II shows the same during the 
non-lock-up operation of the torque converter. 
As shown in FIG. 2A, in the prior arts, the boost pressure control is 
carried out regardless of whether the torque converter is in the lock-up 
position. Namely, the supercharger is always started when the engine load 
becomes larger than a predetermined first value (i.e., TH1 in FIG. 2A), 
the air bypass valve is then gradually closed as the engine load 
increases. When the engine load increases to a predetermined second value 
(i.e., TH2 in FIG. 2A), the air bypass valve is fully closed to achieve 
maximum boosting of the engine. 
Curve I in FIG. 2A, which shows the condition of the lock-up operation of 
the torque converter, indicates the supercharger is started at point B and 
the maximum boosting of the engine is achieved at point C. The driving 
torque obtained in the range between points B and C is relatively low 
since the torque converter is operated in the lock-up position without 
supercharging. Accordingly, if a larger driving torque is required, the 
lock-up clutch must be turned OFF to increase the driving torque since 
operation of the supercharger is not initiated in this operating range, 
which increases the fuel consumption of the engine because of slippage 
loss during the non-lock-up operation of the torque converter. 
On the other hand, assuming that the torque converter is operated in the 
lock-up position at point C on curve I (the maximum boosting condition of 
the engine), if a larger driving torque is required, the lock-up clutch 
must be turned OFF during high engine output torque conditions. As seen 
from curves I and II, if the lock-up clutch is turned OFF at point C, the 
operating point shifts suddenly from point C on curve I to point E on 
curve II. Since the difference in driving torque between points C and E is 
very large, the driver of the vehicle must depress the accelerator pedal 
by large amount so as to adjust the engine torque such that the operating 
point shifts from point C on curve I to point C' on curve II and thereby 
avoid torque shock resulting from a sudden increase in driving torque, 
which deteriorates driveability significantly. 
SUMMARY OF THE INVENTION 
An object of the present invention is to solve the aforementioned problems 
by providing a boost pressure control system that can improve fuel 
consumption by extending the range of the lock-up operation of the torque 
converter and also reduce the torque shock during the ON/OFF operation of 
the lock-up clutch, thereby preventing a deterioration of the driveability 
of the vehicle. 
According to the present invention, there is provided a boost pressure 
control system for a vehicle engine equipped with a torque converter 
including a lock-up clutch; said boost pressure control system comprising 
an intake pressure boost means for boosting the intake pressure of the 
engine, and a boost pressure control means for setting the boost pressure 
of the engine by controlling said intake pressure boost means such that 
said boost pressure is determined as a function of engine load and said 
function of the engine load is selected by said boost pressure control 
means from among a plurality of predetermined functions in accordance with 
whether said lock-up clutch of the torque converter is connected or 
disconnected. 
The present invention will be better understood from the description of a 
preferred embodiment thereof as set forth below, with reference to the 
accompanying drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 illustrates an embodiment of the boost pressure control system 
according to the present invention. 
Referring to FIG. 1, reference numeral 1 represents an engine of a vehicle, 
2 is an inlet air passage, and 3 represents a throttle valve that controls 
an inlet air flow through the inlet air passage in response to the 
operation of an accelerator pedal (not shown) by a vehicle driver. 
Reference numeral 5 represents a supercharger disposed on the inlet air 
passage 2 downstream of the throttle valve 3. In this embodiment, a Roots 
type blower is used as the supercharger 5. The supercharger 5 is driven by 
a crank pulley 4a attached to a crankshaft 4 of the engine 1, via a drive 
belt and magnetic clutch 6. The supercharger 5 can be started or stopped 
during the operation of the engine by connecting (ON) or disconnecting 
(OFF) the magnetic clutch 6. 
Numeral 7 denotes an air bypass passage connecting portions of the inlet 
air passage 2 downstream of the throttle valve 3 (upstream of the 
supercharger 5) and downstream of the supercharger 5. An air bypass valve 
9 is disposed in the air bypass passage 7. The air bypass valve 9 is 
driven by an actuator 8 such as a stepping motor, and can be set at any 
position between fully closed and fully open so as to control the air flow 
through the air bypass passage 7. 
When the degree of the opening of the air bypass valve 9 is increased, the 
air flow returning from the outlet of the supercharger 5 to the inlet 
thereof increases, and thereby the boost pressure of the engine decreases 
due to a reduction in the compression ratio of the supercharger 5. On the 
contrary, when the degree of the opening of the air bypass valve 9 is 
decreased, the boost pressure increases due to an increase in the 
compression ratio of the supercharger 5. Accordingly, the boost pressure 
can be controlled by adjusting the degree of the opening of the air bypass 
valve 9. 
Numerals 12 and 19 denote opening angle sensors that detect the degree of 
the opening of the throttle valve and the air bypass valve, respectively. 
Numeral 13 denotes an engine speed sensor detecting the rotating speed of 
the engine crankshaft 4, and 15 denotes an air flow meter detecting an 
inlet air flow of the engine. 
In this embodiment, an automatic transmission 30 that is equipped with a 
torque converter 31 having a lock-up clutch is connected to the output 
shaft of the engine 1. The gear shift operation of the automatic 
transmission and the ON/OFF operation of the lock-up clutch is initiated 
by control signals from an engine control unit 21, which is explained 
later. Also, a running speed sensor 17 that detects a running speed of the 
vehicle is provided on the output shaft of the transmission 30. 
Reference numeral 21 represents an engine control unit (hereinafter called 
ECU) 21 for performing fundamental engine controls, such as ignition 
timing control and fuel injection control. 
In this embodiment, the ECU 21 is composed of a known type of digital 
computer including a CPU 23, a RAM 24, a ROM 25 and an input port 26, and 
an output port 27 that are interconnected by a dual direction bus 28. 
The ECU 21 in this embodiment further performs the boost pressure control 
of the present invention. To perform these controls, the signals from the 
throttle opening sensor 12, the engine speed sensor 13, the running speed 
sensor 17, and the air bypass valve opening angle sensor 19 are input to 
the input port 26 of the ECU 21. The output port 27 is connected to the 
actuator 8 of the air bypass valve 9, the magnetic clutch 6 of the 
supercharger (hereinafter called supercharger clutch) and the automatic 
transmission 30 via respective drive circuits to control the degree of the 
opening of the air bypass valve 9, the ON/OFF operation of the 
supercharger clutch 6 and the lock-up operation of the torque converter 
31. 
The boost pressure control carried out by the ECU 21 is then explained with 
reference to FIG. 2B. 
FIG. 2B is a drawing similar to FIG. 2A, but shows the relationship between 
the driving torque of the vehicle and the engine operating load under 
boost pressure control according to the present invention. In FIG. 2B, the 
horizontal axis represents the degree of throttle opening of the engine 
(i.e., the load of the engine), and the vertical axis represents the 
driving torque of the vehicle (i.e., the torque at the output shaft of the 
torque converter 31). Curves I and II represent the driving torque of the 
vehicle at a constant vehicle speed, where curve shows the driving torque 
during the lock-up operation of the torque converter (i.e., when the 
lock-up clutch is being ON), and curve II shows the same during the 
non-lock-up operation of the torque converter (i.e., the lock-up clutch is 
OFF). 
As shown in FIG. 2B, the operations of the supercharger 5 and the air 
bypass valve in this embodiment are controlled based on different 
functions of the engine load in accordance with the ON/OFF conditions of 
the lock-up clutch. 
In this embodiment, when the torque converter is operated in the lock-up 
condition (curve ), the ECU 21 initiates the operation of the supercharger 
5 at a lower engine load than when the lock-up operation of the torque 
converter is being cancelled (curve II, i.e., in the non-lock-up operation 
of the torque converter). Further, when the lock-up operation of the 
torque converter is carried out, the boost pressure is controlled in such 
manner that in a lower engine load operating range, the boost pressure 
becomes higher than when the lock-up operation of the torque converter is 
being cancelled, thereby increasing the engine output torque. 
Namely, as shown in FIG. 2B, when the lock-up operation of the torque 
converter is being cancelled (curve II), the operation of the supercharger 
5 is started at the engine load TH1 (point D) and the maximum boosting 
condition of the engine is achieved at the engine load TH2 (point E) in 
the same manner as the prior art. On the other hand, when the lock-up 
operation of the torque converter is carried out (curve I), the operation 
of the supercharger 5 is started at the engine load TH3 (point A) which is 
lower than TH1, and the maximum boosting condition of the engine is 
achieved at the engine load TH4 (point D), which is also lower than TH2. 
Therefore, in the operating range between points A and F on curve 1 , 
though the torque converter is operated in the lock-up condition, the 
driving torque is increased by boosting same to nearly the same level as 
that of the non-lock-up operation of the torque converter. Accordingly, 
the lock-up operation of the torque converter becomes possible even in the 
engine operating range between the throttle opening angles TH3 and TH4 in 
which the lock-up operation of the torque converter must be cancelled in 
the prior art. Thus, fuel consumption is improved because of the extended 
lock-up operation range of the torque converter. 
Further, since the driving torque in the lock-up operation of the torque 
converter (curve I) increases and approachs the driving torque in the 
non-lock-up operation of the torque converter, the torque shock caused by 
disconnecting the lock-up clutch at a relatively large throttle angle (for 
example, at point F in FIG. 2B) is reduced significantly. Accordingly, 
only a small depression of the accelerator pedal is required from the 
driver to avoid torque shock, thereby ensuring that a deterioration in 
driveability does not occur. 
FIG. 3 illustrates a routine for achieving the above-mentioned boost 
pressure control of the present embodiment, which is processed by the ECU 
21 by sequential interruptions at predetermined intervals (for example, 16 
msec.). 
Referring to FIG. 3, when the routine starts, parameters such as the degree 
of the opening of the throttle valve TH, the engine speed NE, and the 
running speed of the vehicle VS are read from the RAM 24. The parameters 
TH, NE, VS are input from respective sensors 12, 13, 17 at predetermined 
intervals or at every predetermined rotation angle of the crankshaft, and 
stored into the RAM 24. Accordingly, the values of these parameters stored 
in the RAM 24 are always updated. 
Then, in step 105, it is determined from the running condition of the 
vehicle whether or not the lock-up operation of the torque converter 31 is 
required. The running condition requiring the lock-up operation of the 
torque converter is defined by the parameters such as the degree of the 
opening of the throttle valve TH and the running speed of the vehicle VS. 
FIG. 4 shows an example of the running conditions in which the lock-up 
operation of the torque converter is required. 
As shown in FIG. 4, when the running speed of the vehicle is higher than a 
predetermined value, the degree of the opening of the throttle valve, at 
which an ON/OFF operation of the lock-up clutch is initiated, is set 
higher as the running speed of the vehicle increases. The dotted line in 
FIG. 4 indicates the boundary of the lock-up operation region of the 
torque converter in the prior art. Also, points A and F in FIG. 4 indicate 
the degrees of the opening of the throttle valve corresponding to points A 
and F in FIGS. 2A and 2B. As explained later, the lock-up operation region 
of the torque converter becomes wider in this embodiment by extending the 
boost operation region of the engine to a lower engine load. 
In this embodiment, the running conditions defined by FIG. 4 are previously 
stored in the ROM 25 as a numerical table of TH and VS, and whether or not 
the lock-up operation of the torque converter is required is determined in 
accordance with such a numerical table. 
If it is determined in step 105 that the running condition of the vehicle 
is in the lock-up operation region of the torque converter (which is 
indicated by hatched area in FIG. 4), the routine proceeds to step 110 in 
which the lock-up clutch of the torque converter 31 is turned ON. Then, in 
step 115, it is determined whether or not the operation of the 
supercharger is required. 
The determination of whether or not the operation of the supercharger is 
required is carried out based on the parameters such as the degree of the 
opening of the throttle valve TH and the engine speed NE. FIG. 5 shows the 
conditions for starting or stopping the operation of the supercharger 
defined by functions of TH and NE. The functions in FIG. 5 are previously 
stored in the ROM 25 in the form of numerical tables, and whether or not 
the operation of the supercharger is required is determined in step 115, 
according to that numerical table. 
In this embodiment, two kinds of functions of TH and NE are provided as 
conditions for starting or stopping the operation of the supercharger as 
shown in FIG. 5, and either one of the functions is selected in accordance 
with whether the lock-up clutch is turned ON or OFF. That is, when the 
lock-up clutch is turned ON, the line A is selected as the criteria for 
starting or stopping the supercharger. Conversely, the line B is selected 
when the lock-up clutch is turned OFF. By comparing the lines A and B, it 
will be understood that the operation of the supercharger is started at a 
lower engine load when the lock-up clutch is turned ON (line A) than when 
the lock-up clutch is turned OFF (line B). 
In step 115, line A in FIG. 5 is selected as the criteria for the operation 
of the supercharger, and if it is determined in step 115 that the 
parameters TH and NE fall in the region requiring the operation of the 
supercharger, the routine proceeds to step 120 in which the supercharger 
clutch 6 is turned ON to start the operation of the supercharger 5. 
Step 125 shows an operation for setting the degree of the opening of the 
air bypass valve 9. In this embodiment, the degree of the opening of the 
air bypass valve is determined in accordance with the degree of the 
opening of the throttle valve TH and the engine speed NE. 
FIGS. 6A and 6B show an example of the setting of the degree of the opening 
of the air bypass valve 9. In this embodiment, the setting of the degree 
of the opening of the air bypass valve 9 is given as functions of TH and 
NE. Different functions having different characteristics are selected in 
accordance with whether or not the lock-up clutch is turned ON (FIG. 6A) 
or OFF (FIG. 6B). In both cases, the degree of the opening of the air 
bypass valve is set larger as the degree of the opening of the throttle 
valve increases if the engine speed is maintained constant, and is set 
smaller as the engine speed decreases if the degree of the opening of the 
throttle valve is maintained constant. However, when the lock-up clutch is 
turned ON (FIG. 6A), the degree of the opening of the air bypass valve is 
set generally smaller than when the lock-up clutch is turned ON in a lower 
engine load region. Accordingly, when the lock-up clutch is turned ON, the 
boost pressure is increased in a lower engine load region, thereby 
increasing the engine load during the lock-up operation of the torque 
converter. 
In this embodiment, the functions shown in FIGS. 6A and 6B are previously 
stored in the ROM 25. In step 125, the degree of the opening of the air 
bypass valve is determined according to the function shown in FIG. 6A (for 
the lock-up operation of the torque converter), and at the same time, the 
degree of the opening of the air bypass valve 9 is adjusted to the 
determined value using the actuator 8 and the opening angle sensor 19 of 
the air bypass valve 9. 
By this control, the driving torque of the vehicle changes along the line 
between points A and F in FIG. 2B, thereby increasing driving torque 
which, in the prior art, is obtained only by a non-lock-up operation of 
the torque converter, and becomes available in the lock-up operation of 
the torque converter. 
If it is determined that the engine load condition is in a region in which 
the operation of the supercharger is not required, the routine proceeds to 
step 130 in which the supercharger clutch 6 is turned OFF to stop the 
operation of the supercharger 5. Then after setting the degree of the 
opening of the air bypass valve at a predetermined value (fully opened, 
for example) in step 135, the routine is terminated. 
Further, if it is determined that the running condition of the vehicle is 
in the non-lock-up operation region of the torque converter, a similar 
control operation as set forth above is carried out based on the functions 
for the non-lock-up operation of the torque converter (i.e.,line B in FIG. 
5 and FIG. 6B) in steps 140 to 155. 
According to the present embodiment, by increasing the boost pressure from 
a lower engine load region in the lock-up operation of the torque 
converter, the difference in the driving torque between the lock-up 
operation and the non-lock-up operation of the torque converter is reduced 
substantially. Therefore, even if the lock-up clutch of the torque 
converter is turned ON or OFF under relatively high engine torque 
conditions, the torque shock caused by the ON/OFF operation of the lock-up 
clutch is very small and driveability is not impaired. 
Although the invention has been described with reference to specific 
embodiments chosen for the purpose of illustration, it should be 
understood that numerous modifications could be turned thereto by those 
skilled in the art without departing from the basic concept and scope of 
the invention.