Sequential turbocharger system for an internal combustion engine

A sequential turbocharger system for an internal combustion engine with an exhaust gas recirculation system. The sequential system includes a large-volume turbocharger and a small-volume turbocharger, which are arranged in series in the direction of the flow of gases. A bypass passageway is connected to an exhaust pipe to thereby bypass the turbine of the small-volume turbocharger, and an exhaust switching valve is arranged in the bypass passageway for controlling the flow of exhaust gas to the turbine of the small-volume turbocharger. The EGR system includes an exhaust gas recirculation passageway for introducing an amount of exhaust gas into the intake pipe of the engine. The exhaust gas for recirculation is taken out from the intake pipe at a position located between the turbines of the small-volume and large-volume turbochargers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device for recirculation of an exhaust 
gas in a series-mounted two-stage turbocharged internal combustion engine. 
2. Description of the Related Art 
In a known series-mounted two-stage turbocharged internal combustion 
engine, a large volume and a small volume turbocharger are arranged in 
series along the direction of the flow of gas (see Japanese Unexamined 
Patent Publication No. 50-129815). Such a two-stage turbocharger system is 
used for obtaining a wide range turbocharging operation, from a low engine 
speed area to a high engine speed area. Namely, when the engine speed is 
low, a turbocharging operation is obtained by a turbocharger having a 
small volume, and when the engine speed is high, a turbocharging operation 
is obtained by a turbocharger having a large volume. An exhaust bypass 
passageway is provided in the small-volume turbocharger, to bypass a 
turbine thereof, and an exhaust switching valve is arranged in the bypass 
passageway. The exhaust switching valve closes the bypass passageway when 
the engine is in a low speed condition, at which the large-volume 
turbocharger is not yet fully activated, so that all of the flow of the 
exhaust gas from the engine is introduced into the small-volume 
turbocharger, to obtain a turbocharging operation by the small-volume 
turbocharger. The exhaust switching valve opens the bypass passageway when 
the engine is in a high speed condition, at which the large-volume 
turbocharger is fully activated, and thus the flow of the exhaust gas 
bypasses the small-volume turbocharger and the turbocharging effect is 
obtained only from the large-volume turbocharger. 
Also known is an exhaust gas recirculation system for reducing the amount 
of nitrogen oxide emissions from the gases exhausted from an internal 
combustion engine, with a single turbocharger, where an amount of exhaust 
gas is taken out from an engine exhaust line at a location upstream of the 
turbine of the turbocharger, and introduced into an engine intake line 
(see Japanese Unexamined Utility Model Publication No. 61-151063). 
When carrying out an exhaust gas recirculation (EGR) operation in an 
internal combustion engine provided with a two-stage sequential 
turbocharger system, as in the prior art for an internal combustion engine 
provided with single turbocharger, the exhaust gas to be recirculated also 
can be taken from the exhaust line at a position upstream of the 
turbochargers which are arranged in series. In this case, however, the 
parts of the exhaust gas recirculating system, which are less resistant to 
high temperatures, such as a diaphragm made of rubber of an exhaust gas 
recirculating valve, are often thermally damaged after a relatively short 
use, because the EGR system is subjected to the high temperature of the 
exhaust gas in the two-stage turbocharger system. To avoid this thermal 
damage to parts of EGR system, the exhaust gas for the EGR operation may 
be taken out from the engine exhaust line at a position downstream of the 
turbines of the series of the turbochargers, where the temperature of the 
exhaust gas is lower, but a substantial drop in the pressure of the 
exhaust gas occurs at such a position, and therefore, the pressure 
differential between the pressure at the position where the exhaust gas is 
taken out and the position where the exhaust gas is introduced into the 
intake line becomes small, and this makes it difficult to obtain a desired 
amount of exhaust gas to be recirculated. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an EGR system capable of 
circulating a desired amount of exhaust gas while preventing a thermal 
damage of parts of the EGR system. 
According to the present invention, an internal combustion engine is 
provided, comprising: 
an engine body; 
an intake line for introducing a combustible mixture into the engine body; 
an exhaust line for removing exhaust gases from the engine body; 
a first, large-volume turbocharger wherein a turbine thereof is arranged in 
the exhaust line and a compressor thereof is arranged in the intake line; 
a second, small-volume turbocharger wherein a turbine thereof is arranged 
in the exhaust line at a position upstream of the turbine of the 
large-volume turbocharger, in the direction of the flow of exhaust gases, 
and a compressor thereof is arranged in the intake line at a position 
downstream of the compressor of the large-volume turbocharger, in the 
direction of the flow of the intake air; 
a bypass passageway connected to the exhaust line so as to bypass the 
turbine of the small-volume turbocharger; 
exhaust switching valve means for controlling a flow of the exhaust gases 
in said bypass passageway; 
means, responsive to engine operating conditions, for controlling the 
exhaust switching valve means to obtain a desired pressure of the intake 
air, and; 
exhaust gas recirculation means for taking out an amount of the exhaust gas 
from the exhaust line at a position located between the turbine of the 
large-volume turbocharger and the turbine of the small-volume 
turbocharger, and for introducing the taken out exhaust gas into the 
intake line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, showing a first embodiment of the present invention when applied 
a diesel engine, reference numeral 10 denotes an engine body to which an 
intake pipe 12 and an exhaust pipe 14 are connected. A large-volume 
turbocharger 16 and a small-volume turbocharger 18, which are arranged in 
series, are provided; the large-volume turbocharger 16 being provided with 
a compressor 20, a turbine 22 and a rotating shaft 24, and the 
small-volume turbocharger 18 being provided with a compressor 26, a 
turbine 28 and a rotating shaft 25. The compressor 20 of the large-volume 
turbocharger 16, and then the compressor 26 of the small-volume 
turbocharger 18 are arranged in the intake pipe 12, along the direction of 
the flow of the intake air. An intercooler 29 is arranged downstream of 
the compressor 26 of the small-volume turbocharger 18. The turbine 28 of 
the small-volume turbocharger 18, and then the turbine 22 of the 
large-volume turbocharger 16 are arranged in the exhaust pipe 14, along 
the direction of the flow of the exhaust gas. 
A first bypass passageway 30 is connected to the exhaust pipe 14 so that 
the passageway 30 bypasses the turbine 22 of the large-volume turbocharger 
22, and a swing-door type waste gate valve (exhaust gas bypass control 
valve) 32 is arranged for controlling the flow of the exhaust gas to the 
bypass passageway 30. A diaphragm actuator 34 is provided, for controlling 
the operation of the waste gate valve 32, and the actuator 34 is provided 
with a diaphragm 34a connected to the valve 32, and a spring 34b urging 
the diaphragm 34a such that the valve 32 usually closes the bypass 
passageway 30. The actuator 34 is provided with a chamber 34c to which a 
positive pressure is applied in the intake pipe at a position downstream 
of the compressor 26 of the small-volume turbocharger 18, which positive 
pressure urges the diaphragm 34a to move against the force of the spring 
34b so that the waste gate valve 32 is opened. 
A second exhaust bypass passageway 36 is connected to the exhaust pipe 14 
to bypass the turbine 28 of the small-volume turbocharger 18, and an 
exhaust switching butterfly valve 38 is arranged in the second bypass 
passageway 36. An actuator 40 is provided for operating the exhaust 
switching valve 38, and is constructed as a two-stage diaphragm mechanism, 
whereby the exhaust switching valve 38 is closed to close the second 
bypass passageway 36 before the large-volume turbocharger 16 reaches a 
fully activated condition, and the exhaust switching valve 38 is opened to 
open the second bypass passageway 36 when the large-volume turbocharger 16 
reaches a fully activated condition. The actuator 40 is provided with 
diaphragms 40a and 40b, and springs 40c and 40d; the first diaphragm 40a 
being connected to the exhaust switching valve 38 via a rod 40e and the 
second diaphragm 40b being connected to a second rod 40f. The arrangement 
of the first and second rods 40e and 40f is such that the movement of the 
second rod 40f toward the first rod 40e causes the first rod 40e to be 
moved together with the second rod 40f, and the movement of the second rod 
40f away from the first rod 40e causes the second rod 40f to be moved 
independently from the first rod 40e. A first chamber 40g is formed on one 
side of the second diaphragm 40b away from the spring 40d; the chamber 40g 
being connected to the intake pipe 12 at a position 60 located downstream 
of the compressor 26 of the small turbocharger 18 via a conduit 39. A 
chamber 40h is formed on one side of the diaphragm 40a away from the 
spring 40c, and will explained later, a turbocharged pressure or 
atmospheric air pressure is selectively introduced to the chamber 40h to 
thereby obtain a step-like opening of the exhaust switching valve 38. 
Namely, when the chamber 40h is opened to the atmospheric pressure, the 
opening of the exhaust switching valve 38 is controlled by the 
turbocharging pressure in the chamber 40g, which is opposed by the 
combined strong forces of the springs 40c and 40d, so that the speed of 
opening of the exhaust switching valve 38 is reduced. Conversely, when the 
chamber 40h is opened to the turbocharging pressure, the opening of the 
exhaust switching valve 38 is controlled by the pressure in the chamber 
40h, which is opposed only by weak spring force by the spring 40c, and 
thus the speed of opening of the exhaust switching valve 38 is increased. 
An intake bypass passageway 44 is connected to the intake pipe 12 so as to 
bypass the compressor 26 of the small-volume turbocharger 18, and an 
intake bypass control valve 46 is arranged in the intake bypass passageway 
44. A diaphragm actuator 48 is provided for controlling the operation of 
the intake bypass valve 46; the actuator 48 being provided with a 
diaphragm 48a connected to the intake bypass valve 46, a chamber 48b on 
one side of the diaphragm remote from the valve 46, a chamber 48c on the 
other side of the diaphragm 48a near to the valve 46, and a spring 48d 
which urges the diaphragm 48a so that the valve 46 is closed. As will be 
explained later, the intake bypass valve 46 is closed when the 
small-volume turbocharger 18 is operated but the large-volume turbocharger 
is not yet fully operational. After a fully operational condition of the 
large sized turbocharger 16 is reached, a positive pressure is applied to 
the lower chamber 48c, and thus the intake bypass valve 46 is opened. 
The internal combustion engine is provided with an exhaust gas 
recirculation (EGR) system, which includes an EGR passageway 50 and an EGR 
valve 52 arranged in the EGR passageway. The EGR valve 52 is provided with 
a diaphragm 52a, a valve member 52b connected to the diaphragm 52a, a 
spring 52c urging the diaphragm 52a so that valve member 52b closes the 
EGR passageway 50, and a chamber 52d on one side of the diaphragm 52a 
remote from the valve member 52b. A pressure from a vacuum pump 67 is 
applied to the chamber 52d, to open the valve 52b. According to the 
present invention, the EGR passageway 50 has an upstream end (exhaust gas 
taking-out port) 50A connected to the exhaust pipe 14 at a location 
downstream of the turbine 28 of the small-volume turbocharger 18 and 
upstream of the turbine 22 of the large-volume turbocharger 16, and has a 
downstream end (exhaust gas introduction port) 50B connected to the intake 
pipe 12 downstream of the intercooler 29. 
A first three-port electromagnetic switching valve (VSV1) 54 controls the 
actuator 34 for the waste gate valve 32, whereby the valve 54 is switched 
between a first position (OFF position) at which the chamber 34c is opened 
to the atmospheric air pressure via an air cleaner 54-1, and a second 
position (ON position) at which the chamber 34c is opened to the intake 
pressure in the intake pipe 12 at a location 56 slightly upstream of the 
intercooler 29. When the valve 54 is in the first position, at which the 
chamber 34c is opened to the atmospheric pressure, the waste gate valve 32 
is closed by the force of the spring 34b, and when the valve 54 is in the 
second position, at which the chamber 34c is opened to the turbocharged 
pressure at the location 56, the diaphragm 34a is moved against the force 
of the spring 34b so that the waste gate valve 32 is opened. 
A second three-port electromagnetic switching valve (VSV2) 58 controls the 
pressure of the chamber 40h of the actuator 40 for the exhaust switching 
valve 38. This second electromagnetic switching valve 58 is moved between 
a first position (OFF position) at which the chamber 40h is opened to the 
atmospheric pressure via an air cleaner 58-1, and a second position (ON 
position) at which the chamber 40h is opened to the intake pressure at the 
outlet position 60 of the small-volume turbocharger 18. Note, the chamber 
40g is always opened to the intake pressure at the outlet portion 60 of 
the small-volume turbocharger 18. 
Third and fourth three-port electromagnetic switching valves 64 and 66 are 
provided for controlling the pressure of the actuator 48 for the intake 
bypass valve 46. Namely, the third electromagnetic switching valve (VSV3) 
64 controls the pressure of the chamber 48b formed on the upper side of 
the diaphragm 48a, and thus the valve 64 is switched between a first 
position (OFF position) at which the chamber 48b is opened to the 
atmospheric pressure via an air cleaner 64-1, and a second position (ON 
position) at which the chamber 48b is opened to the output 60 of the 
compressor 26 of the small turbocharger 18. The fourth electromagnetic 
switching valve 66 (VSV4) controls the pressure of the chamber 48c on the 
lower side of the diaphragm 48a, and thus the valve 66 is switched between 
a first position (OFF position) at which the chamber 48c is opened to a 
vacuum pressure in the vacuum pump 67 driven by the crankshaft of the 
engine 10, and a second position (ON position) at which the chamber 48c is 
opened to the outlet 67 of the compressor 20 of the large-volume 
turbocharger 16. 
A fifth three-port electromagnetic switching valve 70 (VSV5) controls the 
operation of the EGR valve 52, whereby the valve 70 is switched between a 
first position (OFF position) at which the chamber 52d is opened to the 
atmospheric pressure via an air cleaner 70-1, and a second position (ON 
position) at which the chamber 52c is opened to the vacuum pressure in the 
vacuum pump 67. As will be explained later, a pulse signal applied to the 
electromagnetic switching valve 70 has a duty ratio which is controlled to 
thus obtain a a desired exhaust gas recirculation (EGR) ratio. 
A control circuit 72 controls the turbocharging operation of this 
invention, and outputs signals to the electromagnetic switching valves 54 
(VSV1), 58(VSV2), 64(VSV3), 66(VSV4), and 70(VSV5). The control circuit 72 
is connected to sensors for carrying out the control according to the 
present invention. Namely, a first pressure sensor 78 is provided for 
detecting the intake pressure P.sub.1 at the outlet 67 of the compressor 
20 of the large-volume turbocharger 16; a second pressure sensor 80 is 
provided for detecting the pressure P.sub.2 of the intake air at the 
outlet 60 of the compressor 26 of the small turbocharger 18; a sensor 81 
is provided for detecting the engine rotational speed NE; and a sensor 83 
is provided for detecting the engine load by determining, for example, a 
position of an adjusting lever (not shown) of a fuel injection pump (not 
shown), which is a well known manner of connecting an accelerator pedal 
(not shown) to a device controlling the amount of fuel injected by the 
fuel injection pump. 
The operation of the control circuit 72 in the first embodiment will now be 
explained with reference to FIGS. 2 and 3. FIG. 2 shows a routine for 
controlling an intake pressure. As shown in FIG. 2, at step 100, it is 
determined if the pressure P.sub.2 at the outlet 60 of the compressor 26 
of the small-volume turbocharger 18 is larger than the pressure P.sub.1 at 
the outlet 67 of the large-volume turbocharger 16. FIG. 4 illustrates the 
relationships between the engine speed NE and the pressures P.sub.1 and 
P.sub.2 at the outlets of the turbochargers 16 and 18, respectively, while 
the degree of depression of the accelerator pedal is maintained unchanged. 
The solid lines indicate the pressure when the accelerator pedal is fully 
depressed, and the dotted lines indicate the pressure when the accelerator 
pedal is partly depressed. As is easily seen, in accordance with increase 
in the engine speed, an increase in the small turbocharger output pressure 
P.sub.2 is faster than that of the large-volume turbocharger output 
pressure P.sub.1. Accordingly, a result of P.sub.2 &gt;P.sub.1 is obtained at 
step 100 in FIG. 2, when the engine speed is low, and thus the routine 
goes to step 102 and the first electromagnetic switching valve 54 (VSV1) 
is made OFF, which causes the chamber 34c to be opened to the atmospheric 
pressure and the spring 34b to urge the diaphragm 34a to close the waste 
gate valve 32. At the following step 104, the second electromagnetic 
switching valve 58 (VSV2) is made OFF, which causes the chamber 40h to be 
opened to the atmospheric air pressure, while the chamber 40g is always 
opened to the pressure P.sub.2 at the small-volume turbocharger output 60. 
As a result, the pressure P.sub.2 at the chamber 40g opened to the 
small-volume turbocharger output 60 is opposed by a set force which is the 
sum of the forces of the spring 40c and 40d. When the pressure P.sub.2 is 
smaller than this set force, i.e., the engine speed is smaller than a 
predetermined value NE.sub.1 or NE.sub.1 ' in FIG. 4., which values 
provide a intake pressure P.sub.SET corresponding to this spring force, 
the exhaust switching valve 38 is kept closed. 
When the pressure P.sub.2 reaches the set pressure P.sub.SET , the force 
applied to the diaphragm 40b becomes larger than the combined set force of 
the springs 40c and 40d, and accordingly, the diaphragm 40b is moved 
against this combined set force, whereby the exhaust switching valve 38 is 
gradually opened. 
In view of the operation of the intake bypass valve 45 during the engine 
low speed condition, at step 106, the electromagnetic switching valve 64 
(VSV3) is made ON so that the chamber 48b above the diaphragm 48a is 
opened to the pressure P.sub.2 at the output 60 of the compressor 26 of 
the small-volume turbocharger 18 so that the spring 48d urges the 
diaphragm 48a downward and thus the intake bypass valve 46 is closed. 
Furthermore, at step 108, the electromagnetic switching valve 66 (VSV4) is 
made OFF, so that the chamber 48d below the diaphragm 48a is opened to the 
vacuum in the vacuum pump 67, whereby a force is applied to the diaphragm 
48a, to move it downward, which assists the positive seating of the valve 
46 when closing the bypass passageway 44. 
As a result of an acceleration operation, the engine speed NE can increase 
to a value of NE.sub.2 or NE.sub.2 ', whereby the pressure P.sub.1 at the 
output of the compressor 20 of the large-volume turbocharger 16 is 
increased to the pressure P.sub.2 at the output of the compressor 26 of 
the small-volume turbocharger (P.sub.1 =P.sub.2). If the result at step 
100 is "NO", the routine goes to step 110, and the first electromagnetic 
switching valve 54 (VSV1) is made ON. As a result, the chamber 34c is 
opened to the port 56, and accordingly, the positive pressure thereat is 
applied to the diaphragm 34a to urge it to move against the force of the 
spring 34b, to thereby open the waste gate valve 32. At the following step 
112, the electromagnetic switching valve 58 (VSV2) is made ON, and 
accordingly, a positive pressure at the outlet 60 of the compressor 26 of 
the small turbocharger 18 is applied to the chamber 40h, which moves the 
diaphragm 40a regardless of the pressure in the chamber 40g. As a result, 
the opening of the exhaust switching valve 38 is now controlled only by 
the pressure in the chamber 40h, which is opposed by a force of the spring 
40c. Namely, the force of the spring 40d cannot move the exhaust switching 
valve 38 to the closed position. Accordingly, a step-like decrease in the 
set force moving the valve 38 in the closing direction is obtained, 
whereby a step-like opening movement of the exhaust switching valve 38 is 
obtained for a rapid opening of the valve 38 to the fully open position. 
Finally, the operation for controlling the intake bypass valve in the first 
embodiment during the ending of the high speed condition (NE&gt;NE.sub.1 ' or 
NE&gt;NE.sub.2 ') is as follows. In FIG. 2, at step 114, the third 
electromagnetic switching valve 64 (VSV3) is made OFF, so that the upper 
side chamber 48b is opened to the atmospheric pressure via the air cleaner 
64-1, and then at step 116, the fourth electromagnetic switching valve 66 
(VSV4) is made ON, so that the lower side chamber 48c is opened to the 
positive pressure P.sub.1 at the outlet of the compressor 20 of the 
large-volume turbocharger 16. As a result the positive pressure in the 
lower side chamber 48d moves the diaphragm 48a upward against the force of 
the spring 48d, and thus the intake bypass valve 46 is opened very 
quickly. 
FIG. 4 illustrates the relationship between the engine speed and the intake 
pressures (turbocharging pressure) P.sub.1 at the outlet of the compressor 
20 of the large-volume turbocharger 16 and P.sub.2 at the outlet of the 
compressor 26 of the small-volume turbocharger 18. As shown by the solid 
lines, which indicate a characteristic obtained when the accelerator pedal 
is fully depressed, the increase of the engine speed NE to the value of 
NE.sub.1 made the pressure P.sub.2 at the small-volume turbocharger 
output 60 equal to a preset value P.sub.SET, and the increase in the 
engine speed NE to the value NE.sub.2 makes the pressure P.sub.1 at the 
large-volume turbocharger output 67 equal to the preset value P.sub.SET 
which is equal to P.sub.2. As shown by the dotted lines, which indicate 
the characteristic obtained when the accelerator pedal is partly 
depressed, the increase of the engine speed NE to the value of NE.sub.1 ' 
(&gt;NE.sub.1) makes the pressure P.sub.2 at the small-volume turbocharger 
output 60 equal to a preset value P.sub.SET, and the increase in the 
engine speed NE to the value of NE.sub.2 ' (&gt;NE.sub.2) makes the pressure 
P.sub.1 at the large-volume turbocharger output 67 equal to the preset 
value P.sub.SET, which is equal to P.sub.2. 
FIG. 3 shows a routine for EGR control. At step 120, it is determined if 
the engine speed NE is larger than a predetermined value NE.sub.0, and at 
step 122, it is determined if the engine load L is larger than a 
predetermined value L.sub.0. When it is determined that NE.ltoreq.NE.sub.0 
and L.ltoreq.L.sub.0, i.e., the engine is under a low speed and low load 
condition as shown by area defined by horizontal and vertical dotted lines 
m1 and m2, respectively, in FIG. 5, when an exhaust gas recirculation 
operation is to be carried out, the routine goes to step 124, and it is 
determined if the exhaust switching valve 38 is open, the routine goes to 
step 126 and a degree of opening of the EGR valve 52 is calculated from a 
first map (MAP1). Alternatively, the routine goes to step 130 and a degree 
of opening of the EGR valve 52 is calculated from a second map (MAP2). The 
reason for this operation of differentiating the degree of opening of the 
EGR valve 52 in accordance with the position of the exhaust switching 
valve 38 will now be described. 
The amount of EGR gas is basically determined in view of the combinations 
of the engine speed NE and engine load L. In FIG. 5, inclined dotted lines 
schematically indicate equal EGR ratio lines for values of the EGR ratio 
as designated. Namely, the EGR ratio is determined from a combination of 
the engine speed NE and engine load L. As is easily seen, the higher the 
engine speed or the higher the engine load, the smaller the EGR ratio. To 
obtain the designated setting of the EGR ratio, the pressure in the 
chamber 52d of the actuator 52 is controlled by changing the setting of 
the duty ratio of the pulse signal directed to the electromagnetic 
switching valve 70 (VSV5). The duty ratio, as is well known, is a ratio of 
a duration of an ON state to a duration of one cycle of the pulse signal 
output for operating the electromagnetic switching valve 70 (VSV5). The 
higher the duty ratio, the longer the duration of the ON state of the 
pulse signal; the longer the duration time, the stronger the vacuum in the 
vacuum chamber 52d of the actuator 52, and the stronger the vacuum 
pressure, the higher the lift of the EGR valve 52b, i.e., the greater the 
amount of recirculated exhaust gas obtained. Note, the amount of EGR gas 
obtained is determined not only by a degree of opening of the EGR valve 52 
but also by a pressure difference across the EGR passageway 50, i.e., a 
difference between the pressure at the EGR gas taking out port 50A in the 
exhaust pipe 14 and the pressure at the EGR gas introducing port 50B in 
the intake pipe 12. According to the embodiment of the present invention, 
the pressure at the exhaust gas taking out port 50A is not affected by the 
opening or closing of the exhaust switching valve 38 located in the 
exhaust pipe 14 between the turbines 28 and 22. Nevertheless, the pressure 
at the exhaust gas adding port 50B is affected by the opening and closing 
of the exhaust switching valve 38, as the pressure at the exhaust gas 
introducing port 50B corresponds to the pressure P.sub.2, if the pressure 
drop at the intercooler is neglected. As will be easily understood from 
FIG. 4, there is a smooth increase in the pressure P.sub.2 up to the 
engine speed of NE.sub.1 or NE.sub.1 ', and then the pressure P.sub.2 is 
maintained substantially constant. Namely, the characteristic curve of 
P.sub.2 has a non-continuous bent portion at the engine speed NE.sub.1 or 
NE.sub.1 ', Contrary to this, the pressure P.sub.4 at the exhaust gas 
taking out port 50A has a non-bending characteristic. As a result, there 
will be an non-continuous change in the value of the pressure difference 
P.sub.4 -P.sub.2 at the location where the closed exhaust switching valve 
38 is opened. This means that the same value of the degree of opening of 
the EGR valve 52, i.e., the same value of the duty ratio (EGRDUTY), can 
change the amount of recirculated exhaust gas at a state where the exhaust 
switching valve 38 is closed and at a state where the exhaust switching 
valve 38 is opened. Accordingly, a single map of values of duty ratio 
corresponding to the desired setting of the EGR ratio is not sufficient to 
obtain a desired EGR ratio for both the closed and opened states of the 
exhaust switching valve 38. Therefore, two maps of duty ratio values are 
provided; one map being used for calculating the EGR ratio when the 
exhaust switching valve 38 is closed, and the other map being used for 
calculating the EGR ratio when the exhaust switching valve 38 is opened. 
As will be understood from FIG. 3, when it is determined that the exhaust 
switching valve 38 is closed at step 124, the first map MAP1 is selected 
at step 126, to calculate the duty ratio EGRDUTY, which indicates a value 
of the duty ratio of the pulse signal for operating the electromagnetic 
switching valve 70 (VSV5), which operates the EGR valve 52. This map MAP1 
has duty ratio EGRDUTY values for obtaining a desired value of the EGR 
ratio determined by values of a combination of the engine speed NE and 
engine load L when the exhaust switching valve 38 is closed and all of the 
exhaust gas is introduced into the turbine 28 of the small-volume 
turbocharger 18. At step 126, the control circuit 72 executes a map 
interpolation calculation from the first map MAP1, to obtain a value of 
the duty ratio corresponding to the value of the engine speed NE and 
engine load L detected by the engine speed sensor 81 and engine load 
sensor 83. Then at step 128, a pulse signal having a value of the duty 
ratio EGRDUTY calculated at step 126 is output to the electromagnetic 
switching valve 70 (VSV5), to control the lift of the EGR valve 52 to 
thereby obtain the desired value of the EGR ratio calculated at the step 
126. 
When it is determined that the exhaust switching valve 38 is open, the 
routine goes from step 124 to step 130, where the calculation of the duty 
ratio EGRDUTY is carried out by using the second map MAP2. This map MAP2 
has duty ratio values needed to obtain desired values of the EGR ratio 
with respect to combinations of the values of the engine speed and engine 
load when the exhaust switching valve 38 is open. A map interpolation 
calculation is similarly carried out, so that a value of the duty ratio 
EGRDUTY corresponds to a value of a desired EGR ratio corresponding to a 
combination of the engine speed NE and load L detected by the sensors 81 
and 83, respectively. As a result, a lift of the EGR valve 52 is 
controlled such that the desired EGR ratio for the open condition of the 
exhaust switching valve 38 is obtained. 
When the engine is under a high speed and/or high load condition, i.e., 
NE&gt;NE.sub.0 or L&gt;L.sub.0, where the EGR operation is canceled, the routine 
goes from step 120 or 122 to step 132, where a zero is moved into the 
EGRDUTY so that a continuous low level signal for the electromagnetic 
switching valve 70 (VSV5) is obtained, whereby the pressure at the chamber 
52d is made equal to the atmospheric pressure, and thus the EGR valve 52 
is fully closed. 
Instead of providing two kinds of maps MAP1 and MAP2 for the values of the 
duty ratio, a correction factor may be employed for compensating the 
relationship between the values of the duty ratio and the values of the 
EGR ratio as actually obtained, which relationship is changed in 
accordance with whether the exhaust switching valve 38 is closed or open. 
In this case, one and the same map will be used at both steps 126 and 130 
in FIG. 3. Nevertheless, the correction factor which is multiplied by the 
duty ratio EGRDUTY is controlled in accordance with the positions of the 
exhaust switching valve 38, and thus a desired EGR ratio is obtained 
regardless of the position of the exhaust switching valve 38. 
As shown in FIG. 5, the EGR operation is carried out in the region of the 
engine speed and the engine load inside the dotted lines m1 and m2 (engine 
low speed or low load condition). Dotted lines n, which correspond to 
solid lines designated P.sub.2 in FIG. 4, show how the intake pressure 
changes with respect to the change in the engine speed when the 
accelerator pedal is fully depressed. At a point A, which corresponds to 
the point where NE=NE.sub.1 in FIG. 4, the exhaust switching valve 38 
begins to open. The line a, which extends from the point A, shows points 
where the exhaust switching valve 38 begins to open at the combinations of 
a designated engine speed and engine load, during the partial load 
condition. Namely, the line a corresponds to the "equal pressure line" 
where the pressure P.sub.2 downstream of the compressor 26 of the 
small-volume turbocharger 18 has reached the preset value P.sub.SET. As 
can be easily understood, the smaller the degree of the depression of the 
accelerator pedal, the higher the value of the engine speed NE, where the 
pressure P.sub. 2 is equal to the preset pressure value P.sub.SET. 
The point B in FIG. 5 shows the point where the pressure P.sub.1 downstream 
of the compressor 20 of the large-volume turbocharger 16 becomes equal to 
the pressure P.sub.1 downstream of the compressor 26 of the small-volume 
turbocharger 18 (No at step 100 in FIG. 2), so that the exhaust switching 
valve 38 is quickly and fully opened when the accelerator pedal is fully 
depressed. The line b, which extends from the point B, shows points where 
the exhaust switching valve 38 is fully opened during the partial load 
condition. Namely, the line b corresponds to the "equal pressure line" 
where a relationship of P.sub.1 =P.sub.2 =P.sub.SET is obtained at the 
partial load condition of the engine. As can be easily understood, the 
smaller the degree of the depression of the accelerator pedal, the higher 
the value of the engine speed NE at which the relationship of P.sub.1 
=P.sub.2 =P.sub.SET is obtained at the partial load condition of the 
engine. 
In FIG. 5, the region I below the line a is an area wherein the 
small-volume turbocharger 18 is mainly operated, to obtain the desired 
intake pressure. A region II between the line a and b is an area wherein 
both the small-volume turbocharger 18 and the large-volume turbocharger 16 
are operated. Furthermore, the region III above the line c is an area 
wherein only the large-volume turbocharger 16 is operated to obtain the 
desired intake pressure. The EGR area depicted by the lines m1 and m2 
extends between the first and second area I and II. 
FIG. 6 shows relationships between the engine speed NE and the exhaust 
pressure at various portions of the exhaust pipe 14, wherein P.sub.3 is a 
pressure at a location upstream of the turbine 28 of the small-volume 
turbocharger 18, P.sub.4 is a pressure at the occasion between the turbine 
28 of the small-volume turbocharger 18 and the turbine 22 of the 
large-volume turbocharger 16, and P.sub.5 is a pressure at the location 
downstream of the turbine 22 of the large-volume turbocharger 16. As will 
be easily understood, the pressures P.sub.4 and P.sub.5 are continuously 
increased as the engine speed is increased. Contrary to this, the behavior 
of the pressure P.sub.3 with respect to the change in the increase in the 
engine speed is rather complicated. Namely, the pressure P.sub.3 upstream 
of the turbine 28 of the small-volume turbocharger 18 is greatly changed 
by the opening or closing of the exhaust switching valve 38 as already 
explained with reference to FIG. 4. Namely, the circled point 1 
corresponds to a point at which the exhaust switching valve 38 is first 
opened, the circled point 2 is a point at which the exhaust switching 
valve 38 is fully opened in a step-like manner, whereby the pressure is 
abruptly decreased to the circled point 3 as to obtain a value that is 
same as P.sub.4. As well known, the amount of recirculated exhaust gas is 
affected by the pressure of the exhaust gas. Namely, the amount of EGR gas 
is proportional to the difference of the pressure of the intake pipe from 
the pressure in the exhaust pipe. This means that the degree of opening of 
the EGR valve 52 should be determined to obtain the desired amount of the 
EGR gas regardless of the pressure characteristic determined in accordance 
with the position at the exhaust pipe at which the exhaust gas for EGR 
operation is taken out. 
Further, the measures must taken to prevent a reduction of the thermal 
durability of the parts constructing the EGR system. Namely, the removal 
of the exhaust gas for recirculation from the exhaust pipe 14 at the 
position upstream of the turbine 28 of the small-volume turbocharger 18 
causes parts of the EGR system which are less resistant to a high 
temperature, such as the diaphragm 52a, to be quickly damaged, since the 
position is located near the exhaust port of the engine 10, which has a 
very high temperature. Furthermore, the removal of the exhaust gas for 
recirculation at the position downstream from the turbine 22 of the 
large-volume turbocharger 16 causes difficulties in the introduction of 
the exhaust gas into the intake pipe 12, since the pressure P.sub.5 at 
this portion is small. Contrary to this, the removal of the exhaust gas 
for recirculation at the position 50A between the turbines 28 and 30 
enables thermal damage to be suppressed while maintaining a relatively 
high pressure P.sub.4 of the exhaust gas. 
In a modification of the first embodiment, instead of connecting the EGR 
pipe 50 to the position 50B downstream of the intercooler 29 as shown in 
FIG. 1, the EGR pipe, as shown by a dotted line 50', can be connected to 
the intake pipe 12 at a position between the compressor 20 of the 
large-volume turbocharger 16 and the compressor 26 of the small-volume 
turbocharger 18. According to this modified construction, the pressure in 
the exhaust passageway 50 is not affected by the opening or closing of the 
exhaust switching valve 80, not only at the position (50A) where the 
exhaust gas for recirculation is taken out but also at the position (50') 
where the exhaust gas for the recirculation is added. This means that the 
pressure difference between the exhaust gas taking out port and exhaust 
gas adding port is not affected by the opening or closing of the exhaust 
switching valve 38, and thus, in order to control the EGR ratio, instead 
of the two duty ratio maps MAP1 and MAP2 used in the embodiment in FIG. 3, 
only one map for the duty ratio is sufficient to obtain a desired EGR 
ratio. 
FIG. 7 shows the second embodiment of the present invention, wherein the 
exhaust switching valve 38 is controlled by detecting the intake pressure, 
as in the first embodiment, when the engine is under a low speed 
condition, and by the intake air amount when the engine is under a high 
speed condition. FIG. 7 is different from FIG. 1 in that an 
electromagnetic switching valve 300 (VSV6) is additionally provided for 
controlling the pressure to the diaphragm chamber 40g of the actuator 40 
for the exhaust switching valve 38. The electromagnetic switching valve 
300 (VSV6) is switched between a position at which the diaphragm chamber 
40g is connected to the intake pipe 12 at the position 60 downstream of 
the compressor 26 of the small-volume turbocharger 18 and a position at 
which the diaphragm chamber 40g is connected to the atmospheric pressure. 
Furthermore, an air flow meter 302 is provided in the intake pipe 12 
upstream of the compressor 20 of the large-volume turbocharger 16, for 
detecting the intake air amount Q. 
FIG. 8 shows an intake pressure control routine. At step 350 it is 
determined whether the engine speed NE is larger than a predetermined 
value NE.sub.x. The NE.sub.x is a threshold value between the exhaust 
switching valve control based on the intake pressure and the exhaust 
switching valve control based on the intake air amount. As shown in FIG. 
9, the NE.sub.x is a value of the engine speed when the exhaust switching 
valve 38 is fully open and the accelerator pedal is fully depressed. When 
it is determined that NE&gt;NE.sub.x, the routine goes to step 400, where it 
is determined if the intake air amount Q is larger than a predetermined 
value Q.sub.1, and to step 402, where it is determined if the intake 
amount Q is larger than a predetermined value Q.sub.2 (&gt;Q.sub.1). The 
value Q.sub.1 is a threshold value above which the exhaust switching valve 
38 is opened. When it is determined that Q&lt;Q.sub.1 at step 400, the 
routine goes to step 403, where the electromagnetic switching valve 54 
(VSV1) is made OFF so that the diaphragm 34c is opened to the atmospheric 
pressure, whereby the spring 34b urges the diaphragm 40a to close the 
waste gate valve 32. At step 404, the second electromagnetic switching 
valve 58 (VSV2) for controlling the exhaust switching valve 38 is made 
OFF, so that the diaphragm 40h is opened to the atmospheric pressure. At 
step 406, the third electromagnetic switching valve 64 (VSV3) is made ON, 
and at step 408, the fourth electromagnetic switching valve 66 (VSV4) is 
made ON. The steps 406 and 408 are the same as steps 106 and 108 in FIG. 
2, and thus the intake bypass valve 46 is closed, and accordingly, the 
intake pressure is lower than the predetermined value P.sub.SET. At step 
410, the sixth electromagnetic switching valve 300 (VSV6) is made OFF, so 
that the chamber 40g is opened to the atmospheric pressure and the exhaust 
switching valve 38 is closed. 
When it is determined that the intake air amount Q.gtoreq.Q.sub.1 at step 
400, and Q&lt;Q.sub.2 at step 402, the routine goes to the steps following 
step 420. The steps 420 to 426 are the same as steps 403 to 408. At step 
428, the electromagnetic switching valve 300 (VSV6) is made ON, so that 
the chamber 40g is opened to the intake pressure, while the OFF position 
of the electromagnetic switching valve 58 (VSV2) at step 422 allows the 
turbocharging pressure acting on chamber 40h to urge the exhaust switching 
valve 38 to open against the strong combined force of the springs 40c and 
40d. As a result, a controlled or gradual opening of the exhaust switching 
valve 38 is obtained. 
When it is determined at step 402, that Q.gtoreq.Q.sub.2, the routine goes 
to steps below 430, which are the same as steps 110 to 116 in FIG. 1. 
Namely, at step 430, the electromagnetic switching valve 54 (VSV1) is made 
ON, so that the chamber 34c is opened to the intake positive pressure, 
whereby the diaphragm 34a is moved against the force of the spring 34b and 
the waste gate valve 32 is urged open. At step 432, the second 
electromagnetic switching valve 58 (VSV2) is made ON, so that the chamber 
40h is opened to the intake positive pressure, and thus the force urging 
the exhaust switching valve 38 open becomes independent of the force of 
the spring 40d and is opposed only by the weak force of the spring 40c 
alone, and as a result, the actuator 40 allows the exhaust switching valve 
38 to be instantaneously and rapidly opened. Steps 434 and 436 are for 
opening the intake bypass valve 46, and are similar to steps 114 and 116 
in FIG. 2. Furthermore, at step 438, the electromagnetic switching valve 
300 (VSV6) is made OFF, so that the chamber 40g is opened to the 
atmospheric pressure. 
When it is determined at step 350, that the NE.ltoreq.NE.sub.x, the routine 
goes to steps 420 to 428, which are the same as steps 102 to 108 in the 
first embodiment of FIG. 2. Namely, the engine is in a low speed condition 
and the exhaust switching valve 38 is fully closed when intake pressure 
P.sub.2 at the position downstream of the turbine 26 of the small-volume 
turbocharger 18 is lower than a predetermined value P.sub.SET. When the 
intake pressure P.sub.2 reaches the predetermined value P.sub.SET, the 
exhaust switching valve 38 is gradually opened in accordance with an 
increase in the engine speed. 
FIG. 9 shows the operation of the exhaust switching valve 38 with respect 
to the engine speed and load in the second embodiment. A vertical line X 
corresponds to the engine speed, which is equal to NE.sub.x. In the area 
where the engine speed is higher than this value NE.sub.x, the control of 
the exhaust switching valve 38 is carried out by the intake air amount Q. 
The exhaust switching valve 38 begins to open when the line Q=Q.sub.1 is 
crossed, and is quickly opened toward the fully open position when the 
line Q=Q.sub.2 is crossed. When the engine speed is lower than NE.sub.x, 
the control of the exhaust switching valve 38, as in the first embodiment, 
is carried out by the intake pressure. When the line P=P.sub.2 =P.sub.SET 
is crossed, the exhaust switching valve 38 begins to open. It should be 
noted that the setting is 
such that the line P.sub.2 =P.sub.SET in the area I is smoothly connected 
to the line Q=Q.sub.2 in the area II in FIG. 9. 
The control of the exhaust switching valve by the intake pressure during 
the low engine speed and by the intake air amount during the high engine 
speed can prevent a drop in the intake pressure when the engine speed is 
high, which often occurs in a conventional turbocharger system. 
The EGR control in this second embodiment is substantially the same as in 
the first embodiment. Namely, the same routine as in FIG. 3 can be 
employed. The range for executing the EGR operation can be same as that in 
the first embodiment, i.e., can be located between the regions I and II. 
Nevertheless, as shown in FIG. 9, the EGR region can be limited to the 
first region I as shown by the dotted area, and in this case, steps 124 
and 130 can be eliminated. 
Although embodiments of the present invention are described with reference 
to the attached drawings, many modifications and changes can be made by 
those skilled in this art.