Method and apparatus for detecting pneumatic pressure in an internal combustion engine

The absolute intake manifold pneumatic pressure and the atmospheric pressure are selectively detected by a single absolute pneumatic pressure sensor, and the relative intake manifold pneumatic pressure is calculated from the detected values. The detection of the atmospheric pressure by the sensor is carried out only when the engine is operated under a predetermined operating condition.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for detecting 
pneumatic pressure in the intake passage downstream of the throttle valve 
of an internal combustion engine. 
In an internal combustion engine, the relative pneumatic pressure (negative 
pneumatic pressure) in the intake manifold must be detected in order to 
execute ignition timing control or exhaust gas recirculation (EGR) control 
(in the case of using an EGR value controlled by the atmospheric pressure 
and the negative pneumatic pressure in the intake manifold). For this 
purpose, the relative pneumatic pressure in the intake manifold is 
detected by a relative pneumatic pressure sensor. In the internal 
combustion engine of the fuel injection type in which the amount of fuel 
injection is controlled depending upon the running speed of the engine and 
the absolute pneumatic pressure in the intake manifold, however, it is an 
essential requirement that the absolute pneumatic pressure in the intake 
manifold be detected. The engine of this type, therefore, must be equipped 
with both a sensor for detecting relative pneumatic pressure in the intake 
manifold and a sensor for detecting absolute pneumatic pressure in the 
intake manifold. Otherwise, the engine must be provided with either one of 
the two types of sensors, and the detected value must be utilized for 
finding the other value. The former method makes it possible to maintain 
good control precision in the control operations but requires two sensors 
and, hence, necessitates increasing the manufacturing cost. According to 
the latter method, on the other hand, no compensation is effected when the 
atmospheric pressure is changed and, hence, control precision is greatly 
decreased. Consequently, it is difficult in the case of the latter method 
to cope with the stringent regulations placed on exhaust gases in recent 
years. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a method 
and an apparatus for detecting pneumatic pressure in an internal 
combustion engine, whereby only a single absolute pneumatic pressure 
sensor need be employed without decreasing the control precision of the 
engine. 
According to the present invention, a method for detecting pneumatic 
pressure in an internal combustion engine which has a single absolute 
pressure sensor for detecting the absolute pneumatic pressure of an 
applied fluid comprises the steps of: detecting the absolute pneumatic 
pressure in the intake passage downstream of the throttle valve and the 
atmospheric pressure by selectively communicating the absolute pressure 
sensor to a pressure detection port open to the intake passage downstream 
of the throttle valve or to the open air, the above detection of 
atmospheric pressure being executed only when the engine is operated under 
a predetermined operating condition; and calculating the relative 
pneumatic pressure in the intake passage downstream of the throttle valve 
from the detected absolute pneumatic pressure in the intake passage and 
from the atmospheric pressure. 
Furthermore, according to the present invention, an apparatus for detecting 
pneumatic pressure in an internal combustion engine comprises: a single 
absolute pressure sensor for detecting the absolute pneumatic pressure of 
an applied fluid; means for selectively communicating the absolute 
pressure sensor to a pressure detection port open to the intake passage 
downstream of the throttle valve or to the open air so as to detect the 
absolute pneumatic pressure in the intake passage downstream of the 
throttle valve and the atmospheric pressure, the absolute pressure sensor 
communicating with the open air only when the engine is operated under a 
predetermined operating condition; and means for calculating the relative 
pneumatic pressure in the intake passage downstream of the throttle valve 
from the detected absolute pneumatic pressure in the intake passage and 
from the atmospheric pressure. 
The above and other related objects and features of the present invention 
will be apparent from the description of the present invention set forth 
below, with reference to the accompanying drawings, as well as from the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, reference numeral 10 denotes an internal combustion 
engine, 12 denotes an air cleaner, 14 denotes a throttle valve, 16 denotes 
an intake manifold, and 18 denotes an exhaust manifold. The intake 
manifold 16 is communicated with a port of a three-way electromagnetic 
switching valve 22 via a conduit 20. Another port of the switching valve 
22 is open to the open air through a conduit 24 and an air filter 26. A 
further port of the switching valve 22 is communicated with a single 
absolute pressure sensor 30 via a conduit 28. The output signal of the 
absolute pressure sensor 30 is fed to a control circuit 34 via a line 32. 
Responsive to the drive signals sent from the control circuit 34 via a 
line 36, the switching valve 22 selectively communicates the conduit 28 
with the conduit 20 or with the conduit 24. In other words, the switching 
valve 22 selectively applied the pneumatic pressure in the intake manifold 
16 or the atmospheric pressure to the absolute pressure sensor 30 
responsive to the drive signals from the control circuit 34. 
The intake manifold 16 is provided with a plurality of fuel injection 
valves (or often a single fuel injection valve) 38 which inject the 
compressed fuel from the fuel supply system (not shown) into the engine 
responsive to injection signals sent from the control circuit 34 via a 
line 40. 
The absolute pressure sensor 30 detects the absolute pneumatic pressure of 
the gas applied thereto and generates signals of a power or a current that 
corresponds to the detected absolute pneumatic pressure. FIG. 2 
illustrates one example of the absolute pressure sensor consisting of a 
semiconductor. In FIG. 2, reference numeral 42 denotes a housing, 44 
denotes a sealed chamber of which the internal side is maintained as a 
vacuum, 46 denotes a semiconductor chip disposed in the sealed chamber, 48 
denotes a conduit which is mounted so that one end comes into intimate 
contact with one surface of the semiconductor chip 46, and 50 denotes an 
electric circuit. The conduit 48 receives the pneumatic pressure to be 
detected, and the pneumatic pressure in the conduit 48 acts upon one 
surface of the semiconductor chip 46. The semiconductor chip 46 will have 
a plurality of resistance layers that are connected in a bridgelike 
fashion on the surface of a silicon chip. The semiconductor chip 46 is 
distorted when the pneumatic pressure is exerted on the surface thereof, 
and resistances of the resistance layers undergo variation depending upon 
the degree of distortion. By converting the change in the resistances 
through the bridge circuit and the above-mentioned circuit 50, it is 
possible to obtain signals having a voltage value or a current value that 
corresponds to the applied pneumatic pressure. Further, since the other 
side of the semiconductor chip 46 is maintained as a vacuum, the signals 
possess a value that corresponds to the absolute pneumatic pressure. 
FIG. 3 illustrates another example of the absolute pressure sensor of the 
bellows type (Gulton type), in which reference numeral 52 denotes a 
housing, 54 denotes a pressure chamber which receives the pneumatic 
pressure to be detected, 56 denotes a vacuum bellows installed in the 
pressure chamber 54, 58 denotes a moving core coupled to an end of the 
bellows 56 via a rod 60, 62 denotes a differential transformer disposed 
around the core 58, and 64 denotes an electric circuit. The vacuum bellows 
56 expands and contracts depending upon the pneumatic pressure in the 
pressure chamber 54, and the core 58 moves in the axial direction. The 
differential transformer 62 is provided with an exciting coil that is 
served with an a-c current of a predetermined frequency and a detection 
coil for removing, in the form of electric signals, the a-c magnetic flux 
that changes depending upon the position of the core 58. By converting the 
detected signals through the circuit 64, it is possible to obtain signals 
having a voltage value or a current value that corresponds to the absolute 
pneumatic pressure. 
FIG. 4 is a block diagram which schematically illustrates the control 
circuit 34 of FIG. 1. As will be obvious from FIG. 4, the control circuit 
34 chiefly consists of a programed digital computer (microcomputer). 
The output terminals of the absolute pressure sensor 30 have been connected 
to the input terminals of a predetermined channel of an analog multiplexer 
70. Input terminals of other channels of the multiplexer 70 receive 
signals of various sensors (not shown) that generate analog detection 
signals. The multiplexer 70 selects the channels responsive to the 
instructions from a central processing unit (CPU) 84, described later, and 
sends the signals of the selected channel to an analog-to-digital 
converter (A/D converter) 72 which converts the input signals into digital 
signals according to the instructions for initiating A/D conversion sent 
from the CPU 84. An input interface 74 receives signals from the sensors 
that generate digital detection outputs. A fuel injection control circuit 
78 and a drive circuit 80 are connected to an output interface 76. The 
output data related to the fuel injection time sent from the CPU 84 is 
converted into injection signals through the fuel injection control 
circuit 78 and is applied to the fuel injection valves 38. Further, the 
output data related to changing the port of the switching valve 22 is 
amplified through the drive circuit 80 to form drive signals that will be 
sent to the switching valve 22. 
The A/D converter 72, input interface 74, and output interface 76 are 
connected to the CPU 84, to a read-only memory (ROM) 86 and to a random 
access memory (RAM) 88 via a data bus 82. The ROM 86 stores a program for 
controlling the digital computer as well as the data used for performing 
the calculation. 
FIGS. 5, 6A and 6B are flow diagrams illustrating parts of the 
above-mentioned control program. Opration of the embodiment of the present 
invention will be described below in conjunction with FIGS. 5, 6A and 6B. 
When the ignition switch (not shown) of the engine is turned on, an 
initializing signal is applied to the CPU 84 via a line 90; the CPU 84 
executes the initial routine which is shown in FIG. 5. First, at a point 
100, the CPU 84 instructs to select the atmospheric pressure port in order 
to switch the switching valve 22 toward the side of the open air. This 
instruction is performed by producing, for example, an output "1" to a 
predetermined bit position of the output interface 76. Therefore, the 
switching valve 22 is energized so that atmospheric pressure is applied to 
the pressure sensor 30. At a point 101, the CPU 84 instructs the 
multiplexer 70 so as to effect selection of the channel that corresponds 
to the pressure sensor 30. Then, at a point 102, the CPU 84 instructs the 
A/D converter 72 to initiate A/D conversion of output signals produced by 
the pressure sensor 30. At a point 103, the CPU 84 repetitively 
discriminates whether A/D conversion is finished or not until A/D 
conversion is completed. When the notice of completion of A/D conversion 
is sent to the CPU 84 via a line 92, the program proceeds to a point 104 
where the data after it was A/D converted is introduced. Then, at a point 
105, the thus introduced data is stored as the data Pat of atmospheric 
pressure in a predetermined area of the RAM 88. Then at a point 106, the 
CPU 84 produces instructions to select the intake pressure port in order 
to change the switching valve 22 toward the side of intake pressure. This 
instruction is performed by setting "0" to the bit of the output interface 
76 which is related to the instructions for selecting the atmospheric 
pressure port. Therefore, the switching valve 22 is de-energized, and the 
pneumatic pressure (intake manifold pressure) in the intake manifold 16 is 
applied to the pressure sensor 30. At the succeeding points 107, 108 and 
109, the flag that will be used hereinafter and the timer are set to the 
initial conditions. That is, the atmospheric pressure flag and the time 
flag are turned off, and a counted value T, which is the content of the 
timer, is set to "0", i.e., O.fwdarw.T. Then the CPU 84 executes a variety 
of other processings required for the initial routine; the routine of FIG. 
5 is completed. 
According to the processing routine of FIG. 5 mentioned in the foregoing 
paragraph the data Pat of atmospheric pressure is detected when the power 
supply is turned on and the program is initialized, and the data is stored 
in the RAM 88. 
When the engine is continuously operated thereafter, the CPU 84 executes 
the interrupt processing routine of FIGS. 6A and 6B after every completion 
of A/D conversion is reported from the A/D converter 72 via the line 92. 
Namely, the processing routine of FIGS. 6A and 6B is executed after every 
interrupt period that nearly corresponds to the time of A/D conversion. As 
the interrupt is generated, at a point 110, the CPU 84 introduces the data 
which has been subjected to A/D conversion. Then, at a point 111, the CPU 
84 discriminates from the channel selected by the multiplexer 70 whether 
the data subjected to A/D conversion is the data from the pressure sensor 
30 or not. When the channel is not related to the pressure sensor 30, the 
program proceeds to a point 112 where it is discriminated what data was 
subjected to A/D conversion. Then the A/D converted data is stored in the 
corresponding area of RAM 88 at a point 113. Thereafter, the program 
proceeds to a point 114. When it is discriminated at a point 111 that the 
channel is related to the pressure sensor 30, the program proceeds to a 
point 115 where the CPU 84 discriminates whether the atmospheric pressure 
flag is on or not. The atmospheric pressure flag is turned on when the 
atmospheric pressure port is selected and is turned off when the intake 
pressure port is selected. Therefore, when the atmospheric pressure flag 
is on, the input data (A/D data) indiates the detected atmospheric 
pressure. At a point 116, therefore, the detected data is stored as the 
atmospheric pressure data Pat in a predetermined area of the RAM 88. Then, 
at a point 117, the CPU 84 produces instructions to select the intake 
pressure port, at a point 118, turns the atmospheric pressure flag off, at 
a point 119, resets the content of the timer to zero, i.e., sets T.rarw.0, 
and thereafter the program proceeds to the point 114. When it is 
discriminated at the point 115 that the atmospheric pressure flag is off, 
the program proceeds to a point 120 where the input data (A/D data) is 
stored as the intake manifold pressure data Pab in a predetermined area of 
the RAM 88. Then, at a point 121, the CPU 84 calculates the difference Pre 
between the atmospheric pressure data Pat and the intake manifold pressure 
data Pab based upon a relation Pre=Pat-Pab. Since the pressure sensor 30 
is the absolute pressure sensor as mentioned earlier, the intake pressure 
data Pab represents the absolute pneumatic pressure in the intake 
manifold, and the calculated result Pre represents the relative pneumatic 
pressure in the intake manifold. At a point 122, the thus calculated data 
Pre of the relative intake manifold pressure is stored in a predetermined 
area of the RAM 88. 
At a point 123, the CPU 84 discriminates whether the data Pre of the 
relative intake manifold pressure stored in the RAM 88 is on the side of 
the atmospheric pressure compared to a predetermined pressure value, for 
example, compared to a value that corresponds to -60 mmHg. When 
Pre.gtoreq.-60 mmHg, i.e., when the data Pre is on the side of the 
atmospheric pressure, the program proceeds to a point 124 where it is 
discriminated whether the timer flag is on or not. The program proceeds to 
points 125 through 127 only when the relative intake manifold pressure is 
on the side of atmospheric pressure compared to -60 mmHg and only when the 
timer flag is on. In other cases, the program proceeds to the point 114. 
At the point 114, the CPU 84 discriminates whether a counted value T, which 
is the content of the timer, is greater than a predetermined value A. When 
T.ltoreq.A, the program proceeds to a point 128 where to value T is 
increased by one. Regarding T.ltoreq.A, the program necessarily passes 
through the point 128 for every execution of the interrupt processing 
routine of FIGS. 6A and 6B. Therefore, the counted value T becomes greater 
than the predetermined value A after predetermined periods of time have 
passed. In this case, therefore, the program proceeds from the point 114 
to a point 129 where the timer flag is turned on. 
At a point 130, the channel of the multiplexer 70 is changed to the next 
channel which is different from that of the pressure sensor 30. At a point 
131, the CPU 84 produces instructions to initiate A/D conversion of the 
data of the changed channel, whereby the interrupt processing routine is 
completed. 
When the relative intake manifold pressure is on the side of the 
atmospheric pressure compared to -60 mmHg, and when the timer flag is on 
at the points 123 and 124 as mentioned above, processing of the points 125 
through 127 is executed. When the relative intake manifold pressure is on 
the side of the atmospheric pressure compared to -60 mmHg, the operation 
conditions of the engine remain relatively stable, and the intake manifold 
pressure does not change suddenly. Under the above-mentioned condition in 
which the timer flag is on, the CPU 84 produces instructions to select the 
atmospheric pressure port at the point 125, turns the atmospheric pressure 
flag on at the point 126, turns the timer flag off at the point 127, and 
the program then proceeds to the point 131, where the CPU 84 produces 
instructions to initiate A/D conversion without changing the channel of 
the multiplexer 70. Namely, the data Pat of the atmospheric pressure is 
detected in the above-mentioned case. As mentioned above, the timer flag 
is turned on only when the counted value T is greater than the 
predetermined value A. Further, the counted value T becomes zero only when 
the atmospheric pressure data Pat is being detected. Therefore, the timer 
flag is turned on when a predetermined period of time has passed after 
since the atmospheric pressure data Pat of the previous time detected. In 
other words, the pressure sensor 30 usually detects the absolute pneumatic 
pressure in the intake manifold 16 and detects the atmospheric pressure 
only when the relative pneumatic pressure is more on the side of 
atmospheric pressure than on the side of the predetermined value and only 
after a predetermined period of time has passed since the atmospheric 
pressure was detected the previous time. The atmospheric pressure is 
detected after a period of time longer than the predetermined period of 
time, since the atmospheric pressure does not suddenly change, and it is 
desirable to detect the intake manifold pressure more frequently than to 
detect the atmospheric pressure. 
Here, the value -60 mmHg at the point 123 of the processing routine of FIG. 
6B may be changed to any value ranging, for example, from -10 mmHg to -60 
mmHg. 
According to the present invention as described in detail in the foregoing 
paragraphs, the absolute intake manifold pressure and the atmospheric 
pressure are selectively detected by the absolute pressure sensor, and the 
relative intake manifold pressure is calculated from the detected value. 
Therefore, the absolute intake manifold pressure and the relative intake 
manifold pressure can be detected by a single pressure sensor, and the 
engine can be controlled based upon these pressures while maintaining 
increased control precision, without causing the manufacturing cost to be 
increased. Further, since the atmospheric pressure is detected only when 
the engine is operated under a predetermined operating condition, the 
detection precision in the case of intake manifold pressure does not 
decrease and the control precision relying upon the intake manifold 
pressure does not decrease either. Furthermore, the absolute pressure 
sensor features increased precision for detecting the intake manifold 
pressure. If it is attempted to detect the relative pneumatic pressure 
using a relative pressure sensor, the detection precision tends to 
decrease. According to the present invention, however, the detection 
precision does not decrease, and, even in this sense, the present 
invention makes it possible to obtain detection values while maintaining 
high precision and to increase control precision. 
As many widely different embodiments of the present invention may be 
constructed without departing from the spirit and scope of the present 
invention, it should be understood that the present invention is not 
limited to the specific embodiments described in this specification, 
except as defined in the appended claims.