Apparatus for detecting pressure in cylinder of internal combustion engine

An apparatus for detecting the pressure in a cylinder of an internal combustion engine is disclosed which includes a piezoelectric pressure sensor and a current input circuit. The sensor detects the pressure in the cylinder. A current input circuit receives the output signal of the sensor in the form of a current and generates an output corresponding to the value of the current in the form of a voltage or current.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for detecting pressure in a 
cylinder of an internal combustion engine, which is used to measure 
information on the pressure in the combustion cylinder of an internal 
combustion engine. 
2. Description of Related Background Art 
In general, the pressure in a combustion cylinder is measured in order to 
determine the state of combustion of an internal combustion engine and the 
operation of each cycle of rotation. A piezoelectric cylinder pressure 
sensor is ordinarily used as a cylinder pressure sensor in such a pressure 
measurement. In such a sensor, the pressure in the cylinder is directly or 
indirectly applied to a piezoelectric element which generates electric 
charges in response to the pressure, and a charge amount corresponding to 
the applied pressure is then output. FIG. 1 shows an example of a 
piezoelectric cylinder pressure sensor. In the diagram, reference numeral 
11 denotes piezoelectric elements; 12 indicates an electrode which is 
sandwiched between the two piezoelectric elements 11 and leads an output 
signal to a lead wire 13; and 14 is a casing which covers the internal 
structural parts of the sensor. The sensor has a ring-like shape and is 
placed between a cylinder head 2 forming an upper wall of a combustion 
cylinder of an engine and a spark plug 3, as shown in FIG. 2. The pressure 
in the combustion cylinder is propagated through the spark plug to the 
piezoelectric elements 11 of a piezoelectric cylinder pressure sensor 1, 
whereby charges corresponding to the cylinder pressure are generated. 
Since an output signal corresponding to the cylinder pressure of the sensor 
represents a charge amount, the charge amount needs to be converted into 
the value of a voltage which can readily be electrically processed. For 
this purpose, hitherto, a charging amplifier has generally been used as 
means for converting the charge amount into the voltage value. FIG. 3 
shows a fundamental circuit diagram of a charging amplifier. In FIG. 3, 
reference numeral 41 denotes an operational amplifier and 42 is a 
capacitor. An output of the sensor 1 is input to an inverting input 
terminal of the operational amplifier 41. The capacitor 42 is connected 
between the inverting input terminal of the operational amplifier 41 and 
an output terminal thereof. A non-inverting input terminal of the 
operational amplifier 41 is connected to the ground. The output of the 
operational amplifier 41 is controlled so as to equalize the levels of the 
voltages at the inverting and non-inverting input terminals. Accordingly, 
when an electric charge value Q from the sensor 1 is input to the 
operational amplifier 41, the amplifier operates to charge the capacitor 
42 with the same amount as the charge value Q. 
Therefore, assuming that the electrostatic capacity of the capacitor 42 is 
set at C, the voltage of V=Q/C is output from the operational amplifier 
41. Since the charge amount Q is proportional to the cylinder pressure, 
the output voltage V of the operational amplifier 41 has a value 
corresponding to the pressure in the cylinder. During the operation of the 
engine, a combustion pressure signal as shown in FIG. 4 is output. 
However, since the foregoing charging amplifier is of the type in which the 
charge amount is directly converted into the voltage value by the 
capacitor and the electrostatic capacity of the capacitor 42 is set to a 
small value in accordance with the electrostatic capacity of the 
piezoelectric elements 11 of the sensor 1, the following problem is 
encountered. If charges other than the cylinder pressure signal move, that 
is, if leakage currents flow through the sensor 1, the output signal line 
thereof, the input section of the charging amplifier, or the like, or if 
an input bias current flows through the operational amplifier 41 or the 
like, the output voltage of operational amplifier 41 fluctuates and hence 
the cylinder pressure cannot be accurately measured. 
Further, the pressure/charge amount converting characteristics in the 
sensor 1 vary in accordance with temperature changes, and so-called pyro 
effect acts to cause charges to be generated in accordance with changes in 
the temperature of the piezoelectric element. In particular, when the 
sensor is attached near the combustion chamber as shown in FIG. 2, a 
problem is encountered in that the temperature change which occurs each 
cycle due to increases in the temperature of the cylinder head 2 or spark 
plug 3 or any temperature transfer in the combustion chamber is large. 
This means the output signal waveform of the charging amplifier is greatly 
influenced by such temperature changes and the cylinder pressure cannot be 
accurately measured. An additional problem is that, even if attempts are 
made to provide for temperature compensation by certain means, the 
construction of such means will inevitably be complicated because the 
output charge amount in the circuit of the charging amplifier is directly 
converted into the voltage value by the capacitor 42. 
There is still another problem in that although the output voltage of the 
charging amplifier 41 changes in accordance with variations in cylinder 
pressure, it cannot represent the absolute pressure value of the pressure 
in the cylinder. 
SUMMARY OF THE INVENTION 
The present invention was conceived in consideration of the foregoing 
problems and it is the first object of the invention to realize accurate 
measurement which is not influenced by leakage currents in the conversion 
of an output signal of a piezoelectric cylinder pressure sensor into a 
voltage value. 
The second object of the invention is to realize accurate measurement of 
pressure in a cylinder which is not influenced by changes in temperature 
of a cylinder pressure sensor when a signal corresponding to the cylinder 
pressure is obtained from an output signal of a piezoelectric cylinder 
pressure sensor. 
The third object of the invention is to enable generation of a signal 
corresponding to the absolute value of the pressure in a cylinder by 
utilizing an output signal of a cylinder pressure sensor. 
The fourth object of the invention is to provide a cylinder pressure 
detecting apparatus which can detect any failure of a cylinder pressure 
sensor in which the occurrence of such a failure is discerned by checking 
whether or not a cylinder pressure signal from a piezoelectric cylinder 
pressure sensor exceeds a predetermined reference level. 
To accomplish the above objects, the present invention incorporates the 
following features. 
In an apparatus for detecting pressure in a cylinder of an internal 
combustion engine according to the present invention, an output signal of 
a piezoelectric pressure sensor is received in the form of a current and 
converted into a voltage value or a current value in a current input 
circuit. Thereafter, it is integrated by an integrator to thereby obtain a 
cylinder pressure signal. Since the integrator is connected through the 
current input circuit to the sensor, the integrator can be constructed 
without taking into account the electrostatic capacity of a piezoelectric 
element. Thus, a cylinder pressure signal which is not influenced by 
leakage current can be output. 
According to the invention, the cylinder pressure detecting apparatus is 
provided with a temperature sensor for measuring the temperature of a 
cylinder pressure sensor. An output signal from a cylinder pressure signal 
output circuit is obtained from an output signal of the cylinder pressure 
sensor and corresponds to the pressure in the relevant cylinder. The 
output signal is corrected on the basis of an output value of the 
temperature sensor, and the cylinder pressure signal can thus be corrected 
to allow for any temperature change. 
According to the present invention, in the cylinder pressure detecting 
apparatus, an output signal of the piezoelectric pressure sensor is 
received in the form of a current and converted into a voltage value or a 
current value and, thereafter, it is integrated by an integrator and the 
integrated value is set at a predetermined value at a predetermined 
timing. Thus the absolute pressure value can be obtained on the basis of 
an output from the integrator. 
The cylinder pressure detecting apparatus according to the invention 
further comprises a comparator adapted to compare the output signal from 
the integrator with a predetermined reference level and a failure 
detecting circuit adapted to detect an output pulse from the comparator, 
thereby enabling the occurrence of any failure of the sensor to be 
detected.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 5 shows an embodiment of the present invention. In FIG. 5, reference 
numerals 51 and 52 denote operational amplifiers; 53, 55, and 57 indicate 
resistors; and 54 and 56 are capacitors. An output of the piezoelectric 
cylinder pressure sensor 1 is input to an inverting input terminal of the 
operational amplifier 51. The resistor 53 is connected between the 
inverting input terminal of the operational amplifier 51 and an output 
terminal thereof. The capacitor 54 and resistor 55 are connected in series 
between the output terminal of the operational amplifier 51 and an 
inverting input terminal of the operational amplifier 52. The resistor 57 
and capacitor 56 are connected in parallel between the inverting input 
terminal of the operational amplifier 52 and an output terminal thereof. 
Non-inverting input terminals of the operational amplifiers 51 and 52 are 
connected to the ground. 
The operation of the embodiment shown in FIG. 1 will now be described. When 
a charge value Q corresponding to the pressure in a cylinder is output 
from the sensor 1, a current of -dQ/dt is output from the operational 
amplifier 51 and flows through the resistor 53 due to the feedback control 
of the operational amplifier 51. Now, assuming that a resistance value of 
the resistor 53 is set to R.sub.1, an output voltage V.sub.1 of the 
operational amplifier 51 can be expressed by the following equation due to 
a voltage drop brought about by the resistor 53. 
##EQU1## 
That is, the output current dQ/dt of the sensor 1 is converted into the 
voltage value. In FIG. 6(a), a solid line indicates a signal waveform of 
the output voltage V.sub.1 of the operational amplifier 51 which is based 
on the crank angle. The signal waveform corresponds to the result which is 
obtained by differentiating the cylinder pressure over a period of time. 
An output current of the operational amplifier 51 is transferred through 
the capacitor 54 and resistor 55 to the inverting input terminal of the 
operational amplifier 52. 
The electrostatic capacity of the capacitor 54 is set at a high enough 
value to be used for AC coupling. Thus the impedance of the capacitor is a 
fairly small value suitable for an appropriate current change ratio with 
respect to changes in cylinder pressure. Therefore, as shown by the 
following equation, the current I.sub.2 flowing through the capacitor 54 
is determined by the output voltage V.sub.1 of the operational amplifier 
51 and a resistance value R.sub.2 of the resistor 55. 
##EQU2## 
The current I.sub.2 flows from the operational amplifier 52 by virtue of 
feedback control. The resistor 57 is provided to reset the output voltage 
V.sub.2 of the operational amplifier 52 to zero. The resistance value 
R.sub.3 of the resistor 57 is set at a high value so that the current 
obtained will be at a level that can be ignored in terms of the current 
flowing through the capacitor 56. Therefore, the output voltage V.sub.2 of 
the operational amplifier 52 is determined by the output current -I.sub.2 
and the electrostatic capacity C.sub.2 of the capacitor 56 as shown by the 
following equation. 
##EQU3## 
That is, the output voltage V.sub.2 of the operational amplifier 52 is 
proportional to the output charge value Q of the sensor 1 and the 
associated output signal V.sub.2 corresponds to the cylinder pressure as 
shown in FIG. 6(b). 
In this manner, the cylinder pressure signal can be obtained by inputting 
the output signal of the piezoelectric cylinder pressure sensor as a 
current and by integrating the signal corresponding to the input current. 
On the other hand, if a stationary leakage current I.sub.L were generated 
on the output line of the sensor 1, the output voltage V.sub.1 of the 
operational amplifier could be represented as follows; 
##EQU4## 
and the associated output signal waveform is shown by a broken line in 
FIG. 6(a). However, since the electrostatic capacity C.sub.2 of the 
capacitor 56 can be set at a relatively large value, the influence caused 
by the input leakage current of the operational amplifier 52 is reduced. 
Further, since the input signal of the integrator including the 
operational amplifier 52 in the next stage is coupled through the 
capacitor 54 in an AC manner to the output of the amplifier 51, the amount 
of change -R.sub.1 I.sub.L caused by the leakage current is blocked by the 
capacitor 54 and only the signal component 
##EQU5## 
brought about by the change in cylinder pressure is integrated. 
Consequently, the waveform of the output of the integrator, i.e., the 
operational amplifier 52, becomes a stable cylinder pressure waveform as 
shown in FIG. 6(b), irrespective of the presence or absence of leakage 
current. 
In the embodiment described above, the current/voltage converting circuit 
is used as the current input circuit by using the operational amplifier 
51. However, a current amplifying circuit which receives a current and 
outputs a current can also be used. In such a case, a resistor 55 is not 
provided in the integrator including the operational amplifier 52, and it 
is sufficient to directly integrate the output current by means of 
integrator. 
Additionally, although an AC coupling capacitor is only interposed between 
the current input circuit and the integrator in the above-described 
embodiment, it is also possible to dispose one between the current input 
circuit and the piezoelectric cylinder pressure sensor 1. 
Next, the means for compensating for temperature changes in this invention 
will be described. The pressure/charge amount conversion coefficient of 
the piezoelectric cylinder pressure sensor 1 has a positive temperature 
characteristic. When temperature rises, the output charge also rises. 
Therefore, the output signal of the operational amplifier 51 increases as 
shown by the broken line in FIG. 11(a). To accurately obtain cylinder 
pressure, it is necessary to perform an adjustment to allow for the change 
in temperature of the sensor 1. In this embodiment, therefore, a 
thermistor 6 is provided in the sensor 1, as shown in FIG. 7. In FIG. 7, 
an electrode 16 and an insulative plate 15 are provided in the sensor 
shown in FIG. 1 in addition to the thermistor 6. The amount of any 
temperature change is detected from the change in resistance of the 
thermistor 6. As shown in FIG. 9, an amplifier 58 is connected to the 
thermistor 6 and the change in resistance of the thermistor 6 is thereby 
converted into a change in voltage. The resistor 53 for feedback of the 
operational amplifier 51 changes the resistance value in accordance with 
the voltage output of the amplifier 58 so that the resistance value of the 
resistor 53 decreases as the voltage output from the amplifier 58 
increase. Therefore, even if the level of the voltage output from the 
operational amplifier 51 tends to increase as shown by the broken line in 
FIG. 11(a), due to an increase in the output charge caused by the increase 
in temperature of the sensor 1, the resistance value of the resistor 53 is 
reduced because of the change in resistance of the thermistor 6. Thus, 
despite the change in temperature of the sensor 1, a signal as shown by 
the solid line in FIG. 11(a) can be obtained and a cylinder pressure 
signal which is consistently stable can be derived as shown in FIG. 11(b). 
Further, it is also possible to provide an arrangement in which the 
thermistor 6, insulative plate 15 and electrode 16 are assembled in the 
sensor 1 in the manner shown in FIG. 8. In this case, the thermistor 6 
itself functions as a part of the feedback resistor of the operational 
amplifier 51, as shown in FIG. 10. In such a case, the thermistor 6 is 
constructed as a part of a current/voltage converting element and any 
change in resistance caused by a change in the temperature of the 
thermistor 6 immediately changes the current/voltage conversion 
coefficient. 
Therefore, even in the arrangement shown in FIG. 10, if the output signal 
of the operational amplifier 51 tends to increase, as shown by the broken 
line in FIG. 11(a), due to an increase in output charge caused by the 
increase in temperature of the sensor 1, the sum of the resistance values 
of the resistor 53 and thermistor 6 is reduced due to the change in 
resistance of the thermistor 6. Thus, in spite of the change in 
temperature of the sensor 1, the signal shown by the solid line in FIG. 
11(a) can be obtained and a cylinder pressure signal which is consistently 
stable can be derived therefrom as shown in FIG. 11(b). 
In the second and third embodiments shown in FIGS. 7 to 10, the thermistor 
is used as a temperature measuring device. However, other devices such as 
a semiconductor, thermocouple element, or the like can also be used. 
Additionally, the temperature detecting device can be arranged adjacent to 
the cylinder pressure sensor; the position in the sensor is not critical. 
A fourth embodiment of the invention will now be described with reference 
to FIGS. 12 and 13. The output waveform of the operational amplifier 52 
shows a change in cylinder pressure and does not show the absolute value 
of the cylinder pressure. Therefore, in the embodiment indicated in FIG. 
12, an analog switch 68 is connected in parallel with the integrating 
capacitor 56 and an integrated value (charge voltage of the capacitor 56) 
can be reset to zero by turning on the analog switch 68 at a predetermined 
timing. The absolute value of the cylinder pressure at the predetermined 
timing may be measured by any method and the change value of the output of 
the operational amplifier 52 after resetting is added to the absolute 
value at the predetermined timing and an absolute value of the cylinder 
pressure at any given timing after resetting can be obtained as the summed 
value. As a practical method for use in such a case, when, for instance, 
the piston of the cylinder whose pressure is to be measured is located at 
bottom dead center (hereinafter, abbreviated as BDC) in the air intake 
step, the pressure value in the intake air pipe is measured, the pressure 
value is set to the cylinder pressure absolute value at the intake BDC, 
and the integrated value is reset to zero at that timing. This is because 
the intake air valve is open at the intake BDC and the pressures in the 
cylinder and the intake air pipe are equalized. To realize the above 
method, which is arithmetically operated by using an output signal from a 
crank angle sensor attached to the cam shaft of the engine or the like, a 
pulse signal is input to a control input terminal 59 of the analog gate 68 
at the timing of the intake BDC. On the other hand, to measure the 
pressure in the intake air pipe, a pressure sensor for converting the 
pressure value into an electrical signal can be used. According to the 
above method, the output signal waveform of the operational amplifier 52 
is set to zero at the intake BDC as shown in FIG. 13(b), and a change in 
cylinder pressure is shown by using that point as a reference. Hence, the 
absolute value of the cylinder pressure can be represented by the sum of 
the pressure value of the pressure sensor and the pressure value based on 
the output signal of the operational amplifier 52. 
In this embodiment, the integrated value is reset at the timing of the 
intake BDC. However, it can also be reset to another timing in the period 
during which the intake valve is open. 
Although the integrated value is set to zero in this embodiment, it can 
alternatively be set to a value corresponding to the pressure value of the 
pressure sensor, and the absolute pressure value can also be directly 
obtained from the output value of the operational amplifier 52. In such a 
case, the voltage corresponding to the pressure value of the pressure 
sensor is output from the power source and is transmitted to the capacitor 
56 through an analog switch, whereby immediate charging of the capacitor 
56 is executed. 
FIG. 14 shows a fifth embodiment of the invention in which means for 
detecting any failure of the cylinder pressure sensor is added. In FIG. 
14, reference numeral 9 denotes a comparator connected to the second 
operational amplifier 52 and 10 indicates a failure detecting circuit 
connected to the comparator 9, although the diagram shows a construction 
in which the failure detecting means is added to the embodiment shown in 
FIG. 5, the failure detecting means can obviously be added to any of the 
embodiments shown in FIGS. 9, 10, and 12. 
The signal waveform V.sub.2 shown in FIG. 15(b) is applied to the 
comparator 9 and is compared with a predetermined reference level. When 
the piezoelectric cylinder pressure sensor 1 normally operates as shown by 
a solid line in FIG. 15(a) and the signal V.sub.2 is output as shown by 
solid line in FIG. 15(b), the comparator 9 generates an output pulse as 
shown in FIG. 15(c). However, when the sensor 1 fails as shown by the 
broken line in FIG. 15(a) and the voltage V.sub.2 as shown by the broken 
line in FIG. 15(b) is output, no output pulse is generated by the 
comparator 9. The presence or absence of the output pulse is detected by 
the failure detecting circuit 10, thereby determining whether the sensor 1 
has failed or not and the result of the determination as to the presence 
or absence of any failure is displayed by display means (not shown). 
It will be further understood by those skilled in the art that the 
foregoing description is of preferred embodiments of the disclosed device 
and that various changes and modifications may be made to these 
embodiments of the invention without departing from the spirit and scope 
thereof.