1. Field of the Invention
The present invention relates to an alcohol content detecting apparatus for detecting or measuring a content of alcohol such as methanol of an alcohol-admixed liquid such as a liquid fuel for an internal combustion engine of a motor vehicle or the like.
2. Description of the Related Art
In recent years, a mixture of gasoline with methanol tends to be increasingly used as a fuel for internal combustion engines (hereinafter also referred to simply as the engine) of motor vehicles in U.S.A and European countries among others with a view to saving petroleum resources and reducing the air pollution ascribable to the exhaust gas of the motor vehicles.
When such methanol-admixed fuel is used as it is in the engine which is designed to operate with a gasoline fuel, the air-fuel mixture becomes lean, making it difficult or impossible to operate the engine, because the methanol-admixed fuel has a smaller theoretical air-fuel ratio when compared with the gasoline fuel. Under the circumstances, it is generally practiced to detect the content of methanol in the methanol-admixed fuel to thereby regulate correspondingly the air-fuel ratio, the ignition timing or other control quantities for the engine operation.
In conjunction with the detection of the methanol content of the fuel, there has heretofore been proposed a method which is based on detection of a dielectric constant of the methanol-admixed fuel and a method based on detection of a refractive index of the fuel. The applicant has already proposed an apparatus for detecting the methanol content on the basis of detection of the dielectric constant of the fuel (refer to Japanese Patent Application No. 22488/1991). For a better understanding of the background techniques of the present invention, this apparatus will be described by reference to FIG. 6.
Referring to the figure, the methanol content detecting apparatus includes a sensor unit denoted generally by A which is comprised of a cylindrical insulation tube 1 formed of an insulation material such ceramic, an oil-resistive plastic material and having an inner cavity or space in which a fuel passage is defined, as mentioned below. Disposed within the inner space 2 is a cylindrical electrode 3 having a cylindrical outer surface extending substantially in parallel with an inner cylindrical wall surface of the insulation tube 1 and disposed coaxially with the latter. A coil of a single layer winding 4 is provided as wound around the outer surface of the insulation tube 1 in opposition to the electrode 3. A fuel passage 2 is defined between the outer peripheral surface of the electrode 3 and the inner periphery of the coil 4 with the wall of the insulation tube 1 being interposed therebetween.
The electrode 3 is mounted to a flange 5 which in turn is coupled fluid-tightly to the insulation tube 1 with a fuel seal 7 interposed therebetween, whereby a fuel container, so to say, is realized as a whole. In the case of the illustrated example, the flange 5 is formed integrally with the electrode 3. Nipples 6 are provided for introducing the fuel into the fuel container of the sensor unit A.
A detection circuit for processing the output signal generated by the sensor unit A is generally denoted by a reference symbol B. The single-layer-winding coil 4 has a lead wire 4a connected to one end of a resistor 10 which constitutes a part of the detection circuit B and the other lead wire 4b which is grounded.
Signals appearing at opposite ends of the resistor 10 are supplied to a phase comparator 11 to be compared with each other. The output signal of the phase comparator 11 is supplied to a comparison integrator 13 through a low-pass filter 12. The integrator 13 is additionally supplied with a reference voltage V.sub.ref corresponding to a phase difference of 0.degree.. A difference between the output signal of the filter 12 and the reference voltage V.sub.ref is integrated by the comparison integrator 13, the output of which is supplied to a voltage-controlled oscillator 14 as a control signal therefor. An oscillation signal S.sub.vco of a high frequency outputted from the voltage-controlled oscillator 14 is applied to the other end of the resistor 10 via an output amplifier 15. Further, the oscillation signal S.sub.vco outputted from the voltage-controlled oscillator 14 is supplied to a frequency divider 16.
Next, operation of this methanol content detection apparatus will be described.
FIG. 7 shows an equivalent circuit of the sensor unit A. In the figure, L represents an inductance of the single-layer winding coil 4, C.sub.f represents a capacity which is effective between the coil 4 and the electrode 3. This capacity C.sub.f will vary in dependence on a dielectric constant .epsilon. of a fuel flowing through the fuel passage 2. Further, C.sub.s represents a capacity provided by the insulation material forming the tube 1 which serves to protect the single-layer-winding coil 4 from the fuel, and C.sub.p generally represents stray capacitance parasitic to the lead wire 4a, input capacitance of the phase comparator 11 and so forth which are insusceptible to the influence of the dielectric constant .epsilon. of the fuel.
When the frequency of a voltage signal applied to the lead wire 4a of the sensor unit A is varied, the sensor unit A exhibits a parallel LC-resonance characteristic, wherein a parallel resonance frequency f can approximately be given by the following expression: EQU f=1/[2.pi.{L(C.sub.p +1/(1/C.sub.s +1/C.sub.f))}[=k/(a+b.times..epsilon.)(1)
where k, a and b represent constants determined by structural and geometrical factors of the sensor unit A such as the diameter and thickness of the insulation tube 1, the dielectric constant of the insulation material of the tube 1, distance between the electrode 3 and the single-layer-winding coil 4, self-inductance thereof and so forth.
As can be seen from the expression (1), the resonance frequency f depends on the dielectric constant .epsilon. of the fuel. Consequently, as the dielectric constant .epsilon. of the fuel increases, the resonance frequency f becomes lower. In the experimental measurement of methanol content of a fuel mixture of methanol and gasoline conducted by the inventors, the resonance frequency f exhibited a change illustrated in FIG. 8 as a function of the content of methanol. Thus, by detecting a signal corresponding to the resonance frequency f, it is possible to detect the dielectric constant .epsilon. of the fuel and hence the methanol content of the methanol-admixed fuel.
The detection circuit B is so configured as to detect the resonance frequency f mentioned above. More specifically, when the oscillation signal S.sub.vco is applied to the other end of the resistor 10 from the voltage-controlled oscillator 14 through the amplifier 15 in the state in which a methanol-admixed fuel is flowing through the fuel passage 2, there are obtained high-frequency voltage signals at both ends, respectively, of the resistor 10 (one from the coil 4 and the other from the series circuit of the resistor 10 and the single-layer-winding coil 4). These two high-frequency voltage signals are supplied to the phase comparator 11 for phase comparison.
In this case, when the frequency of the oscillation signal S.sub.vco outputted from the voltage-controlled oscillator 14 is equal to the resonance frequency f mentioned above, the current is in phase with the voltage, resulting in that the difference in phase between the two high-frequency voltage signals appearing at both ends of the resistor 10, respectively, becomes zero. Consequently, a signal corresponding to the phase difference of zero is outputted from the phase comparator 11 and thus from comparison integrator 13, whereby the oscillation frequency of the voltage-controlled oscillator 14 is held constant as it is.
In contrast, when the frequency of the oscillation signal S.sub.vco outputted from the voltage-controlled oscillator 14 is deviated from the resonance frequency f of the sensor unit A, the current will then be out of phase with the voltage, as a result of which the difference in phase between the two high-frequency voltage signals appearing at both ends of the resistor 10 assumes a value not equal to zero. Consequently, a signal corresponding to the phase difference is outputted from the phase comparator 11 and hence from the comparison integrator 13, whereby the voltage-controlled oscillator 14 is so controlled that the oscillation frequency thereof becomes equal to the resonance frequency f and that the phase difference between the two high-frequency voltage signals mentioned above becomes zero degree.
In this manner, the voltage-controlled oscillator 14 is controlled so that the oscillation frequency thereof remains constantly equal to the resonance frequency f, whereby an output signal S.sub.out of a frequency bearing one-to-one correspondence to the resonance frequency f is obtained from the frequency divider 16.
The hitherto known methanol content detection apparatus of the structure described above however suffers from a problem that because thickness of the insulation tube 1 as well as distance between the insulation tube 1 and the electrode 3 and hence the parasitic capacitance are inevitably susceptible to variance from one to another apparatus, incurring corresponding change in the resonance frequency f and hence in the frequency f.sub.out of the output signal S.sub.out in dependence on the apparatuses as used even for a same methanol content (refer to FIG. 8), whereby difficulty is encountered in detecting accurately the methanol content of the methanol-admixed fuel without any appreciable influence of the apparatus-dependent variance.