1. (Field of the Invention)
The present invention relates to a hot film type air flow meter having a temperature sensing exothermic resistor, and more particularly to a hot film type air flow meter having a high temperature sensing exothermic resistor for measuring a flow rate of the suction air in an internal combustion engine. The hot film type air flow meter having a high temperature sensing exothermic resistor is used to control an internal combustion engine provided with a fuel injector, having high horsepower and low fuel consumption and capable of controlling an exhaust gas with high accuracy and excellent quick response speed characteristics.
2. (Description of the Prior Art)
A conventional hot wire type air flow meter is constructed so that a bobbin-like hot wire forming an exothermic resistor and a bobbin-like cold wire forming a temperature compensating resistor are inserted in a bypass passage as shown in the specification of Japanese Patent Laid-Open No. 104513/1984.
As shown in FIG. 18, a prior art hot wire forming an exothermic resistor 136 and a prior art cold wire forming a temperature compensating resistor 142 are disposed in a bypass passage 137, respectively. Since a suitable guide is not provided in a portion 138 at which the flow of the air branches from a main passage 139 into the bypass passage 137, the flow varies delicately. The main passage 139 is made of a metallic material such as die cast aluminum.
The hot wire forming an exothermic resistor 136 has two straight line leads 140a and 140b at both ends thereof. In the conventional method, the leads 140a and 140b are bonded to an alumina pipe to each other and overcoated with a glass material. The hot wire forming an exothermic resistor 136 is formed by winding a thin platinum wire onto a bobbin-like alumina pipe and is overcoated with a glass material.
The straight line leads 140a and 140b are disposed along in the lengthwise direction of the exothermic resistor main body and connected to support terminals 141a and 141b, respectively, as shown in FIG. 19. The prior art hot wire forming an exothermic resistor 136 is called a hot wire form constant temperature exothermic resistor or a bobbin-like high temperature sensing exothermic resistor for use in a hot wire type air flow meter in a fuel supply system which is adapted to supply a fuel at a flow rate matching the flow rate of the suction air in the internal combustion engine.
The diameter of the bypass passage 137 is comparatively large as compared with the dimensions of the exothermic resistor 136 having an outer diameter 0.5 mm, such that the exothermic resistor 136 is apt to receive a surface peeling effect of the air current. The exothermic resistor 136 is positioned close to the support terminals 141a and 141b, and the turbulence of the flow of air is propagated toward the exothermic resistor 136 due to the influence of the viscosity of the air as shown in FIGS. 20 and 21.
As mentioned previously, the shape of a conventional hot wire forming an exothermic resistor 136 is determined on condition that the exothermic resistor 136 is inserted in the bypass passage 137 of a comparatively large diameter of 8-10 mm. Accordingly, the ratio of the outer diameter d.sub.o of the exothermic resistor main body to the length l.sub.o thereof is set to a comparatively low level of l.sub.o /d.sub.o =4.
Therefore, the momentary responding capability of the hot wire air type flow meter, which was measured by momentarily charging the flow rate of the air being ejected, by using an electromagnetic valve becomes as in the example shown in FIG. 22. In FIG. 22, the curve line B.sub.1 shows the rising response speed characteristics, and the curve line B.sub.2 shows the falling response speed characteristics, respectively. Namely, the momentary response time from 5 kg/h to 160 kg/h is 211 msec with respect to the momentary 95% response, and 2 sec (not shown) with respect to the momentary 100% response.
In recent years, demands have been placed on an air flow meter to have high accuracy rather than to serve as a main engine control sensor. To be more exact, it is primarily necessary that the 95% responding capability of the hot wire type air flow meter and the acceleration responding capability thereof during an operation of the engine be improved. Secondly, it is necessary that the variations in the output level with lapse of time, which occur due to the dust deposited on the air flow meter, be minimized. Thirdly, it is necessary to minimize the noise in an output signal of the air flow meter. Fourthly, it is necessary to minimize the temperature dependency of the air flow meter.
A conventional hot wire type air flow meter in which the hot wire forming an exothermic resistor is provided in the bypass passage is characterized in that the occurrence of a so-called binary, i.e. a decrease in the level, which has increased monotonously, of an output from the air flow meter, which decrease occurs when a throttle valve is opened gradually with the number of revolutions per minute of the engine kept constant, can be prevented by the air flow meter alone.
Therefore, this air flow meter is used extensively in practice mainly for 4-cylinder engines. It has become possible to easily prevent the occurrence of a binary by utilizing the data obtained by correcting range of occurrence of a binary stored in a microcomputer in advance. Therefore, it has been urgently demanded that the hot wire type air flow meter be improved with respect to its performance and manufacturing cost.
While an engine in which an air flow meter is provided is controlled by a microcomputer, a slight response delay of the air flow meter adversely affects the operation of the engine, and many complaints are made about the bad operational condition of the engine, especially, in a low speed operational region. Under such circumstances, it has been demanded that the high temperature sensing exothermic resistor in the air flow meter have excellent high speed responding capability.
Specimens of air flow meters having different response speeds were prepared, and the degrees of influence of these response speeds upon the operation of an engine were compared. It was ascertained from the results of the comparison that a 100% rising and falling response speed and a 95% rising and falling response speed were 0.5 sec and not more than 0.1 sec, respectively.
On the other hand, it has been pointed out that variations of the output level with a lapse of time due to dust deposited on the high temperature sensing exothermic resistor are close to the limit of the flow rate variation percentage of .DELTA.Q/Q.ltoreq..+-.4% on the target specifications.
A conventional hot wire forming an exothermic resistor is formed by winding a thin platinum wire around a ceramic bobbin, and coating the resultant product with a glass material. Therefore, it is impossible to burn off the deposited dust, and the rate of deposition of earth and impregnated oil from an air cleaner must be minimized.
It is pointed out that an output signal of the air flow meter has a large amount of noise and low accuracy, and it is said that the level of noise must be reduced to less than 1/2 of the noise in a signal from a currently available air flow meter.
Regarding the temperature dependency of the air flow meter, has been reported that engine troubles occur in a low speed region wherein, for example, a vehicle moves out of its garage in winter after the engine is heated. Accordingly, it is necessary that the temperature dependency of the air flow meter be improved.