1. Field of the Invention
The present invention relates to an air flow meter which has a bypass passage in an air flow passage and measures air flow amount in the air flow passage by measuring air flow amount in the bypass passage.
2. Related Art
Conventionally, air flow meters having a sensor unit disposed in an air flow passage of an internal combustion engine are well known. These air flow meters measure intake air flow amount of the engine by the sensor unit so that amount of fuel supplied to the engine is controlled with high accuracy.
However, when both frequency and amplitude of intake air pulsation generated by engine operation in the air flow passage of the engine are high, air flow amount in the air flow passage may not be measured accurately due to response delay of the sensor unit or the like. For example, a thermal air flow meter having an exothermic resistor and a temperature sensing resistor as the sensor unit has nonlinear characteristics in which the heat radiation amount of the resistors is in proportion to a square root of air flow velocity Therefore, the thermal air flow meter may detect an air flow velocity smaller than an actual value due to the response delay of the resistors.
To eliminate these detection errors, JP-B2-6-17810, JP-A-8-193863 and JP-A-9-43020 disclose an air flowmeter which has a bypass passage formed in an air flow passage to measure air flow amount in the main air flow passage by measuring air flow amount in the bypass passage through a sensor unit disposed in the bypass passage. In the air flow meter having the bypass passage, pulsation in the bypass passage can be decreased and measurement accuracy of air flow amount can be improved by adjusting a ratio of a length of the bypass passage to a length between an inlet and an outlet of the bypass passage in an air flow direction in the air flow passage.
However, air flowing through the bypass passage and air flowing through the air flow passage collide with each other in a joint portion between the bypass passage and the air flow passage, thereby decreasing air flow velocity in the bypass passage and generating turbulence in the bypass passage. As a result, air flow amount of the air flow passage may not be measured accurately.