Contaminant Free Backflow Reducing Insert for Mass Air Flow Sensors

1. Field of the Invention

This invention relates to the field of mass air flow sensors and, in particular, to a housing unit for such a sensor.

2. Prior Art

There exists a condition in many internal combustion engines, when the air intake valve is open during the initial part of a compression stroke or prior to the completion of an exhaust stroke, which produces a momentary backflow of air through the engine's air intake manifold and mass airflow sensor.

There are known thermal mass air flow sensors such as taught in U.S. Pat. No. 4,974,455, which are not capable of distinguishing between normal air intake flow and backflow. Further, these sensors do not address the issue of particulate contamination on the mass air flow sensor element itself. Backflow within the intake manifold will result in an erroneous mass air flow signal which in turn will result in an incorrect response by the engine control systems dependent upon the signals. Particulate contamination of the air flow sensor results in readings that are not representative of the actual conditions. This is a result of the accumulation of particulates which reduce and obstruct the air flow passing by the sensing element.

Thermal mass air flow sensors of the type taught in U.S. Pat. No. 4,215,565, U.S. Pat. No. 3,352,154, and U.S. Pat. No. 4,434,656 acknowledge the issue of particulate contamination. The first two aforementioned patents, while acknowledging the issue of contamination, do not demonstrate a device which may effectively prevent it. U.S. Pat. No. 4,434,656 teaches a sensor system whereby the sensor is located along side a bluff body significantly larger than the diameter of the sensor wire such that the wire is neither in the stagnation zone where the particles are trapped or in the turbulent zone where faulty readings will occur due to turbulence. This device although adequately addressing the issue of particulate contamination, does not resolve the issue of faulty readings due to backflow over the sensing element.

Thermal mass air flow sensor taught in U.S. patent application Ser. No. 08/043,719, filed on Apr. 4, 1993, on behalf of inventors L. A. Zurek and L. J. Clowater, addresses the issue of backflow within the air intake system. The device includes a main air passage which leads to the engine, and an auxiliary passage which runs parallel to the main passage and leads into a recovery chamber, which subsequently is joined with the main air passage further downstream. Results for this device indicate significant improvement with respect to backflow induced faulty readings. However, this device does not accommodate the requirement of a contaminant-free sensor. These are some of the problems this invention overcomes.

The invention includes a housing for the mass air flow sensor which will reduce the contamination of the sensing element due to air-born particles and vapors and also will eliminate the effect of backflow on mass air flow readings. The housing has a structure that is fastened at the base to the exterior of the air flow passage housing. The body of the housing extends through a fitted opening in the wall of the air flow passage into the air flow passage itself, transverse to the flow along a longitudinal axis of the air flow passage. The exterior shape of the body of the housing is such that there is high pressure at the upstream surface, the side surfaces, and the bottom surface.

The backside, which constitutes the downstream portion of the housing body, has a surface with a plane which is, during forward flow, perpendicular to the flow of the air within the air passage which results in turbulence and low pressure. At the upstream side of the bottom surface there exists an opening to an internal auxiliary passage that extends in a direction perpendicular to the flow of the air. At the end of the auxiliary passage, the sensing element is situated, along with a venturi that directs the air over the sensing element and through an opening which leads to an exit chamber. The exit chamber has two exits, one at the downstream side of the bottom of the housing and the other located on the backside of the housing body, near the bottom surface.

The advantage of such a housing structure is that it allows a portion of the air flowing around the structure to be redirected into the auxiliary passage while heavier particles and vapor continue downstream, thereby effectively providing a clean air sample for the sensing element. In the condition where there exists backflow, the housing structure acts as a shield against the majority of the backflow air while the opening on the back side of the housing bleeds most of the air to the opening adjacent to it on the bottom side, thereby eliminating the effect of backflow on the sensing element. This device has the added benefit over prior art in that there is no maintenance involved with regards to cleaning the sensing elements or housing structure. This is by virtue of the fact that the contaminants do not accumulate on the housing structure itself but continue downstream with the main air flow.

FIG. 1 is a side view of the housing showing forward direction air flow in and around the housing in accordance with an embodiment of this invention;

FIG. 2 is a side view of the housing showing air backflow in and around the housing in accordance with an embodiment of this invention;

FIG. 3 is a top view of a housing in accordance with an embodiment of this invention;

FIG. 3A is a section view along cross section A--A of FIG. 3; and

FIG. 3B is a section view along cross section B--B of FIG. 3.

FIG. 1 shows a side view of a sensor housing structure in accordance with the preferred embodiment of this invention for providing contaminant free air flow for sampling and significant resistance to the effect of back flow. The arrows indicating air flow are for the condition of flow in the forward direction. A housing structure 1 has a base 2 and a body 3. Base 2 is fastened to an exterior surface of a wall 4 of an air flow passage such that body 3 extends into the interior of an air flow passage 10. Air flowing in the forward direction creates a high pressure area 5 on the front side and bottom surfaces of body 3. A perpendicular component 20 of housing structure 1 is positioned perpendicular the air flow so that it receives first contact with such air flow.

A portion of the air is diverted through an opening 6 into an auxiliary passage 7, while heavier contaminants by-pass opening 6 and continue downstream. The air within auxiliary passage 7 continues upward in body 3 until it is redirected by a venturi 8 over mass air flow sensing elements 9, which are positioned at the end of auxiliary passage 7, through an opening on base 2 of the preferred embodiment. The air then passes to a recovery chamber 11 and then exits recovery chamber 11 through an opening 12 on a back wall 13 of body 3 and also through an opening 14 on a bottom surface 15 of body 3 to a low pressure area 16 downstream. Because a portion of the total air flows within auxiliary passage 7, calibration of sensing elements 9 will be required such that the data taken from the sampled air may be extrapolated to account for the total mass air flow within the air flow passage.

FIG. 2 shows a side view as in FIG. 1 except the arrows show air flow in the reverse direction, or backflow. During backflow, air from a high pressure area 17 flows around a body 3 of the housing while a portion of the air enters an opening 12 in a back wall 13 of body 3. The air entering opening 12 exits through an opening 14 on a bottom surface 15 of body 3 where it continues downstream through a low pressure area 18.

FIGS. 3, 3A, and 3B are top and cross section views of the preferred embodiment. By way of example, base 2 is 55 mm.times.73 mm, has two holes 19 of diameter 4.5 mm located at diagonally opposite corners, and has a flush opening 20 in the center of the base of a diameter of 31 mm which leads to a smaller opening 21 of diameter 11 mm at the top of body 3. A front surface 22 is described as a semicircular plane of height 28 mm, radius 10 mm, and arc 175 degrees. Sides 23 of body 3 consist of two non-parallel planes that extend from the breaklines on the front plane. They extend towards the back of body 3 a distance 15 mm at which point there exist breaklines. From these breaklines, planes with height 35 mm extend in parallel 5 mm towards the back of body 3, thereby completing the sides. Back wall 13 of body 3 is 35 mm high and 24 mm wide. It contains exit opening 12 being 1.5 mm high by 10 mm wide. Bottom surface 15 has a plane that follows the bottom breakline geometry of the front, side, and back planes. Bottom surface 15 has an angle 17.65 degrees to the horizontal such that a high pressure area exists on the outside surface for the case of forward air flow. Towards the rear of bottom surface 15, there is an exit opening 14 15 mm long by 10 mm wide. Opening 6, whose shape is generally a half-circle with radius 8 mm, is on the front side of bottom surface 15, creating an auxiliary passage 7 extending upwards, which has a wall thickness of 1.5 mm consisting on the front surface plane, and a wall thickness of 2 mm consisting of the wall thickness internal to body 3. There exists in the wall of auxiliary passage 7 corresponding to the front plane, at the top, venturi 8 having a depression generally shaped as an elliptical-hemisphere 10 mm wide and 5 mm high.

The angle of at least a portion of the bottom surface 15 may be between 7 degrees and 35 degrees such that there is significant high pressure on said surface and a portion of air flow is diverted at an angle to the main flow such that separation of the contaminants from the air within the sample will occur. The material can be plastic, however any material (i.e., glass, metal, ceramic, wood) that will maintain the geometrical integrity in the conditions of intended use of the device is considerable.

It is recognized that those skilled in the art of such housings will be aware of variations of the angle of the bottom surface and of the axis of flow within the auxiliary passage, the size and location of the venturi, the location of exit openings, and other parameters of the housing device such that there will be improvements in the cleanliness of the air passing over the sensing elements while at the same time maintaining the housing's resistance to backflow in the condition for flow in the reverse direction. Those and other such variations are considered to come within the scope of the appended claims.