Variable geometry fluid flowmeter

Abstract

A low pressure drop air flowmeter for an automotive engine is disclosed. The meter includes a main air flow passage, a relatively small venturi to receive a portion of the air flow, a pressure port for sensing stagnation pressure in the main passage, a pressure port for sensing static pressure in the throat of the venturi, and a set of swirl vanes disposed adjacent the outlet of the venturi and operative to impart a swirl to the remainder of the air in the main passage, thereby creating a reduced pressure at the venturi outlet for amplifying the pressure difference between the stagnation and static pressure without appreciably increasing the pressure drop across the flowmeter. A geometrically reconfigurable element is included in the main passage resulting in a swirl rate which increases with increasing air flow but at a decreasing rate. This characteristic results in an output pressure differential signal which varies substantially linearly with flow rate over a relatively large range of operation. The reconfigurable element can be of the iris type which varies the effective tip radius of the swirl vanes, or alternatively, can comprise flexible swirl vanes which present a variable pitch to the remainder of the air flowing in the main passage. Also disclosed is a solenoid valve having a multiple valving arrangement operative to communicate the stagnation pressure to an absolute pressure transducer in one position, to communicate the static pressure with the transducer in another position, and to communicate engine manifold vacuum pressure with the transducer in a third position. Finally, pressure relief means are disclosed which provide a path for fluid communication between the main passage and the atmosphere at a point downstream of the swirl vanes when the direction of the air flow through the meter is reversed, such as when the engine backfires.