Air flow measurement techniques for air fuel ratio control

Abstract

Measurement of air flow into an engine is the basis for precision control of the engine air/fuel ratio. Such control is needed both to maximize fuel economy and minimize pollutant emissions. Recent developments have focused on air flow meters for electronic engine controls but the performance specifications which an air flow meter must meet are stringent. The flow range is typically forty to one with the greatest accuracy required at the low end of the flow range. The meter must also be able to accurately follow flow transients. Mass flow measurement is preferred, but volumetric flow measurement may be acceptable in adaptive control systems. Stringent meter specifications make common approaches such as differential pressure techniques or turbine meters undesirable. Three measurement techniques which have been the subjects of development programs are vortex shedding, hot wire anemometry and acoustic velocimetry. These three approaches are analyzed in detail to show strengths and weaknesses. The vortex shedding and acoustic approaches are feasible for volumetric flow measurement and have the potential with further development to become valid air mass flow meters. The hot wire meter is an excellent device, but the fragility of the wire and its susceptibility to contamination may prevent it from being a viable approach for long term underhood operation. A near term approach would be to produce a volumetric meter while pursuing evolution of a mass meter based on the volumetric meter design. Mass air flow measurement continues to be a difficult task requiring further developments.