Calibration and modelling of ejector dilutors for automotive exhaust sampling

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Abstract
Ejectors are widespread dilutors for automotive exhaust sampling. Their dilution ratio depends on their geometrical characteristics, the properties of the sample and the dilution gas, and the conditions at the ejector dilutor outlet. In this paper we present a detailed calibration of a typical ejector dilutor, and we model its operation under steady-state conditions. Experiments showed that the dilution ratio can shift by 10-20% when changing the sample or outlet pressure by only 5 kPa or the dilution gas pressure by 50 kPa. Increase of the sample temperature by 140degreesC also leads to a dilution ratio increase of 20%. Diluting pure CO2 instead of ambient air may increase the DR by 20%, due to the different CO2 properties. These are typical changes in the operating conditions when sampling automotive exhaust. Hence, in order to predict and correct for these changes, we developed a model to calculate the actual value of the dilution ratio as a function of the ejector operating parameters. The model assumes compressible flow which reaches supersonic velocity at a variable cross-section duct. This terminates with a normal shock wave occurring at the mixing area. The model can be used with any calibrated ejector dilutor to predict changes in the dilution ratio in actual field measurements.