Analysis of Pulsating Flow Measurement of Engine Exhaust by Pitot Tube Flow Meter

Abstract

Pitot tube flow metering technique has been used to measure pulsating flow from a vehicles engine exhaust. In general, flow metering techniques that utilize differential pressure measurements based on the Bernoulli's theory likely to show erroneous reading when measuring an average flow rate of pulsating flow. The primary reason for this is the non-linear relationship between the differential pressure and the flow rate, i.e. the flow rate is proportional to the square root of the differential pressure. Thus, an average of the differential pressure does not give an average of pulsating flow. In the present study, fast response pressure transducers have been used to measure the pulsating pressure as raw as possible. Then, the pulsating differential pressure is converted to flow rate keeping the pulsation not averaged. An average of the flow rate is then calculated in flow rate domain in order to maintain the linearity of before and after the averaging. The peak amplitude of a pulsation measured here was about 1800 LPM at the average flow rate of 70 LPM when an engine was idling. This measurement has been confirmed by measuring the pulsation with a gas analyzer. The result here implies large amount of back and forth gas movement in the exhaust tube. Such amount of pulsation can cause maximum five times higher erroneous result with the pressure domain averaging when compared to a flow rate domain averaging.