Evaluation of response time of a portable system for in-use vehicle tailpipe emissions measurement

The objective of this paper is to quantify and evaluate the effects of response time of a portable emission measurement system (PEMS). The PEMS measures tailpipe emissions and vehicle dynamics on a second-by-second basis. Response times of the PEMS for exhaust concentrations were quantified on the basis of fixed periods of measurement of calibration gases for NO, hydrocarbons (HC), CO, and CO2. The time constant was quantified on the basis of the time to reach 63% of the maximum measured value when calibration gas was continuously administered for a period of typically 20 s or more. The time constant was found to be 6 s for NO and 3 s each for CO, HC, and CO2. Measurement errors associated with the response time of the PEMS were quantified. A first-order dynamic discrete model was developed to simulate the instrument measurements. Simulations showed that correction improves the measurement accuracy. Correction with smoothing better improves the measurement accuracy, especially when the noise is relatively large. On a trip level, the average error of the simulated measurements relative to the simulated signal before correction is -4%, which is deemed to be acceptable. For real-world data, smoothing and correction is recommended for major peaks to improve the measurement accuracy.