The Vehicle Exhaust Emissions Simulator was developed to evaluate the performance of vehicle emissions sampling and analytical systems. The simulator produces a representative tailpipe volume flow rate containing up to five emission constituents, injected via mass flow controllers (MFCs). Eliminating the variability of test results associated with the vehicle, driver, and dynamometer makes the simulator an ideal quality control tool for use in commissioning new test cells, checking data correlation between test cells, and evaluating overall system performance.
Earlier vehicle emissions simulators being used in the industry were primarily for checking Constant Volume Samplers (CVSs) and Bag Benches but they did not have the ability to properly simulate tailpipe volume. Therefore, these systems were unable to check the other components of the test cell sampling and analytical equipment, including modal benches, mini-diluters, ultrasonic flow-meters, and other exhaust flow measurement techniques, such as CO2 Tracer and Remote Mix Tee (RMT) dilution air volume subtraction from total diluted exhaust volume. The simulator system described here contains a squirrel cage blower with a stepper valve and sub-sonic venturi, enabling variable tailpipe exhaust flow-rates in the range of 0.2 to 4.2 SCMM (standard cubic meters per minute).
Numerous improvements have been made in order to address other limitations that have plagued previous simulator systems. Accuracy of CO2 mass injection has been a challenge because of MFC calibration and gas temperature stability. We have worked with Colorado Engineering Experiment Station Inc. (CEESI) to establish a primary flow standard for CO2 MFCs, and have incorporated the use of a thermostatically controlled gas heater for reliable CO2 temperature control over the flow range of 0-150 SLPM (standard liters per minute). We have evaluated MFC drift and differences in results when using different gas compositions and the system computer, a PC running Windows™ and LabVIEW™ software, has undergone several software enhancements, such as automatic MFC calibration, MFC zero correction, and flexible gas injection profiles.
This paper, besides describing all the functionality and features of the simulator system, also details the performance data, which has been collected and analyzed at several different test sites over the last year. In addition, recommended future investigations and enhancements are discussed.