Instantaneous emissions measurements on chassis dynamometers and engine test benches are becoming increasingly normal for environmental emission factor calculation. Modern exhaust gas analysers allow measurement at a rate of one sample per second, thus creating the impression that the emissions information recorded reflects that precision. Normally, the time delay between the car and the analysers is considered to be constant and compensated for accordingly. In reality, this time delay varies by more than three seconds depending on the engine load. Moreover, emission peaks are smoothed by turbulence during transport. The analyser's dynamics smooth the signals even more. For example, an emission peak of one second is flattened to a 3-second peak in raw gas measurement, and to a very low hill of about ten seconds in diluted gas measurement. If emissions measurements are to be correlated to actual driving conditions, the variable time delays and the smoothing must be compensated for. To achieve this, the time delay at a given time can be calculated by dividing the volume of the exhaust system by the actual exhaust flow. "Smoothing" is compensated for by adopting a system theory-based approach. It can be described with a linear differential equation, which can be "inverted" in an offline procedure. (A)
Abstract