Cold start emissions of petrol vehicles - emissions measurements and updated emission

Introduction. With widespread implementation of effective aftertreatment systems the exhaust emissions of roadgoing vehicles have decreased significantly in the last two decades. However, aftertreatment systems designed for reducing gaseous emissions typically require an operating temperature that is above ambient conditions. Especially during a so-called cold start, gaseous emissions such as NOx can be severely elevated and thereby have a significant impact on the total emissions of a vehicle trip. With improving aftertreatment systems in newer vehicle models, the high emissions that can be attributed to a cold start happen within increasingly short distances. For this reason a point source is now considered the best representation of the cold start extra emissions, and has now been implemented within the VERSIT+ emission factors and thus also within the Dutch Emission Inventory. As the point-source implementation deviates significantly compared to how cold starts were approached in the past, broader research was needed to support currently inferred emission factors for cold start extra emissions. Goal of the work. The goal of the work as presented in this report is not only to further support the currently inferred emission factors for cold start extra emissions of petrol vehicles, but also to broadly investigate cold starts while idling. Method. A total of 31 petrol vehicles from the Dutch fleet have been measured on their emission. The majority of the tested vehicles were Euro 6 approved and had odometer readings up to 150,000 kilometres. The PEMS (Portable Emission Measurement System) data from cold start measurements of the tested vehicles has been used to gain further insight into exhaust gas temperature behaviour, emission levels over time after cold start and after restart.

Results, conclusions and recommendations. The results of this work show the significance of the cold start emissions on the total emissions during a vehicle trip and provide insight into additional testing parameters:
• Temperature levels of the exhaust gas were observed to remain below 60°C, even after extended periods of idling. Cold exhaust gas does not rise to the same extent that warm exhaust gas does, so this would likely lead to the gasses remaining lower around the car. Because the gasses remain low, this can lead to increased exposure for people around or close to the vehicle.
• The majority of the NOx-emissions were observed in the first 30 seconds after engine start, further supporting the current point source representation of the cold start.
• Emission levels of NOx, THC and PN were observed to be significantly elevated during cold start testing. When comparing the average NOx, THC and PN emission levels, cold start levels were observed to be the highest, warm engine levels to be the lowest, and engine restart levels to be slightly elevated compared to warm engine levels. We have not investigated the elevated emissions at restart to this level of detail before; we recommend including restarts in our future campaigns where possible.