Investigation of Emissions Impacts from Hybrid Powertrains

The National Renewable Energy Laboratory (NREL) under California Air Resources Board (CARB) Agreement Number 14-613, NREL Contract Number FIA-15-1802, has performed a series of chassis dynamometer and portable emissions measurement system (PEMS) studies to better understand how tailpipe NOx emissions are affected by the addition of a hybrid propulsion system as compared with conventional medium-duty and heavy-duty diesel vehicles operating in the same vocations. Previous work has demonstrated the addition of an aftermarket hybrid propulsion system can provide a significant fuel economy advantage under the right drive cycle conditions. However, many vehicles also exhibited an increase in tailpipe NOx emissions. This work, under CARB Agreement Number 11-600, NREL Contract Number FIA-11-1763 demonstrated an increase in tailpipe NOx across a number of vocations and vehicle body styles. The emissions increase was also observed using various test methods including chassis dynamometer, PEMS, and in-use on-board diagnostics (OBD) NOx sensors (Thornton et al. 2014). The work presented in this report represents a continued effort to better understand the cause of this emissions increase and steps that can be taken to ensure the issue is addressed for future vehicles going forward.