Investigation of Emissions Impacts from Hybrid Powertrains

Research output: NRELTechnical Report


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.
Original languageAmerican English
Number of pages71
StatePublished - 2020

NREL Publication Number

  • NREL/TP-5400-75782


  • emissions
  • emissions controls
  • fuels and combustion science
  • medium and heavy-duty hybridization


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