Evaluation of Exhaust Emissions from Three Diesel-Hybrid Cars and Simulation of After-Treatment Systems for Ultralow Real-World NO_<i>x</i> Emissions

Hybridization offers great potential for decreasing pollutant and carbon dioxide emissions of diesel cars. However, an assessment of the real-world emissions performance of modern diesel hybrids is missing. Here, we test three diesel-hybrid cars on the road and benchmark our findings with two cars against tests on the chassis dynamometer and model simulations. The pollutant emissions of the two cars tested on the chassis dynamometer were in compliance with the relevant Euro standards over the New European Driving Cycle and Worldwide harmonized Light vehicles Test Procedure. On the road, all three diesel-hybrids exceeded the regulatory NO_x limits (average exceedance for all trips: +150% for the Volvo, +510% for the Peugeot, and +550% for the Mercedes-Benz) and also showed elevated on-road CO₂ emissions (average exceedance of certification values: +178, +77, and +52%, respectively). These findings point to a wide discrepancy between certified and on-road CO₂ and suggest that hybridization alone is insufficient to achieve low-NO_x emissions of diesel powertrains. Instead, our simulation suggests that properly calibrated selective catalytic reduction filter and lean-NO_x trap after-treatment technologies can reduce the on-road NO_x emissions to 0.023 and 0.068 g/km on average, respectively, well below the Euro 6 limit (0.080 g/km).