Gaseous and Particulate Exhaust Emissions of Hybrid and Conventional Cars over Legislative and Real Driving Cycles

Abstract

Road transport exhaust emissions represent the main sources of atmospheric pollution in urban areas, due to the growing number of circulating vehicles and travelled distances. In order to reduce this pollution source, stricter emission standards are periodically set by governments throughout the world. Consequently, the concentrations of gaseous pollutants and particulate mass to be measured during type-approval tests of new vehicles are becoming progressively lower; moreover from 2011, diesel cars have to comply with particle number limit. In order to assess emission levels of different technology vehicles and investigate the use of a particulate number measurement technique at the exhaust of very low-emitting vehicles, an experimental activity was carried out on three in-use vehicles: a diesel car equipped with a particulate trap (DPF), a hybrid gasoline-electric car and a bi-fuel passenger car fuelled with compressed natural gas (CNG). Cold and hot gaseous and particulate emission factors and fuel consumption were measured during the execution of real and regulatory driving cycles on a chassis dynamometer. Particulate was characterized in terms of mass only for the diesel car and of particle number for all vehicles. The emissions measured over the NEDC show that all three vehicles comply with their standard limits, except CO for CNG passenger car and NOx for diesel car. Cold start influences CO and HC emissions and fuel consumption for all the tested vehicles and in particular for the hybrid car. The real driving cycle is the most critical pattern for the emissions of almost all pollutants. During constant speed tests, the emissions of particles of hybrid car are an order of magnitude lower than those of the CNG car.

Strategies for improving the emission performance of hybrid electric vehicles

“…In a high-power cold start, immediate power from the engine is required to assist the electric drivetrain while neither the engine is at its optimal operating condition (Pham and Jeftic, 2018) nor the TWC is at its light-off temperature (Carlson et al, 2007). Several studies have reported elevated regulated emission levels as well as PN emissions from HEVs compared to CVs (Cao et al, 2018;Costagliola et al, 2015;Duarte et al, 2014;Huang et al, 2019;Ng et al, 2001;Pham and Jeftic, 2018;Russell et al, 2015;Varella et al, 2016;Wei and Porter, 2011;Yang et al, 2019). In addition, there are also studies that claim that HEVs' emission rates are comparable to those of CVs (Christenson et al, 2007;Huang et al, 2019;Kontses et al, 2020).…”