Abstract
The impact of passenger cars’ emissions on urban air quality is currently a newsworthy topic and an urgent research direction. Alternative powertrain technologies including hybrids are often mentioned as a solution to urban air quality issues, but quantifying the real world emissions and fuel consumption advantage of such vehicles over conventional vehicles is an important question. This study examined two comparable vehicles (similar size, type and weight), which differed in terms of their powertrain architecture. The aim was to compare emissions in heavy urban traffic; to examine and quantify the emissions advantage of the hybrid powertrain under such operating conditions. This study reports emissions testing of two very similar vehicles (which differed in terms of powertrain type) under hot stabilized urban driving conditions. A PEMS system measuring gaseous exhaust emissions was installed in the test vehicles, which were tested in series. Tests were carried out in heavy urban traffic conditions, although the test runs did include some driving speeds in the range 60-90 km/h. Two laps of a defined test route were run, giving a total trip distance of around 12 km. Hot running emissions were measured (the vehicles had been warmed up before emissions measurements commenced). In general, the EU RDE testing methodology was employed, with the exception of the customized test route and the fact that emissions at idle (vehicle standstill) were also analyzed. Weather conditions during testing were dry and moderate. Regulated emissions from both vehicles were low under urban driving conditions, easily meeting the applicable limits, apart from CO emissions from the conventional vehicle, which were almost three times the Euro 5 limit. Overall, HC and NOx emissions from both test vehicles were < 15% of the Euro 5 limit – not unexpected, as hot running emissions were measured and both vehicles’ TWCs had warmed up before testing commenced. In this context, the excess CO emissions from the conventional vehicle were surprising. Emissions of CO2 (and hence FC) from the hybrid vehicle were less than half those from the conventional vehicle, but the vehicle’s battery level barely changed, implying a genuine long term real-world advantage for the hybrid vehicle when used in the type of driving employed in this study; indeed the hybrid vehicle’s engine was turned off for more than half of the duration of the trip. With the exception of CO from the conventional vehicle, regulated emissions were found to be very low under urban hot running conditions. The hybrid vehicle’s ability to run with the combustion engine turned off caused anunequivocal CO2/fuel consumption advantage. Further testing of the same test vehicles and other pairs of vehicles of similar size and mass is warranted in order to gain further insight into vehicles’ emissions/consumption behavior in urban areas. Testing the same vehicles over a different route with a significantly lower mean speed could also be a promising research proposal.