Green buses may not improve air quality as much as previously thought – new study 10 December 2013

Whole vehicle testing is the best way to ensure air quality improvements from hybrid buses and HGVs – which may not be as green as originally thought.

That's the key finding of a new study from the Low Carbon Vehicle Partnership (LowCVP), which commissioned a review of the air quality impacts from the growing numbers of low-carbon buses in the UK.

With around 1,300 low-carbon buses now in operation, the report – prepared by engineering consultancy Ricardo – recommends that legislation needs to consider hybrid technology impacts in the test processes in order to avoid unintended consequences, in terms of local emissions in urban areas.

Reviewing worldwide test processes for HGV engines, the report says that Euro 6 emissions levels for diesel and gas engines should be roughly the same.

However, limited whole vehicle test data shows that improvements, in terms of regulated emissions often don't match test expectations, due to the actual operating cycles of engines on the road.

Transport for London's bus fleet had the most robust data, according to the study. It showed that carbon emissions, fuel consumption and local air quality emissions were lower for green buses. But, in some cases, hybrid vehicles showed higher emission levels per unit of fuel burned than conventional buses.

The study suggests that although technologies such as hybridisation offer the prospect of significant reductions in fuel consumption and CO2 emissions, compared to conventional vehicles, the improvement in terms of regulated emissions may not be so great.

That's why hybrid technologies should be considered during the legislative test cycle, according to Ricardo's researchers.

It's also why they recommend that buses – both conventional and hybrid, and fossil and alternatively fuelled – should be optimised over drive cycles more representative of their real operational use.

"Using appropriate test methods for every new technology and application – such as the process we have implemented for the low-carbon bus – will help us make sure that low carbon is in all ways low emission," states Andy Eastlake, LowCVP managing director.

Author
Brian Tinham

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