Air quality impacts of low carbon buses

Whole vehicle testing is the best way to ensure air quality improvements from hybrid buses, according to a review prepared by Ricardo for the Low Carbon Vehicle Partnership (LowCVP). With around 1,300 low carbon buses now in operation, the new report recommends that the legislation needs to consider how hybrid technology impacts in the test processes to avoid potential unintended consequences in terms of local emissions. The most robust data available (supplied by TfL’s bus fleet) showed that carbon emissions, fuel consumption and local air quality emissions were reduced for the low carbon vehicles compared with conventional buses. However, using an alternative metric of emissions intensity, in some cases the hybrid vehicles showed higher emission levels per unit of fuel burned than conventional buses. This indicates, researchers said, that there is further scope to optimise emissions control and after treatment systems around the operating cycle.

According to the study, technologies such as hybridisation offer the prospect of significant reductions in fuel consumption and CO2 emissions compared to conventionally powered vehicles, but the improvement in terms of regulated emissions may not be as great. The researchers recommend, however, that consideration of hybrid technologies in the legislative test cycle is needed to facilitate further air quality reduction. It is also recommended that buses – both conventional and hybridised, and both fossil and alternatively fuelled – should be optimised over drive cycles more directly representative of their operational use. The report also notes that unregulated emissions remain a concern to legislators and are likely to become regulated over time where they are seen to have an air quality impact. Unregulated emissions of current concern are: ammonia, N2O/NO2, aldehydes, benzene (from diesels) and methane, ammonia and aldehydes (from gas engines).

LowCVP’s MD, Andy Eastlake, said, ‘The primary focus of our work has been, and will remain, the reduction of carbon and greenhouse gas emissions to mitigate climate change. It’s important, though, that we always keep an eye on other effects of the low carbon shift, including any impacts on local air quality. 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.’


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