Legislators on both sides of the Atlantic believe their latest heavy-duty emission regulations – Euro VI and EPA 2010 – effectively close a number of previous loopholes in the requirements, as well as dramatically tightening the statutory limits, most notably on NOx (oxides of nitrogen) and PM (particulate matter). Euro VI provisions include, for the first time, a particle number ceiling and they demand compliance with the newly-defined world harmonised transient cycle (WHTC) test procedure which, importantly, incorporates a cold-start phase designed to replicate real-world truck and bus operating conditions more closely.
But some incongruities remain, as pointed out by Petter Asman from Sweden’s transport ministry, at the recent Heavy Duty Diesel Emissions conference staged in Gothenburg by the US Society of Automotive Engineers. Asman chairs a United Nations Economic Commission for Europe (UNECE) working group striving to develop a set of emission requirements for commercial vehicles with hybrid powertrains and those with automatic stop-start systems.
Euro VI provisions include a particle number ceiling and demand compliance with the newly-defined world harmonised transient cycle (WHTC) test procedure
It is widely acknowledged that the main fuel-saving and CO2 emission-reducing potential of hybrids and stop-start features is accompanied by important advantages in reducing real-world pollutant emissions, including NOx and PM. Those advantages are not measurable in current heavy-duty emissions legislation, where limits are related to engine horsepower. Uprating an engine’s output from 450 to 500hp, for example, allows it to emit more NOx and PM.
Passenger car and light vehicle emission rules, in contrast, are framed on a per-kilometre or per-mile baseline, where the merits of a hybrid in cutting real-world emissions are more readily identified. So why not apply the same emission limit ‘units’ for heavier vehicles, based on the mass (weight) of pollutants generated over a set distance travelled? The answer is that the gross (all up) weight of the trucks and buses subject to current ‘per horsepower’ legislation varies hugely, in Europe from 3.5 up to effectively 44 tonnes.
For the law-makers to give due legislative advantage to the cleaner footprints of hybrid and stop-start vehicles, by switching to emission limits based on distance travelled, they will have to apply a similar approach to that now used in Europe for under-3.5 tonne vans, based on a so-called ‘limit value curve’, where permitted emission levels are determined by vehicle ‘reference’ mass which, in commercial vehicle terms, is related to plated gross weight.
A progressive move away from engine-with-aftertreatment limits towards whole-vehicle limits looks likely to follow the path being suggested initially for hybrids
However, the principle of a Europe-wide fleet average imposed on every car and van manufacturer – where emissions from heavier models above the fleet average can be offset by lighter, less polluting models – is clearly untenable, bearing in mind the very different weight ranges covered by the truck manufacturers. Most notably Scania builds only heavy trucks of 18 tonnes gross and above. It could not hope to achieve the same fleet average as say Mercedes-Benz, which produces vans and chassis-cabs from 3.5 tonnes upwards.
A progressive move away from engine-with-aftertreatment limits towards whole-vehicle limits, for both pollutant and greenhouse gas truck and bus emissions, looks likely to follow the path being suggested initially for hybrids.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
Alan Bunting has a background in engineering, and has been writing on commercial vehicle and powertrain related topics since the 1960s. He has been an Automotive World contributor since 1996.
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