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Different units needed for truck and bus CO2 standards

Pressure is growing from legislators, both in Europe and North America, for the introduction of CO2 emission standards to cover heavier – mainly over 3.5 tonnes gvw – commercial vehicles, as well as cars and vans. But the issue is mired in controversy, arising largely from the much greater diversity in the size and, more especially, the … Continued

Pressure is growing from legislators, both in Europe and North America, for the introduction of CO2 emission standards to cover heavier – mainly over 3.5 tonnes gvw – commercial vehicles, as well as cars and vans. But the issue is mired in controversy, arising largely from the much greater diversity in the size and, more especially, the weight of trucks, in the EU, effectively stretching from 3.5 up to 40 or even 44 tonnes gross, which at the same time rules out a car producer ‘fleet average’ approach.

As with existing statutory limits on truck and bus pollutant emissions, principally particulate matter (PM) and oxides of nitrogen (NOx), diversity in vehicle weight and size inhibits the use of grams-per-kilometre values as meaningful units in the setting of CO2 standards. The most obvious ‘limit setting’ strategy for the legislators to adopt is to follow the PM/NOx precedent by relating permitted CO2 emissions to engine output – using grams per kilowatt-hour (g/kWh) units – allowing more powerful commercial vehicles to emit greater amounts of greenhouse gases.

Why should a 40 tonne rig powered by a 500hp diesel engine be allowed to create more climate change damage than another 40-tonner equipped with only say a 350hp engine?

But it is being argued in some quarters that such an approach would effectively penalize more modestly powered trucks and buses when compared with their higher-performance counterparts of the same weight (and size). Why, for example, should a 40 tonne rig powered by a 500hp diesel engine be allowed to create more climate change damage than another 40-tonner equipped with only say a 350hp engine?

In any case, under real-life operating conditions, assuming both those 40-tonners were operating at their maximum allowable weight, the fuel being consumed and hence the CO2 being emitted, would differ less than their respective rated outputs would suggest. That is because the average percentage load factor of the lower-powered engine is inevitably greater. It has to work harder, running at higher revs, so that fuel economy suffers in consequence.

So what could be a more equitable approach to truck and bus CO2 standard settings? Mercedes-Benz has suggested, for interested parties to debate and comment upon, that a new unit of measurement be applied for the setting of over-3.5 tonnes gvw CO2 standards. Limit values would be expressed in terms of grams per tonne-kilometre (g/tkm). In other words the standard would be related more closely to vehicle productivity.

If PM and NOx emission limits together with CO2 standards could be rationalized around the use of g/tkm measurement units, then commercial vehicle certification and type approval procedures could eventually be simplified.

Purists might argue that payload rather than gross tonnage should be used in any g/tkm evaluation. But that would bring unacceptable complication in the case of special-purpose commercial vehicles – for example bulk tankers and refuse collectors – whose payload is low in relation to their all-up weight.

If the EU in due course adopts the Mercedes-Benz proposal, it seems quite possible that the European Commission will take the opportunity to review the existing framework of truck and bus pollutant emission limits, most notably the g/kWh values agreed for Euro 6 legislation due to come into force in 2013. If PM and NOx emission limits together with CO2 standards could be rationalized around the use of g/tkm measurement units, then commercial vehicle certification and type approval procedures could eventually be simplified.

The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.

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