Environmental legislators are turning their attention increasingly to the matter of exhaust emission levels as vehicles age. Type approval requirements are designed to ensure compliance with statutory emission limits when trucks, buses, vans and indeed passenger cars roll off the production line. But what of their emissions as the kilometres build up?
Thanks to improvements in manufacturing quality control, backed by the accompanying upgrades in material specifications seen over the last decade or two, the rate of wear and/or distortion of critical engine, fuel system and turbocharger componentry is far less than it was. Especially at the cylinder liner/bore interface and at the valves/valve-seats, wear rates have been hugely reduced, to the extent that most 21st Century heavy-duty diesel engines, at least those employed on long-haul truck operations, can run for up to 800,000km without the need for what used to be called major overhaul.
Type approval requirements are designed to ensure compliance with statutory emission limits when trucks, buses, vans and indeed passenger cars roll off the production line. But what of their emissions as the kilometres build up?
It has been proposed that, with the introduction of Euro 6 emission laws in 2013/14, heavy trucks and buses should be required to maintain their certified green credentials for at least 700,000km, or seven years. At first sight this is a tall order indeed, seemingly making no allowance at all for powertrain wear or aftertreatment system deterioration. It has been suggested that, from an engineering perspective, it is based on impossible expectations but that it has nevertheless been given the nod by the European Commission because it creates a favourable public relations image to the ordinary citizens of EU member states.
At a practical level the emissions of thousands of trucks and buses in everyday service cannot be verified by means remotely comparable to those applied for type approval certification, where engines – complete with their aftertreatment packages – have to be run under a varying dynamometer load and through a range of speeds decreed in EU-approved transient and steady-state test cycles. The new particle number limit, set at Euro 6, requires even more sophisticated and costly equipment to verify.
Not for the first time, the laudable aspirations of legislators, as they strive to impose higher environmental standards, will come up against tough engineering realities.
Measurement accuracy of OBD (on board diagnostic) equipment or that used at roadworthiness testing stations falls far short of such ideals. Today’s requirements therefore allow NOx (oxides of nitrogen) levels for example to be up to 100% more than the statutory limit before the driver warnings kick in, because of the poor accuracy of even the best on-board sensors currently available. It means that in reality there is plenty of margin for deterioration as vehicles get older.
Sensor technology continues to advance, with the Japanese company NGK a leading contender. The key challenge is to improve accuracy in measuring the NOx in diesel exhaust, not just when a vehicle is new, but over its operational lifetime. Clearly the verification of overall emissions durability from an engine/aftertreatment installation cannot be left to a tailpipe sensor system which is itself susceptible to progressively worsening accuracy.
Not for the first time, the laudable aspirations of legislators, as they strive to impose higher environmental standards, will come up against tough engineering realities. Many would argue that is just how it should be, with legal requirements becoming the main technology driver.
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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