It used to be said, and not just by sceptics, that more complex vehicle technology too often led to fresh reliability problems, as an unwelcome accompaniment to performance or fuel efficiency improvements. Over the last two decades that assertion has, however, been largely confounded, notably in the field of electronic engine management on diesel engines, where continuous fuel metering and injection timing control are now taken for granted.
But a new JD Power report suggests that the latest emissions control measures being implemented in response to legislation are bringing new reliability concerns in the US. The report’s headline assertion is that “technology changes related to revised emissions standards for heavy-duty trucks are causing an increase in heavy-duty engine problems and a decrease in overall satisfaction with the powertrain”.
A new JD Power report suggests that the latest emissions control measures being implemented in response to legislation are bringing new reliability concerns in the US
Because the JD Power survey is based on the in-service experience of truck operators, albeit major fleets – or ‘primary maintainers’ – no attempt is made to analyse in-service reliability problems in technical detail. But it is clear that the main headaches for fleet engineers are occurring on EPA 2010 compliant engines. They are all, with the exception of Navistar, equipped with ‘belt and braces’ NOx (oxides of nitrogen) emission control; that is to say a combination of in-cylinder EGR (exhaust gas recirculation) and SCR (selective catalytic reduction) aftertreatment.
European truck manufacturers will be keen to avoid the same kind of in-service reliability problems, as Euro VI-compliant engines get into customers’ hands. The permitted pollutant limits and certification test methods for EPA 10 and Euro VI are very similar. The EGR-SCR combination for NOx reduction, together with a diesel particulate filter (DPF), is being adopted by almost every market contender in Europe, with the exception of Iveco, whose diesel engineers say they can meet Euro VI requirements without using EGR.
Where EGR and SCR are working in tandem to curb NOx emissions, there must necessarily be an interface between their respective electronic control algorithms. The level of NOx generated in the cylinder rises and falls, broadly in line with peak combustion temperature. The percentage of exhaust gas needing to be recirculated through the engine, to restrict NOx formation, is accordingly regulated by progressively opening and closing the electronically-managed EGR valve, which can now take several forms.
The EGR-SCR combination for NOx reduction, together with a diesel particulate filter (DPF), is being adopted by almost every market contender in Europe, with the exception of Iveco, whose diesel engineers say they can meet Euro VI requirements without using EGR
Further constraint on the amount of NOx reaching the tailpipe is imposed downstream by the SCR system, which also requires electronic control to determine the urea solution (AdBlue or diesel exhaust fluid) dosing rate – that is, the amount of urea being sprayed into the exhaust stream ahead of the SCR catalyst.
The microprocessor overseeing the two NOx control systems must, in layman’s terms, perform a balancing act, ‘deciding’, in real time, the percentage ‘contributions’ required, from EGR and from SCR, where different manufacturers are adopting different strategies. It is little wonder that teething troubles are emerging, especially bearing in mind the stringency of EPA 10 and Euro VI permitted emission levels, which are so challenging to verify accurately.
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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