‘Fluid economy’ is a term newly coined by Navistar in the US to encompass not only the fuel usage of today’s diesel-engined trucks and buses, but also their requirement for other consumable liquids, most notably what is known in North America as diesel exhaust fluid (DEF). In Europe the same urea/water solution is marketed as AdBlue. It is an essential ingredient of the SCR (selective catalytic reduction) systems now almost universally adopted by manufacturers for controlling NOx (oxides of nitrogen) emissions on current heavy-duty diesel vehicles on both sides of the Atlantic. Navistar is the exception, claiming to meet the latest EPA 2010 requirements using exhaust gas recirculation (EGR) alone to control NOx levels.
Real-life operational costings should, argues Navistar, include the DEF consumed by its competitors’ vehicles as well as their fuel usage. No one is disputing that basic assertion. Where the other manufacturers take issue with Navistar is in its suggestion that the monthly fuel-plus-DEF expenditure faced by truckers running EPA 2010 compliant Freightliner, Volvo–Mack and Kenworth–Peterbilt chassis is higher than the ‘fuel only’ bills of Navistar customers.
Real-life operational costings should, argues Navistar, include the DEF consumed by its competitors’ vehicles as well as their fuel usage.
It has been shown conclusively in Europe that, through the adoption of SCR, diesel engines run more fuel efficiently. The system allows net injection timing, nowadays under continuous electronic control, to be advanced, to the benefit of both fuel consumption and particulate matter (PM) emissions. That does increase the NOx emissions coming out of the engine, but they are neutralised downstream in the urea-activated SCR catalyst built into the exhaust system.
Those European heavy truck builders which formerly relied solely on EGR to control NOx, notably Scania and MAN, have added SCR to achieve Euro 6 compliance, without any loss of fuel efficiency compared with their non-SCR Euro 5 engines. Euro 6 standards, it should be added, are roughly equivalent in stringency to those of EPA 2010. Various different claims have been made for the fuel savings accruing from SCR; they range from 3% to a probably over-optimistic 7%. But all its protagonists insist that the reduction in fuel bills far outweighs the cost of the AdBlue/DEF.
Where the other manufacturers take issue with Navistar is in its suggestion that the monthly fuel-plus-DEF expenditure faced by truckers running EPA 2010 compliant Freightliner, Volvo-Mack and Kenworth-Peterbilt chassis is higher than the ‘fuel only’ bills of Navistar customers.
In the US, diesel fuel has risen steadily in price in recent months; it now costs in excess of US$4 per US gallon. DEF prices vary greatly, according to the customer’s supply arrangements. If it is purchased in bulk for tanker delivery to a transport fleet depot, it is necessarily much cheaper than if bought in small containers at a truckstop or vehicle dealership. But an average retail price recently quoted by a Mack Trucks spokesman was US$2.8 per US gallon.
Critical to the cost of DEF to the trucker is of course the ‘dosing rate’, that is the amount of urea solution required in relation to fuel usage. Mack says the rate is typically 2.2% of fuel consumption, which amounts to a cost of less than 1 cent per mile. It is likely that more truck buyers will be deterred from buying SCR-equipped trucks by the servicing chore of keeping the DEF tank replenished and by the payload penalty of 100kg or so incurred through the weight of the catalyst, tank and dosing unit of the somewhat cumbersome SCR installation.
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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