Experience with 2007 engines springboard for 2010

Diesel Progress North American Edition, Sept, 2007 by W. Addy Majewski

We should start this forecast with a quick look back at the 2007 engines, which are the starting point for meeting the challenges of the coming EPA 2010 requirements. The 2007 engines represent the first-ever mass introduction of diesel particulate filter (DPF) technology on heavy- and medium-duty diesel engines, ranging from Class 8 truck engines to buses and pickup trucks.

Compared to the introduction of cleaner diesel engines in the past (October 2002), the debut of DPF technology can be described as smooth. While most manufacturers were fine-tuning their DPF regeneration software after the engines were launched, few problems have been reported, and DPF-related vehicle recalls were incidental. Long-term durability of DPF materials will be demonstrated in the coming years.

Particulate filters were launched to meet the 2007 particulate matter (PM) emissions limit of 0.01 g/bhp-hr, which represents a 90% PM emissions reduction from the previous standard. All of the DPF systems are believed to utilize a catalytic wall-flow filter with active regeneration support. Through the use of a catalyst, the filters can regenerate passively during high engine load operation (thus, minimizing the fuel economy penalty associated with active regeneration). When exhaust temperatures become too low to sustain passive regeneration, active regeneration is triggered by the control software. The use of DPF catalysts was made possible by the availability of ultra-low sulfur diesel fuel.

Active DPF regeneration is conducted by increasing the exhaust temperature by combusting an additional quantity of fuel, which can be done in a number of ways. A common method in heavy-duty 2007 engines is to inject fuel into the exhaust system, followed by catalytic combustion over an oxidation catalyst positioned upstream of the DPF. Other 2007 engines employ flame combustion, using a fuel burner positioned upstream of the DPE The fuel burner method provides more flexibility in performing regeneration independently from engine conditions, including light-load operation, at the expense of more complex hardware.

Engine manufacturers are making their final technology choices for meeting the 2010 N[O.sub.x] emissions standard of 0.2 g/bhp-hr--the most stringent N[O.sub.x] standard in the world. Selective catalytic reduction using urea solutions (urea-SCR), combined with exhaust gas recirculation (EGR), appears to be the technology of choice for heavyduty engines. N[O.sub.x] adsorber catalysts, also referred to as [NO.sub.x] traps, still have insufficient thermal durability to meet the [NO.sub.x] reduction demands under higher engine loads (to meet the NTE testing requirements). In light-duty engines, both SCR and [NO.sub.x] adsorber catalysts will be used.

The EPA has opened the door for the application of urea-SCR--a technology that it had previously opposed--by the recent publication of guidelines for emissions certification of urea-SCR engines. In the SCR technology, a water-based solution of urea (which is injected upstream of the catalyst as a [NO.sub.x] reductant) must be carried in an onboard tank and periodically replenished. The guidelines include a number of requirements to ensure that (1) urea solution is available for purchase when needed; and (2) vehicle operators do indeed fill up the urea tank once it gets empty. The latter objective is to be achieved by a warning system that will alert the driver when urea level is low, coupled with a "driver inducement" system that will make it impossible to drive the vehicle without urea in the tank.

Considering the pressing timeframe, it will be interesting to see if urea infrastructure can be developed on time. European experience indicates that more than three years may be needed to make automotive-grade urea solutions commonly available--and we are still not aware of a North American stakeholder consortium that would coordinate industry efforts to establish urea infrastructure.

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It may also be noted that the launch of urea-SCR technology on heavy-duty Euro 4/5 engines in early 2005 was made possible de facto, accepting a certain degree of noncompliance in the initial period, as the availability of urea was rather limited at the time the technology was launched and the European onboard diagnostics (OBD) requirements for SCR engines become effective only from October 2007.

Several manufacturers intend to launch diesel-powered light-duty vehicles--passenger cars, SUVs and light trucks--in the 2008-2010 time-frame in the North American market. These vehicles will meet the EPA Tier 2 Bin 5 emissions standards by using advanced combustion combined with [NO.sub.x] aftertreatment and particulate filters. German manufacturers (Mercedes-Benz, BMW) apparently prefer urea-SCR for N[O.sub.x] control, with N[O.sub.x] adsorbers possibly used in smaller engines (VW).

Japanese manufacturers, on the other hand, are developing advanced N[O.sub.x] adsorber catalysts for diesel cars. Honda intends to use a combined N[O.sub.x] adsorber/SCR catalyst system without urea to meet Tier 2 Bin 5. Nissan announced a complex catalyst concept that combines hydrocarbon and N[O.sub.x] traps for meeting the even more stringent Tier 2 Bin 2 and the California SULEV standards.