Is SCR the latest cumbersome emissions technology or a cost-effective cure for excessive EGR? Most engine makers believe the latter. One sticks with EGR.
All heavy-duty engine makers but one will employ selective catalytic reduction to achieve the required levels of nitrogen-oxides to meet the squeaky clean 2010 emissions standards set by the U.S. Environmental Protection Agency. Only Navistar, in its new line of MaxxForce engines, will rely solely on exhaust gas recirculation.
Some in the engine market say, SCR will be a lot like turning the clock back to the happier, more fuel-efficient days of the late 1990s, before the advent of EGR. While SCR offers some advantages, its overall value versus EGR-only technology is complicated by having to account for its higher costs and its dependence upon an onboard exhaust additive, diesel exhaust fluid.
Proponents of SCR say it will be like a breath of fresh air to diesel owners in terms of dialing back EGR’s intrusive effects. When EGR entered the picture in 2002, it brought variable geometry turbochargers, increased fuel consumption and stresses on the engine oil, and a jacket water EGR cooler and EGR valve that sometimes gave trouble. In 2007, an increase in the percentage of the exhaust that was recirculated was combined with a diesel particulate filter that uses a significant amount of fuel to regenerate itself in some applications. But with SCR, less exhaust will be recirculated, so engines will behave more like they did in the past.
Turning back the clock, however, comes at a price – for the initial cost and in operation. Schneider National’s Steve Duley, vice president of purchasing, says his company’s research shows DEF is likely to cost $2 to $3 per gallon.
Duley’s heard the increased cost of engines with SCR systems will be $5,000 to $8,000. Eric Starks, president of Freight Transportation Research Associates, says his clients are hearing $5,000 to $10,000. Such estimates are similar to the hikes that accompanied the introduction of 2007 technology.
Part of that cost will be the upgrade to the onboard diagnostics – also necessary with Navistar’s system – needed to control the system and signal trouble. However, “There may be some integration between the various components, and that may help with the cost,” Duley says. “Also, the DPF is likely to perform better, going longer between regeneration cycles.”
For most SCR systems, the only change in maintenance will be replacing a simple filter near the dosing valve once every year or two. In Mack’s case, “The only predicted SCR system maintenance is the actual DEF injector annually and the DEF tank filter which is also on an annual PM schedule,” says David McKenna, marketing manager for Mack drivetrains.
Cooling SCR engines will not be a problem, say Stephen Morelli of Daimler Trucks North America, and Preston Feight, Kenworth’s chief engineer. “Cooling system capacities and under-hood temperatures will be very similar to those seen in our current product,” Feight says.
One key to making SCR work economically is continuing to use some EGR, as well as the other tricks diesel manufacturers use, like multiple injections, to keep engine-out NOx low. This will be done so that only a small amount of DEF will be needed.
Volvo’s Ed Saxman, director of powertrains, says, “By adding SCR to EGR, we can meet the standard so handily that we can actually back off the EGR from EPA ’07 levels. Because EGR decreases fuel efficiency, reduces power density and increases heat rejection, this means that, with SCR, we can reduce the level of EGR and increase fuel efficiency, improve power density and reduce heat rejection all at the same time.”
At Detroit Diesel, says Marketing Director David Siler, “The engine will be tuned to provide the best combination of engine-out NOx and DEF consumption rate. Our customers should see roughly a 3 percent fuel economy improvement.”
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