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.”
With Cummins’ new engines, “Customers will see up to a 5 percent improvement on some ratings,” says spokesman Lou Wenzler. Cummins’ plan to increase the displacement of its ISX engines from 15 liters to 16 liters for the higher power ratings in 2010 has been scrapped since it will no longer be necessary for the engine to swallow more exhaust than in 2007.
Paccar’s new heavy-duty engines will make their debut as 2010-compliant. “Our tests and experience with SCR in Europe indicate the improvement in fuel economy can be as high as 5 percent,” says Allan Treasure, Paccar marketing manager. Paccar expects DEF consumption will be about 2 percent of diesel fuel in normal conditions. Cummins also puts DEF usage at 2 percent.
Cummins uses the example of a truck running 120,000 miles per year and getting 6 mpg, which means 20,000 gallons of diesel fuel. That truck would use 400 gallons of DEF (at a 2 percent consumption rate), requiring 20 fill-ups of a 20-gallon DEF tank, which means a refill every two to three weeks. DEF is forecast to cost the same as or less than diesel fuel.
Mack’s McKenna says that EPA wants the DEF tank to hold enough fluid to last through two diesel refueling cycles. A 100-gallon diesel capacity and a DEF dosing rate of 3 percent would dictate a six-gallon DEF tank. A truck that achieves 6 mpg will get 200 miles per gallon of DEF.
Operational costs and inconvenience is one reason Navistar, maker of International Trucks, is staying away from SCR and using only enhanced EGR, says Tim Shick, director of engine marketing. “One estimate is that DEF will cost half of what diesel fuel costs,” he says. “But, we asked ourselves, how variable is the cost?”
Also, Navistar believes the volume of DEF consumed is typically very close to the amount of fuel saved by an SCR system, meaning little or no net reduction in overall fuel cost.
SCR also has the handicap of “250 to 500 pounds added weight,” Shick says. Detroit Diesel estimates 250 to 400 extra pounds, says spokeswoman Hallie Fisher.
Shick also says EGR alone is simpler and “transparent to the customer. There were initial problems with EGR valves, but now they are reliable.”
Because “maintenance and related problems would go up without high-pressure common rail,” Shick says, the 2010 MaxxForce engines will use a high-pressure common-rail injection system developed even beyond what’s on the engine today to optimize performance and minimize engine stress. This system will also allow the engine to cruise efficiently at very low rpms.
Navistar’s strategy will be to introduce an engine that doesn’t quite meet the EPA standard but will be legal because it uses emission credits the company accumulated from exceeding standards with prior engines. Meanwhile, engineers will continue to calibrate functions inside the cylinder to achieve a fully compliant engine.
In 2010, Cummins will introduce its new XPI common-rail fuel system, which, at low rpms, operates at a much higher pressure than the present system and allows multiple injections. It enables the engine to burn cleanly while ingesting less air. Wenzler says, “This allows for flexibility in tuning for fuel economy and low emissions, while also minimizing engine stresses.”
Detroit Diesel’s DD15 has what it calls an amplified common rail fuel system. It has all the same advantages as other common-rail systems, but the piping that carries the fuel around the engine handles much less pressure. A double-ended amplifier piston located near the injector increases the fuel pressure to the desired value hydraulically.
The DD15 also has a turbocompounding turbine in place of the variable geometry used on most engines’ turbochargers. This device returns exhaust energy to the crankshaft through a
geartrain, and builds the backpressure needed to make the EGR system work in a less wasteful manner than a variable geometry turbo.
Paccar’s Treasure says, “The DEF will be injected as the exhaust flows through a mixing tube or pipe. This will ensure a complete mixing of the DEF in the exhaust gas and will occur just before the exhaust enters the SCR catalyst.”
Where will the other components be located?
The DEF tank will always be near the left-hand fuel tank for convenient refilling when refueling.
Morelli says most Daimler-built trucks will have everything located under the right-hand cab entry steps, a typical location for all brands. But there will be other, very different Daimler options, too. McKenna says, “The pump that supplies the DEF will continue to run after the engine is shut off, returning unused DEF back to the tank to prevent supply line freeze-ups.”
While all new technologies usually start with minor problems, the U.S. market has the advantage that SCR has been used in Europe for years. McKenna says, “There has been nothing in the way of negative performance issues, whether we are referring to system robustness or consistency of emission and fuel economy performance.”
In Europe, Cummins has more than 200,000 SCR systems in service. Paccar has more than 100,000. Company officials say this experience should minimize major headaches a year from now when SCR engines hit the road here.
Keeping an eye on that other tank
Truck makers using SCR engines plan to have a dash gauge and warning system to ensure drivers don’t let the diesel exhaust fluid level drop too low. Your engine would never shut down due to lack of DEF, but neither would you want to test the limits too much.
David McKenna of Mack Trucks gives a preview of Mack’s progressive warning system:
“When the DEF level drops to around 20 percent remaining volume, a dash indicator lamp will inform the driver that a DEF fill up is in the future. At something around 5 percent to 10 percent, an audible tone will sound that informs the driver that there is now a minimum reserve of DEF remaining and a small engine de-rate may occur. What happens when the DEF runs out? The engine will go into a substantial de-rate and the road speed of the truck may be limited to 15 mph. At no time will the engine completely shut down and leave you stranded on the road.”
Finding DEF on the road should be no more difficult than finding a truck stop. Aggressive plans are under way to make DEF universally available at truck stops.
OOIDA, Navistar call for postponing deadline
The Owner-Operator Independent Drivers Association has called for a re-evaluation of the U.S. Environmental Protection Agency’s tougher 2010 heavy-duty engine emissions standards.
The call came after publication of a study conducted by NERA Economic Consulting that concluded the price of trucks employing 2010 technology would be significantly higher. Given the slowing economy, OOIDA says, this price premium will be an undue hardship on all parties. Navistar, which helped fund the NERA study, said it stands with OOIDA.
“When those new engines come out, they’re going to be more expensive,” says Navistar spokesman Roy Wiley. “Many of the truckers won’t be able to afford the 2010s.”
A gradual phase-out of 2007 emissions standards would be the goal of changing the Jan. 1, 2010 deadline, Wiley said.
An EPA spokeswoman says there is no plan to change the deadline.
Volvo, Mack and the Engine Manufacturers Association have come out against making any change to the 2010 requirements.
- Todd Dills