It’s not too early to start thinking about which EPA-compliant engine you’ll be buying in 2010 or later: a slightly smaller, lighter one that requires urea as an additive, or a somewhat larger, heavier one that doesn’t.
The U.S. diesel engine industry will divide into two camps in 2010 to meet the U.S. Environmental Protection Agency’s new emissions standard of only two-tenths of a gram of nitrogen oxides per horsepower hour. That’s just a tenth of the NOx allowed in 2004, a percentage reduction comparable to the drop in soot enabled by the diesel particulate filter.
At least three manufacturers – Detroit Diesel,Volvo and Mack – will use selective catalytic reduction, a form of aftertreatment to reduce NOx outside the engine. At least one other manufacturer, Cummins, will use an enhanced version of cooled exhaust gas recirculation. International, likewise, has said it won’t go the SCR route with the heavy-duty engines that debut this year. Neither Caterpillar nor Paccar, which is introducing heavy-duty engines in 2009, has announced a decision as of press time.
SCR feeds a small amount of ammonia-containing urea solution into a catalytic exhaust chamber, where the ammonia combines with NOx to form harmless byproducts. Urea occurs naturally in the urine of humans and other mammals, which is one reason some in the industry prefer to call it “diesel exhaust fluid” or DEF, but it’s also manufactured as an industrial chemical.
SCR will be similar to turning the clock back to the days before EGR engines, says Ed Saxman, Volvo’s drivetrain product manager. “Urea and NOx become nitrogen and water,” Saxman says. “That’s how SCR aftertreatment works. Why are we going to use it to kill NOx? Because everything else the engine makes can be reduced by making combustion more efficient.
“When you don’t have to reduce the engine-out NOx, you can do amazing things with today’s technology, which includes 35,000-psi injectors in our case. That includes greater power density, greater thermal efficiency and reduced heat rejection when compared with higher levels of EGR.”
Volvo’s 2010 engines will reverse the trend of worsening fuel economy ushered in by the 2002 emissions standards, Saxman says. “You can turn the knobs back,” Saxman says. “For somebody who uses a lot of fuel, it puts dollars into his pocket.”
What about the cost of adding a new chemical to the mix? Urea will be consumed at a rate of roughly 3 percent of the diesel fuel burned, Saxman says. Urea is made using natural gas rather than crude oil, so urea costs only 75 percent as much as diesel fuel, Saxman says. Moreover, “Diesel fuel (prices) will go up faster than urea,” he says.
Detroit Diesel’s 2010 DD15 and its sister 13-liter and 16-liter engines will consume urea at a rate of about 2 percent of the diesel fuel burned, says David Siler, Detroit’s marketing director. While SCR engines potentially have higher up-front costs, the long-term benefits more than make up for that, Siler says. “The financial implications of our choice were in the center of our discussion on which path to take in 2010.”
“It’s premature to talk about front-end costs quantitatively because several influencing factors are still in rapid flux,” says Rakesh Aneja, Detroit program manager for 2010 heavy duty engines. He adds, however: “We are confident that our new DD series engines and 2010 aftertreatment will provide the customer with the lowest possible life-cycle costs.”
That’s because Detroit’s 2010 system is based on proven, mature technologies already in production, which “allows us to effectively break away from the three-year product release cycle that we have witnessed in this decade,” Aneja says. “SCR provides us the opportunity to break the classic trade-offs associated with NOx, PM and fuel economy.” Plus, SCR reduces component stress and heat rejection and extends oil-drain intervals, he says.
Urea should be readily available to the diesel engine industry because other sectors already demand it in large amounts: Agriculture uses it in fertilizer, and utilities use it to reduce NOx emissions at power plants. The amount needed by engine manufacturers will be “a very small percentage of present production,” Saxman says. Siler agrees, saying urea producers “are investing in their capacity to meet the demand in plenty of time for the 2010 engines.”
Cummins’ heavy-duty emissions strategy for 2010 mainly consists of enhanced EGR. When exhaust is cooled to about 230 degrees Fahrenheit and fed back into the engine with the intake air, it becomes a very potent way to kill NOx. But meeting the 2010 standard with more EGR is not as simple as it might appear.
You can’t just increase the size of the EGR system components. Your engine would have to endure excessive cylinder and turbocharger boost pressures, and the total volume of air and exhaust the turbo would have to jam in would be much higher than now. Increasing the size of the engine block would get these stresses under control, but such a beast would be difficult to fit under an aerodynamic hood; it also would be too heavy and costly to be practical.
There are other, more subtle ways to put in a lot more EGR, says Steve Charlton, Cummins executive director of heavy-duty engineering. What matters, he says, is not the amount of EGR going in, but the total volume of gases: air plus exhaust.
Since diesels need more air than combustion requires, one possibility is to improve “air utilization,” or how effectively the air and fuel mix in the combustion chamber. A more perfect mixing process can bring down the amount of needed air. In Cummins’ case, half the equation was the XPI High-Pressure Common Rail injection system.
Present injectors trap a tiny volume of fuel under an injector plunger that is driven down by a rocker lever and the camshaft. The camshaft and rockers are mounted on the cylinder head and driven off the crankshaft by a geartrain.
Common rail, on the other hand, uses a crankshaft-driven pump and routes the fuel from the pump to all the injectors via small, high-pressure passages, running at about 31,900 psi. Common rail has one big advantage: The pressure always is there.
In the unit injector system, the speed at which the injector plunger moves downward drops with engine rpm because the crankshaft and geartrain slow down. The slower motion means less pressure produced. Injection pressures drop considerably as the engine lugs down.
The unit injector system also must build up pressure every time the engine fires because the fuel in the injector is un-pressurized once injection ends. A short time lag ensues at the beginning and end of each injection cycle, as low-pressure fuel enters the combustion chamber.
Common rail guarantees maximum injection pressure even at 1,100 rpm and throughout every injection cycle. The higher pressure causes the fuel to better mix with the air.
Cummins also redesigned the combustion bowl. Helped by computer simulations, the designers have increased the force of the air motion and the turbulence so the air moves more rapidly through the sprays and does a better job of supplying needed oxygen. The engine needs less air to burn the same amount of fuel.
Because of these improvements, less air goes in, and the total exhaust and air going in is only slightly higher than what was needed in 2007, Charlton says. The exhaust coming out contains less oxygen, which minimizes the increase in exhaust volume needed to get the NOx down, he says.
Thanks to this improved combustion system, Cummins says, its 2010 ISX will use the current bore and stroke for the lower power ratings, and the engine will increase in size only from 15 liters to 16 liters for the maximum power ratings – all while keeping stresses in line with today’s engines.
Cummins also will add a close-coupled catalyst to increase the effectiveness of active DPF regeneration, which has to occur more frequently when the exhaust contains less NOx.
Nor will International use SCR in 2010, but it has not revealed details of its system. That likely means EGR, though not necessarily the same EGR setup as Cummins.
A different means of dealing with EGR is demonstrated in the SCR camp, in the new Detroit Diesel D15: The fuel will be pumped in slowly at first, then much faster later, during each injection cycle. This means the engine needs less air while achieving the same smoke limit, says Edward Crawford, Detroit director of product engineering.
Caterpillar officials say they will build on the proprietary ACERT technology for 2010, but are not ready to announce the final details.
So how will the choice between SCR and EGR affect your bottom line? Using SCR is likely to reduce fuel cost, even counting the cost of the urea. But plenty of truckers who use increased horsepower to haul more freight more miles know that cutting fuel cost is not the only way to make money. Stopping regularly for urea may add miles and delay deliveries, especially if a careless driver lets the juice run out, which means reduced power, or if the truck runs in remote areas where urea might be difficult to find.
Urea poses problems for truck stop operators
Yes, your favorite truck stops will be carrying urea in time for the 2010 engines, but that doesn’t mean they’re wild about doing so.
“So far, we are looking at this as a cost and a condition of being able to carry diesel fuel,” says Sarah Dodge, NATSO vice president of government affairs. “It may prove to be a profit burden for us, rather than a profit center.”
Urea, which has been used in Europe for years, is sold in much smaller volumes than diesel fuel at a much lower price. Yet, it requires its own infrastructure, including heated or underground tanks and dispensing and metering equipment.
U.S. truck stops are “anticipating more headaches than anything,” Dodge says.
The fluid will be needed not just for big rigs but for a variety of vehicles, including large pickups. Some distribution will begin by summer because of diesel-powered cars and SUVs.
Most truck stops will keep urea in a storage tank because customers will need more than a washer-fluid-sized package. “We can’t figure how much we will need to store,” Dodge says, which makes it challenging to develop a business plan.
Urea also has a short shelf life that varies with the weather; it deteriorates rapidly when too hot and freezes at about 12 degrees Fahrenheit.
Some of the truck stop problems posed by urea, of course, are the same ones posed by diesel fuel and gasoline: how to ship it in, how to clean it up if it spills, how to deal with liability issues if it runs out or if bad urea is dispensed into customers’ tanks – and how to do all the above affordably.
Along with the American Petroleum Institute and the U.S. Department of Energy, NATSO plans to develop an online service to help customers find truck stops, dealerships, service centers and retailers that sell urea. The listing for each location will include whether urea is sold at the pump or by the bottle, what types of vehicles the facility services, and whether the dealer participates in a urea quality-control program.
Mack, Volvo, Detroit Diesel
Selective catalytic reduction (SCR) using urea solution and a catalyst to help reduce NOx. Ideal where fuel economy and weight are primary considerations and trucks operate on main travel lanes.
Exhaust gas recirculation (EGR). Ideal for users running less traveled routes and those eager to avoid replenishing urea.
Unannounced. Caterpillar likely will build on present ACERT technology. Paccar, which will use its engine in Kenworths and Peterbilts, produces engines in Europe that will use SCR.
SCR could yield lower fuel costs
Diesel fuel @ $3.41/gallon
Urea @ $2.56/gallon
Dosage rate: 3%
Fuel economy improvement for 2010 of 3%
1,000 miles @ 6.2 mpg: 161.29 gallons fuel
Fuel cost = $550.
1,000 miles @ 6.386 mpg: 156.59 gallons fuel
Fuel cost = $533.97
Urea for 2010 engines:
156.59 gal. of fuel x 3% (dosage) = 4.7 gal. urea.
Urea cost = $12.03
Fuel + urea cost: $546
SCR’s annual savings at 120,000 miles a year, based on $4 per 1,000 miles
Selective catalytic reduction (using urea)
- It enhances thermal efficiency and fuel economy.
- It reduces heat rejection and cooling system stresses, allowing for a smaller radiator and cooling fan.
- The smaller, lighter engine may equate to increased payload and less expensive injection
- Searching for urea suppliers may add out-of-route miles.
- Consumption of urea is unpredictable, since its mixing ratio varies with driving conditions.
- If urea level is neglected, engine is de-rated, slowing delivery.
Exhaust gas recirculation
- Urea refills are convenient, no risk of experiencing a power-down event.
- Technical risk related to the SCR catalyst and doser is eliminated.
- The simpler fueling requirements are easier for hired drivers, thus good for small-fleet driver retention.
- Engines are larger and possibly heavier, depending on power rating.
- Larger radiator and fan are needed to handle small increase in heat rejection.
- The fuel cost is higher than the cost of fuel plus urea in an SCR system.