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MELROSE PARK, Ill. — In a testing cell tucked deep in the bowels of Navistar International Corp.'s engine plant and technical center in this Chicago suburb, a hulking prototype of a truck engine sits behind a window like a patient on an operating table.
A web of sensors and wires is attached to nearly every part of the engine, feeding data to a battery of computers and a group of engineers in the adjacent control room.
One measurement — for emissions of nitrogen oxide, or NOx — is of particular concern to Navistar. From 2010 onward, all new truck engines must achieve near-zero limits for NOx, a chief ingredient of smog.
Virtually all truck makers besides Navistar chose to use an add-on system to their existing engines that uses a fluid cocktail to help neutralize the pollutant in the engine's exhaust.
Navistar decided to spend hundreds of millions of dollars to refine an engine that produces minimal NOx in the first place.
At the same time, the company attacked the competing systems, suing federal air quality regulators and contending that the add-on technology was so flawed that it failed to meet the clean-air requirements.
If Navistar's engine works — the company recently submitted test results for the latest version to the U.S. Environmental Protection Agency for certification — it could be the simplest, most elegant solution to the vexing engineering problem of how to reduce smog created by diesel truck exhaust.
But the company would have to persuade fleet owners to buy the engines. If those owners do not see a clear advantage in operating costs and fuel efficiency, Navistar could be the only engine maker promoting an alternative to the rest of the industry's EPA-approved approach.
The company has paid a price for choosing the road less traveled. While its engines have been in development, its share of the U.S. market for the heaviest trucks fell to 20.2 percent this year from 28.5 percent in 2009, according to research by JPMorgan Chase.
Many of Navistar's fellow truck makers and some environmental groups have dismissed the company's complaints as the desperate grumblings of a company that painted itself into a corner with a system that some have written off as impractical.
Navistar says its approach will ultimately be justified.
"We didn't make this decision lightly," said Jack Allen, North American truck group president. "We've been making diesel engines for 75-plus years, and … we're confident that our solution is what customers will want."
Operators of truck fleets say they are keeping an open mind.
But so far, the add-on systems made by other truck companies offer the best combination of "fuel performance, up-front purchase price, and running costs," said Art Vallely, senior vice president of rental and vehicle management for Penske Truck Leasing, which operates 200,000 vehicles.
Diesel engines in the nation's 18-wheelers, buses, and other heavy-duty vehicles power less than 10 percent of all vehicle traffic in the United States, but they account for as much as 25 percent of the haze of pollution that hangs over many U.S. cities.
Reducing those emissions is an engineering challenge in which every tweak has consequences. Lowering the temperature of combustion, for example, is an easy way to reduce NOx emissions. But it also generates more soot. Soot levels can be reduced by altering combustion timing, but that raises fuel consumption.
On the floor of Navistar's engine facility here, Luis C. Cattani, a chief engineer, recalled that he was discouraged by colleagues from joining Navistar after a career designing high-performance engines in Detroit.
"They said, ‘Oh, you'll be so bored,'?" Mr. Cattani said. "But as an engineer, you really love this sort of challenge."
Recognizing the complexity of the issue, the EPA gave truck makers a decade to engineer a solution when it issued new NOx limits in 2001.
Navistar was an early proponent of a technique called exhaust gas recirculation, a well-established process in which burned gases from the engine exhaust are routed back to the cylinder, diluting the mixture and lowering the temperature of combustion, which in turn cuts NOx.
Most other U.S. heavy-duty-truck makers — along with makers of diesel passenger cars including BMW and Mercedes-Benz — elected to primarily control NOx with a technology called selective catalytic reduction.
It uses liquid urea, a chemical compound commonly synthesized for a variety of industrial uses, which is injected into the exhaust to break down the pollutant.
Although the EPA does not prescribe technologies for meeting its standards, it did suggest in 2001 that liquid urea would be an unlikely candidate. The system would require a whole new infrastructure for making urea available at truck stops across the country and would rely on drivers monitoring and refilling an on-board tank.
Still, as the 2010 deadline approached, the industry had failed to come up with any alternative to the urea technology that could meet the emission standards.
So the EPA allowed truck makers to use it as long as other components, such as warning lights on the dashboard, audible alarms, and even engine slowing, were included to alert drivers that the urea supply was low.
Meanwhile, when the deadline hit, Navistar's engines were producing NOx levels at almost double the limit. Even as it struggled to refine its technology, Navistar attacked its rivals' approach.
It argued that the urea-based technology was ineffective in some circumstances, such as stop-and-go traffic. In addition, it said the urea system could be easily bypassed with simple tricks, including putting water into the storage tank.
Navistar sued the EPA and the California Air Resources Board, accusing them of relaxing the legal standards. The suits were settled last summer after both agencies agreed to conduct a public review of their policies regarding the urea technology.
Rich Kassel, a senior lawyer with the Natural Resources Defense Council, said there was no evidence that drivers were bypassing trucks' urea systems.
"And the reason why there's no evidence that they're doing it is if you run the truck with low levels of urea, you start getting a series of signals, both lights and audible signals," he said.
If the signals are ignored, the engine slows down. "For a driver who is carrying a full load for any length of time, they're not going to risk losing the power," Mr. Kassel said.