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    Most manufacturers use selective catalytic reduction (SCR), seen here, to remove nitrogen oxides (NOx) from exhaust. The process injects a urea solution over a catalyst to separate the toxic gases into their relatively harmless components: nitrogen and oxygen. Photo: Cummins Inc.

By Paul Abelson

Engine manufacturers have a number of options, including alternative fuels, for meeting U.S. EPA emissions requirements. But for heavy-duty applications, one thing's certain: For the foreseeable future, the power plant of choice remains diesel.

Diesel has two main advantages over all other fuels:

Greater energy density. Diesel produces 132,000 Btu/gallon to 144,000 Btu/gallon compared to gasoline's 112,000 Btu/gallon to 118,000 Btu/ gallon.

More efficient use of intake air. More air compressed more tightly equals more power.

The ratio of air to gasoline must be 14.7:1; if too lean or too rich, the mixture won't burn. A diesel engine, on the other hand, uses as much air as engineers can deliver.

The problem is that what lowers particulate emissions — unburned fuel in the form of smoke or soot — increases noxious gaseous emissions (NOx); and what lowers NOx increases particulates. You can burn diesel more completely with higher combustion temperatures, but within the pressurized confines of the engine's cylinders the heat fuses nitrogen and oxygen to form NOx.

Manufacturers address this Catch 22 in one of two ways. All except Navistar International Corp. treat exhaust after the fact with a process called selective catalytic reduction (SCR), while Navistar's MaxxForce engines use a proprietary “in-cylinder solution.”

The difference is notable because four manufacturers — Cummins Inc., Detroit Diesel Corp., PACCAR Inc., and Volvo Power/Mack Trucks Inc. — are taking the same approach while a fifth is going it alone. It's the “War of the Words” in advertising and at truck shows that's adding excitement to what would otherwise be fairly routine competition.

To understand the controversy, let's examine the two approaches.

All engines made after Oct. 1, 2002, dilute oxygen-rich intake air by recirculating exhaust, which is inert. This lowers combustion temperature, thus limiting the formation of NOx. To meet 2007 regulations, manufacturers introduced diesel particulate filters (DPFs) to remove particulate matter.

By the 2010 model year, engines were allowed to release no more than 0.2 gram NOx per horsepower-hour (0.2 g/hp-hr).

The process most manufacturers use, SCR, injects urea into the exhaust, where heat converts it to ammonia that breaks nitrous oxides into their harmless components: nitrogen and oxygen. This allows for more complete combustion, reducing particulate generation and improving fuel economy. Because the catalysts are downstream of turbochargers, the process doesn't compromise engine components and accessories by increasing under-hood heat.

Navistar opted to save the cost, weight, and complexity of exhaust after-treatment by using “Advanced EGR." Although the process recirculates more exhaust, advances in fuel and air management limit particulate formation to keep filters from clogging.

The bad news is that the process achieves 0.5 g/hp-hr NOx, which is 0.3 gram short of EPA's requirement. But because Navistar engines surpassed requirements for years, they have enough credits to be certified as compliant for 2010. In fact, using credits, they comply through 2012.

While that protects you, the buyer, it also gives Navistar's competitors some ammunition.

First, their product doesn't have to resort to credits.

Second, they claim that Advanced EGR aggravates the two complaints customers have been making since the process was introduced in 2002: less fuel economy and more under-hood heat. Instead, they say, their process is easier on components and accessories and 4% to 6% more fuel-efficient.

Navistar's response is to cite tests and customer experience showing comparable fuel economy gains. The company says engines achieve results by carefully metering fuel injection multiple times each piston stroke. High-pressure injections atomize fuel more finely, giving it greater surface area to absorb heat, vaporize, and combust. Director of Business and Product Strategy Tim Shick says that slows the rate of burn, thus lowering NOx formation. Shick also told PUBLIC WORKS that Navistar has compliant (0.2 g/hp-hr) engines in operation now, pending EPA certification.

This particular war's been both entertaining and informative. I enjoy the debate between proponents and opponents of Advanced EGR. The ultimate winner will be you, the customer using diesel power.

— Paul Abelson (truckwriter@anet.com) is a former director of the Technology and Maintenance Council of the American Trucking Associations, a board member of Truck Writers of North America, and active in the Society of Automotive Engineers.