For 2010, North American bus customers will have two choices to be compliant: they can either buy an all-new, add-on after-treatment system using SCR in addition to EGR, which will be offered by most bus manufacturers. Or, they can continue to use an in-cylinder solution with advanced EGR, offered by Navistar in its IC Bus brand school and commercial buses.
On Jan. 1, 2010, the U.S. Environmental Protection Agency (EPA) will fully implement the 2007 Heavy Duty Diesel rule, which will require all diesel engines to be at 0.20 grams per brake horsepower-hour (g/bhp-hr) of NOx, or oxides of nitrogen. Two methods that likely will be used to meet these new standards are classified as Exhaust Gas Recirculation (EGR) and Selective Catalytic Reduction (SCR).
The entire commercial vehicle industry utilized EGR to meet the first phase of the EPA’s 2007 emissions regulations. For 2010, North American bus customers will have two choices to be compliant: they can either buy an all-new, add-on after-treatment system using SCR in addition to EGR, which will be offered by most bus manufacturers. Or, they can continue to use an in-cylinder solution with advanced EGR, offered by Navistar in its IC Bus brand school and commercial buses.
SCR vs. EGR
Proponents of SCR, which uses the organic compound urea as the reductant, cite that its chief benefit is that it enables the engine to run more efficiently at higher temperatures and generates a relatively lower amount of particulate matter. Yet, Navistar counters that SCR engines produce more NOx, which is very difficult to eliminate once it is formed.
To reach 0.20 g/bhp-hr, a mixture of 32.5 percent urea and 67.5 percent de-mineralized water is required. This mixture is marketed in the U.S. as diesel exhaust fluid. As European bus fleet operators know, the urea mixture, known as “AdBlue” in Europe, needs to be replenished frequently. “We feel that advanced EGR provides the best solution for our customers, both in terms of operator convenience and overall ownership costs,” says John McKinney, president and general manager, Global Bus Operations at Navistar.
SCR takes place outside of the engine and, as a result, requires special software and an after-treatment system that comprises hundreds of pounds of added equipment to properly utilize urea. “That’s a lot of hardware to find a place for on our buses. We just don’t have a lot of room for it,” says Ed Hartung, director, IC Bus North American Operations.
SCR systems also have to contend with challenges presented by environmental temperatures. Urea freezes at 12 degrees F, requiring buses to be equipped with tank and line heaters. At low operating temperatures, the catalyst can clog. While urea does not require a hazardous materials (HAZMAT) handling certificate to haul, it does begin to decompose at 130 degrees F, resulting in ammonia emissions.
“Evaluate” miles per dollar
North American bus manufacturers opting for SCR tell customers that it improves fuel economy, that it has no effect on service and oil change intervals, that a urea distribution network is being established, and not to worry about its price because widespread use of SCR systems will only increase the total demand for urea by a few percent.
But, with SCR, it takes diesel fuel plus urea to get that improvement, and the greater the improvement in fuel economy, the more urea required, offsetting fuel economy gains. Urea may become widely available over time, but its price may be significantly higher where it is not available in bulk. “Bus operators will need to evaluate carefully their total operating costs and calculate their ‘miles per dollar,’ which takes into account the amount and cost of urea in addition to diesel fuel. Data that includes only traditional miles per gallon of diesel is misleading,” says Randy Ray of Navistar Engine Group.
Navistar’s advanced EGR strategy is to prevent NOx from forming in the engine in the first place. This in-cylinder approach, in combination with higher EGR rates, is key to Navistar’s 2010 solution. They have eliminated the need for complex after-treatment hardware and procedures, simplifying the way for customers to achieve compliance. “This further adds to our ability to keep emissions compliance on our shoulders, while SCR puts the burden of compliance on bus fleet operators.”
So, how can Navistar achieve full compliance without after-treatment? “Navistar has long planned for this level of emissions reduction, following a strategic path toward an in-cylinder solution for the past six years,” Ray says.
Four specific technologies enable Navistar to differentiate from the industry and offer what McKinney calls “a truly customer-focused solution:”
