In less than two years, when bus builders supplying motorcoaches to the North American market begin introducing their 2010 models, there could be major differences among the competing brands unlike anything operators have seen before.

Thats because the coach manufacturers will be using two completely different engine technologies to power their new models.

And there are predictions the best technology for meeting the 2010 federal emission standards could have a dramatic long-term impact on the over-the-road commercial vehicle industry.

Unlike 2004 and 2007, 2010 will mark the first time engine manufacturers and their coach and trucking-building customers will be offering two distinctly different diesel engine emissions-reduction technologies.

Fleets will have to choose which of the technologies is best for them, and that has the potential to trigger shifts in market shares among the OEMs. If coach buyers dont feel comfortable they may move their purchases to another brand.

In recent months, executives from the major diesel engine suppliers have vigorously defended their companies choice of technology, either selective catalytic reduction, or exhaust gas recirculation. In some cases, there has been pointed criticism of one technology over the other.

The battle over diesel engine emissions technology was instigated by the U.S. Environmental Protection Agency, which set standards for 2010 engines that reduce nitrogen oxide emissions to practically zero.

To cut NOx, selective catalytic reduction mixes urea into the engine exhaust, where it becomes a catalyst in a filter that converts NOx into nitrogen and water.

Exhaust gas recirculation, on the other hand, circulates some engine exhaust gas back into the engine to cool combustion, reducing NOx.

The cost to develop both technologies has been enormously expensive.

Daimler, owner of Setra and Detroit Diesel, and Volvo AB, which owns Prevost Car and produces Volvo-brand engines, have said they will use SCR, which is used widely in Europe but is new to North America, for their 2010 heavy-duty engines.

International Truck and Engine Corp. will use only exhaust gas recirculation, which has been used in motorcoaches and trucks since 2002, while engine maker Cummins Inc. will use SCR for its medium-duty engines and EGR for its heavy-duty models.

Caterpillar has not yet announced what technology it will use. Cats delay has spurred widespread speculation, ranging from talk its having trouble meeting the new exhaust standards, to suggestions it may be planning to bail out of the on-highway engine market. Stay tuned.

International, which supplies chassis and midsize buses to the motorcoach industry, has been among those companies touting its decision to stay away from SCR and use only exhaust gas recirculation.

In briefings, executives from International have pointed out the significant differences between the two technologies, noting specifically the "extra components" used by SCR: an extra tank and extra emissions system, in addition to a diesel particulate filter. The International executives contend there are going to be more maintenance issues and more parts that can break down with SCR.

Navistar, which is Internationals corporate parent, has published a pamphlet that asserts, "There is no delivery infrastructure in place to support urea distribution in North America for 2010 ... the need for urea demands a separate tank that will add weight and reduce payload ... SCR urea must be a high grade and a precise solution to avoid catalyst damage and assure system integrity."

Navistars claim of urea availability problems is disputed. Spokesmen from Volvo and Daimler point out that only a tiny amount of urea is needed for each gallon of diesel.

According to one calculation, if a motorcoach has a 230-gallon diesel tank, the urea tank would only have to be 5.75 gallons to run the coach for two-and-a-half to three fill-ups.

Daimler, other truck makers and independent suppliers are working to establish a nationwide urea distribution system.

Meanwhile, Cummins contends that long-distance fleets "do not need another component to worry about" in running their businesses, especially something that can shut them down, such as being unable to find urea.

2010 SCR-equipped vehicles must have a system to de-power and eventually shut down the engine if the system runs out of urea.

Meanwhile, supporters of SCR note the technology is being used successfully in Europe and they decry the "misinformation and scare tactics" about SCR.

Before SCR was introduced in Europe earlier in this decade there were doubts urea would be available but most of the perceived SCR issues vanished after it hit the road.

And, despite some reports, drivers will not have to add urea to fuel and will not have to fill their vehicles urea tank every time they buy fuel. One tankful of urea will be enough for several tanks of diesel fuel.

SCR supporters also note that -- by using the technology -- they will be able to cut NOx to acceptable levels without increasing the amount of exhaust gas recirculation in their engines. Because EGR causes engines to run hotter and decreases fuel mileage, use of SCR will improve efficiency, they say.