The nation’s long-haul truck industry is suddenly seeing an explosion of orders, as trucking fleets rush to beat a new environmental rule aimed at big diesel engines. Starting Oct. 1, the Environmental Protection Agency (EPA) will require new big-rig engines to meet lower limits on emissions of nitrogen oxides. The biggest trucking fleets say that the technology that will make the engines cleaner also will add between $3,000 and $5,000 to the current cost of an engine. They also expect the new engines to be less fuel efficient, costlier to maintain, and possibly more prone to breakdowns on the road.