The deal on diesel emissions is done. Now we have to wait and see what it will really mean out on the road.

After 10 months of arm twisting, the Environmental Protection Agency (EPA) believes it has finally solved the diesel engine conspiracy. Unfortunately, it looks like the EPAs solution adds about $1-billion to the cost of making truck engines for the dubious benefit of cutting one particular exhaust emission nitrogen oxides, or NOx. I say dubious because this solution could well come at the expense of increased fuel consumption and a jump in another greenhouse gas emission carbon dioxide.

Putting aside questions about the real environmental benefit of the settlement, the most immediate concern for trucking is how it will affect your operations. The deal worked out with engine makers moves the deadline for stricter NOx standards from 2004 to 2002. (For details on the technologies that will be used to meet the requirement, see Engine Technology: The Drive For Cleaner Air, page 43 of this issue.)

While that two-year change is in itself a fairly significant challenge, its the second part of the agreement that has many observers worried. By January 1, 1999, diesel engines will have to be modified to reduce NOx emissions at highway speeds by up to 40%. That can only be accomplished by reprogramming electronic engine controls to change injection timing.

How will that change in timing affect engine performance? This column is being written only a few days after details of the EPA deal were announced, but speculation on its effect has already divided into two camps: the optimists and the pessimists.

The pessimists say the timing change will increase fuel consumption in over-the-road operations by at least 2% and possibly much more. Since the timing change increases heat and soot generated by the engine, they believe that it will require more frequent oil-change intervals and also present cooling problems, especially in newer aerodynamic truck models that have limited frontal area. On top of that, they say it raises questions about long-term engine durability and about operation of smart components such as automated mechanical transmissions that share data with the electronic engine controls.

As for the optimists, which include many of the engine manufacturers, they say fleets wont notice any significant changes when they start taking delivery of the re-timed engines. In their view, there will be no measurable impact on fuel economy.

They also say that oil-drain intervals will remain unchanged, although the engines will require a new oil formulation. However, the optimists point out that a new appropriate API (American Petroleum Institute) classification is already under development and will be ready for the new year at prices no different than current PC-7 oils.

Any increased heat rejection will be minimal, in their view, and easily handled by minor changes to fan speeds. As for smart component operations, those will simply require minor programming modifications to handle the changes in engine operating characteristics.

Both sides make some convincing arguments. The pessimists ask, If cutting NOx at highway speeds is so painless, then why did engine designers choose the original timing maps that brought down the wrath of EPA? The optimists respond by pointing out that in Europe many of their engines are already running with the revised timing without problems.

As it does in most cases, the truth probably lies somewhere in the middle of these two opposing views. At this early date, only two things are certain diesels will have to meet the intermediate NOx requirements if theyre going to be sold in the U.S. after January 1, and only experience will tell us how much the control revisions will impact fleet operations and profitability.

All you can do at this stage is wait and see how it all plays out.