In the march toward ultra-clean air, we sometimes forget that diesel exhaust in the U.S. wasn't regulated at all until the 1980s, with the federal Environmental Protection Agency gradually tightening limits through the '90s. It then decreed the giant leap scheduled for January 2004, but pulled it back to October '02 to punish many domestic builders the agency accused of cheating by setting engine controls to pass emissions tests while burning dirtier running down the road. Builders protested, but went along with the pull-back and huge monetary fines when the only alternative was to quit making engines.

Meanwhile, California's Air Resources Board and other authorities were setting even more stringent standards, including some for trailer refrigeration units, and set up roadside exhaust inspections. The state's insistence on real-life checks has led to establishment of federal standards for on-board diagnostics, where engines' electronic controls and memories are quizzed about engine performance. It's a long step from an officer holding up an opacity meter near a tailpipe to judge the exhaust's cleanliness by its smokiness, or lack of it.

Exhaust pollutants have been cut to a minute fraction of what they once were. Particulates, oxides of nitrogen, carbon monoxide and unburned hydrocarbons were all targets. Most were dealt with by enhancing combustion performance through higher fuel-system pressures and more precise injection, advanced turbocharging and charge-air cooling.

All this required use of electronic controls, which began appearing in the early '90s and were standard in all truck diesels by the end of the decade. Electronics are both a blessing and a curse, because they allow engines to meet the standards and run well, but fail far too often to please truck users. Everything from sophisticated control modules to simple wiring and connectors suffer malfunctions or simply break.

Perhaps the toughest pollutant to cut was NOx, a component of smog, because it's formed in the heat of combustion that's needed to completely burn the oily fuel. The October '02/January '04 regs aimed primarily at NOx, and the only way to reduce it was to reduce cylinder temperatures. Most builders did this with cooled exhaust-gas recirculation, where special piping sends varying quantities of exhaust gas back to the cylinders, where it pushes out like amounts of heat-generating oxygen.

Exhaust gas first must be cooled, which is done by surrounding parts of the piping with water jackets. Thus the engine's cooling system carries off more heat, which must be dealt with by bigger radiators and tougher hoses and belts. Caterpillar avoided EGR by using other methods, including double turbocharging on some engine models, and the highly compressed intake air needed its own special cooling.

Although enhanced cooling systems and their fluids work harder, much more heat radiates off the engine and its accessories. High heat under the hood ruined bearings and cooked grease in key parts like water pumps, fan hubs and alternators, whose manufacturers had to revise their designs. Turbochargers failed sooner than before, and this happened with every make of engine, according to some users. Some turbos failed when brand new.

Darry Stuart, current general chairman of the Technology and Maintenance Council of ATA, said turbo life in heavy-duty diesels dropped from a typical 500,000 miles to 300,000 or less, and a variable-geometry turbo, which most builders use, now costs thousands of dollars instead of a few hundred.

EPA regs for 2007 further clamped down on NOx and particulates, requiring builders to use aftertreatment devices that are poisoned unless most sulfur is wrung from fuel and the ash from motor oil. Ultra-low-sulfur fuel was mandated by EPA and now is generally available. Since the early '90s, refiners have been revising their motor oil formulations every few years to accommodate tougher combustion and lube conditions, and the latest one, called CJ-4, is made to help protect oxygen catalysts, particulate filters and other devices that are now a part of exhaust systems.

Along with further advanced electronics and combustion schemes, the '07 changes and expensive development work prompted stiff price increases. Those, along with general fear of the new complexity, have kept most buyers out of the market. A massive pre-buy of trucks powered by less-costly and complex EPA '06 diesels has led to a steep slump in new-truck sales.

Therefore, few EPA '07 diesels are now on the road, so it's impossible to know how well or badly they're working, Stuart says. Anecdotal information indicates they are reliable, and drives by HDT editors indicate the engines run strongly and emit no smoke or odor – major benefits to those who work in and around diesel trucks and to society in general.

But the clean-air benefits will be few until more of the latest diesels get on the road. That might be fairly soon, as those in the industry predict another pre-buy leading up to January 2010, when even tougher emissions limits go into effect. They will keep particulates at the 2007 level, but reduce oxides of nitrogen from today's 1.2 grams per horsepower-hour down to 0.2 grams. There are ways to do it, but they won't be cheap. Some say the upcharges will be similar to what the builders are trying to get for EPA '07 diesels, which ranges from $2,000 to $10,000 per truck, depending on weight class and engine used.

Two builders – Detroit Diesel and Volvo Powertrain, which includes Volvo and Mack engines – have announced that they'll use the aftertreatment method employed in Europe since '05, called selective catalytic reduction, or SCR, along with current EGR and other high-tech methods. SCR is also used in Japan, so importers of Japanese-made medium-duty trucks and engines also expect it will come here for 2010, though they've made no announcements as yet. Caterpillar, Cummins and International's Engine Group say they continue to explore available methods, and expect to decide soon, probably by the end of this year.

SCR injects a urea solution into the exhaust stream where it reacts to form ammonia. This combines with NOx to form nitrogen gas and water vapor, both harmless components of the atmosphere. In Europe, the urea solution is dubbed Ad Blue for its blue color; it's sold in bulk and in packages at fueling stops.

One variant of SCR involves the direct injection of ammonia gas. Another, being developed by Eaton Corp., generates ammonia aboard the truck to treat NOx before it leaves the tailpipe. Eaton is trying to interest engine makes to use it, but has announced no deals thus far.

Detroit Diesel (through its parent company Freightliner LLC), Volvo and Mack have teamed up to address the topic of using SCR to meet the 2010 deadline. The consortium is open to any manufacturer with a declared intention to use SCR. Topics the group will work on include development of a Diesel Exhaust Fluid infrastructure, working with the EPA, facilitating DEF distribution to fuel retailers, and creating and enforcing DEF quality standards. Invitations to join the consortium have been extended to all members of the Engine Manufacturers' and Truck Manufacturers' Associations.

While SCR looks like the path to be taken by most diesel builders for 2010, there are alternate methods that concentrate on reducing nitrogen oxides in the cylinders but also rely on aftertreatment. Some were described by engine representatives in a well-attended session earlier this year during the annual meeting of the Technology and Maintenance Council of ATA. The methods include homogeneous charge compression ignition (HCCI) and hydrogen enabled combustion (HEC).

HCCI involves mixing engine fuel with air, as with a spark-ignition gasoline engine, but with a greater amount of air. The mixture ignites by compression heat from the piston stroke, as with a diesel. But it burns at a far lower temperature. The reduced temperature plus the high ratio of air significantly reduce NOx. HCCI is being studied in a project involving General Motors, Bosch and Stanford University, as well as by other engine makers, according to Chuck Blake of Detroit Diesel.

One of HCCI's mechanical features is variable compression produced by changing the pistons' stroke; this is accomplished by an eccentrically cradled crankshaft. Low compression ratios at startup and under high load eliminate various problems while still allowing compression braking. Electronically variable valve timing is also a requirement.

HCE uses a reformer to produce a hydrogen-rich gas from the diesel fuel on board the vehicle. The gas enhances combustion, also at lower temperatures, for NOx reduction. The reformer works by injecting minute amounts of fuel into a canister, where it's mixed with air and ignited by a spark plug. The resulting ionized gas, called plasma, glows when burned in the cylinders, helping to preliminarily break down fuel into hydrogen and carbon monoxide, which is sent to NOx traps, where further chemical reactions strip out oxygen to prevent formation of NOx.