Engine-out emissions: First cut, then after-treat

Diesel Fuel News, April 1, 2002

Tempe, Ariz. - Engine makers should first try every means to cut engine-out emissions for lowest-cost pathway to U.S. EPA's 2007-2010 heavy-duty diesel emissions limits, according to a leading investigator.

"Though exhaust after-treatment systems could by themselves achieve much of this reduction [to future EPA limits], they are potentially more expensive, bulky and less reliable than the alternative in-cylinder strategies," according to Nigel Gale, Southwest Research Institute (SwRI) VP-Engine & Vehicle Research.

Combining engine and aftertreatment controls would be the most-efficient strategy, he explained, but starts with the engine first.

"Control of emissions by combustion is usually less expensive and less intrusive into the design of the truck," Gale told the Clean Heavy-Duty Vehicles Conference here. "The engine manufacturer who achieves more of emission control within the engine will have a strong competitive advantage," he said.

Recapping research results presented by U.S. EPA on experimental versions of combined PM/NOx traps, Gale pointed out that such systems are extremely sulfur-sensitive even with a 3-ppm ultra-low sulfur diesel (ULSD). This tends to degrade NOx traps permanently.

SCR on the other hand is rather complex and bulky if all EPA and performance targets are to be met, including avoidance of ammonia slip, he showed. Meantime, while PM filters show good filtration performance, regeneration under all conditions is still an issue.

By contrast, advanced fuel injection - high injection pressure, high mixing rates, reduced fuel residence time in the rich pyrolysis zone of fuel jet - can produce less PM, although it's not been shown as able to hit EPA's 0.01 grams/brake horsepower-hour 2007 limits.

A combination of "partial pre-mixed charge compression ignition" (PCCI) also can reduce NOx, he showed.

Meantime, "smart" model-based control development like that at SwRI continues apace, enabling engine makers to optimize transient control and infer unmeasurable signals, he showed. Such models could generate operational setpoints for EGR and variable-geometry turbo (VGT) and thus avoid smoke emission.

On a related front, Ricardo's VP-Engineering Graham Weller showed here that urea-SCR isn't necessarily a slam-dunk winner versus EGR to hit Euro 4 NOx limits, even though Europe's high diesel fuel prices would seem to favor SCR combined with high fuel economy! high engine-out NOx.

The critical SCR cost advantage, Weller showed, comes only if the truck carries a modest amount of reductant and refills the tank at every diesel refuel, rather than carrying around a lot of heavy reductant that impacts cargo capacity and hurts revenue weight limits.