Precise sensing key to meeting future emissions challenges

Diesel Progress North American Edition, June, 2004

As emissions regulations have gotten more stringent, engine manufacturers have adopted a variety of technologies to meet the regulatory targets. These technologies have included everything from exhaust gas recirculation (EGR), used by most of the major truck engine builders to Caterpillar's ACERT system.

No matter what technology is used, however, a new emphasis has been placed on "knowing the exact operating condition at any given moment and being able to control and regulate the function of engine systems. This is especially true concerning critical functions such as engine cooling.

In just about every case, the new emissions control strategies have led to increased heat rejection--up by as much as 30%--which has led to the redesign of cooling systems. While the simple solution would have been to mount a larger radiator up front, truck hoods have become more sloped as truck designers try to reduce aerodynamic drag. Indeed in many cases, the space available for radiators and fans has actually decreased rather than grown.

Instead, truck manufacturers have sought to come up with new ways to pull additional air through the radiator and over the engine. Some are achieving this by mounting the main radiator alongside the charge-air heat exchanger, rather than in series. Radiators have also been mounted lower by redesigning front frame rail brackets.

All truck makers have had to make some changes to fans and fan controls. By increasing fan pitches, fan diameters, drive ratios and the number of fan blades, more air could be pulled through the radiator and charge air heat exchanger.

But more aggressive fans create higher parasitic loads on the engine, increasing fuel consumption and reducing engine power. To reduce the impact on truck users, the cooling package needs to be designed so that the fan is only on when it needs to be.

"Optimizing the flow of air through the various heat exchangers requires precise temperature sensing and switching systems," said Gary Wilson, marketing director at Index Sensors. "If truck engine fans are turned on too soon, they may be overcooling the engine. If they aren't turned on soon enough, it will take longer to pull engine temperature down." Index Sensors has been a leading player in engine temperature sensing technology beginning with the development of its bi-metal thermal switch technology in 1976.

In 1982, the company introduced electronic switches with multiple switching and control functions. This has allowed engine and truck engineers to create better systems to control fan on-time. Nine years later, Index launched the K-7 fan control kit to improve operating link between engine cooling fan and air conditioning system. That system is still in use today in various applications.

Wilson said that some of the experience Index engineers had working with engine and truck OEMs helped spur the development of the company's latestsensing product, the Universal Temperature Switch (UTS)."We were seeing a growing number of customers interested in smart sensor technology," he says. "They wanted a multifunction switch which could easily be programmed to initiate specific functions at a set temperature. That's why we came out with the UTS."

The need for smart sensors and switches such as the UTS is expected to grow as engine manufacturers prepare for the next emissions cuts. In 2007, 50% of all on-highway diesels sold can emit no more than 0.20 g/hp-hr of N[O.sub.x], a 90% drop from 2002 levels. By 2010, all on-highway engines must be at 0.20 g/hp-hr.

On top of the N[O.sub.x] reductions, particulate emissions of 2007 engines cannot exceed 0.01 g/hp-hr. That creates an enormous challenge for engineers trying to adjust combustion processes and manage heat. Ordinarily, decreasing exhaust particulate levels increases N[O.sub.x] levels and vice versa.

"With 2002 engines, reductions were achieved primarily by adjusting combustion," said Wilson. "With the upcoming 2007 emissions levels, it's a whole new ball game. At this stage, it looks like they aren't going to extract that much extra N[O.sub.x] in the cylinder. More needs to occur after combustion."

Engine manufacturers have a number of strategies to accomplish this, but it seems clear that selective catalytic reduction (SCK) and N[O.sub.x] adsorbers are the leading candidates.

SCR, using liquid urea injected into the exhaust stream ahead of a catalyst to break down the N[O.sub.x] into nitrogen and water, is the clear choice in Europe. But in the U.S., questions remain over the urea infrastructure, the cost of the technology and the impact on fuel economy.

N[O.sub.x] adsorbers, also known as lean N[O.sub.x] traps (LNT), are also mounted on the exhaust stream and adsorb N[O.sub.x] on the surface of a catalyst. When the trap is filled, the air/fuel ratio is richened briefly to increase the exhaust temperature and regenerate the catalyst, neutralizing the stored N[O.sub.x] and turning it into nitrogen and oxygen. A passive system, it seems to be the likely winner in North America. But the downside is the fact that it is new and relatively untested technology and must be used with low-sulfur diesel fuel. And as fuel is used to regenerate the trap, there is clearly a fuel economy penalty.