Green by Design
Part 3: A New Day for Diesel?

For years, diesel passenger vehicles have been criticized for their high levels of tailpipe emissions. Although diesels are more efficient (and thus emit lower levels of carbon dioxide) than their gasoline counterparts, their high tailpipe pollution has placed them at an overall disadvantage to gasoline vehicles in terms of eco-friendliness.

That may soon be changing. Diesel manufacturers have been working hard to develop systems capable of meeting stringent U.S. air quality standards that are currently being phased in. And whereas a couple years ago the question was whether diesels could meet such levels, the question now is at what cost? Thanks to EPA's recent set of stringent fuel and vehicle emissions requirements, the auto and oil industries are now better able to produce diesel vehicles and fuel. They have undertaken a threefold approach to controlling diesel emissions: cleaning up the fuel itself, modifying engine operation to minimize the amount of pollutants being generated, and controlling pollutants that do get created with robust "aftertreatment" systems.

The two primary pollutants emitted by diesels are nitrogen oxides (NOx) and particulate matter (PM). Controlling both of these pollutants simultaneously has been a challenge to automakers, as high temperatures in engine cylinders create NOx, but are required to minimize PM. Further complicating matters has been the fact that the high level of sulfur in diesel fuel has prevented aftertreatment systems from working effectively.

The federal government has mandated that in the fall of 2006, diesel fuel be dramatically cleaned up nationwide to create a version containing very low levels of sulfur, known as Ultra-Low Sulfur Diesel (ULSD). This requirement has enabled automakers to produce vehicles with the confidence that their emissions control systems will behave as designed.

DaimlerChrysler, which currently produces diesel versions of the Mercedes E320 and Jeep Liberty for the U.S. market (meeting two of the least-stringent emission standards legally available today), will be the first to produce a diesel that meets the cleaner Tier 2 bin 5 and California LEV II standards frequently seen on gasoline vehicles. DaimlerChrysler's system, known as BlueTec, will arrive on a Mercedes E-class sedan in late 2006, and is expected to branch out into their M-, R-, and GL-class SUVs shortly thereafter. In time, it may be seen in Chrysler, Dodge, and Jeep products as well.

How does the BlueTec system work? It uses a diesel oxidation catalyst to control CO, HC, and, to a degree, PM emissions; a particulate filter to control PM; and a selective catalytic reduction (commonly known as "SCR") system that converts harmful NOx emissions to nitrogen and water. SCR systems have for years been used in stationary applications with steady-state operation (such as power plants), but only recently been regarded as a viable control technology for vehicles. SCR catalysts use a nitrogen-containing compound, such as urea or ammonia, as a reductant rather than a hydrocarbon like diesel fuel. This avoids a roughly 5% fuel economy penalty associated with using fuel as a reductant. AdBlue, the commercial name of the BlueTec system's NOx-reduction agent, is a 32.5% urea solution that Mercedes claims reduces NOx emissions in the vehicle's exhaust stream by up to 80 percent. AdBlue is stored in a separate tank on the vehicle and, to accommodate urea infrastructure plans, is expected to be refilled by the dealer during routine service checks.

While systems such as this open doors for diesels to be sold in all 50 states, environmental experts are concerned about the potential harm from SCR-based vehicles operating without a reduction agent. Widespread availability—and use—of the agent is critical to these vehicles' achieving certified emissions levels. Vehicles operating when the reduction agent runs out—or being driven by their second or third owners, when the cars are not serviced by dealers—will yield higher emissions levels. One possible solution may be outfitting SCR systems with ignition or fuel door locks tied to sensors in the urea tank.

In time, more surefire alternatives to handling NOx emissions in diesels may become available. Lean NOx traps (LNTs) are one such example. Operating in two phases, LNTs first cause nitrogen oxides in the exhaust to adhere chemically to "storage sites" on the device, where they become trapped. Then, when the storage sites become full, a hydrocarbon reductant is injected into the exhaust stream to regenerate the device by releasing the trapped NOx and converting it to nitrogen gas. LNTs have certain shortcomings, however. The use of diesel fuel as the hydrocarbon reductant means a fuel economy penalty will be incurred. Furthermore, LNT technology is more expensive than competing technologies like SCR, because LNT devices require precious metal coatings. Also, temperature boosts necessary to regenerate LNTs affect the devices' long-term durability. At this time, LNTs do not meet the necessary durability requirements. Research efforts are underway to design LNTs that regenerate less often, for shorter periods of time, and at lower temperatures.

While a number of technical achievements have been made in controlling diesel emissions in recent years, equally critical to diesel vehicles' success will be automakers' ability to provide them at reasonable cost. New diesel vehicles today are more expensive than gasoline powered models with similar performance characteristics. On top of that, advanced engine controls, such as high-pressure ("common rail") fuel injection systems, cylinder modifications, and electronic fuel injection, all come at a cost that will play into clean diesel technology's commercial viability. Competing technologies, such as clean-and-efficient gasoline-electric hybrids like the Tier 2 bin 3/PZEV Toyota Prius or Tier 2 bin 2/ PZEV Honda Civic Hybrid, will vie for the same passenger car market. In the coming years, diesels may even have to compete with non-hybrid gasoline vehicles that have adopted certain diesel-like elements: gasoline direct injection, or GDI, uses direct injection of the fuel into the cylinders to improve vehicle efficiency. In summary, automakers seeking to carve out a niche for clean, efficient diesels will have to do so in an increasingly competitive market.

For years, diesel vehicles have been criticized for their high levels of tailpipe pollution. A number of new diesels, led by the Mercedes E320 BlueTec, hope to change that. The BlueTec system shown here consists of four main emissions control components: an oxidation catalyst, DeNOx catalyst, particulate filter, and selective catalytic reduction (SCR) device.
Photo: DaimlerChrysler Corp.

Continue to Part 4: Engines of Change