A Fresh Start

Heavy-duty pickup owners love their diesels. According to Dodge, more than 80 percent of its heavy-duty pickups are sold with the diesel
engine option, so don’t expect Chrysler Group to retreat from this
market in the face of escalating emission standards. In fact, Chrysler’s answer is the 2007 Dodge Ram Heavy-Duty pickup, built with the new Cummins 6.7-liter (L) diesel, which replaced the 5.9L engine in January of this year and meets 50-state heavy-duty emissions standards through 2010.

The 2007 Dodge Ram Heavy Duty with BLUETEC technology meets two goals: Limit engine-out NOx using a high-pressure common-rail injection system and EGR, and then achieve further reductions using NOx adsorber after-treatment.
(Photo: DaimlerChrysler)

Huge progress has been made in diesel engine emission control technology during the past three years. Many of the advances have been pioneered in Europe, where diesel engines comprise half of the new light-duty vehicle market. North America presents a greater challenge for light-duty diesels however, as U.S. emissions standards are currently the world’s toughest on oxides of nitrogen (NOx).

The primary diesel emissions are particulate matter (PM) and NOx. Of these, NOx is the most difficult to deal with after it leaves the engine cylinders. The PM/NOx tradeoff is widely recognized; any measures taken to limit one leads to increases in the other. With this in mind, the solution adopted by domestic manufacturers has been to take measures to decrease NOx output in the engine cylinders, and then use hardware in the exhaust system to deal with the increased PM. Note that all domestic diesel-powered pickups have now incorporated diesel particulate filters (DPFs) as part of this strategy.

The diesel combustion environment is exceptionally lean in nature. In other words, a great deal more oxygen is available than what is required to completely burn the injected fuel. Air-fuel ratios of between 50:1 and 100:1 are common during normal operation.

When fuel is introduced into the combustion chamber, it mixes with the air and burns very rapidly. Because there is always excess air available for combustion, the diesel engine is able to burn the fuel very efficiently. However, combustion in some areas of the cylinder is much hotter than in others, and it is in these high-temperature areas that nitrogen combines with oxygen to form NO and NO2, commonly expressed as NOx.

The BLUETEC System The BLUETEC system uses a close- coupled diesel oxidation catalyst (DOC), which plays a critical role in the operation of the NOx adsorber. Any remaining particulate matter is trapped in the diesel particulate filter (DPF), which also relies on the DOC to help with the regeneration process.1 Diesel Oxidation Catalyst (DOC)
2 Oxygen Sensor Mount Locations
3 Exhaust Temperature Sensors
4 NOx Adsorber Catalyst (NAC)
5 Diesel Particulate Filter (DPF)

(Illustration: DaimlerChrysler)

NOx generation can be controlled to some degree using the high-pressure common-rail injection system, which is capable of producing multiple injection events during the combustion cycle to limit peak combustion temperature. Further reductions can be achieved using exhaust gas recirculation (EGR).

The BLUETEC system on the Dodge Heavy Duty pickup uses liquid-cooled EGR to limit NOx. Note that the 5.9L Cummins diesel installed in 2007 Dodge Heavy-Duty pickups through December 2006 did not use an EGR system.

This basic concept is known as“high-pressure EGR.” The exhaust gases enter the intake air downstream from the turbocharger and the intercooler, thereby increasing the durability of these components.

In order to enhance EGR flow, a lower pressure may be created in the intake manifold where the EGR valve is located. This is accomplished using an electronic throttle mechanism in the air intake upstream from the EGR valve. It should be noted that conventional diesel engines do not use intake air throttle valves. Control of the engine has traditionally been accomplished through fuel metering by the injection system. A side benefit of using a throttle valve in the air intake is that it can be closed during engine shutdown, making the process smoother and quieter.

When higher EGR flow is desired, the EGR Airflow Control Valve is partially closed to lower the air pressure downstream from the valve. The EGR valve is operated by a brushless DC motor that is controlled by the powertrain control module (PCM).

The operation of the NOX adsorber catalyst is monitored using wideband oxygen sensors. These are especially suited for this application because of their ability to measure air-fuel ratios over a very wide spectrum.(Photo: T. Martin)

Three Hall-effect sensors are used to determine the exact location of the EGR valve. System operation is very different from an EGR system found on a gasoline engine. For instance, the EGR system is active during idle conditions, and the flow is greater at idle and part throttle than it is under cruise conditions.

While it is possible to meet current NOx emission standards for heavy-duty pickups using in-cylinder techniques only, most agree that some level of after-treatment will be necessary to meet upcoming regulations. NOx after- treatment has been the focus of much research and development in recent years as diesel engine manufacturers
look for ways to design effective systems that are durable and require little or no maintenance.

The solution that has been adopted for use in the BLUETEC pickup is the NOx “adsorber.” NOx adsorbers are also known as lean NOx traps (LNTs) or NOx Adsorber Catalysts (NACs). Ad-sorb means to hold the NOx on the surface of the brick (substrate) like a magnet, as opposed to “absorb” into the brick like a sponge.

“The brick materials and construction of a NOx adsorber catalyst is the same as that used in catalysts on millions of gasoline products,” says Don Altermatt, director of Diesel Engine Engineering
for DaimlerChrysler. “The technology which makes a NOx adsorber work is in the wash coat and precious metals formulations.”

The brick is a flow-through design with a wash coat containing precious metals applied to its surface. The precious metals create “catalyst” and “storage” sites that work together to store and treat NOx in the exhaust stream.

The basic idea is to “trap” or adsorb NOx on the storage sites during normal engine operation, and then use the catalyst to convert it into less harmful gases during a process called “regeneration.” Because the diesel exhaust stream is lean (has an abundance of oxygen) under nearly all operating conditions, the NOx adsorber is able to store NOx very efficiently. The period when storage is taking place is known as the “load phase,” and this can last anywhere from 30 to 300 seconds depending on the mode of vehicle operation.

The NOx adsorber has a limit as to the amount of NOx it can store on the catalyst surface. When this limit is reached, the engine management system must “richen” the exhaust gases for a short period. This regeneration phase is relatively short, usually between two and 10 seconds.

The presence of hydrocarbons in the exhaust stream “empties” the NOx adsorber and reduces the NOx into nitrogen (N2), H2O, and CO2, which are then sent on to the diesel particulate filter (DPF). At this point, the engine returns to normal operation and the NOx adsorber re-enters the load phase.

In order for diesel-powered light-duty vehicles to be sold in all 50 states, they must meet EPA Tier 2 Bin 5 emission standards. Current light-duty (less than 8,500 pounds GVWR) diesel offerings have only been able to achieve Tier 2 Bin 8, which means they can only be sold in the 45 states that have not adopted California’s emission standards.

The major problem with urea SCR is the level of maintenance that is required to keep it operating. An on-board reservoir must be refilled at regular intervals, so availability of the AdBlue urea solution, as well as ensuring the urea tank doesn’t run dry, are concerns. Researchers are working on solutions to these problems to make the 50-state diesel passenger car a reality once again.