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Report on Control Technologies for Medium-Duty Diesel-Powered Vehicles

SYNOPSIS: 

In the Rule 1191 – Clean On-Road Light- and Medium-Duty Public Fleet Vehicles, adopting resolution, the Board directed staff to report on the state-of-science of control technology for diesel-powered medium-duty vehicles and to compare emissions from engines powered by diesel and other transportation fuels. The staff has summarized the necessary technical information on medium-duty vehicles. Although significant progress has been made in diesel engine and control technology, these vehicles are still emitting more NOx and PM emissions than other transportation fuels. Low sulfur diesel fuels and further advancements in engine design and control devices will be necessary for these vehicles to comply with LEV II emission requirements. No changes are recommended for Rule 1191, subject to technological development in the future.

COMMITTEE: 

Technology, September 22, 2000, Reviewed

RECOMMENDED ACTION:

Receive and file.

Barry R. Wallerstein, D.Env.
Executive Officer

Background

Rule 1191 – Clean On-Road Light- and Medium-Duty Public Fleet Vehicles, adopted in June 2000, requires public fleet operators with 15 or more fleet vehicles to purchase California Air Resources Board (CARB) certified low-emission vehicles when adding, replacing or forming a new fleet of light- and medium-duty vehicles. During rule development, affected operators were concerned with availability of compliant medium-duty vehicles. Consequently in the adopting resolution, the Governing Board directed staff to prepare a report to the Board on the state-of-science of control technology for diesel-powered medium-duty vehicles and compare emissions from engines powered by diesel, gasoline, liquefied petroleum gas (LPG), and compressed natural gas (CNG).

Introduction

Diesel engines are commonly used in trucking, service, and delivery industries, and, generally, among businesses and individuals requiring vehicles with larger carrying capacities and capable of handling a more rigorous operating environment. There is, however, growing public concern about diesel emissions from trucks and buses, including:

• Obnoxious diesel exhaust odors, white clouds of vapor emitted under cold starting, idling, and low loads, and black smoke emissions particularly at maximum loads.
• Potential adverse health effects of diesel particulate emissions.
• Finally, the possibility that medium- and heavy-duty diesel powered vehicles would account for an increasing portion of mobile source generated nitrogen oxides (NOx) emissions, as high fuel-efficient diesel engines replace low fuel efficient gasoline engines in pickup trucks and sport utility vehicles.

The AQMD, CARB, and U.S. EPA are aware of the potential health risks posed by diesel particulate matter (PM) emissions, and the significant contribution of mobile sources to the Basin’s NOx emissions inventory. As a result, the U.S EPA, followed by CARB, established technology-forcing requirements for controlling emissions from mobile sources. These requirements are generally the basis of the AQMD program for mobile sources.

AQMD Program Authority

The AQMD is an extreme nonattainment area for ozone. Under the California Health and Safety Code (HSC) Section 40919(a)(4), "serious" or worse ozone attainment areas may require "the use of a significant number of low-emission motor vehicles by operators of motor vehicle fleets." In addition, under HSC Section 40447.5 the AQMD may adopt rules requiring public and private fleets with 15 or more vehicles to acquire vehicles which are capable of operating on "methanol or other equivalently clean burning alternative fuel" when adding to or replacing vehicles in an existing fleet or purchasing vehicles to form a new fleet and to require that these vehicles be operated substantially on alternative fuels. Based on these legislative authorities, the AQMD also developed specific short- and long-term emission reduction measures to address air pollution problems from mobile sources, and recently adopted clean on-road fleet vehicle rules to reduce mobile sources emissions within the AQMD's regulatory authority.

State Program

In 1990, CARB adopted a low-emission vehicle (LEV I) program establishing a different set of tailpipe emission standards for passenger cars (PCs), light-duty trucks (LDTs), and medium-duty vehicles (MDVs), beginning with the 1994 model year. CARB recognized that heavier light-duty trucks (HLDT) and MDVs emit more because they have larger capacity, and are generally used in more strenuous work activities. Consequently under this program, HLDTs and MDVs are subject to less stringent emission standards than PCs or smaller light-duty trucks (SLDTs). An example of these differences is shown in Table 1.

Table 1¹:         LEV I Exhaust Emission Standards

In November 1998, CARB established LEV II regulations to continue the progress of reducing mobile source emissions, beginning with the 2004 model year. This regulation tightened emission standards for most vehicle emission category (i.e., LEV, ultra low-emission vehicles-ULEV, super ultra low-emission vehicles-SULEV) and extended PC emission standards to heavier sport utility vehicles, minivans, and LDTs which were formerly regulated under less stringent emission standards. Specifically, under this program, the LDT category include SUV and trucks under 8,500 pounds (Gross Vehicle Weight Rate - GVWR) such as Toyota RAV4, Ford Ranger, Jeep Grand Cherokee, all minivans, Ford Explorer, Ford F150, Ford Expedition, Chevrolet Suburban, and Dodge Ram 1500. The MDV category (8500 – 14000 GVWR) includes Ford Excursion, Ford F250 and F350 Super-Duty trucks, Dodge Ram 2500 and 3500 trucks, and many full size vans. Table 2 lists the corresponding LEV II emission standards for PCs, LDTs, and MDVs.

Table 2¹:        LEV II Exhaust Emission Standards

¹ A complete listing of LEV I and LEV II emission standards is contained in Title 13 of the California Code of Regulations.

Strategies to Reduce Diesel Emissions

In response to CARB and U.S. EPA tighter emission standards, vehicle and engine manufacturers, emission control manufacturers, fuel suppliers, and researchers have continued to direct considerable efforts and resources to developing strategies to reduce NOx and PM emissions from diesel engines. These efforts have resulted in many options available for (1) improving engine design and (2) developing exhaust aftertreatment devices to achieve higher emission reductions.

Diesel Engine Design

Engine manufacturers have improved or are improving their engine designs to meet low-emission standards. Generally, these improvements involve electronic engine controls, changes in fuel injection systems, handling of intake air, combustion chamber modifications, exhaust gas recirculation systems, and reducing oil consumption. Engine manufacturers are implementing combinations of these improvements to achieve low-emission levels under the CARB LEV I program. So far, five diesel-powered medium-duty engine families are LEV certified under the CARB LEV I program. There is no current LEV II-certified diesel-powered medium-duty engine.

Exhaust Aftertreatment Devices

Under LEV I and LEV II programs, diesel-powered engines are required to comply with the same emission standards as engines powered by other transportation fuels. Gasoline-, LPG-, and CNG-powered engines are equipped with existing technologies that are already capable of achieving much lower emission levels than most LEV I and some LEV II emission standards. Based on Tables 1 and 2, NOx and PM emissions from a certified LEV 6,000-lb diesel-powered MDV would have to be reduced by at least 92% for the vehicle to meet LEV standards under the LEV II program. To meet these targets, manufacturers of diesel-powered medium-duty engines and vehicles must to (1) implement most of the demonstrated options for improving engine performance, (2) use improved diesel fuel quality, and (3) develop exhaust control devices, capable of reducing at least 92% of NOx and PM engine-out emissions.

Several NOx and PM emission control devices are being developed and tested for use with diesel engines in the United States. Most of these aftertreatment devices can be used for controlling NOx and PM emissions from medium-duty diesel engines, but those control technologies require diesel fuels with sulfur levels below 30 ppm or 10 ppm in most cases. Emerging diesel aftertreatment technologies are briefly discussed below.

• Lean-NOx Catalytic Converters: A lean-NOx converter employs a platinum-based or copper-based catalyst operating within a narrow temperature range of 200 to 300°C or 350 to 500°C, respectively. It is subject to a fuel economy of approximately 3% penalty because it requires the addition of diesel fuel or on-board hydrocarbon (HC) reductant into the exhaust gas upstream of the catalytic converter. Although it has been incorporated in European diesel-powered PCs and achieves NOx control efficiency in the range of 15%, further improvements are being developed for durability (especially against sulfur poisoning), broader operating temperatures, and control efficiency.
  
  
• NOx Adsorbers: NOx adsorbers are newly developed control systems with an operating temperature window of 200 to 550°C, and capable of reducing up to 90% of engine-out NOx emissions from low sulfur diesel-powered engines. During typical diesel engine operation, NOx is oxidized to nitrogen oxide (NO₂) over a precious metal catalyst and temporarily trapped as basic oxides. Before the NOx adsorbent becomes saturated, the stored NO₂ are released in a fuel-rich exhaust and simultaneously catalytically reduced to nitrogen (N₂). It has been estimated that there is an approximate fuel economy penalty of 3% for using NOx adsorber because it requires injection of diesel fuel to the exhaust to produce a fuel-rich exhaust. This system requires diesel fuels with sulfur levels below 30 ppm.
  
  
• Selective Catalytic Reduction (SCR): SCR is an effective strategy for controlling NOx emissions, especially from industrial and marine applications. It involves chemical reactions of NOx with ammonia (NH3) or urea over a catalyst. Although SCR systems have been successfully used in stationary industrial applications, they have not yet been commercially used for diesel-powered vehicles because of safety concerns, large size, complexity, and operating transients in vehicle applications.
  
  
• Non-Thermal Plasma Discharge (NTPD): NTPD system is a promising advanced technology, which uses electric discharge to generate reactive free radicals which in turn selectively reduce NOx to N₂ and simultaneously oxidize PM to CO₂ in conjunction with a catalyst. It has been demonstrated in laboratory studies to be capable of reducing both NOx and PM emissions by at least 70%. This technology is still in a developmental stage.
  
  
• Diesel Oxidation Catalytic Converter (DOCC): In the U.S., several diesel-powered vehicles have been equipped with oxidation catalytic converters to control PM emissions. DOCC oxidizes CO, HC, and the soluble organic fraction (SOF) of the total particulates to CO₂ and water. It is estimated that DOCC is capable of reducing 25 to 50 percent of total PM emissions. Although DOCC is an effective control for PM emissions, its performance/durability could be significantly improved if sulfur levels were lower than 30 ppm.
  
  
• Diesel Particulate Filters (DPF): These devices are capable of achieving over 80% reduction of the insoluble portion of the total exhaust particulates, mostly carbonaceous, sulfate, and ash. To avoid plugging over time, the filters are periodically regenerated or cleaned by oxidation of the particulate matter to CO₂. During the regeneration process, an exhaust temperature ranging from 600 to 650°C is required to cause auto-ignition and sustain combustion of the particulate matter. Because the engine-out exhaust temperature is not sufficient, control manufacturers equip DPFs with a heat source, such as in-line burners and electric heaters, to raise the exhaust temperatures to levels that would cause auto-ignition and support combustion.
  
  
• Catalyzed Particulate Traps (CPT): These devices are similar to DPFs except that the heat sources are replaced by catalysts in front of the DPFs. The catalyst lowers the particulate ignition temperature, allowing the filter to self-regenerate during periods of high exhaust gas temperature. Although CPTs are effective in controlling (>90%) PM emissions, their performance/durability could be significantly improved if sulfur levels were lower than 30 ppm.
  
  
• Fuel-Borne Catalyst (FBC): FBC uses fuel additives such as iron, cerium, copper, and platinum to lower the ignition temperature of the particulates thereby facilitating the regeneration of the DPF. Although FBCs are effective in controlling (>90%) PM emissions in conjunction with DPF, their performance is significantly affected by high sulfur content in diesel fuels.

Emissions from Engines Powered by Diesel Versus Other Transportation Fuels

The AQMD staff had discussions with CARB staff, engine and vehicle manufacturers, and other organizations working on projects involving comparing emissions from engines powered by diesel, gasoline, CNG, and LPG. Based on these discussions, staff reviewed extensive emissions data, including the CARB LEV I and LEV II certification list. This analysis is based solely on emission data from the CARB certification list because most of the technologies used in past or present projects on diesel fuels are either still in the developmental stage or not available for commercial applications. Table 3 provides emission data and other information, as contained in the CARB certification list, for medium-duty engines (8,501 – 14,000 GVWR) considered in this analysis. There are currently five diesel-powered LEV-certified engine families and over eleven gas (gasoline, CNG, and LPG) powered LEV/ULEV-certified engine families under the CARB LEV I program.

Table 3:        LEV I Exhaust Emission Certification Levels at 120,000 miles

As shown in Table 3, NOx and PM emissions from gasoline-, CNG-, and LPG-powered engines are consistently lower than diesel-powered engines because they are equipped with commercially available technologies that are capable of reducing NOx and PM emissions to low levels. The control strategies for these engines generally involve combination of sequential multiport fuel injection, one or two heated oxygen sensors, three-way catalytic converters, engine control modules, and EGR systems. Generally, these technologies with some refinements are used in two gasoline- and one CNG-powered PCs, which have been SULEV-certified under the CARB LEV II program.

NOx and PM emissions from diesel-powered engines are higher than those from other transportation fuels because oxidation catalysts are the only applicable aftertreatment currently used to control diesel engine exhaust. There are many promising technologies which are capable of achieving lower NOx and PM emissions that are still being developed and optimized for diesel applications, but are not yet available for commercial applications.

Conclusion

With tighter regulations and the efforts of engine and vehicle manufacturers, and researchers, today’s diesel engines are much cleaner than when they were first regulated in 1960. However, diesel-powered MDVs are still emitting much more NOx and PM emissions than gasoline-, CNG-, and LPG-powered vehicles. In conclusion,

1. Five diesel-powered medium-duty engine families are LEV certified under the CARB LEV I program, and none under the LEV II program.
   
   
2. Engine manufacturers will still need to reduce NOx and PM emissions from the LEV-certified diesel-powered medium-duty engines by 92% beyond LEV I requirement to comply with the LEV emission standards under the LEV II program.
   
   
3. Integration of engine modification, electronic controls, low-sulfur fuels, and aftertreatment emission control devices is likely to be required to achieve LEV II standards for medium-duty diesel vehicles.
   
   
4. The advanced catalyzed PM trap technology with low-sulfur fuel is likely to reduce PM emissions to the LEV II standards. However, cost would have to be significantly reduced for this technology to be introduced into commercial applications.
   
   
5. Further technological advancements are required to achieve LEV NOx standards under the LEV II program. Staff will continue to monitor the development of these technologies and report significant future developments if they may impact rule implementations.

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