Advanced emission controls paramount for diesel engines
Jun 1, 2003 12:00 PM
Increasingly stringent federal emission standards for diesel engines have become a major headache for trucking. Like most new technologies, the post-October 2002 emissions controls have their share of challenges — fuel consumption, maintenance, durability, and reliability for heavy-duty diesel engines.
As engine manufacturers refine their various current technologies, more emission control deadlines mandated by the Environmental Protection Agency take effect in 2007. To meet those deadlines, manufacturers will have to use an integrated approach in developing technology that reduces diesel emissions, said speakers at the 2003 Dairy Distribution and Fleet Management Conference in Savannah, Georgia.
“It's very important to go beyond technology that will meet EPA numbers,” said Bruce Bertelsen, senior advisor, Manufacturers of Emission Controls Association (MECA), Washington DC. “Technology must maintain the power and performance of the engines, provide low impact to fuel economy and maintenance requirements, and make the technology as transparent as possible to the end user. All of this must be achieved to develop a successful strategy.
“The 2007-2010 standards require an integrated systems approach — the best in engine design, high-quality fuels, and advanced emission controls technology. The most important issue regarding advanced emission controls is low-sulfur fuel.”
Between 1970 and 2000, the United States Gross Domestic Product increased 147%. During the same time, vehicle miles traveled rose 140%, US population increased 33%, and six principal pollutants that include oxides of nitrogen and particulate matter decreased 31%, Bertelsen said.
“That is a significant reduction,” he said. “While the United States emissions control program is recognized as one of the world's most successful environmental stories, we can't rest on our laurels. The EPA reports that 120 million people live in areas that have unhealthy air. So we need to do more.
“EPA has focused on standards that are performance-based, so the government is open to any technology,” he said. “EPA has leaned in the direction of NOx (oxides of nitrogen) adsorber technology while European authorities have focused on selective catalytic reduction. Our companies are involved with both technologies. Performance-based standards encourage competition and diversity in the marketplace.”
MECA is a consortium of more than 40 companies that manufacture emission control technologies for mobile sources, such as automobiles, trucks, buses, off-road machinery, and lawn and garden equipment. In simple terms, if it moves and has an engine, EPA classifies it as a mobile source, he said.
In 2002, MECA surveyed its membership. Twelve member companies responded that they will invest more than $1.5 billion to produce the necessary technology in research and development or in capital expenditures to meet the 2007-2010 emission standards, Bertelsen said. An independent review panel was created by the EPA in 2002 to examine the progress of meeting the 2007-2010 emission standards.
“The review panel concluded that there are remaining challenges, but the technology is on track to be ready in 2007 and 2010,” Bertelsen said. “Particulate matter emissions have been identified by the Bush administration as being the top priority for reduction to protect public health. Our company is extremely optimistic that diesel engines will meet future emission standards. The investments are being made, not only by us, but also the engine manufacturers and the refining industry to bring about the products and fuels that will be necessary.”
EPA regulations for diesel engines have focused primarily on two emission components — NOx and particulate matter (PM). NOx levels increase when flame temperatures rise in the combustion chamber. Emissions from particulate matter are made of tiny particles, such as soot, hydrocarbons, and sulfates — all products of the combustion process.
Examples of advanced emission control technologies for particulates are filters, diesel oxidation catalysts, and crankcase emission controls. Diesel particulate filters already are receiving wide application with installation in trucks, buses, and off-road vehicles, Bertelsen said. More than 400,000 diesel particulate filters have been installed on passenger cars in Europe without one report of a fuel-related failure.
During the 2003 Geneva Auto Show, Renault unveiled its Vel Satis 2.2 dCi (direct common-rail injection) luxury sedan equipped with a diesel particulate filter system, based on a catalyzed filter technology to be supplied by Engelhard Corporation.
The filter works by capturing particulate matter emitted from the engine. The filter's catalytic coating enables the device to burn the soot during normal driving conditions, thereby regenerating itself. The regeneration process is initiated by the car's engine management system. In combination with the oxidation catalyst, Renault says the filter system removes more than 95% of the hydrocarbons, carbon monoxide, and particulate matter created by the engine.
Particulate filters were introduced on diesel cars by Peugeot in 2000. They were introduced in the mid-1980s on cars sold in California by Mercedes-Benz, but use of that technology at the time was abandoned.
Toyota is testing its Diesel Particulate-NOx Reduction (DPNR) system, which reduces particulates and NOx at the same time. The DPNR catalytic converter is mounted close to the exhaust manifold, and an oxidation catalytic converter is farther downstream in the exhaust system. The DPNR converter features a highly porous ceramic filter coated with a catalyst for its NOx storage reduction catalytic converter, initially designed for Toyota's high-oxygen gasoline engines. To avoid catalyst deterioration, DPNR requires diesel fuel with less than 10 parts per million of sulfur.
Used with a heavy-duty truck engine, the diesel particulate filter, which resembles a honeycomb, can reduce particulate emission by 85% with ultra-low sulfur fuel, Bertelsen said. Exhaust flows through a series of parallel chambers. Alternate ends of the filter are plugged, so exhaust gas enters, but can't escape through the other end. It must pass through the ceramic walls, where the particulates are captured, and the exhaust gas passes out the other side.
Another particulate control product is the diesel oxidation catalyst, which is typically a flow-through design. It is composed of a catalyst wash coat, a substrate — ceramic or metallic — and canning, separately as a converter or in the muffler. The catalyst interacts with the exhaust as it passes through the converter, causing the particulate to burn at normal exhaust temperatures.
“This technology is capable of reducing particulate emissions by 20% to 50%,” Bertelsen said. “It depends on the type of particulates coming from the engine. Obviously, you get more particulate reductions from older, dirtier engines. Carbon monoxide and hydrocarbons can be reduced by 90%.” Oxidation catalyst operating experience extends to 250,000 off-road engines, 100,000 heavy trucks and buses, and 1.5 million Class 1 and 2 vehicles.
NOx emission control technologies include adsorbers, selective catalytic reduction, and exhaust gas recirculation. The physical structure of a NOx adsorber is similar to a three-way catalytic converter or diesel oxidation catalyst — a flow-through substrate coated with catalyst washcoat. The challenge is to control NOx emissions in the oxygen-rich exhaust environment of a diesel engine.
“NOx is basically nitrogen molecules with oxygen attached,” Bertelsen said. “You need to pull the oxygen away from the nitrogen, which is difficult to do with an oxygen-rich exhaust. But the solution can occur while the engine is operating lean — you capture the NOx molecules and store them on the walls of the adsorber. During fuel-rich engine operation, NOx is released, and nitrogen and oxygen are separated to create carbon dioxide or water.”
The current focus of this technology is to expand the temperature range over which the NOx adsorber is effective and increase its durability so that it can survive hundreds of thousands of miles, he said. The use of low-sulfur fuel is an absolute must for success, because even at low levels, sulfur interferes with the functioning of this technology.
Selective catalytic reduction
Selective catalytic reduction (SCR) has been used successfully in stationary applications since the mid-1980s, primarily in Germany, Japan, and the United States. In 1985, the US Department of Energy and private industry sponsored the Clean Coal Technology Demonstration Program to evaluate the performance and economics of the SCR process for removing NOx from the flue of gas of boilers fired with high-sulfur coals. These systems now are recognized by EPA as one technology for meeting on-highway heavy-duty diesel engine emissions standards for 2007.
“Heavy-duty trucks operating in Europe with SCR since the mid-1990s have performed effectively, and some of these vehicles have achieved significant mileage,” Bertelsen said. “Some demonstration programs are ongoing in the United States. SCR uses a reductant to help make the NOx control function work effectively. The leading candidate as a reductant agent is urea, which typically is needed onboard the vehicle.”
Mack Trucks completed its urea-SCR demonstration program on a heavy-highway truck in 2000. The program objective was to evaluate the feasibility of SCR for meeting the EPA 2007 standard, which presently calls for a .20 g/hp-hr (grams per brake horsepower hour) NOx maximum emission from heavy-duty diesel engines.
The demonstration was conducted on a Class 8 truck with a loaded trailer of 53,500 pounds for a total 70,000-lb GVW. The truck was equipped with a 12-liter, six-cylinder diesel engine rated at 350 horsepower at 1800 rpm. Siemens Westinghouse Power Corporation developed the exhaust aftertreatment technology under the name SINOx, which originally was used to reduce NOx emissions in power plants. A SINOx SCR catalyst system including 63-liter catalyst, an on-board urea solution tank, and a control system were installed on the vehicle.
Over-the-road measurements showed an average NOx reduction of 65%. Engine tests on the FTP and ESC cycles showed NOx reductions between 55% and 85%.
Retrofitting existing diesel engines with emission control technology can be an important part of a comprehensive strategy to reduce pollution from mobile sources, Bertelsen said. Retrofit options are available for diesel engines to reduce hydrocarbons, carbon monoxide, particulates, and toxic emissions. NOx retrofit controls, such as exhaust gas recirculation and SCR, are just beginning to emerge.
“NOx adsorber technology is still being developed and won't be ready for retrofit applications for some time to come,” Bertelsen said. “Initial retrofit efforts have focused on municipal and school buses. The EPA is very interested in cleaning up emissions from school buses, and a nationwide voluntary program will be announced soon.”
To meet EPA 2007 emission standards for heavy-duty diesel engines, several manufacturers, including Cummins, decided to use cooled exhaust gas recirculation (EGR) aftertreatment systems, said Steven Goss, Cummins southern division manager.
“We think cooled EGR is the foundation for where we need to go in 2007,” Goss said. “We will continue to optimize our cooled EGR platforms and introduce aftertreatment technologies. With the addition of these aftertreatment systems, we feel confident that Cummins will achieve new emissions standards and excellent performance at good operating costs for our buyers. Naturally, the infrastructure for low-sulfur diesel fuel and urea has to be in place for these new technologies to be successful.”
Cummins decided on a combination of cooled EGR and a variable-geometry turbocharger developed by its subsidiary Holset for the best balance of performance and fuel economy. The technology is used in the ISB, ISM, and ISX engines.
“Comparing the ISX to the Cummins N14, Detroit Series 60, or Caterpillar engines, we're seeing a 1%-3% loss in fuel economy,” Goss said. “Comparing the current ISX to the previous ISX engine, it's about a 3%-5% reduction.”
A diesel engine uses EGR to reduce exhaust emissions with the help of engine coolant and a heat exchanger. Exhaust gases are cooled from about 1200°F to 250°F. Exhaust gas replaces some of the oxygen in the cylinder to lower peak flame temperature, reducing the level of NOx. EGR systems are designed to allow more advanced fuel injection timing for better fuel economy, but they also introduce an acidic environment from sulfur in the fuel and nitrogen compounds in the cylinder.
One problem associated with EGR-equipped engines is the high level of soot introduced into the engine oils, which can lead to higher soot-related wear and increases in oil viscosity. API CI-4 diesel oil and specially designed diesel oils from equipment manufacturers are expected to handle these increased levels of heat and soot. They also must neutralize additional acids to prevent corrosive wear of cylinder components.
Long change intervals
For the ISX and ISM, Cummins recommends oil changes every 25,000 miles for normal duty and 35,000 for light duty, Goss said. The company continues to recommend the 15,000-mile oil change for the ISB engine. He emphasized the use of the CI-4 oil with an additive package to maintain these oil drain intervals.
To strengthen its support of EGR technology, Cummins took an additional step and issued an Uptime Guarantee, under which anyone buying an ISX or ISM engine between October 2002 and March 2003 will receive a guarantee that any repairs under warranty will be completed in less than 24 hours. If repairs cannot be finished within that time, Cummins will reimburse truck rental fees for up to three days.
Fleetguard Emission Solutions, a subsidiary of Cummins, in April announced the availability of the first retrofit system that reduces emissions levels from diesel engines. The company reported that the Longview system reduces NOx by 25%, while simultaneously reducing particulate matter by 85%.
Cummins also said it plans to introduce new ISC and ISL engines this summer that will use a high-pressure common-rail fuel system. Pressurized fuel is pumped to the common rail for storage and is injected by an electronically controlled injector at the optimal time regardless of the rotary speed of the engine. A series of injectors is connected to the rail, and each is opened and closed by a solenoid driven by an electronic control unit.
The new ISL engine has a 24-valve centered fuel injection — four valves per cylinder for better breathing characteristics. Injectors are positioned vertically for optimum combustion. The company said a combination of pilot fuel injection and revised gear cover will make the ISL 50% quieter.
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