The following article provides an overview on off-highway diesel engine after treatment. The overview provides readers a composite reference. A composite reference will feature key up-to-date markers which aggregate information on the following key subject markers. Each key subject marker relates to a specific aspect of off-highway diesel engine after treatment technology. The key subject markers include:
• Strategic Situation
• Emissions Standards and Challenges for Manufacturers
• Clean Diesel and After Treatment
• EGR vs. SCR After Treatment Technologies
Strategic Situation. Back in the mid-1990s, clean air concerns had reached a point by various parties around the world where technologists, scientists, and manufacturers faced new regulations. To address perceived needs of mainly urban centers to have clean air, specific attention was brought to bear on diesel engines and the role these devices played in fouling the air with noxious emissions. As discussions, research and debates on policies evolved, a terminology for managing solutions to the problem came into existence. The technological solution to problems of air quality brought on by diesel technology became known as “Clean Diesel Technology.”
Clean Diesel Technology- What Is It? What we’re talking about is clean diesel technology or CDT. Within CDT are three main areas of research, development, and manufacturing. These areas have three handy names under which all of the related areas fit. These are:
• Cleaner Burning Fuel Systems
• Advanced Engine Technology
• After Treatment
Cleaner diesel fuel addresses the obvious fuel-related issues which, in the aggregate constitute how the air quality problem will be solved by using a more appropriate fuel and system for managing the flow of inputs into the diesel power unit.
Advanced engine technology addresses the mechanical and electronic components of the diesel power unit which must be in harmonic balance to burn new grades of fuel while producing power and exceeding dynamic standards set by new demands of modern off-highway situations.
After treatment deals with the complexities of reducing emissions by volume and substance into quantities and gases which are not net pollutants to the ambient air quality.
Standards for all three areas, inputs, power output, and emissions, are referred to around the world as “Tier 4” standards. Global refers to four of the world’s largest economies of scale in the European Union, the United States, Canada, and Japan. Of particular relevance to the United States manufacturer are Tier 4 standards developed and set by the federal government’s Environmental Protection Agency (EPA), and an influential state appointed environmental board from California known as the California Air Resources Board (CARB).
CDT standards came on the scene more fully back in the late 1990s. The focus of this article takes a look at Tier 4 standards relative to after treatment, moving forward from 2010 onward. This is seen as the maturational arc of the policy development cycle. In this arc of the policy development cycle, the regulation/manufacturer compliance relationship has now yielded the dividends of concentrated research and development investment by the private sector. Such investment in research and development has brought about marketable technologies that are part of defining a new breed of clean diesel engine available now in markets worldwide.
Evolving Standards for Emissions: Ongoing Challenges for Manufacturers. The federal Environmental Protection Agency (EPA) sets emission standards for all technologies manufactured around the world. If a technology wants to be marketed and sold in the USA then it must meet emission standards. Clean diesel technology standards are no exception. The emissions for clean diesel technology have been a rolling target since 2001. By 2008, certain horsepower ratings classifications for different manufactured diesel off-highway power units began coming under the most advanced, most pollution restrictive emission regulation standards known as Tier 4.
Tier 4 regulations require manufacturers to build diesel engines that produce significantly less amounts of particulate matter and nitrous oxide. The amount of emissions allowed is far less than what was allowed under Tier 1, 2 and 3 regulation standards. Tier 4 requirements reduce particulates by a full 50% less than Tier 3 equipment predecessors. Nitrous oxide emissions under Tier 4 requirements restrict an additional 96% compared to Tier 3 regulations.
Tier 4 regulations are not retroactive to equipment models in compliance with Tier 1, 2 and 3 regulations. The purpose of the tiered regulations structure was to phase into manufacturing standards the clean diesel technology compliance standards for emissions, optimizing research and development planning and investment to ensure full compliance by 2015.
Since 2001, Tier 1 standards for clean diesel emissions have set the compliance challenge for all new diesel engines coming to market in the USA from around the world. The EPA used 10 horsepower ratings classifications in setting the Class I through Class IV standards for the years 2001 through 2015, the final year by which manufacturers will need to meet the requirements.
There are 10 general ratings classifications: 3 classifications within the 0-49 range; 3 classifications within the 50-174 range; and 4 within the 175-750 range.
In the 300- 599 range units were exempted from Tier 1 regulations and came under full compliance requirements of Tier 2 in 2011, the only class of diesel engines to be exempt.
Timelines vary for regulatory compliance standards per horsepower rating classifications. Tier 1, 2, 3, and Interim Tier 4 standards have been in effect for different ratings classifications for different amounts of time during different years between 2001 and 2011.
For example, small horsepower unit ratings at 0-10 and 11-24 were held to T1 regulatory standards for 4 years, from 2001 through 2004. Tier 2 standards for those same classifications went into effect for 2005 through 2007. In 2008, Tier 4 standards went into effect, a much shorter compliance cycle. Comparatively speaking, the 50-99 class of power units will be under Tier 3 standards until the end of 2011. At the beginning of 2012, the same class will enter into Interim Tier 4 standards through 2014, with Tier 4, Class IV standards going into effect for them in 2015. That is a full seven (7) years of different regulatory impact between horsepower rating classifications.
The largest horsepower ratings classification, 750+ hp, has had the longest T1 and T2 regulatory compliance period, and is not scheduled to begin its Interim T4 period until this year in 2011. Full stage IV, T4 compliance is not scheduled until the very last year of the EPA policy compliance cycle 2015. Ratings classifications within the 25-99 hp range begin full-on compliance with Stage IV T4 regulations as early as 2012.
The “tier” system for regulatory compliance standards has been around for about 13 years. To put the technology challenge in perspective, a “Tier 0” diesel engine was basically unregulated. It was a purely mechanical power unit. What have evolved quickly within the past 13 years is a complex engine design, with a sophisticated set of subsystems for fuel delivery, and an exhaust control system complete capable of controlling heat exchange and polluting emissions beyond comprehension just over 1 decade ago.
Clean Diesel Technologies and After Treatment. The quest to reduce emissions required a full re-think of diesel power unit operating systems. Fuel, mechanics, and exhaust all play a pivot role in the final outcome of importance- reduced emissions of particulate matter and nitrous oxide.
There have been different schools of thought and engineering, research and development, design, and testing among manufacturers who were in search of the best methodologies for producing a new generation of clean diesel technology for the off-highway market of Europe, Canada, Japan, and the United States.
Fuel Systems and “Burn” Efficiency. At the heart of diesel combustion engines are the fuel source and its delivery system. The composition of diesel fuel and its many properties needed to handle all types of operating environments and its capacity to burn for optimal combustion is a subject that could take volumes to report on how the fuel impacts emissions.
The delivery system is a high precision technology which must repeatedly, hundreds of trillions of times over, meter, deliver, atomize, and inject fuel to the engine with crack timing.
Any loss of efficiency in the fuel delivery system will compound exponentially the amount of unburned fuel propelled into the exhaust system, with higher amounts of pollutants per million. Industry leaders such as Caterpillar, John Deere, Volvo, Navistar, Cummins, and Detroit Diesel have all focused research and development on developing electronically assisted components for clean diesel fuel system. Precision combustion control improvements have been harmonized with electronically-integrate machine systems to improve the overall efficiency of the new generation clean diesel engines for off-highway use.
Increasing fueling efficiency, the theory goes, will increase the “clean” burn on the front end of the power system, reducing the pressure on the exhaust- or after treatment system- to clean the exhaust. Increasing injector pressurizing capacity has required improving injector technology to meet the 30,000 pound per square inch pressure threshold. Improved pressuring improves the atomizing or “misting” pattern of the fuel being injected into the cylinder. This assures two critical functions occur which greatly reduce the N Ox emission volumes per cylinder stroke: 1) complete burning on impact; and, 2) more effective cold starting when unburned fuel and higher emissions are at peak levels.
Air Intake and Exhaust Systems: Blurring the Lines of Tradition. Clean diesel engines will have evolved into more complex, cleaner burning power units as compared to their predecessors of the last three decades. Besides key innovations in electronic controls on mechanical engine functions, clean diesel technology uses enhanced air intake systems and new technological configurations for treating and recycling diesel engine exhaust. Innovations at both ends of the system, clean diesel technology emphasizes cleaner, more complete fuel combustion, and the recycling and chemical scrubbing of exhaust.
Emission regulations present specific challenges for manufacturers. Clean air is an important priority and social dividend to industrialism which manufacturers have embraced under EPA policies. However, they soon learned the tradeoffs were immense: regulating off-highway emissions can negatively impact- and perhaps profoundly impair engine performance at all levels of the horsepower rating continuum.
Fortunately, innovation has offset any setbacks. Manufacturers like John Deere have improved air-to-air intercooling systems to cool intake air. Cooler intake air means more oxygen by volume can be pushed into the combustion chambers on the down stroke, heightening compression due to the lower air mass, leading to a more “violent” ignition and a better burn. The net result is low-speed torque maintenance, keeping transient response times at standards, and sustaining peak torque under load. The fuel consumption rates are uncompromised and unit installation costs are insignificant relative to the total sticker price for power units across the board.
Exhaust Gas Recirculation (EGR) systems serve as a reliable means for cooling and integrating precise ratios of exhaust gases cooled with intake air. The cooler peak combustion temperature lowers the key pollutant in the exhaust which is the central target of Tier 4 regulations: Nitrous Oxide (NOx). After treatment components reduce the NOx volume even further through adsorbent filtration, while particulates are filtered out by high performance filters (DPFs).
Selective Catalytic Reduction (SCR) is not a new technology although it is fairly new to North American diesel power unit markets. The desired end result of an SCR system is to convert nitrous oxide to CO2, or carbon dioxide. Specifically, the SCR system is designed to take N Ox-rich diesel exhaust gases and mix these with a catalytic agent, changing nitrous oxide into diatomic nitrogen (N2) and water (H2O). The standard industry catalytic agent is urea, a form of diatomic nitrogen, which can be suspended in a 32% aqueous solution, or diesel exhaust fluid (DEF). DEF is pumped from its system holding tank near the power unit into a DEF injection pump assembly where it is injected directly into the exhaust system. The atomized DEF mixes with the low pressure exhaust gases and catalyzes with the nitrous oxide, reducing emissions almost to zero.
Why SCR is not the Industry Standard. Manufacturers using advanced EGR technology as the cornerstone of their front end emissions control system on new generation clean diesel power units talk about how EGR is a “known” technology which has been proven adaptable in meeting the growing demands of EPA clean air regulations. The technology is the simplest approach to the problems faced with clean air regulations, and EGR has results that are comparable to SCR along with its high degree of compliance with EPA standards under Interim Tier 4. Simpler technology means EGR requires simpler maintenance over the longer term and is easy on the system operator(s).
EGR in its most current enhanced configurations does not require extreme and complex hardware rearrangements and system handling modifications. Extra fluid/catalyst storage units, injector pumps, regulating systems or electronic controls are not needed for advanced EGR to give thousands of trouble-free hours of operation. Availability of DEF is a non-issue for EGR systems. Service technicians across the country are already familiar with EGR maintenance and service procedures.
EGR-Based Systems After Treatment Mechanisms. After treatment mechanisms on an EGR-based system will rely on a dual-filtration process using catalytics and particulate filtration. The direct oxidation catalyst operates under normal conditions to reduce certain pollutants. Once reduced by catalytic reactions within the diesel oxidation catalyst, the remaining gases and resident particulates flow into the diesel particulate filter. DPF elements have walls with porous channels. The filtering maze traps and holds any free-moving particulates. Exhaust heat creates a cleaning dynamic within the filter to oxidize the exhaust particulates.
SCR Systems After Treatment Mechanism. Particulates are trapped using standard particulate filtration. However, the significant difference between SCR and EGR emission control systems is the use of urea-based DEF to catalyze engine out N Ox pollutants. The reduction of engine out N Ox emissions is the interactive function of the SCR catalyst component with the control module. Electronically controlled the module monitors NOx concentrations in the engine out exhaust gas stream. When needed, the control module signals the DEF injector assembly to inject DEF into the stream. As the injection and exhaust mixture pass over the catalytic unit the NH3-N 0x reaction occurs, producing N2 (non-toxic) and H2o vapor.
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