High-horsepower Diesel Design Revs Up to Meet Tighter Emissions Standards
Engine builders are confident they have access to the technologies needed to meet upcoming EPA Tier 4 emissions standards for large, off-highway diesel engines. The challenge will be to implement these systems without degrading engine performance or reliability
By Russell A. Carter, Managing Editor
As 2009 draws to a close, off-highway equipment engine suppliers are gearing up for a new era in diesel design. The world of high-horsepower, off-highway diesels is about to be challenged to an unprecedented degree by what could be called the “three C’s”—compliance, complexity and cleanliness.
In order to comply with increasingly stringent emissions standards for off-highway diesels rated 751 hp or higher, engine builders must find ways to reduce diesel emissions to a fraction of the levels allowed by the U.S. Environmental Protection Agency (EPA) and European Union under previous Tier 1 and 2/Euro Stage I and II standards, respectively. Although >751-hp diesels were exempt from Tier 3/Stage IIIA requirements, a new set of emissions-control standards will soon apply, beginning with the EPA’s Tier 4 Interim rules in 2011 for the U.S. (and Canada), leading up to Tier 4 Final standards in 2014.
Regulatory agencies have primarily focused on the reduction of particulate matter (PM) and oxides of nitrogen (NOx). Carbon monoxide (CO) and hydrocarbons (HC) are also regulated but are inherently low from diesel engines.
According to Blake Larson, executive engineer, Cummins High Horsepower engine division, Tier 4 Final regulations “present the most significant change in mining engine technology in recent history” by stipulating a 62% reduction in NOx and 93% decrease in diesel particulate matter (DPM) from previous regulated levels. Larson spoke earlier this year at the 2009 Haulage & Loading Conference sponsored by Mining Media, publisher of E&MJ and Coal Age. The magnitude of emissions reductions called for by the upcoming standards is most easily understood by looking at the chart Larson displayed in his presentation.
Not only are compliance emission levels drastically reduced by these regulations, but testing and certification will be more complex. As shown in the accompanying table, high-horsepower diesels will be subject to more involved test procedures and monitoring throughout their service life.
The result: off-highway diesels complying with the new standards will be much “cleaner” but more complex—and the technology employed to achieve the necessary levels of emissions control will require greater attention to engine fluid contamination. Newer diesel engines are much more sensitive to dirty fuel or oil, and internal damage may occur much more quickly than with previous engine designs. According to some experts, the mandated transition to ultra low-sulphur diesel fuel (ULSD) exacerbates potential problems in this area.
There are a number of available technologies that will enable engine builders to meet future emissions control standards. Key to their implementation, said Larson, is an approach that takes into account the concerns of three major engine stakeholder groups: owner/operators, original equipment manufacturers (OEMs), and the engine suppliers themselves. Owner/operators, for example, are mainly interested in obtaining high engine reliability/durability, power density and fuel efficiency. OEMs want minimal impact from new-engine installation requirements, a more-or-less universal design to allow engine use in different areas of the global market and robust features that can stand up to the toughest duty cycles. Engine manufacturers, for their part, must deliver a proven engine platform that can meet regulated emissions levels while providing equal or improved performance.
For some diesel suppliers, this has meant working from a clean sheet to develop a new family of engines. Caterpillar, for example, determined that it couldn’t meet its future engine-power growth and emissions control objectives with its high-horsepower 3500 series platform and turned to a new design—the C175 family, which has the capability of providing up to 4,000 hp for mining applications and even more power in other industrial and marine applications, along with the potential for meeting Tier 4 emissions levels by employing selected features of Cat’s ACERT technology—originally developed for emissions control on its over-the-road engines—in combination with exhaust aftertreatment technologies. In the process, Cat also incorporated a number of design features into the C175 line designed to improve overall durability and efficiency. These include an optimized cylinder block with cross-bolted main bearings caps, a more robust crankshaft design, drilled connecting rods featuring a beefier piston pin joint for increased cylinder pressure capability, forged steel pistons, and a stronger cylinder head.
Cat has equipped its newest haul truck models, the 797F and 793F, with 20- and 16-cylinder versions of the C175 platform. On both trucks, engine design includes four turbochargers, an air-to-air aftercooler and an electronically controlled common-rail fuel system. And, as reported in last month’s issue, Cat notes that every new 793F and 797F will have the same emissions footprint regardless of where the truck is sold. “We could have placed noncompliant engines in less-regulated regions, but that goes against our code of social responsibility,” according to David Rea, global marketing manager-mining trucks, Cat Global Mining.
Aftertreatment Lies Ahead
High-horsepower engine manufacturers have depended on electronic controls, air-flow improvements, internal combustion optimization, advanced fuel delivery systems and other engine-design solutions to comply with Tier 2 standards, but the consensus among diesel suppliers is that exhaust aftertreatment technologies will be necessary to lift emissions control to Tier 4 Final levels.
Aftertreatment methods include Catalyzed Diesel Particulate Filters (DPF); Diesel Oxidation Catalysts (DOC); Selective Catalytic Reduction (SCR) and NOx adsorbers. The technologies can be used in various combinations; Cummins, for example, has assessed combinations such as the use of SCR aftertreatment for NOx reduction, combustion optimization for PM control and particulate aftertreatment; or using combustion optimization and cooled EGR for NOx reduction along with a catalyzed DPF for PM control. For the Cummins series of high-horsepower diesels, however, key engine systems such as variable-geometry turbocharging, high-pressure common-rail fuel injection systems and electronic controls will be critical components in any final design.
Not surprisingly, there are tradeoffs involved in each technological choice. For example, Larson explained, Cooled Exhaust Gas Recirculation (CEGR) can help control engine NOx emissions. In CEGR, gas is collected from the exhaust manifold and cooled, and then a mixture of the cooled exhaust gas and fresh air is introduced into the combustion cycle. This approach requires an exhaust gas cooler or heat exchanger, an exhaust gas valve and mixer, and sophisticated turbocharging to provide precision control of airflow at all engine speeds and loads while also furnishing the necessary pressure differential to drive the exhaust gas recirculation process.
The tradeoffs: High-capacity cooling fan and radiator/charge air packages will be required to dissipate the additional heat absorbed by the engine coolant in the CEGR process. There is also increased complexity of air handling equipment, and other concerns include increased cylinder pressure and higher fuel system injection pressure to control PM emissions.
Another promising technology, Selective Catalytic Reduction (SCR), uses ammonia and a catalyst to transform NOx in exhaust into nitrogen and water. A reductant is added to exhaust flow to create the reactions in a catalyst chamber. The process employs liquefied urea and has been proven in on-highway European applications. Key components for successful SCR include an air handling system optimized to accommodate an increase in exhaust back pressure, plus urea handling and injection equipment, a catalyst chamber and temperature/NOx sensors.
Issues with SCR range from the packaging and installation of the required equipment and urea availability and handling, to SCR catalyst life in off-highway applications and unwanted “ammonia slip,” or release of unreacted ammonia.
Ran Archer, mining sales manager for engine builder MTU Detroit Diesel, asserted in his H&L conference presentation that SCR generally provides better fuel economy than EGR. SCR technology is less sensitive to sulphur content in diesel fuel, which can vary widely around the world. On the downside, SCR systems use urea which must be stored onboard and will require routine replenishment. Urea consumption is approximately 5% by volume of fuel consumed. Although urea is relatively common in Europe for on-highway applications, distribution of urea for mining applications is virtually nonexistent. And, he noted, all aftertreatment components increase the engine’s “space claim” within the equipment.
These are all major concerns for engine builders, who generally agree that DPF and DOC technology will be required to attain emission levels mandated by Tier 4 Final regulations. A DPF captures particulate matter (PM) in a semi-porous medium as it flows through the exhaust system. DPFs are available in “passive” or “active” configurations; active DPFs use a control system to promote required filter regeneration. A DOC consists of a catalytic coating on a honeycomb substrate for oxidizing PM. It operates in a passive-only mode without active regeneration, and thus is less efficient at PM reduction than the DPF. A DOC is used in sequence with the DPF to drive up the exhaust stream temperature in the presence of exhaust-borne hydrocarbons. The DPF is regenerated—PM is burned off—by injecting hydrocarbons into the exhaust stream ahead of the DOC.
Although the DPF-DOC combination can be used in association with NOx-reducing technologies such as CEGR and SCR to meet Tier 4 Final emission limits, engine designers also must deal with other issues, such as coping with variable exhaust-backpressure levels as the DPF fills and regenerates; as well as requirements for precise, duty cycle-based control of temperatures and dosing frequency for regeneration. Overall DPF life in off-highway applications is also an unknown variable at this point.
Bulking Up for Fuel Efficiency
Roger Miller, Director of OEM Sales, Kaydon Filtration, warned attendees at the H&L conference that issues with haul-truck engine reliability may proliferate due to engine-design changes aimed at improving fuel efficiency and emissions control, along with a transition to ultra-low sulphur diesel (ULSD) fuel and possible inattention to engine-fluids anti-contamination measures.
Potential problems, said Miller, begin at the fuel-specification level. Some ASTM diesel fuel specifications, for example, haven’t changed since 1947, and current specs allow a level of solids and water contamination that can have significant effects on engine reliability. Miller highlighted, as an example, a large copper mine in the U.S. that uses 30 million gallons of diesel fuel annually; with specification-allowable solids contamination and water content levels, this would amount to as much as 125,000 lb of dirt and 30,000 gallons of water reaching mine-equipment fuel tanks annually.
In addition, mandated low-sulphur diesel (LSD) and ULSD fuels are more water-friendly than previous fuels, and provide less lubricity due to the extra refining needed to remove sulphur content. This reduction in fuel lubrication capability can be deleterious to high-pressure pumps and injectors.
Potential problems are compounded by high-horsepower engine design changes required to meet EPA Tier 2 and Tier 4 emission levels. These changes include high-pressure common-rail (HPCR) fuel injection systems with injector and internal system pressures reaching as high as 30,000 psi or more—and requiring very tight tolerances between system components to maintain these pressure levels. Critical particle size leading to malfunction from contamination in these systems is generally less than half (2–3 µm) that of non-HPCR systems (6-7 µm), and water-in-fuel tolerance of HPCR systems is nil, according to Miller. As little as one spoonful of abrasive dirt in a fuel tank can lead to injector failure within eight hours, he explained, and component replacement costs are high, ranging from $35,000 to replace a high-pressure injector pump to $10,000–$17,000 for a new injector set, plus the considerable costs of unplanned downtime and lost revenue.
Even though oil companies claim to filter fuel from their refineries to remove contamination down to 2 µm, the opportunities for recontamination in the distribution chain are many, according to Miller. Poor on-site housekeeping practices add to the problem, and can be multiplied manyfold in remote regions or developing countries. Putting full faith into onboard filtration systems can backfire as well. With filter component size limited by space requirements on mobile equipment, a filter’s dirt-retention and water-removal capacity can be rapidly depleted and the filter may go into bypass mode. Most modern fuel control systems will not allow a machine to run with plugged filters.
As a wide-ranging solution to these problems, Miller recommended bulk diesel fuel conditioning, which he defined as “filtration of diesel fuel in a single pass at high flow rates in order to deliver ultra-clean and dry fuel to on-site terminal storage tanks and/or, most importantly, to the working fleet of equipment.”
Kaydon supplies both fuel and oil conditioning systems suitable for use in high-volume mining applications. According to its product literature, the use of bulk fuel conditioning in fuel offloading and fuel forwarding lines removes the burden of contamination removal from onboard filters, thus providing fewer fuel-related maintenance incidents and extending vehicle operational time.
Recently, said Miller, Kaydon assisted a large South American mine in upgrading its fuel conditioning strategy, with highly positive results. After implementing a two-part strategy that included 1) installation of a bulk fuel conditioning system with a coalescing unit for water removal, strict use of OEM-provided fuel filters, close adherence to recommended filter changeout intervals, and fuel tank drainage every 500 hours or three months; and 2) onboard protection including a 10-µm primary filter, 4-µm high efficiency secondary filter and onboard coalescer, the customer reported a drastic drop—from 96 to 22 over a 90-day period—in low-power maintenance calls arising from plugged fuel or air filters, while actual downtime dropped from 287 to 30 hours in the same period. This, reported the customer, represented 3.5 “truck days” recovered per month, or 43 truck days per year.