Setting the Standard
SwRI's Heavy-Duty Engine Benchmarking Program provides a means to obtain comprehensive performance and design data on new diesel engines
By Michael G. Ross
Benchmarking competitive products is a standard practice by manufacturers to gauge the performance and features of their products against their competitors. Having up-to-date benchmarks is crucial for effective product development and marketing. Without reliable data for reference, relative strengths and weaknesses in a manufacturer’s own products cannot be easily identified. Access to accurate information allows manufacturers to see where their product stands, allowing them to establish Ňbest-in-classÓ targets for future designs.
Thorough benchmarking requires large commitments of time, money, staff and facilities Ń resources that otherwise could be used for product development or improvement. Simply performing tests generates vast amounts of data, but not necessarily knowledge. For the data to be meaningful, analysis and interpretation by a team of experienced engineers is essential. Collectively, the team must be knowledgeable about emission regulations, competitive engines, engine performance, engine design, electronic controls and exhaust emissions aftertreatment devices. Although most engine manufacturers have these specialists, often they are not available for time-consuming projects such as benchmarking because of their production development schedules. Even when in-house analysis is performed, the critical documentation step may not be completed, so the knowledge is not disseminated or retained within the company.
The situation is even more difficult for component suppliers who do not have the resources to perform complete engine or vehicle benchmark testing. These suppliers must rely on engine manufacturers to provide design reference information such as flow rates, temperatures, pressures, dimensions and mass. Accurate information is essential for designing competitive products. If the parts are underdesigned, they will not have adequate performance or durability. If they are overdesigned, the parts will have excessive cost, size or mass. Because engineers are typically conservative by nature, the tendency is to overdesign. Likewise, engine manufacturers may provide design criteria with excessive factors of safety or combinations of maximum temperature, pressure and flow specifications that do not occur simultaneously in actual use, such as when the maximum flow rate through a heat exchanger does not occur at the same engine operating condition as the maximum inlet temperature. If the heat exchanger manufacturer assumes that the maximum flow and temperature occur simultaneously, the heat exchanger will be oversized. A third-party source of engine performance information would give component manufacturers the data needed to properly, cost-effectively design products for new applications.
Recognizing the broad need for engine benchmark data on heavy-duty engines, Southwest Research Institute (SwRI) started the Heavy-Duty Diesel Engine Benchmarking Program in 2003. New emission regulations by the U.S. Environmental Protection Agency (EPA), which took effect in late 2002, generated interest in establishing a benchmarking program. Seven engines were tested during the first year, four for tractor-trailers and three smaller engines for pickup and delivery trucks. At first, the number of test conditions and instrumentation systems were limited. Based on subscriber feedback, additional tests and instrumentation were added, including emission certification cycles and cylinder pressure measurements. Today, the tests include several different emission test cycles, steady-state mapping at up to 200 different engine speed/load conditions, throttle response testing, engine friction and brake curves, and tests for power reduction at high coolant temperature or at high altitude.
Engine performance is only part of the story. After testing is completed, SwRI completely tears down the engine to study the parts to better understand the design features that enable the engine to perform as it does. Each part is cleaned, weighed, measured and photographed. In addition to being valuable to subscribers, this measurement database is an important reference for SwRI’s own engine design activity.
New emission controls for 2007
Tighter emission standards made 2007 another important year for engine benchmarking. The new regulations require a 90 percent reduction in particulate matter Ń mostly composed of soot. To capture the soot, a highly efficient diesel particulate filter (DPF) eliminates the black smoke traditionally associated with diesel exhaust. The DPF gradually fills up until the soot is consumed in a process called regeneration, during which high exhaust temperatures are used to cleanly burn off the soot like a self-cleaning oven. Regeneration can be passive or active. During continuous high-load operation, the exhaust is hot enough to passively burn off some of the soot. During lighter loads and idling, the exhaust temperature is not naturally high enough for passive regeneration, so active controls are used to increase the exhaust temperature. The most common method of active exhaust temperature control is to inject a small amount of raw fuel into the exhaust ahead of an oxidation catalyst. The fuel is consumed in the catalyst, releasing a large amount of heat that initiates the DPF regeneration process.
One of the engines tested during the 2007 program, a 6.7L Cummins diesel that is used in the Dodge Ram pickup truck, includes an additional exhaust aftertreatment device called a NOx Adsorber Catalyst (NAC). Oxides of nitrogen (NOx) in the exhaust gas stream are adsorbed by the NAC , which means they adhere to the surface of the catalyst material, substantially reducing tailpipe emissions. The NAC requires two additional regeneration modes, a DeNOx mode in which the adsorbed NOx is chemically converted to harmless gases by briefly injecting extra fuel to produce a rich air-fuel ratio, and a DeSOx mode in which accumulated sulfur is purged from the NAC by injecting extra fuel for an extended period of time to produce high exhaust temperatures with stoichiometric exhaust composition. Without periodic desulfation, the NOx capture efficiency would rapidly deteriorate. The DeSOx mode restores the performance of the NAC and keeps emissions low.
The fuel consumption challenge
One of the biggest challenges facing engine manufacturers has been to maintain fuel efficiency while producing lower NOx emissions. Lower in-cylinder combustion temperatures reduce NOx, but engine efficiency is best when combustion temperatures are high. Increases in peak cylinder pressure have traditionally been used to maintain or increase engine efficiency and performance; however, further increases in cylinder pressure capability are limited using current engine design methodology and materials (see sidebar).
In addition, the DPF and NAC regeneration modes require extra fuel to achieve the required exhaust temperature and exhaust gas composition. During an active DPF regeneration or NAC desulfation, the exhaust temperature is increased to more than 600ˇ C (over 1100ˇ F) for up to 30 minutes, which uses a significant amount of extra fuel. Under some conditions, the fuel consumption penalty for exhaust temperature management exceeds 10 percent. It is generally worse at light loads when the exhaust temperature is naturally lower.
In spite of these challenges, engine manufacturers have done a remarkable job of maintaining fuel efficiency. In general, the 2007 heavy-duty engines have the same or better fuel efficiency than their 2003Đ2004 counterparts. Part of this is optimization of the hardware and combustion process, but much of it is due to advancements in engine controls.
Engine controls — a quiet revolution
Electronic controls were introduced on heavy-duty diesel engines in the late 1980s. At first, only injection timing and duration were controlled to optimize fuel consumption and performance under all operating conditions; however, with each new round of emission reductions the controls have expanded in scope and complexity. Today, the engine control unit (ECU) communicates with various other modules in the truck to receive information on accelerator, brake, and clutch pedal positions, gear selection, and vehicle speed. The engine controls can limit torque to prevent transmission or axle overload in lower gears, provide cruise control functionality, and even reward driver behavior by providing a higher governed speed when fuel economy goals are met. With the new common-rail injection systems, the ECU is able to control injection pressure and up to five injection events per firing cycle for each cylinder with precision to a fraction of a millisecond. Beyond injection, the ECU controls exhaust gas recirculation and the vane or nozzle position of variable geometry turbochargers. Some new diesel engines even have intake throttle valves, EGR cooler bypass valves, or variable intake valve timing that must be controlled by the ECU, but the biggest change for 2007 is the addition of aftertreatment regeneration control.
The ECU must determine when regeneration is required and be able to initiate it without driver intervention. Maximizing the time between regenerations is desirable to reduce the fuel consumption penalty, but aftertreatment performance degradation or even permanent damage can occur if regenerations are not performed frequently enough. During regeneration, fuel flow, injection timing, EGR flow, boost and air/fuel ratio may be radically altered, but the mode changes must be automatic and transparent to the driver. The output torque must remain constant, engine responsiveness and performance must be maintained, the engine sound must not change objectionably, and exhaust temperature must be maintained within a narrow range regardless of changes in speed or power demand. Overall, the 2007 benchmarking program has shown that engine manufacturers have done an excellent job in implementing complex regeneration control strategies.
Fuel consumption has always been important to long-haul truckers, but even more so now with the rapid rise in diesel fuel prices. Also, the 2007 program has clearly shown that a great deal of attention has gone toward maximizing fuel economy. One way this is done is by optimizing air/fuel ratio and injection timing to produce exhaust temperatures that promote passive DPF regeneration, especially during typical cruise conditions. By increasing passive regeneration, the fuel consumption penalty for active regeneration can be delayed as long as possible.
The 2007 Heavy-Duty Diesel Engine Benchmarking Program also found that engine manufacturers have risen to the challenge of reducing particulate emissions by 90 percent and NOx emissions by 30 percent or more while maintaining fuel consumption at 2004 levels. New exhaust aftertreatment devices with sophisticated regeneration strategies have been successfully integrated. To reach these conclusions, SwRI’s benchmarking team has produced an enormous amount of data regarding fuel consumption, emissions, control strategies and engine design features. These data provide subscribers with a complete picture of each engine’s attributes and a valuable reference for design and marketing activities.
As of 2008, SwRI is offering benchmarking subscriptions for the new Detroit Diesel DD15™ and International MaxxForce™ 13 engines. The 14.8L DD15 is the first in a new, worldwide family of engines that will eventually include four different displacements. The unique features of the DD15 include turbocompounding, in which a turbine in the exhaust delivers power directly to the crankshaft through a geartrain, and an amplified common-rail injection system that can modify injection pressure during an injection event to provide rate shaping of combustion. The MaxxForce 13 includes two turbochargers in series with interstage intercooling. A unique liquid-cooled intercooler system is expected to provide optimized control of boost temperature under all operating conditions. Will either of these new engines be the new benchmark for state-of-the-art heavy-duty diesel engines? Subscribers to SwRI’s Heavy-Duty Diesel Engine Benchmarking Program will soon know. Subscriptions are available through the web at benchmarking.swri.org.
Questions about this article? Contact Ross at firstname.lastname@example.org or (210) 522-2690.
Published in the Summer 2008 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.