Impact of Advanced Technologies on Medium-Duty Trucks Fuel Efficiency

Transcription

1 Impact of Advanced Technologies on Medium-Duty Trucks Fuel Efficiency Copyright 2010 SAE International Antoine Delorme, Ram Vijayagopal, Dominik Karbowski, Aymeric Rousseau Argonne National Laboratory ABSTRACT Rising fuel costs, increased regulations, and heightened customer sensitivity to energy efficiency has prompted the evaluation of numerous powertrain technology improvements to introduce into production. The actual impact of such technologies can differ broadly, depending on the technology or application. To evaluate the fuel consumption impact, various baseline vehicles have been created and simulated by using Argonne National Laboratory's vehicle modeling and simulation tool, the Powertrain Systems Analysis Toolkit (PSAT). This paper provides a quantitative evaluation of several technologies or combinations of technologies. First, we assess the impact of single technologies, including vehicle/chassis characteristics, such as weight, aerodynamics, or rolling resistance. Next, we consider advanced powertrain technologies, ranging from dieselization to transmissions with a higher gear number, and hybridization. Finally, we examine the effects of combining each technology, such as aerodynamics and tire improvements. INTRODUCTION To improve the fuel efficiency of medium- and heavy-duty vehicles, a significant amount of research is ongoing to develop new technologies for various components of the vehicle powertrain. In particular, truck manufacturers have reduced vehicle aerodynamic losses as well as tire rolling resistance, and several hybrid vehicle architectures have appeared on the medium- and heavy-duty truck market. All these technologies could potentially save significant amounts of fuel compared with a current vehicle. In this study, we consider the example of a class 2b pickup truck. A model of this vehicle was created in the Powertrain Systems Analysis Toolkit (PSAT), which is the state-of-the-art, flexible, and reusable simulation package developed by Argonne National Laboratory. PSAT is designed to serve as a single tool that can be used to meet the requirements of automotive engineering throughout the development process, from modeling to control. PSAT is the primary vehicle simulation tool to support FreedomCAR and the Vehicle Technologies Program. In 2004, PSAT received an R&D100 Award, which highlights the 100 best products and technologies newly available for commercial use from around the world [1]. Using test data from Argonne s Advanced Powertrain Research Facility, both conventional and hybrid PSAT models have been validated within 2% to 5% for fuel economy and battery state-of-charge on several drive cycles [2]. In this paper, different improvement options are explored with more or less aggressive assumptions. First, we estimate the fuel consumption reduction that could be achieved by each individual technology (i.e., reducing the drag coefficient alone, improving engine peak efficiency alone, etc.). In the second part, we consider several combinations of these technologies. 1

2 VEHICLE CHARACTERISTICS The vehicle model assumptions for the pickup truck were based on the GMC Sierra 2500 HD [3]. The vehicle has a gross vehicle weight rating (GVWR) of 4172 kg and consequently belongs to the class 2b. This particular pickup also presents the advantage of offering specifications for both gasoline and diesel configurations, which will be used when evaluating the impact of fuel type on fuel consumption. Table 1 summarizes the main assumptions used for the baseline pickup truck. In the remainder of this paper, these values will be used as the baseline for any individual technology improvement. Although Table 1 contains both gasoline and diesel engine assumptions, the pickup truck model in this study uses a gasoline engine, except for specific gasoline/diesel fuel efficiency comparisons. Table 1. Vehicle Specifications for the Class 2b Pickup Truck Component Model Characteristics Engine Diesel: Cummins 6.7 L, 272 kw Gasoline: GM LM7 5.3 L, 276 kw Transmission Automatic 6-speed Tire P245/75/R16 Radius = m Rolling Resistance = Vehicle Losses Drag Coefficient = 0.44 Frontal Area = m2 Curb Weight 2659 kg GVWR 4172 kg Max. Payload 1513 kg Since the engines specified by the manufacturer (Duramax and Vortec) were not available in PSAT, similar engines were used and scaled to match the specifications. The gasoline engine was a GM V8 LM7 5.3L with overhead valves, two valves per cylinder, and sequential fuel injection, which offers specifications very similar to the Vortec engine in terms of rated maximum torque and power [4]. The closest available diesel engine was a Cummins ISB 6.7L calibrated for vehicle test procedure purposes (i.e., suitable for a class 2b pickup truck, contrary to other heavier duty versions of the ISB engine calibrated for emissions). The engine power and torque curves of the Duramax can be found in the literature [4]. After scaling, the engine had a peak torque of 940 Nm at 1600 rpm and a peak power of 272 kw at 2950 rpm. No cylinder deactivation technology was considered for this study. IMPACT OF INDIVIDUAL TECHNOLOGIES ON FUEL CONSUMPTION In the following sections, the impact of several technologies on fuel consumption is assessed. For each of them, only one parameter at a time will be changed. We particularly focus on aerodynamics, the type of fuel used, vehicle weight, engine efficiency, tire losses, the number of gear ratios, and hybridization. Since the technology improvements could have different impacts on fuel consumption when they are applied to a conventional vehicle or a hybrid vehicle, we show the fuel consumption reductions in comparison to both baselines. (For a description of the hybrid vehicle used in the simulation, refer to the hybridization section.) In bold in the tables are the assumptions that were selected to be used for package comparison in the next section. The results were generated based on the combined drive cycle composed of the Urban Dynamometer Driving Schedule (UDDS) and the (Highway Fuel Economy Test) (HWFET) cycles (see Figure A.1 in the Appendix for drive cycle descriptions) [5]. 2

3 AERODYNAMICS Table 2 shows the fuel consumption savings that can be expected when reducing the drag coefficient of the pickup truck. The values considered ranged from 0.44 (for the default vehicle) to 0.33 for the most advanced improvement. These improvements are very aggressive and would require significant technology development in the long term. If the drag coefficient is reduced from 0.44 to 0.34 (-22%), we could expect up to 3% fuel savings for the conventional and almost 4% for the hybrid. This is consistent with light-duty fuel consumption reduction estimates, which predict a 1.5% fuel savings for each 10% reduction in drag coefficient. However, these figures highly depend on the drive cycle considered. On low speed cycles (e.g., lower than 40 mph), the fuel savings could be very limited, whereas promising fuel consumption reductions could be expected at high vehicle speeds. Indeed, when lowering the drag coefficient from 0.44 to 0.34 for the conventional vehicle, the fuel savings on the UDDS cycle (low average speed) is 1.4%, whereas it is 6% on the HWFET cycle. Table 2. Drag Coefficient Reduction Fuel Savings on the Combined Drive Cycle Drag Coefficient Saved for Conventional (%) Saved for Hybrid (%) DIESELIZATION The gasoline and diesel versions of the GMC Sierra 2500 HD are described in Table 1. Four different drive cycles were used to offer various aggressiveness conditions. Due to the similarities with light-duty pickup trucks, the UDDS, HWFET, LA92, and US06 cycles were selected. The LA92 is mainly an urban drive cycle with occasional higher vehicle speeds than the UDDS, more aggressive accelerations, and fewer stops. The US06 is a very aggressive cycle with long highway driving conditions at maximum vehicle speeds of 80 mph (see Appendix and reference [5]). All vehicles were simulated at gross vehicle weight. Table 3 shows the fuel consumption results with values in gallons/100 miles and unadjusted. For the diesel vehicles, the first value is the volumetric fuel consumption (gallons of diesel consumed per 100 miles), and the second value in parenthesis is the gasoline fuel consumption equivalent. Table 3. Fuel Consumption Comparison of Gasoline and Diesel Pickup Trucks on Various Drive Cycles UDDS HWFET LA92 US06 Gasoline Diesel 7.0 (7.8) 4.0 (4.4) 7.2 (8.0) 6.4 (7.1) Percent of 18.3% (9.1%) 25.3% (16.9%) 20.4% (11.5%) 27.7% (19.5%) Fuel Saved The amount of fuel saved by a diesel pickup truck compared with its gasoline counterpart ranges from 18.3% to 27.7% when comparing volumetric fuel consumptions (9.1% to 19.5% when in gasoline equivalent). The trend shown by the different cycles is that the more aggressive/faster the driving pattern, the greater the advantage of diesel. These results are lower than the common diesel/gasoline comparison for light-duty vehicles (commonly used value of 30% for volumetric fuel consumption). Since class 2b trucks were simulated at GVWR, and these vehicles were designed to be able to tow an additional 5900-kg trailer, both engines did not operate at full load. Both engines were operated at low loads (typically lower than 200 Nm), where their efficiency maps are similar. These are the typical conditions observed on the UDDS cycle, which explains why the fuel 3

4 consumption advantage of the diesel is only around 19%. On the other hand, when the engine is operated at higher loads (more than 200 Nm), the diesel engine operates at higher efficiencies than the gasoline engine. As a consequence, the US06 cycle shows a greater fuel consumption advantage for the diesel engine (around 28%). TIRE ROLLING RESISTANCE Several rolling resistance improvements were considered (Table 4). A 10% reduction in rolling resistance could be achieved in the future [6]. With such tire improvements, the fuel savings would range from 1% to 2%, with higher reductions for the hybrid. These simulation results are consistent with the figures presented in [6]. Table 4. Rolling Resistance Reduction Fuel Savings Saved for Conventional (%) Saved for Hybrid (%) Rolling Resistance TRANSMISSION Although 6-speed automatic transmissions are now widely available on most class 2b pickup truck models, some of them still offer optional 4-speed automatic transmissions. We will thus evaluate the fuel consumption reduction from 4- to 6-speed gearbox. Due to the recent introduction of an 8-speed gearbox in passenger cars (e.g., Lexus LS), it is also reasonable to expect them to be available for pickup trucks in the future. Table 5 shows the fuel savings associated with the use of advanced transmissions for a gasoline truck. If the baseline is a 6-speed automatic, only 1.7% of savings could be expected by switching to an 8-speed transmission. However, if the reference vehicle is equipped with a 4-speed automatic gearbox, then the amount of fuel saved could reach 6.2%. Table 5. Advanced Transmissions Fuel Savings Saved if reference is 6-Speed (Conventional) Saved if reference is 4-Speed (Conventional) Saved if reference is 6-Speed (Hybrid) Gearbox 6-Speed Automatic Speed Automatic ENGINE EFFICIENCY The modeling assumptions for the engine technology focused on the improvement of the peak efficiency through linear scaling of the entire engine fuel map. The baseline reference gasoline vehicle had an engine peak efficiency of 34.7%. Based on the engine improvement technology presented in [6] for class 2b engine technologies (in particular, the use of variable valve timing, variable valve lift, and direct injection), a 38% peak efficiency was considered in simulation. As a result, roughly 9% of fuel savings can be expected for both conventional and hybrid vehicles. For all the package simulations where engine improvement was applied, a 38% peak efficiency value was used. 4

5 Table 6. Increased Engine Peak Efficiency Fuel Savings Engine Peak Efficiency (%) Saved for Conventional (%) Saved for Hybrid (%) VEHICLE WEIGHT For light-duty vehicles, weight reduction usually involves engine downsizing. For class 2b pickup trucks, because of payload, towing, and grade performances, it is very likely that the engine power would remain the same, thereby resulting in either carrying more load or performing better in acceleration and grade. As a result, we considered a light reduction in vehicle curb weight without changing the payload weight. In other words, the baseline was the same truck described previously, simulated at 4172 kg, 4072 kg, 4036 kg and 3972 kg. Table 7. Reduced Vehicle Weight Fuel Savings Weight Reduction (kg) Saved for Conventional (%) Saved for Hybrid (%) Overall, a 100 kg weight change leads to 1% fuel saved. For the package simulation, we will use a weight reduction of 136 kg. HYBRIDIZATION The hybridized class 2b truck was a pre-transmission parallel hybrid electric vehicle (HEV) equipped with a 50- kw/100-kw electric machine. The features offered by such technologies are engine start/stop operations, regenerative braking, electric launch at low vehicle speeds, and a blend of engine power and electric machine power, depending on the battery state-of-charge. The engine was not downsized to ensure the same grade and towing capabilities as the conventional vehicle. Table 8. Hybridization Fuel Savings Saved Compared with Conventional Rolling Resistance (%) Parallel HEV 50-kW motor Parallel HEV 100-kW motor Drive cycles such as UDDS or HWFET do not require an electric machine with a power greater than 50 kw to capture most of the braking energy. Indeed, as shown in Table 8, using a 100-kW electric motor would bring very limited improvements compared with the 50-kW electric motor, and it would be more expensive. Hybridization leads to a 15% fuel consumption reduction, as shown in Table 8. IMPACT OF COMBINED TECHNOLOGIES ON FUEL CONSUMPTION After looking at the fuel savings of single technology improvements, we will now identify packages that include two or more combinations of advanced technologies. For each scenario, we will add the individual 5

6 percentages given in the previous part (the line in bold in each previous table) and compare that with the fuel savings of the simulated package. Both the conventional and the hybrid baselines will be used for the comparison, since the technology improvements show different results when they are applied to one or the other. In most cases, the sum of the fuel consumption benefits of individual technologies is greater than their combination. It is also interesting to notice that, in most cases, the fuel consumption reduction is higher for the hybrid than for the conventional vehicle when the same technologies are applied. This is due to the fact that reducing vehicle losses (i.e., rolling resistance, aerodynamics, etc.) not only lowers the component energy losses, but also lowers the energy provided by the motor. Thus, the battery will require less charging from the engine. Figure 1 shows an example of the improvement package. In this case, a change in the drag coefficient (reduced from 0.44 to 0.34) and weight (136 kg lighter) is applied to both the conventional and hybrid baseline vehicles (with gasoline engines). The sum of individual technologies either equals the combination (for the conventional) or is greater than the combination (for the hybrid). This figure shows that by combining a lighter weight with improved aerodynamics, fuel consumption could drop by 4 to 5%. Figure 1. Impact of Drag Coefficient and Weight Reduction on Fuel Consumption Figure 2 considers an additional improvement with the reduction of rolling resistance (lowered by 10%, from to ). For reasons similar to those previously described, the hybrid fuel savings are greater than for the conventional when considering individual technologies or packages. The combination of a light-weighted vehicle with low rolling resistance and improved aerodynamics could save between 5.5% and 6.6%. Figure 2. Impact of Drag Coefficient, Rolling Resistance, and Weight Reduction on Fuel Consumption 6

7 Figure 3 adds the impact of an improved transmission to the previous figure. The percentage shown is for an 8- speed transmission in comparison to a 6-speed baseline vehicle. Since the reference vehicle already has an efficient transmission, this package does not reduce fuel consumption by a significant amount. Overall, a 7% fuel consumption reduction could be reached by the conventional vehicle and 8% by the hybrid. Figure 3. Impact of Advanced Transmission, Drag Coefficient, Rolling Resistance, and Weight Reduction on Fuel Consumption As shown in Figure 4, adding the improved engine efficiency (peak efficiency raised from 34.7 to 38%) to the combination package increases the fuel savings dramatically. This package can provide up to 15% fuel savings for the conventional and 16% for the hybrid. Figure 4. Impact of All Combined Technologies on Fuel Consumption Finally, Figure 5 shows the impact of the full improvement package. In this case, we take the conventional vehicle equipped with all the improved technologies shown in Figure 4 and add the hybridization. This combination predicts a 28.4% fuel consumption reduction compared with the reference gasoline conventional truck. 7

8 35.0% 30.0% Percent Fuel Saved 25.0% 20.0% 15.0% 10.0% 5.0% 0.0% 14.8% 8.6% 1.7% 1.2% 3.0% 1.4% Each technology 28.4% Combination Hybrid CS Engine Transmission RR Cd Weight Hybrid improvements Baseline improvements Figure 5: Impact of All Combined Technologies and Hybridization on Fuel Consumption CONCLUSIONS A vehicle model was developed for a class 2b pickup truck, based on the GMC Sierra 2500 HD. Numerous simulations were run to estimate the amount of fuel that could be saved by using advanced technologies for each component. Apart from the aggressive aerodynamic improvement, most assumptions could be obtained in the near future. Among them, hybridization or engine efficiency improvement seemed to provide the greatest fuel reduction. It was found that when the same technology improvement is applied to conventional and hybrid baselines, the hybrid vehicle shows greater fuel savings, mainly due to the reduced amount of charging required by the battery. Several advanced technology combinations were simulated to compare the fuel savings of the packages versus the sum of the individual improvements. In each case, the individual sum of the fuel savings was greater than the package improvement, since the effect of some advanced technologies was attenuated when combined with others. For class 2b pickup trucks, nearly 30% of fuel could be saved by combining aerodynamics, wheel and engine loss reductions, light weighing, a large number of transmission gears, and hybridization. This number, however, is dependent on the duty cycle and would vary if UDDS and HWFET were replaced by different driving conditions. Finally, the impact of package improvements was considered for only the gasoline truck, since class 2b vehicles are sold mainly with that fuel. However, similar work could be done on a diesel vehicle or with other types of fuel (e.g., ethanol and hydrogen). REFERENCES 1. Argonne National Laboratory, Powertrain Systems Analysis Toolkit (PSAT), available at: Rousseau, A., Kwon, J., Sharer, P., Pagerit, S., and Duoba, M., "Integrating Data, Performing Quality Assurance and Validating the Vehicle Model for the 2004 Prius Using PSAT," SAE paper , SAE World Congress, Detroit, MI, GMC Sierra 2500 HD, available at: GM Medium-Duty Engine Specifications, available at: mediumduty/techspecs/engine.pdf, U.S. Environmental Protection Agency, available at: The National Academies, "Technologies and Approaches to Reducing the Fuel Consumption of Medium and Heavy Duty Vehicles," March

9 CONTACT INFORMATION Antoine Delorme, Tel: Ram Vijayagopal, Tel : Dominik A. Karbowski, dkarbowski@anl.gov, Tel: Aymeric Rousseau, arousseau@anl.gov, Tel: Vehicle Modeling and Simulation Team, Center for Transportation Research, Argonne National Laboratory, ACRONYMS AND ABBREVIATIONS Cd drag coefficient Conv conventional DOE U.S. Department of Energy GVWR gross vehicle weight rating HEV hybrid electric vehicle HWFET Highway Fuel Economy Test PSAT Powertrain Systems Analysis Toolkit RR rolling resistance UDDS Urban Dynamometer Driving Schedule ACKNOWLEDGMENTS This study was funded by the National Academy of Science, as part of their project named "Assessment of Fuel Economy Technologies for Medium and Heavy Duty Vehicles", and identified BEES-J-08-A. The development of PSAT and its models was supported by DOE s Vehicle Technology Office under the direction of Lee Slezak and David Anderson. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory ( Argonne ). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. 9

10 APPENDIX Figure A.1. Drive Cycles Used in Simulation* UDDS Cycle HWFET Cycle US06 Cycle * Source [5]. LA92 Cycle 10