Development and Vehicle Demonstration of a Systems-Level Approach to Fuel Economy Improvement Technologies
2013-01-0280 Published 04/08/2013
Keith A. Confer and John Kirwan Delphi Powertrain
Nayan Engineer Hyundai-Kia America Technical Center Inc Copyright ÂŠ 2013 SAE International doi:10.4271/2013-01-0280
required efficiency improvements while also meeting customer expectations for performance, comfort and safety.
Global fuel economy and CO2 reduction mandates are driving the need for a substantial increase in vehicle fuel efficiency over the next several years, with improvements coming from many sources. This paper describes a vehicle demonstration program to improve fuel economy by implementing a systems approach to reduce friction and parasitic losses. The work concentrated on nearer term technologies that can be quickly transferred to production vehicle programs. Major technologies demonstrated on the vehicle included gasoline direct injection (GDi) with cooled EGR, advanced valvetrain, rollerization of both crankshaft and camshaft, and stop-start engine operation. The work described in this paper comprises Phase I of a two phase program funded by DOE contract DE-EE0003258. It includes a hardware overview and a description of the system development activities. Results focus on vehicle fuel economy benefits compared to a production baseline vehicle.
INTRODUCTION Regulatory trends are placing increased pressure on fuel consumption throughout the world. As an example, Figure 1 shows the rollout of fuel consumption targets for the U.S., Europe, and China over the next several years. Dramatic fuel consumption / CO2 reductions are necessary, both near-term and long-term, in order to meet legislative requirements. This is driving the need for substantial innovation to develop and broadly deploy technologies on vehicles that provide the
Figure 1. Rollout of light duty fuel consumption / CO2 targets. A range of options are available to reduce vehicle fuel consumption ( offers one recent assessment of a number of available options). At the vehicle level, reduction in vehicle mass, aerodynamic drag, and tire rolling resistance decrease the net energy required for propulsion. Other technologies concentrate on increasing the overall efficiency of the fuel consumed to provide the useful work required to propel the vehicle. These include engine and transmission technologies to improve the efficiency of the combustion process, as well as methods to reduce parasitic losses due to friction, pumping losses, engine accessory loads, and engine idling. Vehicle electrification is one means to significantly reduce vehicle fuel consumption. However, electrification currently carries a
marked increase in vehicle cost. While the fraction of electrified vehicles, especially hybrid electric vehicles (HEVs), is expected to grow in the coming years, internal combustion engines are projected to continue being a dominant propulsion source for a number of years into the future (see Figure 2). Thus, technologies focused on improving the efficiency of these engines have broad applicability.
FUEL ECONOMY IMPROVEMENT TECHNOLOGIES Table 1 indicates the fuel economy technologies evaluated in this study. The focus of our work was near-term application rather than longer term development. Thus, all technologies evaluated are readily available. As shown in the table, the technologies covered a variety of mechanisms to improve vehicle fuel economy, from improving combustion thermodynamics to reducing parasitic losses due to engine friction, accessory loads, pumping and engine idling. Table 1. Fuel economy technologies evaluated
Figure 2. Projected global market penetration for major powertrain architectures. Forecast developed using market share data from IHS. Delphi Automotive Systems, LLC and Hyundai America Technical Center Incorporated (HATCI) are currently working together in a program partially funded by the U.S. Department of Energy to develop and demonstrate gasoline engine technologies offering substantial fuel economy improvements. A significant gain in fuel economy is expected to come from development and demonstration of a low temperature combustion scheme called gasoline direct compression ignition (GDCI) under development in Phase II of the program. GDCI combustion is showing excellent results to date. Progress in developing GDCI is documented in a number of publications [2,3,4,5], and is not considered further in this paper. The subject of the present paper is to document the Phase I study for this program, which has recently been completed. Phase I focused on the fuel economy impact of nearer term technologies that could be quickly deployed on production vehicles. Two development vehicles were ultimately used to evaluate technology combinations providing more efficient combustion and reduced parasitic losses. The sections that follow offer a description of the technologies implemented on the two vehicles and their observed benefits to vehicle-level fuel economy.
The two development vehicles used in this study were both derived from a production 2011 Hyundai Sonata with a 2.4L Theta II engine and a six speed manual transmission. Both vehicles implemented gasoline direct injection (GDi). Figure 3 shows the major components for GDi fuel systems comprising inwardly-opening, multi-hole GDi injectors, a fuel rail and an engine-driven high pressure fuel pump. GDi is a well-established technology to enable improved vehicle fuel economy, and its worldwide market share is projected to markedly increase as indicated in Figure 2. In many cases, GDi is combined with turbocharging and engine downsizing to improve vehicle fuel economy (see, for example,  for one analysis of the benefits of GDi in downsized, turbocharged engines). Neither vehicle in the present study was turbocharged. However GDi is also a fuel economy enabler in naturally aspirated engines as described, for example, in [7,8,9]. GDi benefits in naturally aspirated engines result largely because it allows increased compression ratio, and because it complements other technologies such as variable valve actuation
production calibration. Engine friction reduction was addressed on Vehicle 1 through piston group friction reduction, camshaft and crankshaft bearing rollerization, exhaust heat recovery, and engine downspeeding. Accessory load reduction was accomplished with a two-step variable displacement oil pump.
Figure 3. Gasoline Direct Injection (GDi) system hardware Both vehicles also applied cooled exhaust gas recirculation (CEGR). The CEGR system consisted of an exhaust gas recirculation valve, an air to liquid heat exchanger and low volume plumbing to route the exhaust gas (see Figure 4). The EGR charge was introduced ahead of the intake throttle body in such a way as to promote even distribution for exhaust gas to each cylinder. Cooled EGR provides charge dilution and lower in-cylinder heat transfer for reduced pumping losses and improved combustion thermodynamics.
Piston group friction reduction comprised reduced spring load and low friction coatings (see Figure 5). Previous work has shown that the oil control ring has a substantial impact on the friction of the total ring pack . Oil ring friction was reduced in the present study by reducing the radial spring load applied by the oil control ring onto the cylinder bore. The ability to reduce the oil ring spring load was partially enabled by engine downspeeding (described in a following section). Optimization work was done to ensure that the oil ring maintained sufficient radial force to cope with bore distortion and thermal expansion such that oil consumption was not adversely affected over the engine's operating range.
Figure 5. Piston Group hardware
Figure 4. Cooled Exhaust Gas Recirculation (CEGR) system. The remaining technologies were split between the two demonstration vehicles. Broadly, Vehicle 1 focused on technologies to reduce engine friction and accessory loads. Vehicle 2 focused on pumping losses and engine idling reduction. A brief description of the technologies on each vehicle is offered below.
Vehicle 1 Vehicle 1 implemented technologies that did not require any major changes to the engine control module or engine control algorithms and only minor changes to the vehicle's
Further piston group friction reduction derived from coatings to the top compression ring and the piston skirt. Chromium Nitride (CrN) coatings were applied to the compression ring using physical vapor deposition (PVD). These coatings have been reported to have the lowest friction coefficients among currently-available wear resistant ring coatings . Additionally, solid lubricant Molybdenum Disuphide (MoS2) coated piston skirts were used. Previous studies suggest that the piston itself contributes up to 30% of the total piston group friction  and that MoS2 offers similar friction reduction as provided by forced lubrication at the piston skirt thrust face to bore interface . Additional engine friction reduction efforts in this study focused on the crankshaft and camshaft. Some reduction was accomplished by implementing a roller-type timing chain with reduced tension. More significantly, stock hydrodynamic support bearings were replaced with needle roller bearings (NRBs) as shown in Figure 6. The split cage NRBs were used with purpose built crankshaft and connecting rod assemblies. Similarly, the camshaft support bearings were modified to NRBs for the direct acting valvetrain. Here each camshaft had 4 bearings converted to NRBs. Only the front #1 location near the cam phaser support
retained a journal bearing. This was required to help maintain pressurized lube supply to the hydraulic cam phasers located near this bearing.
where cooling demands are greatest. Preferential cooling further optimizes oil viscosity to reduce engine friction .
Figure 6. Roller bearing application to camshaft and crankshaft In addition to offering lower bearing friction, the NRBs also enabled lower engine oil flow rates to the main bearings, connecting rod big ends and camshaft bearing locations. Results from previous studies indicate the potential for significant potential benefits due to both bearing friction reduction and reduced oil pump demand . Reduced oil flow to meet the lower demand in the present study was accomplished by using a 2-step variable displacement oil pump. A schematic for this newly-developed pump is shown in Figure 7. Active oil pressure control via an electric solenoid switched the pump between low and high mode based on engine speed and load.
Figure 7. Two Step variable displacement oil pump Exhaust heat recovery was another technology employed for friction reduction. The exhaust heat recovery system (EHRS) employed a heat exchanger in the exhaust downstream of the catalytic converter to provide captured waste exhaust heat to the engine lubricating oil. Additional heating of the lubricating oil reduces its viscosity thereby helping to reduce engine internal friction. Lubricating oil viscosity reduction is particularly effective during cold start-up when engine internal friction is high. Figure 8 shows a schematic diagram of the switchable EHRS used to manage heat flow to the lubricating oil circuit. As a complement to the EHRS, water jacket inserts were used in the coolant flow passages to preferentially channel coolant to the top of the cylinder bore
Figure 8. Exhaust Heat Recovery System (EHRS) Engine downspeeding is the final major fuel economy improvement technology applied to Vehicle 1. Downspeeding was achieved by modifying the transmission ratio. Compared to the baseline engine, rated speed was reduced from 6300 RPM down to 5000 RPM. Downspeeding provided reduced friction both directly via reduced engine speed, as well as by enabling reduced oil ring spring load as described earlier. By increasing engine operating load at a given power requirement, engine downspeeding also reduced pumping losses and improved engine thermodynamics afforded at higher engine loads. Engine downspeeding is often accompanied by engine boosting; however the engine remained naturally aspirated in this evaluation study. To augment the down speeded engine's performance, a taller final gear ratio set was added to give a 6 % lower N/V in 1st, 2nd and 4th gear and 9% lower N/V in 3rd, 5th and 6th gears compared with the baseline ratios. Reducing the rated speed of this engine resulted in a reduction in engine power so that its final value was approximately the same as the PFI baseline vehicle. Consequently the fuel economy difference due to downspeeding reflects an improvement for an engine with similar performance.
Vehicle 2 Fuel economy technologies on Vehicle 2 did require changes to the engine control system. For this vehicle, a new engine control module was selected for the application and project specific algorithms were developed where required. Full powertrain calibration was completed using an engine dynamometer engine and the demonstration vehicle. Vehicle 2 addressed pumping losses via 2-step variable valve lift (see Figure 9) and electric cam phaser (see Figure 10). A
variable valve lift system with cam phasing allowed separate consideration of differing engine operating conditions. Higher loads utilized high valve lift with phasing optimized for good engine torque. Lower loads implemented reduced valve lift and duration to reduce the air mass delivered to the cylinder at a given manifold pressure for reduced pumping losses. The low load scheme implemented in this engine also applied asymmetric lift between the two intake valves for increased charge motion to help combustion. Compared to a hydraulic cam phaser, an electric phaser (ePhaser) offers increased phasing rate and higher precision. An ePhaser also offers the ability to change cam phasing at zero engine speed and at high and low temperature extremes where oil viscosity effects make phasing difficult with hydraulic phasers.
maximum torque capability of 40 N-m. Fourteen volt systems are typically designed for a maximum engine displacement of 1.6L; however this unit was recently applied successfully to a 2.4L engine . It was selected in the present study because it offered an easily packaged mechanization of a BAS system for evaluation purposes. The BAS system was used for restart purposes only and did not add tractive force to the vehicle.
Figure 11. Belt Alternator Starter(BAS) system
Figure 9. 2-step variable valve lift mechanism
EXPERIMENTAL RESULTS / DISCUSSION Vehicle fuel economy and emissions tests were performed at HATCI's emissions site in Superior Township, Michigan. The EPA Federal Test Procedure (FTP) and Highway Fuel Economy Test (HWFET) drive cycles were used for all of the fuel economy testing and combined, unadjusted values were calculated from the test cycle results. All test methods, fuels, vehicle settings and procedures were consistent for Vehicle 1, Vehicle 2 and a baseline PFI vehicle. Both Vehicle 1 and Vehicle 2 met the same tailpipe emissions standards as the original production vehicle from which they were derived.
Figure 10. Electric Cam Phasers (ePhasers) as used on both intake and exhaust cams Stop-start addressed engine idling losses in Vehicle 2 by shutting the engine off when the vehicle was stopped, and then restarting the engine immediately before vehicle driveaway. Stop-start was mechanized in two ways during the study. The control system developed using rapid prototyping was capable of controlling either mechanization. The first mechanization simply comprised using the existing 12V geared starter motor to restart the vehicle. A second mechanization consisted of adding a 14V belt-alternatorstarter (BAS) system mounted to the front accessory drive, as shown schematically in Figure 11. This 14V BAS unit had a
Table 2 below highlights key differences between the 2009 baseline PFI vehicle and the 2011 GDi Sonata vehicles. Higher compression ratio afforded by gasoline direct injection, electric driven power steering, improved Cd with lower friction in the driveline and reduced curb weight are some inherent advantages the 2011 Sonata has over the baseline vehicle prior to deploying fuel economy improvement technologies described above.
Table 2. Vehicle comparison
Figure 13. Fuel Economy results for Vehicle 2 Both vehicles showed broad benefits from the technologies. In both cases the combined fuel economy benefit was in excess of 13% compared to the PFI vehicle. Improvements were similar for the highway and city test cycles. Results for Vehicle 1 were biased slightly in favor on the highway cycle while Vehicle 2 was slightly better over the city cycle.
The overall fuel economy test results for Vehicle 1 are shown in Figure 12. Fuel economy improvements of 13.9% were achieved for the highway drive cycle compared to the PFI baseline vehicle. City fuel economy was improved by 12.6% and a combined unadjusted fuel economy improvement of 13.1% was realized for the vehicle.
Figure 12. Fuel Economy test results for Vehicle 1 The overall fuel economy results for Vehicle 2 are shown in Figure 13. A fuel economy improvement of 13.4% was achieved for city drive cycle compared to the PFI baseline vehicle. Highway fuel economy was improved by 12.8% and the combined unadjusted fuel economy improvement was 13.4%.
Figures 14 and 15 give an estimated breakdown by technology for the overall Vehicle 1 and Vehicle 2 test results respectively. For a given vehicle, exact partitioning by technology of the total fuel economy improvement is not possible due to system level interactions between the individual technologies. For example GDi was implemented with several other changes between vehicle model years therefore the fuel economy improvement due to GDi is combined with the other vehicle level changes. The estimated contributions represented in these figures have been determined through measurements of incremental fuel economy improvements as the technologies were sequentially added. GDi plus vehicle model year upgrades were common to both vehicles. However their fuel economy impact varied between Vehicle 1 and Vehicle 2. As described earlier, EMS changes were required on Vehicle 2 to implement the additional technologies. The increased fuel economy, while not expected, could be due to these changes in the engine control system. Considering Vehicle 1, base engine technologies downspeeding and engine friction reduction - were applied simultaneously to the engine so that determination of their individual contributions is not possible. These base engine technologies combined, however, accounted for about one third of the total fuel economy improvement.
Figure 14. Estimated Fuel Economy contributions by technology for Vehicle 1 Reduction of the accessory load through the use of a two-step oil pump also contributed significantly to the vehicle level improvements. The production oil pump was constrained to over lubricating the engine in most regions in order to meet the engine's oil demand at the high engine speed upper boundary. The two-step oil pump provided lower oil pressures and flow rates under lower engine loads to reduce parasitic losses. Optimization of the two step oil pump allowed a balance between minimizing parasitic oil pumping work while supplying enough oil to minimize friction at interfaces. While crank-train rollerization did result in FE improvements testing within the test cycles, further testing revealed that the benefit did not extend into higher RPM areas that are within normal driving ranges. Additionally, durability concerns for crank-train rollerization were found with the system as mechanized for this project. The rollerized camshaft system did not exhibit bearing problems during this study however engine durability testing was outside the scope of this project. The final two technologies applied to Vehicle 1 were CEGR and EHRS. Cooled EGR applied to Vehicle 1 was found to give significant NOx reductions. This enabled slightly leaner transient engine calibration resulting in fuel economy gains while still meeting emissions targets. The exhaust heat recovery system used to pre-heat the engine oil yielded little fuel economy benefits as mechanized in this study. Estimated fuel economy contributions for each of the fuel economy systems incorporated in Vehicle 2 are shown in Figure 15. The advanced valvetrain technologies (2-step valve lift and ePhasers) yielded the highest fuel economy improvement gain for Vehicle 2. During the vehicle calibration, the low lift valve operation range was maximized to take advantage of the reduced engine pumping work and improved fuel consumption.
Figure 15. Estimated Fuel Economy contributions by technology for Vehicle 2 Considering Vehicle 2, cooled EGR again resulted in notable combined fuel economy benefits. For this vehicle, cooled EGR was verified to have the positive effect of reducing knock due to the relatively low temperature of the inert cooled EGR gas when compared to internal EGR delivered via cam phasing. This knock reduction allowed use of the low lift cam to be extended to higher loads that were previously knock-limited without cooled EGR. Recent work in the literature implementing cooled EGR has generally focused on turbocharged GDi engines [17-18]. Results from the present study indicate its utility for naturally aspirated engines as well. The Stop/Start system contributed significantly during the city drive cycle but was not a factor in the highway portion of the drive schedule since there are no idle conditions during that test. As discussed earlier both a 14V BAS system and the starter motor were mechanized for the stop-start function. Testing generally indicated better NVH performance for the BAS system, with no determinable difference in fuel economy performance between the two mechanizations. However, addition of the 2-step variable valve lift system to the vehicle resulted in an increase in motoring cylinder pressure such that the 14V BAS system did not have sufficient torque capacity for reliable re-start. Consequently, all Vehicle 2 results shown in this paper were obtained using the geared starter motor for stop-start. It is interesting to discuss results from the present study in comparison with a recent report from the National Academy of Sciences (NAS)  that assessed fuel economy technologies for light duty vehicles. Technologies comprising GDi and vehicle model year upgrades in the present study would expect to yield approximately 9% fuel economy benefit based on summation of the average values tabulated in the NAS report. This simple summation offers a rough value for comparison with results from the present work. For Vehicle 1, GDi and vehicle model year upgrades yielded about a 2% improvement in fuel economy compared to the PFI baseline vehicle. The larger 4% improvement offered by GDi and model year upgrades for Vehicle 2 was also noticeably lower than the 9% rough target from the NAS report. Fuel economy benefits will vary vehicle-by-vehicle,
and interactions between technologies affect their impact on fuel economy.
example, [19-20]. The present work estimates about a 5.5% benefit for 2-step combined with eVCP.
Additionally the model year upgrades included increased engine power and torque (see Table 2). These factors likely contribute to the reduced fuel economy benefits in this study compared to the rough target from the NAS report.
SUMMARY AND CONCLUSIONS
Engine downspeeding and friction reduction combined to offer approximately 4.5% fuel economy benefit in the present study. The NAS report attributed an average 1.3% to friction reduction, but did not directly consider downspeeding. However, the report did attribute significant fuel economy benefits to increasing the number of transmission speeds. For example, an average 4% fuel economy improvement is estimated for a 6-speed transmission compared to a 4-speed transmission. Engine downspeeding is one factor that results in improved fuel economy for vehicles with a greater number of transmission speeds. Using the NAS report as a guideline, the measured fuel economy benefits for downspeeding and friction reduction are within a reasonably expected range for this technology combination. Rollerization was not considered in the NAS report. However, the benefits of this additional friction reduction technology in the present study are estimated to be roughly the same as the total average benefit for friction reduction in the NAS report. Nevertheless, based on durability concerns and increased friction at rollerization does not appear attractive at this time for a production vehicle application. Neither cooled EGR nor exhaust heat recovery were considered in the NAS report, so a direct comparison with results for these technologies in the present study is not possible. However, the EHRS was implemented to reduce friction by heating the engine oil to provide more favorable viscosity. The NAS report did consider low friction lubricants, whose estimated fuel economy benefits were only 0.5%. In this light, the marginal benefit attributed to exhaust heat recovery in the present study is not surprising. The last technology groups for comparison with the NAS report are stop-start, the two-step oil pump, and advanced valvetrain (2-step valve lift with ePhaser). Stop-start according to the NAS report (described therein as 12V BAS Micro-Hybrid) offers an estimated 3% fuel economy benefit, while in the present study stop-start contributed an estimated 2% benefit. The two-step oil pump comprises an improved accessory. The NAS report attributes an average 1% fuel economy benefit collectively to improved accessories. This is significantly lower than the approximate 3% benefit estimated for the 2-step oil pump in the present study. Finally, the NAS report estimates a 2.3% benefit for discrete variable valve lift. However, other studies focused on 2-step VVL indicate fuel economy gains of roughly 5% (see, for
This study considered a number of readily available technologies for spark-ignition engines to improve vehicle fuel economy in the short term through improved thermodynamics, reduced friction, improved accessories and lower pumping losses. The technologies were evaluated using two demonstration vehicles. Fuel economy was determined over the EPA FTP and HWFET drive cycles These demonstration vehicles were derived from a production 2011 Hyundai Sonata. Each demonstration vehicle met US tailpipe emissions standards. Using a systems level approach, different technologies were evaluated on the two vehicles, although GDi and cooled EGR were common to both vehicles. Each of the two vehicles demonstrated in excess of 13% improvement in EPA combined fuel economy compared to a 2009 Hyundai Sonata PFI baseline vehicle. Minimal overlap between technologies implemented on the two vehicles suggests that further combination of favorable technologies evaluated in this study would likely offer further improvement on a single vehicle whose fuel economy improvement could approach 20% compared to the PFI baseline.
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CONTACT INFORMATION Keith Confer Engineering Manager Delphi Advanced Powertrain 3000 University Drive Auburn Hills, MI 48326 firstname.lastname@example.org John Kirwan Chief Scientist Delphi Powertrain 3000 University Drive Auburn Hills, MI 48326 email@example.com Nayan Engineer Manager Engine Design and Test Group Hyundai-Kia America Technical Center, Inc. 6800 Geddes Road Superior Township, MI 48198 firstname.lastname@example.org
ACKNOWLEDGMENTS The authors gratefully acknowledge work contributions from Harry Husted, Gregg Roth, Mike Lavan, Gary Fulks, Timothy Henshaw, Andrew Fedewa, Robert Hammond, Edward Joo, Donald Johnson, Jason Eiseman, Jeremy Kraenzlein, Jeff Cowan, Xiaojian Yang, Raymond Parker, and Paul Rau (from Delphi Automotive Systems, LLC), John Juriga, Sung Seo Park, Dong Suk Chae, Sangsik Kim, Paul Arlauskas, Mark Bourcier, Mark Shirley, Thomas Hollowell, Steven Stewart, Jeffrey Hollowell, Joel Cherry, Steven Rathbun and Eric Seaberg (from Hyundai-Kia American Technical Center, Inc.) Research sponsored by the U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy under contract DE-EE0003258
DEFINITIONS/ABBREVIATIONS BAS - Belt alternator starter CEGR - Cooled exhaust gas recirculation CO2 - Carbon Dioxide CrN - Chromium Nitride DOE - Department of Energy EGR - Exhaust gas recirculation EHRS - Exhaust heat recovery system EMS - Engine management system EPA - Environmental Protection Agency FE - Fuel economy FTP - Federal Test Procedure GDCI - Gasoline direct compression ignition GDi - Gasoline direct injection HATCI - Hyundai America Technical Center, Inc. HEV - Hybrid electric vehicle
DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
HWFET - Highway Fuel Economy Test L - Liter MoS2 - Molybdenum Disulphide NRB - Needle roller bearing N/V - Engine Speed [rmp]/Vehicle Speed [mph] NVH - Noise vibration and harshness PFI - Port fuel injection PVD - Physical vapor deposition rmp - Revolutions per minute
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