Recent Hybrid Electric Vehicle Trends and Technologies Eric Rask, Michael Duoba, Henning Lohse-Busch Argonne National Laboratory
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Abstract- In collaboration with the Department of Energy’s Advanced Vehicle Testing Activity, Argonne National Laboratory performs dynamometer testing and evaluation for a wide array of advanced vehicles. Using data obtained from this benchmarking, this paper discusses recently observed trends in hybrid electric vehicle technologies, discussing both overall vehicle trends as well as select component trends. This work discusses both “full” hybrids with a significant amount of electric (EV) or engine-off operation and more “mild” hybrids with a lesser amount of electric operating capability. This paper seeks to summarize some of the high-level operation trends that have been appearing in recent hybrid electric vehicles.
I.
INTRODUCTION
The vehicle dynamometer testing and analysis for this paper focuses on operation over select standard regulatory cycles and seeks to obtain a general overview of how the vehicle performs. The test cycles discussed in this work are the US FTP cycle (UDDS), the Highway (Hwy) Fuel Economy Test cycle, and the US06 cycle, a more aggressive supplemental regulatory cycle. Although only select parameters will be discussed in this report, data collection for this testing includes emissions and fuel measurements from an exhaust emissions bench, high-voltage and accessory current/voltage from a DC power analyzer, and decoded CAN bus data such as engine speed, engine load, and electric machine operation. Although many hybrid vehicles exist in the current market, the focus of this work is to contrast Model Year (MY) 2010 hybrid vehicles to a previous baseline hybrid vehicle. The primary evaluation vehicles for the “full” hybrid evaluation portion of this paper will be the MY 2010 Toyota Prius and the MY 2010 Ford Fusion. The baseline vehicle for comparison is the 2006 Toyota Camry Hybrid which represents a vehicle of roughly similar size and a previous generation of hybrid system. The “mild” hybrid vehicles discussed in this report are the MY 2010 Honda Insight and the MY 2010 Mercedes S400h. For certain comparisons related to the Insight, the MY 2006 Honda Civic will be used. Fig. 1 shows the evaluation vehicles in the laboratory. More information and test data pertaining to these vehicles can be found in [1,2,3,4,5].
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Fig. 1. Vehicles used for evaluation of recent trends. Clockwise from upper left: 2010 Toyota Prius, 2010 Ford Fusion, 2010 Honda Insight, 2010 Mercedes S400h.
To broadly summarize, some of the noteworthy trends observed for full hybrid vehicles are: 1) More EV Operation: Relative to the baseline vehicle, the recent vehicles tested show additional EV operation at higher relative loads during low/moderate speed operation as well as during higher speed, but low load operation. 2) Reduced “cold-start” penalty for UDDS operation: Recent vehicles have shown a reduced penalty associated with vehicle warm-up. Despite this general trend, differences exist between the warm-up operating strategies for recently tested vehicles. 3) Reduced battery pack size through voltage boosting: Recently tested vehicles have continued to reduce battery pack size and voltage boosting has seen increasing adoption. Depending on the effective pack voltage and system design, different boosting strategies have been observed. 4) Refinements for improved real-world fuel economy: Both recent vehicles continue the trend of seeking to improve fuel consumption during real-world driving. Factors such as engine sizing, accessory electrification, driver feedback, and thermal management help enable improved fuel consumption. In contrast to the full hybrid trends discussed above, the predominant trend observed for recent MY 2010 mild hybrids is somewhat reduced component usage and capability. This reduced usage is generally due to other design refinements in the form of reduced system cost, packaging, and mass. More detailed discussion regarding some of these tradeoffs can be found in [5].
II. “FULL” HYBRID OPERATIN NG TRENDS A. More engine-off operation S, Hwy, and US06 During vehicle testing over the UDDS cycles, both recent vehicles exhibited increeased EV operation in terms of relative vehicle load as well as higher vehicle speeds. Fig. 2 illustrates the EV operationn of the respective vehicles in terms of vehicle speed and norm malized load. For this analysis, normalized load is deffined as the vehicle tractive force normalized by the maximum m torque observed over the US06 drive cycle for each speccific vehicle. Since each vehicle has a different road load and associated vehicle test weight, normalized vehicle load wass used so that the relative fraction of EV operation could be visually compared between vehicles on a single plot. Vehicle speed did not need to be normalized since the drive cycles are a defined using a speed versus time trace. From observing the different areas of EV V operation, several notable areas of additional EV operation caan be seen. Area A represents additional EV operation at higgher relative loads and fairly low vehicle speeds. This increasse in EV operation corresponds to more and deeper electrric vehicle launch operation. “Deeper” in this context referss to increasing the vehicle speed at which the engine must turnn on during vehicle launch. Fig. 3 shows an example vehicle launch that illustrates this deeper launch behavior. Enngine-on operation, as represented by nonzero engine speed, begins much sooner for the Camry as opposed to the two more recent r vehicles due to the higher relative load EV operationn. At even higher relative loads during launch, all of the vehhicles must activate the engine in order to meet the power demaanded. Area B in Fig. 2 represents the ability a to operate electrically at higher vehicle speeds (up to roughly 47 mph in the case of the Fusion). While both vehhicles display EV operation at elevated speeds relative to thhe baseline vehicle, this operation is fairly constrained in term ms of relative load and thus is only used during areas of veery low demanded
Figure 2: Comparison of observed EV operatioon relative to overall operation. A represents an increase in EV operation at higher relative loads and low speeds. B illustrates higher EV operating speeeds.
Figure 3: Engine operation duringg moderate vehicle launch. Due to increased EV operation at higher relative tractive loads, the MY2010 vehicles start the engine at higher vehiccle speeds.
tractive effort. Despite this issue, maximum EV operating speed has become an advertiising feature for new hybrid vehicles and is thus likely to conntinue to increase. While not shown in this analysis, a engine-off operation during vehicle deceleration incrreases marginally between the baseline vehicle and the twoo most recent vehicles. This increase is primarily related to t vehicle speed and can be observed as slightly more enngine-off operation (i.e. zero engine speed) during decelerattions between roughly 35 and 45 mph. This additional engine-off operation is a fairly marginal benefit for most drivve cycles since a hybrid can rotate the engine without requiiring fuel and thus only fairly small electrical losses are inncurred. During decelerations above roughly 45 mph, all vehicles appear to keep the engine spinning with fueling dependinng on the deceleration rate and vehicle speed. Although the change in the ovverall fraction of EV operation between the most recent hybbrid vehicles tested and the baseline vehicle is highly depeendent on the particular cycle driven, the relative change forr the UDDS, Hwy, and US06 cycles is worth discussing. Hott UDDS operation (i.e. vehicle is already warmed) shows a minimal m 2-3% increase in the fraction of total cycle time withh the engine-off relative to the baseline vehicle. This would bee expected, since the UDDS is mostly comprised of moderatee to low speeds/accelerations and is thus already mostly covered c by the existing EV operation. The fraction of Highway H engine-off operation increases significantly relativve to the baseline vehicle primarily due to higher sppeed engine-off deceleration capabilities, but remains low, on o the order of 8-12%. US06 operation shows an increase in EV operation of roughly 11% for both MY 2010 vehicles. Unnlike the previous two cycles, this improvement is due to botth the higher load and higher speed EV operating behavior exhibited by recent vehicles. These results are summarized inn Fig. 4.
Fig. 4. Summary of engine-off percentage of total cycle time for UDDS-hot, Hwy, and US06
B. Reduced UDDS “cold-start” penalty An additional trend observed for recent full hybrid vehicles is a reduction of the “cold-start” penalty observed between the first (cold-soaked vehicle) and second (warm vehicle) UDDS drive cycles. For full hybrid vehicles, the fuel consumption impact between the warm and cold UDDS cycles has traditionally been roughly 14% with a fairly significant change in operating behavior in the form of reduced engine-off operation. The operational and fueling differences between these two cycles are generally attributed to vehicle/engine warm-up and cold-start emissions mitigation. Recent vehicle testing has shown a reduced cold start penalty in the range of roughly 8-10%. This is due to warmup strategy improvements observed for the MY 2010 vehicles, which enable improved engine/vehicle warm-up while retaining acceptable emissions. Figure 5 shows the difference in fuel accumulated during cold versus warm operation during the first 350 seconds of vehicle operation for all three full hybrids in this discussion. During this time period, most of the “cold” fuel consumption penalty is accumulated due to increased fueling and reduced engine-off operation in order to activate the catalyst and thermally stabilize the vehicle. Assuming a consistent warm-up, a fairly linear increase in extra “cold” fueling would be expected
Fig. 5: Difference in accumulated fueling between UDDS cold and hot during initial vehicle startup. Both 2010 vehicles show a reduced difference in fueling between cold and warm operation. The trends between the Fusion and Prius are also significantly different.
during this time period (excluding engine-off at idle). This type of behavior is illustrated in Fig. 5 by the dashed orange line of the Camry. As discussed, the difference between cold and warm fueling increases fairly linearly across the entire 350 second range. In contrast, the Prius and Fusion show both a reduced accumulation of “cold” fueling as well as significant alterations in the trend of the difference between cold and warm accumulated fuel. Despite both vehicles attaining Super Ultra Low Emissions Vehicle (SULEV) status [6], the warm-up strategies employed during initial vehicle warm-up are very different. Figures 6 and 7 contrast the cold and warm fueling rates during initial operation for the Fusion and Prius respectively. Although fuel rate is shown in this analysis, it should be noted that both vehicles utilize their engine operational flexibility to operate at nearly constant speed during the initial 130 seconds of cold-start operation. Additionally, both vehicles show a dramatically smaller reduction in engine-off operation during the cold UDDS due to improved vehicle warm-up characteristics. During the first 200 seconds of initial vehicle warm-up, the Fusion can be observed to operate in a similar manner to the Camry: showing additional fueling and reduced engine off. Following the first 200 seconds and unlike the Camry the Fusion shows minimal difference in fueling between cold and warm operation. This observation is also confirmed by the flat slope for the Fusion in Fig. 5 during this period, meaning the difference between accumulated cold and warm fuel is unchanging. During the first roughly 50 seconds of cold Prius operation, the vehicle actually operates predominantly on battery power in order to run a prescribed catalyst activation procedure that produces minimal emissions while quickly warming the catalyst. Following this initial warm-up, the vehicle the proceeds with more conventional approach of additional fueling as indicated by the increasing “cold” fuel penalty observed in Fig. 5. An additional consideration for the Prius is that it utilizes minimal engine-off operation during the first 130 seconds of warm operation and thus the usual impacts of reduced engine-off are not as severe. Nonetheless, this specialized catalyst warm-up procedure is particularly noteworthy because it is very applicable to Plug-in Hybrid Electric Vehicles (PHEVs) due to the typically larger amount of electric propulsion capability available to a PHEV. More
Fig. 6: Fusion fuel rate during initial operation for cold and warm UDDS operation
Fig. 7: 7: Prius Priusfuel fuelrate rate during initial operation for cold and warm UDDS during initial operation for cold and warm operation operation
Fig. 9: Fusion battery terminal and estimated boost converter voltage during US06 operation.
information regarding this and further refined PHEV warmup strategies can be found in [7]. C. Reduced battery pack voltage and voltage boosting Another continuing trend for many full hybrid vehicles is the continued utilization of voltage boosting systems to enable lower pack voltages and thus reduce battery pack cell count when desirable. Additionally, the flexibility of higher voltage afforded by the boost converter allows for some unique system designs and motor configurations [4]. Fig. 8 shows the battery terminal voltage versus current for the three vehicles over the UDDS cycle. It can be clearly seen that operating voltage measured on the Prius is considerably lower compared to the Camry and Fusion. This roughly 20% reduction in battery voltage allows for a smaller pack design, while the 650V maximum capability boost converter (same as Camry) provides higher motor operating voltages for efficiency and power. Although the Fusion’s operating voltage is similar to the baseline vehicle’s, it is worth noting that the Fusion is Ford’s first vehicle to feature a boost converter and has enabled a reduction from the above 300V levels of previous vehicles. For reference, the Fusion uses a voltage boost converter observed to have a roughly 400V maximum capability. Figures 9 and 10 show the boost converter and battery
Fig. 8: Battery pack terminal voltage versus current over the UDDS cycle. Prius operating operating voltage voltage can canbebeobserved observedtotobebe roughly roughly 20% 20% lower compared compared to to Camry Fusion. Camry andand Fusion.
Fig. 10: Prius battery terminal and estimated boost converter voltage during US06 operation.
voltage over the US06 cycle. The US06 is a useful cycle for evaluating boost converter usage because it includes a mix of high power demands over a wide range of vehicle speeds. As might be expected from the different maximum boost capabilities and nominal pack voltage ranges, the Prius and Fusion exhibit fairly different voltage boosting strategies. Namely, the Prius operates using the boost converter for the vast majority of the cycle (90%) whereas the Fusion uses its boost converter for roughly 40% of the operation. In addition to this large increase in duty cycle, the Prius appears to boost mostly to a stable 500V and only uses the 650V boosting for scenarios where higher power or voltage is required. Over less intensive cycles, such as the UDDS and Hwy, the Fusion rarely uses its boost converter (
