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A hybrid powertrain that combines an internal combustion or diesel engine with electric propulsion adds several design options to vehicle manufacturers and several costly components to the vehicle. From some estimates, the electrical/electronic content in the vehicle will almost double if the vehicle is a hybrid, especially in the near term with low volumes. The key added components of the hybrid include an electric motor, inverter, dc-dc converter, control electronics and high-voltage batteries. Ford has indicated that the price of the 2005 Escape Hybrid will be $3,300 more than a comparably equipped V-6 (only) powered Escape [1]. GM's Silverado 42 V mild hybrids cost $1500 more [1].
FROM FUEL ECONOMY ONLY TO PERFORMANCE
Expected sales of existing U.S. hybrid vehicles in 2004 are approximately 36,000 Honda Civics and 47,000 Toyota Prius with the Honda Insight dropping to a mere 1,000 or so units. The addition of Ford's Hybrid Escape with forecast annual sales of 20,000 units (4,000 units in 2004), GM's Silverado and Sierra models (scheduled for 2,500 units in MY2005) and Daimler Chrysler's diesel electric Ram pickup scheduled for 100 units will raise hybrid numbers slightly [1]. However, with sales of 16 million total cars and light trucks expected in 2004, this means hybrids are quite a bit less than 1% of the market — but it's a start.
In spite of record high fuel prices in 2004, the additional upfront cost of the fuel-efficient hybrid would still mean a payback period of two to three years to offset the higher purchase price. In contrast, customers appear to be more than willing to pay an additional $895 for a Dodge Durango with a 345 hp, 5.7-liter hemi engine over the 230 hp, 4.7-liter engine. To further raise the cost impact of their purchase, drivers often trade in a perfectly good low-mileage, one-year-old vehicle to get the additional performance, costing them an additional $7000 to $8000 in depreciation [2].
Hybrids have the potential to provide fuel economy, performance and reduce emissions. Automotive power train designers have options depending on the type of vehicle they want to produce. For highest fuel economy, the engine and electric motor are operated at optimal points to achieve high efficiency. The internal combustion engine (ICE) has average efficiencies of 15% but can achieve 30% when operating in the sweet spot. For performance, different points in the torque and horsepower curves between the motor and engine are exploited. This can provide more low-end torque with increased fuel economy.
The Accord hybrid that will go into production in 2005 provides an example of the change in hybrid design. It uses the Integrated Motor Assist (IMA) hybrid system concept from the Insight and Civic but will provide V-6 performance with 4-cylinder economy. Honda did not reduce the engine size when it added the electric motor to the drive train. A summary of vehicles and their performance increase using electric motors is shown in Table 1. Using historical values, the Honda Insight's 0.033 and Toyota's 2002 Prius at 0.034 hplb are more like the performance of vehicles sold in the early 1980s, limiting the number of people who would be interested in purchasing such vehicles for lack of performance. For comparison, a 2004 Camry 6 cylinder is 0.062 and Ford F-150 5.4L V-8 is 0.061 hp/lb. When vehicles exceed 0.047 hp/lb they appeal to more than 50% of the buyers, and those buyers are frequently willing to pay more money for performance, as indicated in Figure 1 [3].
POWER ELECTRONICS IN HYBRIDS
Two common classifications for today's hybrid are: (1) mild and (2) full hybrid. The 166 V Honda Insight in Figure 2 (a) is a mild hybrid because it always uses the gas engine to drive the wheels and the electric motor provides an extra boost during acceleration. In contrast, the Toyota Prius in Figure 2 (b) is a full hybrid with the ability to use the engine alone, the electric motor alone, or a combination of both to drive the wheels.
Like the Toyota Prius, the Ford Hybrid Escape is a full hybrid. Another classification for hybrids is series, parallel or split hybrid system. The Ford Escape Hybrid and Toyota Prius are both split systems with a planetary gear that allows operation in either the parallel or series mode. As shown in Figure 3, in the parallel path the energy is converted and directly transmitted to the driving wheel through a mechanical path. In the series mode, the energy is converted to electrical energy by the generator.
Ford's Escape Hybrid demonstrates the added system complexity with hybrids. The Escape Hybrid brings together:
- Transaxle from Aisin AW that includes a 70 kW, 400 V electric motor,
- 330 V nickel-metal hydride battery pack from Sanyo,
- 400 V, 300 A generator,
- Regenerative brakes from Continental Teves,
- 14 V electric power steering from NSK,
- 1.5 kW dc-dc converter from TDK,
- Battery software from PI Technology, and
- 2.3 liter, 4-cylinder Atkinson cycle engine from Ford.
Each of these hardware elements is controlled by software that must work together flawlessly to achieve 35 mpg to 40 mpg in city driving and avoid reliability problems [6].
As shown in Figure 3, the inverter, traction motor and traction generator combine for vehicle propulsion, battery charging and regenerative braking. The dc-dc converter provides 14 V power for the vehicle's electrical/electronic components. The key to high efficiency and long component life in hybrid power electronics controlling several kilowatts is integration and ethylene-glycol cooling. This is true in the Hybrid Excape, Toyota's Crown and Prius, Honda's Insight and Civic, and GM's 42 V Silverado.
CONVERTING THE VOLTAGE
Toyota designed a 1.2 kW dc-dc converter for its dual-voltage 42 V Crown THS-M (Toyota Hybrid System — Mild). The layout in Figure 4 shows some of the packaging-related problems associated with switching higher power. Three 6.8 mm × 5.9 mm 75 V trench MOSFETs are used in this converter. Using synchronous rectification instead of the diode improved peak efficiency from 90.8% to 92.8% in the 20 A range and by almost a percent at maximum current (85 A) range. Note the multiple wire bonds used to connect to the power die and to connect the substrate with the external connections. In contrast, the industry standard surface-mounted D2PAK has a footprint of 10.2 mm × 15.4 mm (0.4 inches × 0.6 inches) and a maximum die size of 4.3 mm × 6.9 mm (0.170 inches × 0.270 inches). This package is used for many of the higher power loads in today's 14 V vehicles that are typically 100 W (average) or less [7].
In the new Prius, two dc-dc converters are used. One boosts the voltage from the 202 V battery to provide 500 V power for the traction motor and generator. The other dc-dc converter reduces the voltage from 202 V to 14 V to power traditional vehicle loads. Based on the number of existing 14 V components, high-voltage transitions will be common in higher-voltage vehicles.
OTHER POWER CHANGES
Since the engine is usually off in hybrid vehicles at idle and could be off during braking, vehicle systems that rely on the engine for power must be addressed. These systems include power steering, A/C and braking. The answer to the engine-off situation is frequently the use of electric power steering (EPS) or electrohydraulic power steering (EHPS), electric A/C, and regenerative braking. An electric clutch is added to the system that disengages the components from the engine when it is shut off. Figure 5 shows the clutch for disconnecting the loads normally driven by the engine in GM's belt alternator system (BAS) mild hybrid. The clutch will be electrically operated as it is in the Toyota Crown.
In the smaller-size hybrid vehicles, electric power steering can be implemented with 14 V. However, larger vehicles such as the Chevy Silverado and GM Sienna require higher voltage and take advantage of the 42 V bus and available 42 V EPS systems. Peak current in a 14 V EPS can be as high as 100 A. For the same value motor that value reduces to slightly more, than 33 A for a 42 V supply.
EXPECT CHANGES
With the demonstrated success of Toyota's New Prius and the Honda Civic and new vehicles from Ford, GM, Daimler Chrysler, Toyota and Honda, car makers will soon know even more about what sells and what consumers want from a hybrid vehicle. However, the amount of technical challenges, changing economic outlook impacting development budgets and fuel concerns shifting consumers' interest from fuel-guzzling SUVs to fuel-efficient vehicles have already provided shifts in product plans and launch schedules.
In 2003, GM announced a change in its plans for the Saturn Vue from a full hybrid with two motors and a transaxle that would have achieved 50% improved fuel economy to a less-expensive 42 V belt alternator starter (BAS) mild hybrid system with a continuously variable transmission (CVT) that is a stop-start system and gets 12% to 15% improved fuel economy. A variation of the Allison Hybrid Bus System offered in 2003 will be its first full-hybrid passenger vehicle and will be used in future full-size SUVs and pickup trucks in 2007-2008 [7].
More recently, Toyota and Ford have scrambled to find ways to increase capacity based on higher fuel prices and higher-than-expected consumer interest in fuel-efficient hybrids. This is just the beginning. We are in for an even more exciting ride.
THE ELECTRONIC GAS TANK
The hybrid vehicle needs energy storage for the electric portion of its propulsion system in addition to the gasoline or diesel fuel tank for the engine. Long term this could come from fuel cells, but today batteries provide the energy storage. The 12 V battery has gone through various improvements during its 50-some years as the reigning vehicle electrical system voltage. However, the electrical propulsion in the hybrid requires storage capability beyond traditional 12 V battery technology.
Battery chemistries that are being evaluated and developed for hybrid vehicles include valve-regulated lead acid (VRLA) batteries, nickel metal hydride (NiMH) and Lithium ion (Li-ion) technologies. Key concerns with these batteries in hybrid applications are cost and reliability. As shown in the spider chart, VRLA technology excels in economics, cold cranking and recycling infrastructure because it is an improvement on the established lead-acid battery. In contrast, Li-Ion technology has the highest energy density while NiMH has the highest specific power and charge acceptance.
Today, 42 V vehicles such as Toyota's Crown and GM's Silverado and Sierra models use VRLA technology. Higher-voltage vehicles such as the Toyota Prius and Ford Hybrid Escape models use Ni-MH batteries. In Ford's Hybrid Escape, the Sanyo battery also includes battery control electronics.
Toyota reduced the battery voltage from 275 V in its initial Prius to 202 V for the new Prius and used a dc-dc converter to boost the voltage to 500 V. With 500 V, they use a 50 kW traction motor instead of 33 kW motor used with power direct from the 275 V battery. The voltage conversion provides increased power with lower battery voltage, placing less stress on the high-voltage energy source.
The replacement cost of the battery in the Toyota Prius has been quoted at $3,000. With unproven history, vehicle manufacturers are offering warranties of eight years/100,000 miles on the initial hybrids.
Nissan displayed Li-ion batteries at the 2003 Tokyo Motor Show using laminated cells that are less than half the size of rival systems. A 25 kW battery would have a volume of only 15 liters compared with 45 liters using conventional technology. The thin cell construction also improves cooling [7].
Supercapacitors/ultracapacitors can relieve the stress on the battery and provide peak energy capability when needed. Ultracapacitors have been used in 42 V and fuel cell development vehicles to cover the peak energy needs in portions of the system and avoid increasing the size of other costly components. Maxwell Technologies introduced a standard size D Cell ultracapacitor earlier this year that is expected to drive down the cost and ease the integration of the technology into production vehicles. The first production use of ultracapacitors will most likely occur on a 14 V vehicle in 2005.
REFERENCES:
- Richard Truett, “DCX, GM Go Slow On Hybrids,” Automotive News, July 5, 2004.
- Mary Connelly, “People Keep Asking: ‘That Thing got a Hemi?’” Automotive News, Feb. 16, 2004.
- Hansen Report on Automotive Electronics, March 2004.
- “Light-Duty Automotive Technology and Fuel Economy Trends: 1975 Through 2004,” U.S. Environmental Protection Agency, EPA420-R-04-001, April 2004.
- Venkateswara Anand Sankaran, “Introducing Power Electronics in Ford Hybrid Escape Vehicle,” Power Electronics Society Newsletter, Vol. 16, No. 3, October 2004, pp. 13-15.
- Richard Truett and Amy Wilson, “Systems Don't Mesh so Ford Hybrid is Delayed,” Automotive News, Nov. 3, 2003.
- Intertech's “Power Management in Today's and Future Automotive Systems,” including the 2004 update http://www.intertechusa.com/studies/PowerManagement/PM_Study.htm
- Richard Johnson, “Spiral Wound Lead-Acid Batteries for 42V Applications,” SAE 42 V Challenges Toptec, April 29-30, 2002, Troy, MI.
ABOUT THE AUTHOR
Randy Frank is a freelance writer and president of Randy Frank & Associates, Ltd., a technical marketing consulting firm based in Scottsdale, AZ. He can be reached at [email protected].