Automakers recognize the need for hybrid-electric powertrains to help meet tightening fuel economy standards, but costs of the full-feature designs currently in production are leading two OEMs to produce less expensive configurations.
Starting later this year, Hyundai and Volkswagen will introduce parallel-hybrid configurations that provide all-electric drive, electric assist, regenerative braking, and idle stop/start. They'll connect to production automatic transmissions rather than purpose-built transmissions with two integrated motors. Hyundai begins with the front-drive Sonata hybrid; the Kia Optima follows. VW starts with rear-drive Touareg and Porsche Cayenne hybrids.
Also emerging for industry consideration is a prototype by FEV shown at the 2010 SAE World Congress. In FEV's design, one motor provides all hybrid functions, including A/C compressor operation, in a seven-speed AMT (automated manual transmission).
The acceleration/load electric-assist system with its smaller battery pack is a low-cost design proven by Honda, which calls its system the Integrated Motor Assist or IMA. But the inability to provide all-electric operation limits its effectiveness. And although a two-mode hybrid as developed by Allison, GM, and other makers offers the additionally sought improvement in highway fuel economy, along with the ability to haul payloads and tow, its widespread application is limited by cost.
The Hyundai and VW designs both feature a computer-controlled hybrid clutch between engine and motor—a wet multidisc clutch and 30 kW motor for Hyundai, and a single dry clutch and 38 kW motor for VW.
When the hybrid clutch is disengaged, the motor alone drives through the transmission to power the car in all-electric mode, and both systems launch that way from a stop. When the clutch is engaged and the engine is running, power then flows from engine through motor to transmission. The motor just spins as if part of the flywheel in the Hyundai system; on the VW system, the motor functions as a generator if needed.
If there is the demand, battery current is supplied to the motor for acceleration/load assist. On deceleration, the motor also may operate as a generator for regenerative braking.
Hyundai: 62 mph in EV mode
Hyundai adds a belt-driven high-voltage motor/generator (8 kW) to the 2.4-L four-cylinder, in place of a conventional generator. It provides engine start and charges the 270-volt hybrid battery pack by allowing the engine to run even while the vehicle is moving in all-electric mode.
Compared with the 33 kW and higher generator/motors integrated into other systems, this design appears to be considerably less expensive. However, to enable all hybrid functions with just one motor would require an additional clutch, as in the torque converter of the VW system. Hyundai eliminated the torque converter for packaging and improved efficiency.
Hyundai employs a computer strategy that controls fuel injection for the engine and electric-current feed to the motor to synchronize rpm, for smooth hybrid clutch engagement during the rolling launch, explained Woong-chul Yang, President of the R&D Division of Hyundai-Kia.
A 270-volt pack with lithium-polymer cells is used. Hyundai claims it is the first U.S.-market application for Li-polymer in a non-plug-in hybrid vehicle. It is reportedly intended to give the maker field experience with this type. Combined with the belt-drive generator/starter, the pack apparently enables more all-electric operation while an optimized charging schedule is maintained.
Conventional hybrids typically increase only city fuel economy compared with non-hybrid versions, with the two-mode system being the price-premium exception. However, Hyundai uses the 30 kW motor to drive the car through the six-speed automatic at speeds up to 62 mph (100 km/h). This helps boost hybrid highway fuel economy from the 35 mpg of the conventional Sonata to a claimed 39 mpg.
Hybrid clutch is key to VW's system
VW made only modest modifications (primarily an electric oil pump and new torque converter) to the eight-speed automatic used in the conventional Touareg. The entire hybrid module fits into the space between engine and transmission without modifying the vehicles. A 288-volt Ni-MH battery pack is used.
Although the VW design allows all-electric operation at up to 36 mph (60 km/h), the Touareg/Cayenne still must satisfy customers expecting V8 levels of torque. So the 3.0-L V6 is supercharged, rated at 329 hp and 326 lb·ft (245 kW and 440 N·m, respectively), with a tow capacity of 7700 lb (3493 kg).
The tow requirement dictated use of a torque converter, so VW took advantage of its lockup clutch to permit the single motor/generator to also perform the start function.
From a stop, the vehicle launches in electric drive with the motor/generator—hybrid clutch disengaged and the torque converter lockup clutch closed. The lockup clutch then is control-slipped, the hybrid clutch is engaged, and the motor cranks the engine.
At suffciently high rpm, fuel is injected and the engine starts. The hybrid clutch is released so the engine can rev without load to a computer-requested setpoint to match the speed of the motor/generator, at which the hybrid clutch (and lockup clutch) then can be engaged. It's all instantaneously smooth, according to VW engineers.
When the driver lifts his foot off the accelerator at cruising speed, the computer stops the engine and de-energizes the motor, and the vehicle coasts freely to boost highway mileage, noted Dr. Bernd Stiebels, VW hybrid powertrains manager. Fuel economy numbers have not yet been announced.
Because the engine is supercharged, VW's hybrid requires a more complex cooling system. Motor electronics and charge-air cooler are in one circuit with an electric pump and two small radiators.
The engine and transmission, electric motor, and passenger compartment heater are in another circuit with a large radiator. This circuit includes an electric pump and, to speed warm-up, a vacuum-controlled blocking cover for the engine mechanical water pump, to inhibit coolant circulation through the crankcase.
FEV's 7H-AMT concept
Conventional AMTs typically have been limited to vehicles where smooth shifting isn't a priority. But FEV's one-motor 7H-AMT provides fill-in torque when the electro-hydraulic shifters make gear changes, which eliminates the lurching effect typical in AMTs. Four gears are direct; three are overall ratios from gear pairs.
The FEV prototype has a single dry clutch between the engine and transmission; its electric motor is located on the transmission. With the clutch engaged, the vehicle can operate entirely with the engine or in electric assist; or with the clutch disengaged, it operates in all-electric mode.
The transmission is a three-shaft design, with a 35 kW electric motor on one shaft, simplifying use of the motor to torque-manage shifts. Further, if the vehicle is running entirely on the gasoline engine and the battery pack needs recharging, the motor of course just operates as a generator.
The engine is started by controlled slip of the clutch and computer modulation of motor torque. As the vehicle launches entirely on electric power, the engine will start with the transmission engaged as high as fifth gear. The 7H-AMT permits all-electric operation at speeds as high as 42 mph (70 km/h).
The idle-stop A/C compressor operation is a cost-saving bonus. The 7H-AMT has a belt-driven conventional compressor with a magnetic clutch, which can be engaged to operate with engine power or hybrid motor power. This eliminates the need for the comparatively expensive electric motor-drive compressor.