|The Silverado 2-Mode hybrid pickup.|
At the Los Angeles Auto Show, Chevrolet rolled out the Chevrolet Fuel Solutions initiative designed to aggressively promote the fuel-saving technologies available in the brand’s current and future lineups. An accompanying parade of vehicles included a gasoline-powered subcompact (Aveo); a hydrogen fuel cell crossover (Equinox fuel cell electric vehicle); extended-range electric vehicle (Volt); and three hybrids.
The event included the world premiere of the Chevrolet Silverado Hybrid pickup truck, which will go on sale in North America later next year as a 2009 model. Chevrolet also revealed the all-new Malibu Hybrid sedan, arriving in North American dealerships this month.
Chevrolet also announced a historic partnership with the Walt Disney Company in which 10 Chevrolet Equinox Fuel Cell vehicles will be deployed as shuttles at Disney properties in California.
The 2-mode Silverado. The hybrid Silverado combines the 2-mode Electrically variable Transmission (EVT) with the 6.0L Gen IV V8 engine with Active Fuel Management (GM’s cylinder deactivation) and an Energy Storage System.
The all-new EVT is an assemblage of two 60 kW electric motors, three planetary gearsets and four traditional hydraulic wet clutches. This arrangement allows continuously variable operation, as well as providing four fixed gear ratios (with operation comparable to that of a standard electronically controlled automatic transmission).
The design was selected because of the operational characteristics of electric motors, which are very efficient when turning at low speeds, but much less efficient as motor rpm increases. Current hybrid passenger vehicles run their electric motors almost continuously throughout the entire drive cycle, which can be very inefficient under high loads and at highway speeds. The opposite is true with GM’s EVT, which can activate any of its four hydraulic clutches to allow power to be transferred via the fixed-gear ratios whenever high load conditions are experienced.
A Hybrid Optimizing System (HOS) constantly receives torque-based data from the powertrain and other vehicle systems, and then determines the most efficient means of propelling the vehicle—either via electric power, gasoline engine power or a combination of the two. The EVT is like having two transmissions in one – continuously variable drive for light-load conditions and fixed-ratio drive for high-load situations.
All functions of the EVT are controlled by the HOS, which constantly searches for the optimal transmission operation (using either variable or fixed ratios) to meet current operating conditions. The HOS also bases its decisions on allowing the Vortec 6.0L V-8 to take full advantage of its Active Fuel Management system and, because of an equivalent 30-horsepower (22 kW) boost of electric power, remain in V-4 mode as long as possible for maximum fuel economy. This is the basis of the Silverado Hybrid’s 25% improvement in combined city/highway fuel economy.
Unlike other hybrid systems, this new GM system occasionally shuts down the electric motors, allowing the EVT to function as a conventional automatic transmission. Typically, this fixed-ratio operation occurs at highway speeds or when hauling heavy loads, and can actually be more efficient than operating in electric-gasoline hybrid mode under the same conditions. This is because under high-load situations, when the fixed gears are in use, the electric motors can be used to generate electricity—or, if needed, they can be called on to supply additional torque for improved performance.
Energy Storage System. Providing power to the EVT’s two electric motors is a 300-volt nickel-metal hydride Energy Storage System (ESS). This battery pack is located under the second-row seat, where it takes up virtually no additional space and does not interfere with second- or third-row ingress/egress.
The primary function of the ESS is to provide power (300 volts) to the EVT via the Traction Power Inverter Module (TPIM) and to store captured energy produced during regenerative braking. The ESS can also be charged, when necessary, by the gasoline engine via one of the two electric motors when operated in generator mode.
In addition to supplying power to the EVT, the ESS also provides power to the air conditioning compressor and the Accessory Power Module (APM), which converts the high-voltage supply to 42 volts for the electric power steering system, and 12 volts for the vehicle battery and other 12-volt electrical accessories.
Battery pack durability and reliability is maintained via optimized charge and discharge cycles, as well as a dedicated cooling system that draws air from the passenger compartment. As part of the vehicle’s emission control system, the ESS is warranted for eight years/100,000 miles.
The ESS also has numerous safety features that prevent over-charging, over-heating, unintended access to high-voltage components and infiltration from liquid spills.
Regenerative braking. During braking, the Silverado Hybrid uses one or both of the EVT’s traction motors as a generator, converting the braking energy to electrical energy for storage in the ESS for future use to propel the vehicle.
The regenerative brakes are used along with the standard hydraulic brakes to slow the vehicle and/or bring it to a stop. Depending on the amount of braking force required, the hydraulic brakes may not even be used, such as during mild deceleration when slowing to allow space for merging traffic near a highway on-ramp.
When additional braking is called for, based on a change in the position of and/or the force applied to the brake pedal emulator, the hydraulic braking system will be called on to assist the generator(s) in slowing or stopping the vehicle. The anti-lock braking system (ABS)/Electronic Stability Control (ESC) modulator used on the Silverado Hybrid has been adapted to allow this interaction between the hydraulic brakes and the regenerative braking system.
The use of cooperative control between the regenerative braking system and the hydraulic brakes results in excellent braking control and maximum energy recovery. The system also provides feedback in the form of brake pedal resistance, which gives the driver the same feel as would be experienced with a normal hydraulic braking system.
Regenerative braking has the additional benefit of extending the life of the friction materials used in the hydraulic braking system, as well as improving braking performance in the form of shorter stopping distances.
6.0L V-8 engine. The Silverado Hybrid’s 6.0L V-8 engine is unique. It features Active Fuel Management and late-intake valve closing (modified Atkinson-cycle combustion process) for reduced pumping losses and better overall fuel economy.
The engine uses flat-top pistons, cylinder heads borrowed from GM’s 5.3L high-output V-8 and a 10.8:1 compression ratio, producing 332 horsepower (248 kW) at 5,100 rpm and 367 lb-ft of torque (497 Nm) at 4,100 rpm. It runs on regular unleaded fuel.
A key contributor to the Silverado Hybrid’s fuel economy is the gasoline engine’s Auto Stop mode. Once the vehicle reaches 0 mph, depending on the current state of the battery charge, the gasoline engine can automatically shut down. By leaving the engine off and allowing the vehicle to move only under electric power, such as during heavy stop-and-go traffic, fuel consumption is greatly reduced.
However, when extra power is required, such as for wide-open-throttle acceleration from a standing stop, the Vortec 6.0L V-8 is seamlessly restarted so it can deliver the necessary power and torque. In this case, the engine is restarted effortlessly from the Auto Stop mode using the EVT’s powerful internal electric motors; there is no traditional starter motor.
Fuel economy figures for the Silverado Hybrid are not finalized, but GM projects 40% greater city fuel economy and a 25% improvement in overall fuel economy over the full gasoline version. The 2008 Silverado with 6.0-liter engine is EPA rated at 13 mpg city, 18 mpg highway, 15 mpg combined.