By Jim Kerr; photos courtesy General Motors
2009 Chevrolet Silverado Hybrid
GM’s Two-mode hybrid powertrain first appeared in the full-size 2008 Yukon and Tahoe sport utility vehicles. Now the 2009 GMC Sierra and Chevrolet Silverado pickups are getting this hybrid powertrain, and for those of us who need a pickup, this may be part of the solution to the high cost of fuel.
GM claims this hybrid pickup will achieve a 35 per cent improvement in city fuel consumption and a 25 per cent improvement in overall fuel consumption compared to a conventional pickup. Although they don’t list the improvement in Highway fuel economy, looking at the numbers would suggest an improvement of 15 per cent. Part of this gain comes from the hybrid’s low rolling resistance tires and the engine’s variable displacement design, but the hybrid electric/gasoline powertrain will provide the majority of the savings, especially at low speeds.
The two-mode transmission is the key to the hybrid system. This transmission contains three planetary gear sets and four clutches that give the transmission four mechanical speeds. This is combined with two 300-volt electric motor/generators inside the transmission case that are capable of driving the vehicle by themselves up to 48 km/h, even when towing. (Note that it doesn’t run on electricity only if you run out of gasoline.) The electric motors are also used to start the gasoline engine, which eliminates the conventional starter system and reduces vehicle weight. Like other hybrids, the Sierra and Silverado also operate in “Autostop” mode, where the gasoline engine shuts off when operating conditions are right and the vehicle is at a stop. As soon as power is required from the gasoline motor, it is instantly started again by the transmission electric motors.
2009 Chevrolet Silverado Hybrid. Click image to enlarge
In addition to a 12-volt battery under the hood for accessory power, a 300-volt battery pack is located under the rear seats. Inside this battery pack, there are 40 individual nickel metal hydride battery modules, each putting out 7.2 volts. To produce the voltage necessary to drive the motors, the 40 modules are connected in series to produce about 288 volts DC. This voltage is supplied through relays that disconnect the battery pack during vehicle shut down and also directs power out to the drive motor/generator power inverter module. This module controls power to the transmission motors, as well as converting the voltages to 14 volt and 42-volt levels to operate vehicle lights, accessories and electric motors for power steering, brakes and transmission pressure when the gasoline engine is in AutoStop mode.
The power inverter module also controls charging of the batteries. This can be done during regenerative braking, or they can be charged by the gasoline engine, driving the electric motor/generator units during cruise conditions. Because there is a lot of heat generated in the power inverter module, it has its own cooling system separate from the gasoline engine cooling system. There is a heat exchanger (radiator) at the front of the vehicle and electric pumps to move the coolant through the system. De-ionized water and Dexcool are mixed to provide the coolant; it is important that regular tap water not be used in the system because it can cause corrosion on the switching transistors inside the power inverter module.
2009 Chevrolet Silverado Hybrid. Click image to enlarge
The 6.0-litre Generation IV small block V8 engine has been specially modified to work with the hybrid electric motors. This engine features Active Fuel Management, which deactivates four of the engine’s cylinders during light load operation to save fuel. Variable cam timing is computer-controlled to provide both low-end torque and high-speed power, but this engine also keeps the intake valve open a little longer than a normal engine, allowing a reverse flow into the intake manifold. This reduces the effective compression ratio, allowing the expansion ratio to increase while retaining normal combustion pressures. Efficiency is gained because the high expansion ratio delivers a longer power stroke and reduces the heat wasted in the exhaust. Engine efficiency is improved at the expense of some power, but this is compensated by increasing the engine compression ratio and supplementing low-end power with the power from the electric motors in the transmission.
Brake pedal feel is simulated during brake applications. Instead of the pedal motion pushing brake fluid to all the wheel brakes, it forces the fluid into a simulation chamber that gives the pedal feel a driver expects. The computer then uses regenerative braking provided by the electric motors combined with hydraulic application of the brakes to slow the vehicle.
These hybrid pickups are also designed for work. The 2WD model has a payload of 662 kg (1459 lbs) and a towing capacity of 2087 kg (4600 lbs). While payload is slightly less than a conventional 2WD GM pickup, towing capacity is higher.
Finally, the hybrid is a vehicle that shouldn’t be parked for long periods of time (say, a couple of months). The 300-volt battery automatically disconnects from the vehicle electrical system when the key is off, but if it does become completely discharged it has to be replaced. If it is too low to start the engine, the system can be put into a special Jump Assist mode with a scan tool that uses the 12-volt battery to charge the 300-volt battery, but you can’t do this without a scan tool from the dealership.
There is a lot going on to make a hybrid vehicle work. As part of the vehicle’s emission control system, the hybrid energy storage system is warranted for eight years or 160,000 km. Perhaps the biggest problem you may have with a hybrid pickup is answering all the questions from other drivers you meet, curious about driving a hybrid.