This is what General Motors claims for its GMC Sierra and Chevrolet Silverado hybrid pickups — which are new for 2009

Like other hybrids, the Sierra and Silverado 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 engine, the transmission electric motors instantly start it up.

GM’s two-mode hybrid powertrain first appeared in the full size 2008 Yukon and Tahoe sport utilities. Now the 2009 GMC Sierra and Chevrolet Silverado are getting this hybrid powertrain. 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 when 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 comes from the hybrid’s low rolling-resistance tires and the engine 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 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 it doesn’t run on electric only, so you have to keep some gas in your tank. 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 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 engine, the transmission electric motors instantly start it up.

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.

The power inverter module also controls charging of the batteries. This can be done during regenerative braking or 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. Do not use regular tap water in the system because it can cause corrosion on the switching transistors inside the power inverter module.

The engine

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 cylinders during light load operation to save fuel. Variable cam timing is computer controlled to provide both low-end torque and high-speed power. 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 but at the expense of some power. 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. 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 and a towing capacity of 2,087 kg. While payload is slightly less than a conventional 2WD GM pickup, towing capacity is higher.

Finally, hybrid vehicles shouldn’t be parked for long periods of time — a couple months max. If the 300-volt battery 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 one.

Jim Kerr is an automobile writer based in Saskatoon. Contact him at [email protected]

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