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Parallel hybrids are powered by an electric motor, an internal combustion engine, or both at the same time. General Motors used a parallel powertrain in the discontinued ‘mild’ hybrid GMT 800 Chevrolet Silverado. The truck shut its gas V8 engine off at full stops, running internal systems off a small electric motor, until the truck started moving again solely on V8 power. Toyota uses a parallel system too, for its Hybrid Synergy Drive cars, like the Prius and Camry. It's a sophisticated version of parallel technology, where the electric motor or conventional engine solely powers or moves the vehicle, or both motors work together, depending on engine load conditions. But parallel hybrids only swap or pair engines in low-speed driving conditions, like city traffic, so they are also called single-mode hybrids.

Two-mode hybrids use an advanced automatic transmission that determines whether to use an electric motor or the conventional engine, or both at the same time during low-speed and highway driving - which is why it's called two-mode. This allows the vehicle to behave like it had a continuously variable transmission (CVT) so it can achieve optimal fuel economy at all times while driving. GM’s new GMT 900 Chevrolet Tahoe SUV uses a two-mode hybrid powertrain because the system doesn't compromise much of the vehicle's all-around capabilities. It can still tow up to 6,000-pounds, only 1,000-pounds less than a conventional Tahoe. We'll see a new Chevrolet Silverado two-mode hybrid pickup by December 2008.

Serial hybrids use an electric motor to drive the wheels, while a conventional (e.g. diesel) or non-conventional (e.g. fuel cell) engine powers a generator. The generator either powers the electric motor directly, like a locomotive, or is used to charge a battery pack, which powers the electric motor. Using a battery in a serial hybrid does three things. Batteries can be plugged into the energy grid for recharging, making the vehicle a 'plug-in' series hybrid, like GM's Chevrolet Volt concept. Batteries enable driving for some distance without burning fuel, and they let the generator engine run only at its most efficient point, and any time the generator is running it’s charging the battery. This is unlike an idling locomotive, stopped at a train station and burning fuel without purpose.

The advantage of a serial hybrid over the parallel and two-mode versions is that, within the battery’s charge range, the vehicle is emitting zero exhaust and not using any oil. It also has fewer moving parts, even counting the generator, which means less vehicle maintenance and lower lifetime operating costs.

Mr. Wright favors the plug-in serial hybrid approach for heavy duty pickups.

"The Volt is where it's all going to go, eventually. It's the right development path. Of course, it doesn't have enough power (for a truck). It would be great if it had enough scale for a pickup truck," says Mr. Wright.

It's one thing to power a small family sedan, like the Chevy Volt, off a serial hybrid powertrain. But the large size and demanding power requirements of heavy duty pickups make it a more challenging proposition for use in a truck application.

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