First Look: 2009 Chevrolet Silverado Hybrid
By: Mike Levine Posted: 11-14-07 00:01 ET
© 2007

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General Motors is introducing its all new full size hybrid pickup at the Los Angeles Auto Show.

The 2009 Chevrolet Silverado Two-Mode Hybrid promises gains of up to 40% in city miles-per-gallon and up to 25% in combined mileage, compared to the conventional 6.0-liter Vortec V8 Silverado it supplements in the lineup.

Many are going to only take note of the Two-Mode Hybrid's electrically variable transmission (EVT), but the Silverado Hybrid also depends on a modified version of the 6.0-liter V8 to hit the higher fuel economy numbers.

The V8 uses GM's proven Active Fuel Management (AFM) technology, which seamlessly switches between four-cylinder and eight-cylinder modes depending on engine load, and what GM calls 'late intake valve closing' (LIVC) technology.

LIVC is cool. It's also known as the Miller combustion cycle, invented in the 1940s by American engineer Ralph Miller. But the Miller-cycle has rarely been used in car and truck engines because the fuel efficiency gains (between 10% to 20%) haven't been worth the performance, technical, and cost trade-offs.

Miller-cycle engines improve fuel economy by reducing the amount of energy needed to compress the fuel-air charge during the engine's compression stroke. It's done by leaving the intake valve(s) open longer than a traditional 'Otto-cycle' engine, allowing some of the gases to escape the cylinder during the piston's upward motion. But less gas in the cylinder also means a less efficient burn and less energy from combustion during the expansion or downward piston stroke, which is why Miller-cycle engines have traditionally been paired with a supercharger to pump in extra air after the intake valves have closed to boost the fuel-air pressure right before ignition. The whole process can also be thought of as postponing compression until as late as possible and then making up for the delay by using the supercharger to rapidly pump in extra air to replace the gases pushed out during the earlier energy saving portion of the stroke. And because the gases are igniting later in the compression stroke, they are also under greater pressure, resulting in a better fuel burn.

A Miller-cycle engine's disadvantage has been that the cost and complexity of adding the supercharger has outweighed the cost savings from improved fuel economy. It also generates less torque in the low-RPMs - a critical miss in a truck application where low-end torque is craved for towing and hauling.

The Silverado Hybrid doesn't use a supercharger, though. This is where the new Two-Mode Hybrid transmission assists by managing more than just gear swaps. Two electric motors placed fore and aft in the transmission stand-in as 'digital' superchargers to provide extra power and torque, so the engine can burn fuel using LIVC / Miller-cycle combustion.

The electric motors also use battery-stored energy to help improve fuel economy and drive the wheels, sometimes without need for the 6.0-liter V8 at all.

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