How VVT Helped GM Get Best-In-Class MPG In Light-Duty Pickups


When it comes to eight-cylinder gas engines, GM has some twisted engineers who are hard at work in its labs. No, they're not crazy, unless you consider the almost fanatical obsession they have with improving the fuel economy and performance of GM's workhorse motors — two seemingly conflicting goals that are the automotive equivalent of having your cake and eating it, too.

In that case, yep, they're twisted because one of the biggest underhood improvements for 2010 is the availability of variable valve timing across the entire V-8 engine lineup in GM's full-size pickup trucks, that's helped GM grab the title for .

When it comes to VVT, it's all about the twist. Here's why.

VVT isn't new technology. It's been common in cars since the 1990s. Until then, engineers walked a fine line trying to balance an engine's personality between high performance and daily drivability because one of the most important powertrain components that determined whether an engine would be a pussycat or a tiger was its camshaft.

A camshaft manages the timing of the opening and closing of the intake and exhaust ports in a cylinder. It determines how well an engine breathes and makes power. It's driven by an engine's crankshaft, via a timing belt or chain.

Before VVT, camshafts were locked in a single position. The opening and closing of an engine's valves was predetermined by the cam's profile, its lobe lift (measured in thousandths of an inch) and the duration its intake and exhaust valves remained opened and closed. An engine's behavior was fixed from the moment it rolled off the assembly line.

VVT technology changed engine behavior forever by enabling the camshaft to be rotated slightly based on driving conditions instead of being lock-synched to the rotation of the engine's crankshaft. Using a component called a cam phaser, an engine could adapt on the fly to offer a dynamic mix of power as needed or refinement in low-speed situations. Twisting the camshaft forward (advancing) or backward (retarding) was all it took to change that behavior by changing the opening and closing duration of the valves.

VVT brought other benefits, too, including improved emissions due to better control of airflow through the cylinder and combustion temperatures.

Getting VVT into trucks took a bit longer than cars because of the widely varying applications that trucks are used for, from towing to hauling to running errands. Ford was the first to implement VVT in a full-size pickup, when it was introduced in the Ford F-150's three-valve 5.4-liter overhead cam V-8 . The first GM full-size truck to receive VVT was the with the 6.2-liter overhead valve V-8.

VVT is a technology that ties many recent advances together to achieve improved power and refinement as well as fuel economy, says Jordan Lee, assistant chief engineer for Chevy and GMC V-8 truck engines.

"Variable timing gives us a lot of flexibility," Lee said. "With fixed cam timing, you're always trying to balance low-speed torque with high-speed power, and then fuel economy and idle stability have to balance in accordance with where the cam timing is fixed. With variable cam timing, we're able to optimize the cam timing for low speeds, and in our truck applications it improves idle stability.

“Alone, it also improves fuel economy by about 2 percent, but at higher speeds and peak torque, we couple VVT with Active Fuel Management [which shuts off half the engines during stead-state driving conditions to save fuel]. It's synergistic. It allows us to run in four-cylinder mode longer and achieve a 5 percent improvement in fuel economy in the 5.3-liter V-8 for 2010 over the 2009 5.3-liter V-8."

In practical terms, the 5.3-liter V-8 has seen its EPA mileage ratings improve from 15/21 mpg city/highway in 2009 to 15/22 mpg in 2010, in the XFE model. That's given GM some new ammo to use against its competitors for bragging rights in the ultra-competitive half-ton truck segment.

"We used to really sweat over horsepower," Lee said. "Horsepower was king for a long time, and a lot of energy went into improving horsepower and torque. Now, fuel economy is the new horsepower war. A tremendous amount of engineering effort by GM is going after fuel economy. It's much more difficult than going after power."

GM's entry-level small block 2010 4.8-liter V-8 with VVT keeps the same gas mileage ratings as 2009's. The Chevy Silverado and GMC Sierra Heavy Duty three-quarter-ton and one-ton pickup trucks also use VVT in the 6.0-liter gas V-8, according to GM powertrain spokesman Tom Read, but trucks in that segment aren't required to rate their fuel economy.

GM is also trying to eke out engine-specific fuel economy improvements beyond VVT by using lower friction elements in the valvetrain, optimizing bearing sizes so they're not bigger than they have to be and using new lubricants and thinner weight oils."The biggest gains that we're looking at achieving will be by optimizing our combustion system and reducing friction," said Yoon Lee, design system engineer for GM small block truck engines. "Friction is the biggest enemy of fuel economy."

Jordan Lee and Yoon Lee are also using supercomputers to model airflow within the cylinder, following the combustion process molecule by molecule in the fuel/air charge to get the maximum benefit without compromising the engine's idle stability or combustion performance.

"We want the world's smoothest engines as well as crowd-pleasing fuel economy," Jordan Lee said. "That's where a lot of the optimization work is focused right now."

GM’s engineers are intently studying other engine technologies that are just around the corner for use in truck engines, too.

"I think direct injection is one of the most exciting new technologies," Jordan Lee said. "We're getting into it in a big way. It's not on the small block V-8s (yet), but it's technology that we'd consider for the future. It has lots of advantages. Using direct injection, we're injecting fuel directly into the cylinder instead of through a port. It helps cool the charge, so we can run at a higher compression ratio without having to worry about spark detonation. Anytime you raise the compression ratio of an engine, that's the one parameter that gives you an improvement in fuel economy and in power. It's a great technology that goes hand-in-hand with VVT and Active Fuel Management."


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