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GM Expands Applications of AFM and E85 on 3.9-liter V-6

GM introduced its first V-6 application of its Active Fuel Management (displacement on demand) technology on the 3.9L V-6 offered in the 2007 Chevy Impala. The company is also extending its E85 flex-fuel coverage to the 3.9L V-6 offered in the 2007 Chevy Uplander minivan.

Preliminary testing of the 2007 Chevy Impala equipped with the 3.9L V-6 with AFM indicates an estimated 20 mpg in the city and 29 mpg on the highway—improvements of approximately 5.5% and 7.5%, respectively.

Active Fuel Management enables the engine automatically to operate on half of the engine’s cylinders under light-load conditions, improving efficiency by reducing fuel consumption when the cylinders are deactivated. (Earlier post.)

GM offers this technology in 11 vehicles for 2007, including trucks and SUVs—more than any other automaker. The Impala is GM’s first V-6 application of AFM in North America.

The addition of the E85 flex-fuel engine to the Uplander brings the number of flex-fuel vehicles in GM’s 2007 line-up to 14. The non-flex-fuel 3.9L engine in the Uplander delivers fuel economy of 18 mpg city, 25 mpg highway.

In addition to AFM and E85 capability, the 3.9-liter V-6 engine features variable valve timing (VVT)—a first for cam-in-block V-6 engines, and so recognized by Popular Mechanics with a “Breakthrough Technology Award” in 2005.

Along with optimizing performance and economy, the VVT operation helps the engine maintain an adequate torque load to maximize the benefits of AFM’s fuel-saving mode.



I wonder what type of harmonic dampening/compensation needs to be done in V-6 applications of "displacement on demand" such as is used here and in the Accord V-6. 3 cylinders firing by themselves isn't exactly smooth (one firing every 240 degrees).

John W.

I certainly didn't expect a 50% reduction in fuel usage with half of the cylinders not supporting combustion at times, but I did expect better than 5.5 and 7.5%. Does GM close the valves on those cylinders too, to minimize pumping/throttling losses?

Nevertheless, it's a good thing. I see no reason why this shouldn't or can't be built into almost every I.C.E. out there, no matter what the engine size, because the engines are always rated for the vehicle they are installed in.

Sid Hoffman

GM's AFM system does close the valves by means of deactivating the lifters. Sort of a VTEC in reverse system. The main reason they initially listed doing it was to prevent overcooling the combustion chambers of the inactive cylinders, although obviously you get to avoid pumping losses with the chambers sealed too.

Rafael Seidl

The only reason GM is touting displacement-on-demand is because their engines are so huge to begin with.
There simply is no need for a 3.9L V6 in a minivan.

For comparison, European minivans (e.g. VW Sharan or Opel Zafira) feature engines (gasoline or diesel) with typical displacements of 1.6-2.2L. VW does offer a 150kW NA V6 and Opel a 177kW 2.0L turbocharged variant, but those are the top-of-the-line options.

Less is more. Btw, displacement on demand requires fast-acting hydraulic actuators and sophisticated control logic. It is also tricky to deploy on anything but a V engine in which a whole bank can be dectivated. It is definitely not the only nor the best strategy for improving fuel economy.

Mark A

Will GM ever introduce AFM on four cylinder engines, or is a V-6 the low end?

Bill W

I believe the other reason for deactivating the lifters and pumping on the same charge is for emissions. Pumping all that cool/fresh air into the exhaust stream will play havoc with the catalytic converter (Too much o2 and will cool the unit below operating temp)

It would be slick if they separated the inactive cylinder systems and then reduced the pumping losses by opening the valves. Possible supercharging effect from the non-fired cylinders?


Next GM will be telling us how they exceed CAFE standards, what with all those V6s using E85. Of course, they won't be using E85 but they will be thrown into the rating mix as if they do.

Tim Russell

Bill W. You reduce pumping loses by closing the valves so your not moving any air throught them. With the valves closed the cyl act as air springs, much of the energy that compresses the air in the cyl is returned by the compressed air pushing on the piston. To do the "supercharging effect" would be highly complicated.

Bill W

I figured it would be less energy to let the air pump though rather than compressing/decompressing.

Is there public data that I could have a read so I can learn a little bit?



Rafael Seidl

Bill W. -

when a cylinder compresses & decompresses with valves closed, it is acting as a gas spring. The losses due to heat transfer to the walls are small, especially if the collant circuit can be deactivated separately for each bank. Blow-by losses are also small.

Pumping gas through the cylinders (i.e. simply not injecting fuel and not firing spark plugs) would play havoc with three-way catalyst. It would also eat significant amounts of power (cp. engine brake on hill descents using manual transmission).

It is not possible to use the deactivated cylinders to supercharge the active ones. Cylinder manifolds are designed for minimal interaction between cylinders (i.e. no two in close proximity open at the same time). Exception: V8 with non-flat crankshaft (race V8s have flat crankshaft).

Bill W

A couple of thoughts:
1)Engine braking with a manual is usually with the throttle closed. With the trottle open is there significant drag?
Say an diesel engine, I have heard there is very little engine braking due to the open throttle design, it that true? I am hoping Tim could help with this question.

2)I believe that some motors use fluctuating intake pressures to help fill the cylinders. My supercharger proposal would require a non-standard manifold layout, more than just shutting off cylinders. It was more along the lines of filling a pressurized cylinder for standing start or pressurizing the intake manifold for a boost of power while in a low cylinder count "efficiency" mode.

Working this out could not be that much more complicated that a hybrid braking/assist.

Sid Hoffman

To clarify one of the above statements, AFM does not deactivate all cylinders of a single bank for the V8 implementation. They run the outer two of one bank and inner two of the other bank. This is mainly done just due to the natural firing order of a V8, so you get a cylinder firing every 180 degrees of crankshaft throw. I'm not sure what V-angle their V6's use though and they may run it as a single bank in the case of the V6 motors.

Rafael Seidl

Bill W. -

1) Engine braking is more effective with a closed throttle. Commercial (diesel) vehicles feature throttles at the end of the exhaust for that purpose, though the extra noise they produce has led some places to ban their use.

The other mechanism for dissipating energy on long hill descents is called a retarder, assentially a centrifugal pump that is activated manually (usually pumps transmission oil, though one manufacturer has a model based on coolant). The pressurized liquid is forced through a throttle and the heat generated rejected by a radiatior.

A variant is to use a hydrostatic machine instead and store the energy in a hydraulic accumulator. EPA has been touting such hydraulic hybrids for large vehicles lately. The capacity of the accumulator is finite, though, so you'd still need a retarder as well.

2) The reason cylinders are deactivated is to concentrate the load on the remaining ones, which can then operate more efficiently. Using the unfired cylinders to supercharge the others would be possible in theory, further reducing the number of fired ones. In practice, the overhead involved in making this work would not weigh up against the extra fuel saved in the vehicle's lifetime.

The whole point of cylinder deactivation is to achieve modest fuel economy improvement of ~7% in large-displacement pushrod engines. After all, GM has a lot of money invested in those engine prodution plants.


I also find 7.5% disappointing. Like John W. above, I was not so naive as to think this would lead to a 50% reduction in fuel use, but I did think 20-30% sounded reasonable since, after all, you're shutting down half the engine when cruising on the highway. Why are the savings so little?

My in-laws have a '91 Park Avenue with the 3.8l and they already get 30mpg highway with it. Why does a 3.9l with cylinder deactivation and 15 years of other technological advancement, placed in a smaller car, do worse?

Bike Commuter Dude

Because that's what the consumer demands. In addition, fuel economy is more a function of weight and aerodynamic drag, with only a small portion of the formula contributed by engine volumetric efficiency and reciprocating weight differences. As far as vehicles go, combustion is a thing of the past altogether, but the customer demands products that are "familiar" and "reliable".
What's more efficient, lighter, producing less drag whilst being both extremely "familiar", and "reliable"? Bicycles! That's why I can keep up with traffic by pedaling even though I am using a fraction of the energy.

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