If you are keeping a scorecard, there’s a new automotive acronym in play — DFM. It stands for Dynamic Fuel Management and is GM’s double-down of Active Fuel Management (AFM), which, back in the beginning was called Displacement-on-Demand (DoD). This dizzying alphabet soup anthology is all aimed at one goal — creating a variable-displacement V8 engine.
As you probably are aware, AFM was a relatively simple system which converts a V8 engine into a four-cylinder (operationally) by deactivating four cylinders. DFM expands on this idea by installing the AFM style, spring-loaded lifters in all 16 positions so that all eight cylinders (instead of only four) can now be selectively deactivated. This mode of operation is currently being used in 2019 5.3L and 6.2L truck and SUV engines as a way to enhance fuel mileage.
DFM Compared to AFM
Mechanically, DFM is just an extension of the earlier AFM system. AFM employes special hydraulically activated lifters which essentially becomes what could be called a lost motion device when activated. A normal lifter follows the cam lobe and converts that rotary motion into vertical lift through your pushrod, which then acts on your valves.
When the AFM lifter is activated, it decouples this lifting motion so that while the lifter still follows the cam lobe, it imparts no vertical lift to the pushrod. Of course, fuel is not injected and spark is withheld in the deactivated cylinders. In both systems, this deactivation and subsequent reactivation can be accomplished in milliseconds, allowing a seamless transfer between active and inactive cylinders.
The AFM deactivation process begins by first not opening the exhaust valve, trapping the combusted gas in the cylinder. This gas acts as a spring, compressing as the piston rises in the cylinder, and helps to push down on the piston as it moves away from top dead center (TDC). While there are some pumping losses, the expansion acts like a gas spring. The power required to compress the gas in one cylinder is offset by the expansion of the trapped gas in an opposite cylinder. The DFM system mechanically operates in the same manner.
Just so everybody is on the same page, when activated the AFM system, converts the V8 engine into a four cylinder engine. DFM expands the original V8/V4 mode into 17 separate deactivation configurations ranging through as few as a two-cylinder mode up through its normal V8 operation. The sequencing of the various cylinder firing orders is controlled by the ECU, which can easily process 80 decisions per second – or one every 0.0125-second. GM tells us there are 69,904 lines of code required to control the DFM’s various cylinder firing sequences. It will require a few paragraphs just to explain how all this works.
To begin, GM engineers created what they call “fractions of operation,” expressed in what appear to be the standard fractions we all learned in elementary school. These are really just numerical terms illustrating the number of active versus inactive cylinders. Right away these fractions will seem unusual since they are not expressed as x/8 as you might expect. Instead, the definition of 4/5 means four of the five cylinders are active — leaving only one cylinder in a sequence of five not firing.
GM’s method uses multiple denominators (the number below the line of the fraction) for the various sequences. Examples include 2/5, 3/8, 5/9, 1/3, and others with 16 total fractions with the 17th mode operating all eight cylinders. The denominators are selected because they all divide evenly into the 720 degrees of rotation necessary for a four-cycle engine. As an example, 720 divided by 5 = 144 degrees.
GM sent us a grid that displays 16 of the 17 strings to illustrate how the cylinder deactivation progression occurs. This means cylinders are designed to reactivate in a string to minimize vibration and to maintain heat within all the cylinders for better efficiency. For the 2/3 orientation as an example, the deactivated cylinders for the first 720 degrees of rotation would be 1, 2, and 4. In the next 720 degrees of rotation the deactivated cylinders would be 8, 6, and 3.
GM says the system has been tested to ensure proper operation through a temperature range between -13 and 239 degrees F. Plus, dyno testing has pushed the system to the equivalent of 5 million driving miles in an effort to ensure durability. This calculates out to tens of millions of cycles.
GM points to a typical industry test cycle conducted on an older 5.3L truck’s AFM system. It operated in V8 mode 52-percent of the time and in V4 mode the remaining 48-percent. Driving this same industry test cycle with a 2019 5.3L DFM truck significantly altered the percentages, where V8 mode accounted for 39-percent, 45-percent in V4 mode, and 16-percent in less than four cylinders. Adding the DFM deactivation modes, this represents over half (39 + 16 = 55 percent) of the total driving cycle occurring in a fuel saving mode.
What Does It Do To Power
Power numbers with the DFM engines are the same as the 2018 model-year versions. The 2019 5.3L DFM V8 is rated at 355 horsepower and 383 lb-ft of torque while the 6.2L engines peak at 420 horsepower and 460 lb-ft of torque. The 5.3L engines will be backed by the GM 8-speed automatic while the 6.2L will get the 10-speed. The new system won’t be available on the 4.3L 90-degree V6. Instead, they will continue to run AFM, which deactivates only two cylinders and runs the engine as a V4 in AFM mode.
These new DFM-equipped trucks and SUV’s will also be upgraded with what GM calls a “centrifugal pendulum absorber” in the torque converter. Essentially, this is similar to a dual mass flywheel for a manual transmission. This pendulum absorber employs coil springs mounted to a plate splined to the input shaft of the torque converter.
This pendulum device will isolate noise and vibration between the engine and the transmission. When cylinders are deactivated, the number of crankshaft rotation increases between firing pulses. A V8 firing all 8 cylinders sees 90 degrees between combustion events, while a 4 cylinder engine lengthens this to 180 degrees. That inherently less smooth operation requires additional NVH dampening.
Improving fuel mileage while maintaining a large, pushrod V8 engine in larger vehicles for pulling power is clearly the mandate. From a hot-rodder’s standpoint, this new DFM system represents a dramatic increase in complexity. As these engines become available in the used engine market, it will be interesting to see how they are received. Current AFM engine issues appear to revolve around problems stemming from lifter failures, which can be traced to infrequent oil changes.
It won’t take long for the performance industry to react to this new technology, and the initial response will likely not be overly positive. But this is also the same reaction applied to the Gen III engines back in 1998. Once these engines are integrated into the performance industry playground, they will undoubtedly find their niche.