Active Fuel Management (AFM) is General Motors version of a cylinder deactivation system. Cylinder deactivation is used to increase fuel economy of higher displacement engines while cruising down the highway in light load conditions when the extra power is not needed. During light load conditions the PCM (Powertrain Control Module) can shut down predetermined cylinders by disabling the camshaft lifters and shutting off the fuel injectors. Which prevents those cylinders from consuming incoming air and fuel. Thereby reducing fuel consumption. Think of it this way. First, you give your teenager a soft drink with a large straw. The pop consumption will increase. Now if you give the same kid the soft drink with a tinny tiny coffee straw the pop consumption will decrease.
Why not just keep it simple and install small engines in all vehicles? Well, a four-cylinder engine in your pick-up or large SUV would leave you craving additional power when accelerating. Add a trailer to that same vehicle and you would likely be scared for your safety. Many vehicle manufacturers are now installing smaller engines with forced induction (Turbos and superchargers) to make up for the power loss. Combined with direct fuel injection technology, these engines can put out impressive torque and horsepower numbers while consuming less fuel overall. Forced induction engines are pushed harder to produce similar numbers to a larger displacement natural aspirated engine. Which can affect the overall life expectancy of the engine. The forced induction engines do require specific oils, fuel, and regular maintenance to ensure their integrity. The adage, “There is no replacement for displacement” still holds true when towing or hauling large loads. Large displacement engines use more fuel but make up for it with reliability and low maintenance costs.
The AFM engines dip their toes in each pond. The base engine has the displacement needed to make the vehicle perform as desired while cruising speed fuel consumption is reduced using the AFM system.
Sounds great right! Let’s add cylinder deactivation technology to all pick-ups and SUVs! Well….here is the rest of the story. GM said this technology can improve fuel economy by up to 12%. What we have seen in real world application is an improvement of less than 10%. Which equates to, at best, a 1 MPG improvement in fuel economy. Even though half of the cylinders are shut down, the crankshaft and pistons in those cylinders still turn. Which is an additional load on the engine. It’s like deploying a small parachute behind your vehicle. Not a drastic pull on the vehicle but still a drag that must be overcome by the cylinders that are producing power. The transition from 8 cylinders to 4 cylinders or 6 cylinders to 3 cylinders, depending on the engine, can be noticeable at times.
The next progression in cylinder deactivation technology is DFM (Dynamic Fuel Management) or DSF (Dynamic Skip Fire). The main difference between the two technologies is when AFM engages it shuts down half of the cylinders. They are either on or off. DFM has more control over disabling cylinders. It can shut down any cylinder in any combination of cylinders. Up to 17 different variations. It all depends on how much engine power the driver is requesting at the time. GM reports that DFM technology will have the vehicle running on less than all the cylinders 9% more often as compared to AFM systems. It is also reported that DFM equipped vehicles operate smoother when shutting down cylinders. The driver is usually not aware of the change in active, power producing, cylinders. GM reports that their 6.2L V-8 engine using DFM technology can show an improvement of 2 MPG in fuel economy.
Regular maintenance is key to the long life of these engines. Pressurized engine oil is used to control the cylinder deactivation of the lifters. Extended oil changes can cause sludge build-up in these systems. Which can cause starting and drivability issues. Change your oil at regular intervals, around 5000-6000 miles, to maintain proper system operation. We also recommend performing an oil flush on these engines once per calendar year.