If you want to save fuel, one of the easiest ways is to not to use it. That’s the idea behind cylinder deactivation, used by some automakers to help improve fuel economy and reduce emissions.
Most use trademarked names for their systems, and exactly how they operate may differ slightly between them, but the overall concept is the same: When full power isn’t needed, some cylinders don’t get any fuel.
Deactivation is mostly used on V6 or V8 engines, where, in principle, it reduces the engine’s displacement when it functions: Bigger-engine power when all cylinders are activated, and smaller-engine fuel economy when some are shut off. Some automakers prefer to use small turbocharged engines instead, which force in extra air and fuel to provide more power when needed. Essentially, deactivation is a larger engine that can act like a smaller one, while turbocharging is a smaller engine that can perform like a larger one. (Some automakers combine deactivation and turbocharging on their engines, as well.)
Engines contain pistons that move up and down in their cylinders, powered by the force of a gas-air mixture as it combusts, to turn a central crankshaft. The crankshaft spins, and that force eventually turns the wheels.
When a cylinder deactivates, the system closes its intake valves, which draw in air, and its exhaust valves, which release spent gases. It also stops injecting fuel into the cylinder. The piston still moves up and down – it has to, because it’s attached to the spinning crankshaft – but now it’s just along for the ride.
The extra efficiency isn’t just in only half the cylinders getting fuel. When you’re not asking a lot of your engine, such as when you’re cruising at a steady speed, it isn’t operating at its peak performance. The pistons must overcome air resistance when they pull in and expel air, known as pumping loss.
When you’re light on the throttle, such as that steady speed, pumping losses are higher because of the pressure differences between the intake and exhaust manifolds. When some cylinders are deactivated, no air is going in or out of them, so there’s no pumping loss. Beyond that, as the engine automatically compensates for those “missing” cylinders, it creates less of an intake-exhaust pressure difference. This reduces pumping loss in the active cylinders, making them work more effectively. Although they’re helping to move the deactivated pistons, since they’re all attached to the crankshaft, the engine is still working more efficiently overall.
All of this is controlled by the engine’s main brain, known as its engine control unit (ECU) or engine control module (ECM). As soon as more power is needed, such as when you accelerate, the system brings the deactivated cylinders back online. The transition is usually so smooth that it’s almost impossible to detect. Natural Resources Canada estimates that cylinder deactivation can reduce fuel consumption and emissions by 4 to 10 per cent.
Most engines with cylinder deactivation turn off half of them at a time, such as an eight-cylinder that switches to four cylinders. Honda’s system, which it calls Variable Cylinder Management, can switch a V6 engine to run on three or four cylinders, depending on what’s best for the driving conditions. General Motors, which currently shuts off half the cylinders on its pickup truck engines, will introduce a variable system on its 2019 trucks. Some other manufacturers offer similar systems.
The systems have come a long way from when one was first introduced on Cadillac in 1981. It was called Modular Displacement, and it could switch a V8 engine to run on six or four cylinders. Mitsubishi brought in a four-cylinders-to-two version shortly afterwards. The electronics and fuel systems of the day weren’t up to the task, and neither company kept it very long. Today, there aren’t many disadvantages to deactivation systems, other than they add some cost and complexity to the engine.
Of course, cylinder displacement is only one tool in the fuel-efficiency box. Automakers program in deceleration fuel cut-off (DFCO), which completely shuts off fuel delivery to the engine when your foot is off the throttle and you’re slowing down. The engine continues to operate, and the system starts feeding it fuel if you accelerate, or when the engine speed gets close to idle as you come to a stop.
And some vehicles now include a start/stop function on their engines. While this used to be exclusive to hybrids, it’s now showing up on conventional gasoline vehicles, and even on some light-duty diesels. Automakers are adding it to further reduce fuel and emissions numbers.
When you come to a full stop with your foot on the brake, such as sitting at a red light, the engine shuts off. The vehicle’s lights, stereo, and climate control continue to operate, and certain conditions must be met, including ambient and engine temperature. The engine automatically restarts as soon as you take your foot off the brake.
How it restarts depends on the vehicle. With a hybrid, the car’s electric motor handles it, while conventional cars have a heavier-duty starter. On many vehicles you can deactivate the start/stop system with a button, and if that isn’t possible, putting the car in “Sport” mode usually keeps it off.