Variable Displacement
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Variable displacement, also known as cylinder deactivation (Cylinder Deactivation or Cylinder on Demand - CoD), is an internal combustion engine technology designed to improve fuel efficiency. The principle involves temporarily deactivating a portion of the engine's cylinders when full power is not required, for example at a stabilized cruising speed or under light load. By cutting off the fuel injection and closing the intake and exhaust valves of the affected cylinders, the engine then operates with a reduced effective displacement. This decreases pumping and friction losses, forcing the remaining cylinders to work in a more optimal load range where their efficiency is maximal. The transition between full-capacity mode and economy mode is electronically managed and designed to be completely seamless for the driver. Initially reserved for large engines (V8, V12), this technology has become widespread and is now found on more modest 3- or 4-cylinder engines, actively contributing to the reduction of fuel consumption and CO2 emissions in modern vehicles.
Benefits
- ✓Significant reduction in fuel consumption
- ✓Decrease in CO2 and pollutant emissions
- ✓Imperceptible operation for the driver
- ✓Instant availability of full power when needed
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What is variable displacement technology?
Variable displacement is an ingenious innovation that allows an internal combustion engine to adapt its working capacity in real time according to the driver's needs. Commonly referred to as "cylinder deactivation," its primary objective is to reduce fuel consumption and pollutant emissions without sacrificing performance when called upon. Imagine a V8 engine that can, while cruising at a constant speed on the highway, transform into a more fuel-efficient V4 engine. This is the very essence of this technology: using only the exact number of cylinders strictly necessary at any given moment.
How does variable displacement work?
The system is controlled by the engine control unit (ECU), which continuously analyzes dozens of parameters such as accelerator pedal position, vehicle speed, engine speed, and required load. When low-load conditions are detected (steady-speed driving, slight descent), the system acts on one or more cylinders.
The deactivation process typically operates via electromechanical actuators located at the camshaft. These disconnect the valve rocker arms for the targeted cylinders. Simultaneously, fuel injection and ignition are cut off for those same cylinders. The valves remain closed, trapping a volume of gas that acts as an air spring, thereby minimizing energy losses. As soon as the driver presses the accelerator to overtake or climb a hill, the system reactivates the cylinders in just a few milliseconds, in a completely imperceptible manner.
The tangible benefits of cylinder deactivation
- Fuel economy: This is the number one benefit. By running on a reduced number of cylinders, the engine decreases its pumping and friction losses. The active cylinders work harder, within a range where their thermal efficiency is much better. Fuel savings can reach 10 to 15% under optimal driving conditions.
- Emission reduction: Less fuel burned mechanically means fewer CO2 emissions, the primary greenhouse gas. Emissions of other pollutants such as nitrogen oxides (NOx) can also be reduced.
- Comfort and transparency: Manufacturers have worked extensively to make transitions between modes undetectable. Thanks to active engine mounts and highly sophisticated electronic management, the driver feels neither vibrations nor jolts during cylinder deactivation or reactivation.
- Power on demand: The technology never compromises the engine's maximum power. It offers the best of both worlds: the fuel economy of a small engine under partial load and the power of a larger engine at full load.
Which manufacturers use this technology?
While the technology was popularized on large American V8s, it is now widespread, including on more compact European engines. The Volkswagen Group is one of the pioneers of its democratization with its ACT (Active Cylinder Technology) or CoD (Cylinder on Demand) system on its TSI gasoline engines, notably the 1.5 TSI Evo which can run on two cylinders. Ford also uses it on its 3-cylinder 1.0 EcoBoost engine, which is capable of running on two cylinders. Premium brands such as Mercedes-Benz and Audi also integrate it into their more powerful engines to reconcile performance and efficiency. In conclusion, variable displacement is an elegant and mature technical solution that plays a key role in optimizing modern combustion engines.
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Associated equipment
Gasoline engine
The gasoline engine, also known as a spark-ignition engine, is a type of internal combustion engine that has been the heart of the majority of light vehicles for over a century. Its operating principle is based on the four-stroke cycle: intake, compression, combustion-expansion, and exhaust. During this cycle, a mixture of air and fuel (gasoline) is drawn into a cylinder, compressed by a piston, and then ignited by an electric spark generated by a spark plug. The resulting explosion pushes the piston back down, creating the mechanical energy required to propel the vehicle. Modern gasoline engines have evolved considerably, incorporating advanced technologies such as direct injection, turbocharging (downsizing), or mild hybridization. These innovations aim to improve efficiency, increase power, and reduce fuel consumption as well as pollutant emissions. Appreciated for its flexibility, responsiveness, and rapid revving, the gasoline engine offers high driving pleasure, particularly suited for dynamic driving and urban trips.
Eco Mode
Eco Mode, also known by names such as 'Efficiency' or 'Eco Pro' depending on the manufacturer, is an increasingly common feature in modern vehicles, whether ICE, hybrid, or electric. Integrated into the drive mode selector, its main objective is to reduce fuel or energy consumption by optimizing the operation of several vehicle systems. To achieve this, it adjusts several key parameters. Accelerator pedal response is softened, requiring firmer pressure to achieve the same acceleration as in Normal or Sport mode. Automatic transmission management is also modified to shift up earlier and at lower engine speeds. Additionally, Eco Mode can reduce the power of certain energy consumers, such as air conditioning or seat heating. On electrified vehicles, it maximizes regenerative braking to recover more energy during deceleration. It is a valuable tool for calm, economical driving, particularly effective in urban areas and on highways at steady speeds.
48V Mild-Hybrid
48V Mild-Hybrid technology, also known as mild hybridisation or MHEV (Mild Hybrid Electric Vehicle), is an ingenious solution designed to reduce fuel consumption and CO2 emissions in internal combustion engines (petrol or diesel). Unlike a conventional full hybrid vehicle, a mild-hybrid model cannot drive in 100% electric mode. Its principle relies on assisting the internal combustion engine with a small electric motor, generally a 48-volt starter-alternator. The latter is powered by a small lithium-ion battery that recharges automatically during deceleration and braking phases, thanks to the regenerative braking system. The electric motor steps in at key moments: it provides extra torque during start-ups and acceleration to relieve the combustion engine, thereby reducing its workload and fuel consumption. It also enables the Stop & Start system to operate more smoothly, quickly, and over a wider operating range, switching off the engine even before the vehicle comes to a complete stop to maximise savings. This technology represents an excellent compromise between performance, energy efficiency, and manufacturing cost, making it accessible across a wide range of vehicles.
Cylinder Deactivation
Cylinder deactivation, also known by commercial names such as 'Cylinder on Demand' (COD) or 'Active Cylinder Technology' (ACT), is an advanced engine technology designed to improve the fuel efficiency of internal combustion engines, primarily gasoline engines. The principle involves temporarily interrupting the operation of a certain number of cylinders when the engine is under low load. In these driving conditions, such as at a steady highway speed or during smooth city driving, the full power of the engine is not required. The engine management system then deactivates fuel injection and valve lift for half of the cylinders (for example, 2 out of 4, or 4 out of 8). The engine thus operates like a smaller-displacement engine, reducing pumping losses and optimizing the efficiency of the remaining cylinders, which operate at a higher and therefore more efficient load. The transition between modes is managed electronically in a way that is imperceptible to the driver, ensuring optimal driving comfort without vibrations or surges. This technology represents an ingenious solution for balancing on-demand performance with controlled fuel consumption.
Stop & Start
The Stop & Start system, also known as automatic engine stop and restart, is a technology designed to reduce fuel consumption and pollutant emissions in internal combustion vehicles. Its operating principle is simple: it automatically shuts off the engine when the vehicle is stationary for an extended period, such as at a red light or in a traffic jam, and restarts it instantly as soon as the driver wants to move off again. For a manual transmission, the engine cuts out when neutral is engaged and the clutch pedal is released. It restarts as soon as the driver presses the clutch again. On an automatic transmission, the system activates when the vehicle is immobilized with the brake pedal depressed and restarts as soon as the pedal is released. This technology relies on reinforced components, notably a more robust starter motor and battery (often AGM or EFB type) capable of withstanding a much higher number of starting cycles than a conventional system. An intelligent electronic control unit manages the whole setup, ensuring that optimal conditions (engine temperature, battery charge, etc.) are met before shutting down the engine, thereby guaranteeing safety and comfort.