CYLINDER DEACTIVATION
I recently thought about a basic concept to reduce emissions and fuel consumption by temporarily reducing the number of operating cylinders in an engine. Would it be possible to “switch off” one cylinder from say a four-cylinder engine by the engine control unit or even a button on the dashboard?
The result is when in the deactivated state, the three-cylinder con guration would improve consumption and cut unnecessary power. What is your take on the feasibility of this idea? IWAN DE JONG Pretoria
Cylinder deactivation is not new and has been widely implemented. It is easier to execute in multi-cylinder engines (like a V8), as the induced extra vibration is minimised by cleverly deactivating cylinders to limit the impact on the balance of the combustion pulses. Remember, the balance of an engine is determined by the number of cylinders, layout, crank design and ring order.
Let’s take a step back to see why cylinder deactivation can potentially save fuel. It is a common mistake to assume if one cylinder is deactivated in a four-cylinder engine, 25% of the fuel is saved. For example, a vehicle travelling at a constant speed on a level road needs a xed amount of power (energy) to continue motion. As fuel is the energy source, if one cylinder is deactivated, the remaining cylinders will have to burn more fuel to produce the same amount of power to continue steady-state vehicle motion. The reason why fuel may be saved is that the ef ciency of the engine can be increased by running the remaining cylinders at a higher load.
This is true of a petrol engine – and explains why the technology is almost exclusively employed on petrols – with a xed air-fuel ratio. At low loads, a throttle is used to reduce the incoming air (oxygen) in the stoichiometric ratio according to the fuelling needs. This creates pumping losses. By running the remaining cylinders at a higher load point, throttling is reduced and combustion ef ciency increased.
A four-cylinder engine running as a three-cylinder, for example, would never be as ef cient as a three-cylinder only. The additional piston still has inertial and frictional losses despite the valves remaining closed to eliminate pumping losses. The advantage of the extra cylinder is that, when full power is required, it increases maximum power. Many manufacturers have refrained from cylinder deactivation because the ef ciency gain is rarely worth the additional mechanical complexity in the engine and cost.