Variable displacement

Cylinder deactivation is used to reduce the fuel consumption and emissions of an internal combustion engine during light-load operation.

Operation without variable displacement is wasteful because fuel is continuously pumped into each cylinder and combusted even though maximum performance is not required.

The compression and decompression of the trapped exhaust gases have an equalising effect – overall, there is virtually no extra load on the engine.

In most instances, cylinder deactivation is applied to relatively large displacement engines that are particularly inefficient at light load.

[3] The original multi-displacement system turned off opposite pairs of cylinders, allowing the engine to have three different configurations and displacements.

The cars had an elaborate diagnostics procedure, including showing engine trouble codes on the air conditioning display.

However, the system was troublesome, misunderstood by customers, and a rash of unpredictable failures led to the technology being quickly retired.

[3] In 1981 Alfa Romeo developed in collaboration with the University of Genoa a semi-experimental variable displacement engine version of the Alfa Romeo Alfetta, called Alfetta CEM (Controllo Elettronico del Motore, or Electronic Engine Management), and showed it at the Frankfurt Motor Show.

An initial batch of 10 examples were assigned to taxi drivers in Milan, to verify operation and performance in real-world situations.

[10] Other efforts taken to minimize vibrations and harshness included a section of flexible exhaust pipe, not operating the system until the coolant temperature reached 70C, and a 70 percent heavier flywheel.

The revived MD technology was now in its second generation with improved electronic engine controls enabling the switch from 4 to 2 cylinders to be made almost imperceptibly.

In MD mode, the MIVEC engine utilizes only two of its four cylinders, which reduces significantly the energy wasted due to pumping losses.

[13] In addition to these issues, while the company proposed a hydraulically controlled system that could be switched from within the car, the version they implemented had to be manually changed in the engine compartment using hand tools.

In 2018 GM introduced an improved system called Dynamic Fuel Management[16] that shuts off any number of cylinders, in a variety of combinations, depending on immediate needs.

[21][22] As of the 2020 model year, about 15% of light-duty vehicles sold in the United States used cylinder deactivation, predominantly used by Mazda (64%), GM (44%), Honda (24%), and FCA (23%).

The best known such system was the experimental Saab Variable Compression engine, which used a hinged block to move the pistons closer to or further from the head, thus changing the size of the combustion chambers.