e-CVT

An e-CVT, or electronic continuously variable transmission, is the transmission at the heart of a full hybrid vehicle, and despite its name it works in a fundamentally different way from a conventional belt-driven CVT. It exists to solve the central problem of a petrol-electric hybrid: how to blend the output of an internal combustion engine and one or more electric motors into a single drive to the wheels, while allowing each power source to operate at its most efficient speed independently of road speed.

The mechanism that makes this possible is a planetary, or epicyclic, gearset acting as a power-split device. The engine, a motor-generator and the drive output are each connected to different elements of the planetary gears, typically the carrier, the sun gear and the ring gear. Because a planetary set has this three-way relationship, the speed of any one element is determined by the other two. By electronically controlling the speed of the motor-generator, the system can hold the engine at a chosen, efficient speed while the wheels turn at whatever rate the road demands, effectively varying the ratio with no belts, pulleys or stepped gears at all.

The practical effect for the driver is a transmission with no perceptible shifts. Under acceleration the engine settles at an efficient speed and the road speed rises to meet it, which gives the characteristic sensation of revs that do not climb and fall in lockstep with pace. Electrical energy flows between the two motor-generators and the battery so that surplus engine power can be stored or used to drive directly, and the vehicle can pull away and run at low speeds on electric power alone with the engine switched off entirely.

This architecture is most closely associated with Toyota's Hybrid Synergy Drive and the closely related systems used across Toyota and Lexus models, with comparable power-split designs adopted by other manufacturers. Its great virtue, beyond efficiency, is mechanical simplicity and durability: with no clutches to wear, no friction bands and no hydraulic shifting, an e-CVT has remarkably few parts that degrade, which is a major reason such hybrids have earned a reputation for reliability and long service life.

There are nuances worth understanding. The much-criticised drone, where engine speed seems disconnected from acceleration, is a direct consequence of the way the system holds revs steady, and software has progressively tuned this to feel more natural. Maintenance is modest, largely limited to periodic transaxle fluid changes, though the high-voltage components and battery sit alongside the gearset. Conceptually the e-CVT relates to the ordinary CVT only in the smooth, stepless result it delivers; in its inner workings it belongs firmly to the world of the hybrid powertrain, the planetary gearset and the electric motor.