Many full hybrids use a transmission with no clutches, no gear ratios and nothing to shift. A single planetary gearset and two electric machines replace the entire conventional arrangement.
A planetary gearset has three connected members
The set consists of a central sun gear, an outer ring gear, and planet gears held by a carrier that meshes with both.
The speeds of these three members are locked in a fixed mathematical relationship. Set any two and the third is determined automatically.
Conventional automatics use clutches and brakes to hold one member stationary and produce discrete ratios. The power split transmission holds nothing and produces none.
Two electric machines control the equation
The engine drives the carrier, one electric machine is attached to the sun gear, and the ring gear connects to the wheels through a fixed reduction.
By controlling the speed of the machine on the sun gear, the system can set engine speed to any value it chooses for a given road speed.
The second, larger machine sits on the output side, adding drive torque or recovering energy during braking independently of what the engine is doing.
Electrical power completes the path
Holding the sun gear against engine torque generates electricity, and that electricity is routed to the machine on the output side, where it becomes drive torque again.
Part of the engine's output therefore reaches the wheels mechanically through the gearset and part travels electrically, which is what gives the arrangement its name.
The battery buffers the difference between what is generated and what is used, allowing the engine to be run at a load that suits it rather than the road.
The result explains the unusual driving feel
Engine speed rises to a chosen value under acceleration and stays there while road speed climbs, which drivers often describe as slipping because it resembles a failing clutch.
Nothing is slipping. The engine is being held at an efficient speed while the electrical path handles the mismatch, and the sensation is a consequence of doing exactly that.
Manufacturers now program artificial steps into the control strategy so that engine speed rises and falls in a familiar pattern, purely for the driver's comfort.
Why it is mechanically simple and durable
There are no friction clutches to wear, no bands, no torque converter and no shift events, so the wearing items common to automatic transmissions are absent.
The gearset itself carries only part of the power, with the rest handled electrically, and it operates continuously rather than being engaged and disengaged.
The complexity moves into control software and power electronics, which is why these transmissions have a strong service record while being difficult to repair conventionally.