HYBRID DRIVETRAINS
The drivetrain refers to the group of components used to generate and deliver power to the road. And generally speaking, hybrid drivetrains fall into three broad categories - series, parallel and series-parallel - and differ in the way that their electric
Series
Series hybrids are so-called because only the electric motor is used to drive the wheels, the internal combustion engine is only there to drive a generator which charges the electric motor during long distance driving. Essentially, these are really electric cars with an onboard generator that is powered by an engine.
The series hybrid drivetrain attempts to reconcile the weakness of the two forms of motors. Electric motors are highly efficient and provide good torque over a wide speed range, but unfortunately, batteries takes ages to recharge and charging stations are not as widespread as gas stations. On the other hand, the traditional internal combustion engine draws energy from fuel, which can be replenished quickly and easily. But, it is only at its most efficient when running at a constant speed.
As such, the electric motor provides the motive force, so that the internal combustion engine can be designed to run at maximum efficiency to power the generator.
Series hybrid vehicles are designed primarily for use in urban environments where commutes are generally shorter. This way they can be powered solely by batteries, thus keeping emissions down, and relying only on the internal combustion engines when trips out of town are required.
Parallel
Parallel hybrids get their names because their drivetrains are designed in such a way that the internal combustion engine and electric motor are powering the car at the same time. The premise behind the parallel hybrid drivetrain is for the electric motor to assist the engine in providing energy to the wheels, so that car makers can downsize the engine but still maintain the same level of performance.
Because the electric motor is only there to assist the engine and not take over duties completely, cars with a parallel hybrid drivetrain typically use smaller battery packs. The smaller battery packs along with the smaller engine helps reduce vehicle curb weight, which also helps improve overall fuel efficiency. Since cars with parallel hybrid drivetrains cannot run on batteries alone, they are also known as mild hybrids.
Since the batteries in parallel hybrid systems are small, they are typically recharged using regenerative braking or via the engine during cruising when power demands are low. Alternatively, if the drivetrain is setup such that the electric motor drives one axle and the engine drives the other, the batteries can be charged by loading the electrically driven wheels during cruising. Such a setup is capable of providing four-wheel-drive.
Series-Parallel
The series-parallel hybrid drivetrain, also sometimes known as the power-split drivetrain, is the most common system used in hybrid cars today. This was popularized by the Toyota Prius. It is the most complex, but it is claimed to offer the best efficiency because if designed correctly, the engine and batteries will operate at their most efficient points more frequently.
Unlike pure series and parallel drivetrains, the premise behind series-parallel drivetrain is flexibility in that the car can be powered by the electric motor and engine together or individually. Typically, this is achieved by coupling the engine, electric motor and gearbox and using automatically controlled clutches to control the drivetrain. For instance, if the car was running solely off the electric motor, the clutch between the internal combustion engine and the gearbox would be left open, while only the clutch between the electric motor and gearbox would engage.
In practical usage, what happens is that in low speed or stop-start environments such as in the city, the car will operate only using energy stored in the battery without the engine. This is where the electric motor is at its most effective because it produces maximum torque from a standstill, which is ideal for acceleration. At higher speeds or when cruising, the engine takes over because this is where the electric motor becomes less efficient.
Hybrids that utilize this type of a drivetrain are also known as full hybrids and some even offer plug-in charging capabilities.