Series – Parallel Hybrid Electric Vehicle / Power Split HEV

Power-split hybrid or series-parallel hybrids are parallel hybrids. They incorporate power-     split devices allowing for power paths from the engine to the wheels that can be either mechanical or electrical. The main principle behind this system is the decoupling of the power supplied by the engine (or other primary source) from the power demanded by the driver.

A combustion engine's torque is minimal at lower RPMs and in a conventional vehicle, a larger engine is necessary for acceleration from standstill. The larger engine, however, has more power than needed for steady speed cruising. An electric motor, on the other hand, exhibits maximum torque at standstill and is well-suited to complement the engine's torque deficiency at low RPMs. In a power-split hybrid, a smaller, less flexible, and highly efficient engine can be used. The conventional Otto cycle (higher power density, more low-rpm torque, lower fuel efficiency) is often  modified to a Miller cycle or Atkinson cycle (lower power density, less low-rpm torque, higher fuel efficiency). The smaller engine using a more efficient cycle contributes significantly to the higher overall efficiency of the vehicle.

Toyota Prius is a series-parallel hybrid, technology used for Prius is called Hybrid Synergy Drive by Toyota. It is having a single power-split device, since the power of the engine is split at the input to the transmission so it is known as input-split. General Motors, BMW, and DaimlerChrysler have developed in collaboration a system named "Two-Mode Hybrid" as part of the Global Hybrid Cooperation. The technology was released in 2007 on the Chevrolet Tahoe Hybrid. The Two-Mode Hybrid can operate in two power-split modes, the design can be referred as a multi-regime design. The Two-Mode Hybrid powertrain design can be classified as a compound-split design, the addition of four clutches within the transmission allows for multiple configurations of engine power-splitting. The objective of the design is to vary the percentage of mechanically vs. electrically transmitted power to cope both with low-speed and high-speed operating conditions. This enables smaller motors to do the job of larger motors when compared to single-mode systems. The four fixed gears enable the Two-Mode Hybrid to function like a conventional parallel hybrid under high continuous power regions such as sustained high speed cruising or trailer towing.