Generally, hybrid vehicle of series-parallel type is built around one epicycloid gear train used as a power split device. In the following description, the vehicle has got two epicycloid gear trains with a common satellite carrier driven by a thermal engine, a common ring gear which drives the vehicle wheels in cooperation with an electrical Motor and two planetary gears which selectively drive a Generator. The selection of one of the planetary gears determines the gear train which is active (refer to figure 1). The Motor and the Generator work close to the electric balance what is a condition to get a small Battery.

The selection of the epicycloid train is done by a dog clutch which has 5 axial positions named: a, b, c, d, e. In "d" the train 1 of low gear ratio (= planetary gear teeth number / ring gear teeth number) is activated, In "b" the train 2 of high gear ratio is activated, in "c" both trains 1 and 2 are connected through their planetary gear and by consequence, all shafts rotate at the same speed at a fixed gear ratio of 1, In "a" the planetary gear of the train 2 is locked and the train 2 works at a fixed gear ratio of 1.2 in our example, In "e" the planetary gear of the train 1 is locked and the train 1 works at a fixed gear ratio of 1.6 in our example.

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http://contest.techbriefs.com/2015/entries/sustainable-technologies/5453

The « c » mechanical function can be replaced by an electric shaft between the Generator and the Motor. The “d” and “e” functions can be done by making the Generator rotating slowly.

Refer to figure 2: characteristics diagram and available propulsion powers.
From 0 to 40 km/h, from O to C, "d" function: the system works as a classic CVT, but with a very low gear ratio which provides high torque and which does not need extra power from the Motor and the Battery (the Motor does not require to be over sized).
From 40 to 70 km/h, from C to D, "c" function: the system works at a fixed gear ratio, here 1.
From 70 to 100 km/h, from E to F, "e" function: the system works at a fixed gear ratio, here 1.2.
From 100 km/h, from H to I, "a" function: the system works at a fixed gear ratio , here 1.6.

This arrangement concentrates hybridization effect on low speeds, where it is the most effective. It also improves torque at low speed in the point to exempt of an additional electric propulsion fed by the Battery. For superior speeds the system has 3 fixed gear ratios which no longer consume electrical energy for a better efficiency.

The system allows to maintain the power of the electric chain below 10 KW for an ICE of 90 KW. Consequently, it should be compatible with the 48 V voltage while driving improvements and more safety.