One further option for reducing passenger car CO2 emissions is the use of hybrid propulsion. The hybrid propulsion concept for passenger cars was initially introduced in 1997 by Toyota in the Prius car.
It consists of using the association between a thermal engine and an electric engine for ensuring the propulsion of the vehicle. A battery for electricity storage is required to supply the energy needed by the electrical engine.
Using a hybrid propulsion system makes it possible to reduce energy consumption:
1. By making it possible to stop the thermal engine, when the vehicle is stationary rather than when the engine is running ('stop and start' function). This function requires only a limited power increase of the starter as compared with a standard one and yet makes it possible to achieve a reduction in the energy consumption of 7-10%.
2. By recovering part of the energy lost when braking; the additional consumption reduction is around 4-8 %.
3. By operating the engine close to its optimal operating conditions. The internal combustion engine can be operated at a constant running speed. Part of the energy supplied by the internal combustion engine is used for loading the battery when the power delivered by the internal combustion engine exceeds the power required for moving the vehicle and can be released back by the battery in the reverse case. The corresponding gain varies between 10 % and 30 %, according to the initial efficiency of the internal combustion engine.
Hybrid propulsion requires a more elaborate transmission system for making the most of all these new options.
The internal combustion engine and the electric motor can be linked in a parallel or a series-parallel arrangement, which combines the two possible driving modes. Further possible options arise from the possibility of operating the system in a purely electric mode by using plug-in batteries, which can be recharged by using an external electricity supply source. Such an option will be discussed in Chapter 6.
Hybrid propulsion is typically a transition technology. It helps to significantly improve the performance of the present vehicles, while leading the way for new propulsion systems . It contributes to the acceleration of further progress in the area of electric batteries and should thus facilitate the development of electric cars.
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