Reducing Energy Consumption, while Protecting the Environment - Energy and Climate: How to Achieve a Successful Energy Transition - page 58

The difficulties encountered when trying to reach these objectives have
pushedcarmanufacturers to investigate theuseof alternative fuelsemitting
less carbon of fossil origin per unit of energy produced (biofuels, NGV).
Hybrid propulsion
One further option for reducing passenger car CO 2 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. Bymaking 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 combus-
tion 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 electricmotor can be linked in a
parallel or a series-parallel arrangement, which combines the two possi-
ble driving modes. Further possible options arise from the possibility of
operating the system in a purely electric mode by using plug-in batteries,