There are four types of hybrid vehicles classified according to the size and function of the electric engine.
Micro hybrids are internal combustion vehicles equipped with stop and start systems, such as the Citroën C3.
Mild hybrids, like the Honda Insight, have electric motor of around 15 kilowatts that assist the combustion engine during the ignition and acceleration phases, which consume the most fuel and thus emit the most CO2. During propulsion at stable speeds, the combustion engine runs on its own, since it is at maximum efficiency.
Full hybrids like the Toyota Prius have 35 to 50 kW electric motor that can power the car on its own when the car is started and when driving at low speeds. They are called parallel hybrids since each engine can power the vehicle on its own.
Some hybrid vehicles have a series drivetrain structure, in which the combustion engine that runs a generator is used only to provide additional energy to the powerful electric engine that drives the vehicle. This type of arrangement, taken from railways, is particularly adapted to long-range heavy vehicles that drive at relatively constant speeds. To store the required energy, these vehicles are equipped with batteries that are recharged by the combustion engine. One example is the Chevrolet Volt which can operate in series mode or in parallel mode during acceleration phases.
For the three hybrids above, the user only fills the gas tank and never has to worry about charging the battery.
Plug-in hybrids can recharge their batteries using an external power source, and therefore manage the two energy sources independently.
Some plug-in hybrids are considered Range-Extended Electric Vehicles, with a combustion engine that is used as a complement to the electric engine - for example in stable driving conditions and to recharge the battery (series hybrids).
Current hybrid powertrains use Ni-MH batteries – or more recently Li-ion batteries in the case of the Mercedes hybrid S 400 – which can store around 1 kWh of energy and are recharged by the combustion engine. This safe and proven technology nonetheless has a limited range of 2 kilometers in purely electric mode.
Technologies under development
Diesel hybrids and plug-in hybrids
The first development is the commercialization of hybrid vehicles with optimized diesel engines that are even more efficient than gasoline vehicles, as with the Peugeot 3008, for example. Furthermore, they emit very little NOx thanks to electric engines which power the acceleration phases during which diesel engines would emit the most nitrogen oxides. The Volvo V60 plug-in hybrid is an example of this.
Also currently under development are hybrid plug-ins (rechargeable) with a higher battery capacity which can be recharged using the electric power grid. The new generation lithium batteries have a range of several dozen kilometers. Cars, even larger ones, will be able to run on purely electric energy in the city, before possibly needing to use the combustion engine on roads. Toyota commercialized the plug-in version of its Prius in 2011. This solution is particularly well-adapted to delivery trucks which supply businesses in the city center at night. Mercedes is currently developing the concept in its small trucks (Sprinter and others). Systems to increase the electric range of hybrid vehicles by transforming them into plug-ins are already on sale. A professional replaces the battery with a pack including a "smart" charger, a higher capacity battery of another kind (lithium-ion instead of Ni-MH) and electronic controls. It noticeably increases the vehicle's range in purely electric mode (for example twenty kilometers instead of two for the Toyota Prius).
Kinetic Energy Recovery Systems (KERS): other energies for hybrids
Hydraulic-pneumatic technology: it uses hydraulic pressure accumulators that combine hydraulic and compressed air technologies. During acceleration and deceleration, a hydraulic pump fills a compressed air tank. The energy is made rapidly and frequently available by means of a piston that sends the compressed air to a hydrostatic motor like the ones used in construction machinery. Thanks to strong starting torque and the ability to reach speeds of around 100 km/h, these internal combustion engine-hydraulic hybrids are well-suited to heavy urban vehicles such as buses or delivery vans.
The American Environmental Protection Agency (EPA), along with Eaton Corp. and Morgan-Olson, equipped United Parcel Service (UPS) delivery trucks with this kind of hydraulic hybrid system. They boast the same performances as non-converted models but consume half as much energy, all with very simple technologies that are commercially available.
China, the largest bus manufacturer and user, is experimenting with hydraulic hybrid buses. Poclain Hydraulics offers a stop and start system for city buses.
The French manufacturer PSA announced that it would roll out a small hybrid car combining compressed air and hydraulic technologies in 2016 (the PSA Hybrid-Air). The 3-cylinder combustion engine exists in both gasoline and diesel versions. According to the manufacturer, this technology is particularly well-adapted to small vehicles and costs half as much as a classic hybrid with the same level of CO2 emissions. The "air mode" used in cities at low speeds can cover 60 to 80 % of the driving time depending on traffic density.
Flywheels: It is also possible to store braking energy in a flywheel. Energy that is recovered during braking sets one or two massive rotors into motion. In the absence of friction (they spin in a vacuum), the rotors maintain their speed and therefore their kinetic energy.
The moving parts of the electric motor-generators release kinetic energy stored as electricity which powers a second motor-generator - connected to the tires - that is much smaller since it doesn't store any energy. This second motor-generator drives the energy-storing rotors during regenerative braking. Overall efficiency is very high, around 90 %. China has developed an experimental bus rapid transit system in which each bus is equipped with an enormous flywheel that is re-launched at each bus station. The flywheel's range covers the distance from one station to the next. The combustion engine is thus not often needed.
These kinds of deceleration energy recovery systems are used at the highest technological levels, including Formula 1 racing and the Le Mans 24 Hours race by Audi and Porsche.