Hybrid vehicles are vehicles with two or more power sources in the drivetrain. There are many different types of hybrid vehicles, although only the gasoline-electric hybrid is currently commercially available.
Types by drivetrain structure
Parallel hybrid
Parallel hybrid systems, which are most commonly produced at present, have both an internal combustion engine (ICE) and an electric motor connected to a mechanical transmission. Most designs combine a large electrical generator and a motor into one unit, often located between the combustion engine and the transmission, replacing both the conventional starter motor and the alternator. To store power, a hybrid uses a large battery pack with a higher voltage than the normal automotive 12 volts. Accessories such as power steering and air conditioning are powered by electric motors instead of being attached to the combustion engine. This allows efficiency gains as the accessories can run at a constant speed, regardless of how fast the combustion engine is running.
Parallel hybrids can be categorized by the way the two sources of power are mechanically coupled. If they are joined at some axis truly in parallel, the speeds at this axis must be identical and the supplied torques add together. Most electric bicycles are in effect of this type. When only one of the two sources is being used, the other must either also rotate in an idling manner or be connected by a one-way clutch or freewheel. With cars it is more usual to join the two sources through a differential gear. Thus the torques supplied must be the same and the speeds add up, the exact ratio depending on the differential characteristics. When only one of the two sources is being used, the other must still supply a large part of the torque or be fitted with a reverse one-way clutch or automatic clamp.
Parallel hybrids can be further categorized depending upon how balanced the different portions are at providing motive power. In some cases, the combustion engine is the dominant portion (the electric motor turns on only when a boost is needed) and vice versa. Others can run with just the electric system operating.
Series hybrid
Series or serial hybrid have also been referred to as a Range-Extended Electric Vehicle (REEV); however, range extension can be accomplished with either series or parallel hybrid layouts.
Series hybrid vehicles are more like a battery electric vehicle in design than an internal combustion vehicle or parallel hybrid. In a series hybrid system, the combustion engine drives an electric generator instead of directly driving the wheels. The generator both charges a battery and powers an electric motor that moves the vehicle. When large amounts of power are required, the motor draws electricity from both the batteries and the generator. A transmission may not be needed at all and if it is present it can be far less complex, as electric motors are efficient over a wide speed range. Some vehicle designs have separate electric motors for each wheel. Series hybrids can be also fitted with a supercapacitor or a flywheel to store regenerative braking energy, which can improve efficiency by minimizing the losses in the battery. The vehicle conceptually resembles a Diesel-electric locomotive with the addition of a battery.
Because a series hybrid lacks a mechanical link between the combustion engine and the wheels, the engine can be run at a constant and efficient rate even as the vehicle changes speed. The engine can thus maintain an efficiency closer to the theoretical limit of 37%, rather than the current average of 20%. At low or mixed speeds this could result in ~50% increase in overall efficiency (19% vs 29%). The requirements for the engine are not directly linked to vehicle speed, resulting in more efficient or alternative designs possible, such as a microturbine or a linear combustion engine.
The power from the combustion engine must run through the generator and electric motor and, depending on the design, may also run through the charger and battery pack further reducing efficiency (see illustration). Each transformation results in a loss of energy. The engine-to-transmission efficiency is 70%-80%, less than a conventional mechanical clutch, having an engine-to-transmission efficiency of 98%. During long-distance highway driving, the combustion engine will need to supply the majority of the energy, in which case a series hybrid will be 20%-30% less efficient than a parallel hybrid.
The use of one motor per wheel eliminates the conventional mechanical transmission elements (gearbox, transmission shafts, differential) and can sometimes eliminate flexible couplings. If the motors are integrated into the wheels, the unsprung mass increases and suspension responsiveness decreases which impacts ride performance and potentially safety. If the motors are attached to the vehicle body, flexible couplings are still required. Advantages of individual wheel motors include simplified traction control and all wheel drive, and allowing lower floors, which is useful for buses. Some 8x8 all-wheel drive military vehicles use individual wheel motors. Diesel-electric locomotives have used this concept for over 60 years.
In 1997 Toyota released the first series hybrid bus sold in Japan. BYD Auto's F3DM sedan is the world's first mass-produced series hybrid automobile, which went on sale in China on December 15, 2008. It costs the equivalent of USD $16,062 and has an all-electric range of 68.4 miles. The F3DM is set to debut in North America and Europe in 2011. Meanwhile, GM hopes to introduce the Chevy Volt by 2011, aiming for an all-electric range of 40 miles and a price tag of around $40,000.
Power-split or series-parallel hybrid
Power-split hybrid or series-parallel hybrid 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 also 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.
Interesting variations of the simple design (pictured at right) found, for example, in the well-known Toyota Prius are the
- addition of a fixed gear second planetary gearset as used in the Lexus RX400h and Toyota Highlander Hybrid. This allows for a motor with less torque but higher power (and higher maximum rotary speed), i.e. higher power density
- addition of a ravignaux-type planetary gear (planetary gear with 4 shafts instead of 3) and two clutches as used in the Lexus GS450h. By switching the clutches, the gear ratio from MG2 (the "drive" motor) to the wheel shaft is switched, either for higher torque or higher speed (up to 250 km/h / 155 mph) while sustaining better transmission efficiency.
The Toyota Hybrid System THS / Hybrid Synergy Drive has a single power-split device (incorporated as a single 3 shaft planetary gearset) and can be classified as an Input-Split, since the power of the engine is split at the input to the transmission. This in turn makes this setup very simple in mechanical terms, but does have some drawbacks of its own. For example, the maximum speed is mainly limited by the speed of the smaller electric motor (usually functioning as a generator). Also, the efficiency of the transmission is heavily dependent on the amount of power being transmitted over the electrical path, as multiple conversions, each with their own, less than perfect efficiency, lead to a low efficiency of that path (~0.7) compared with the purely mechanical path (~0.98). Especially in higher speed regimes (>120 km/h or 70 mph) the efficiency (of the transmission alone) therefore drops below that of a generic automatic transmission with hydrodynamic coupler.
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 the fall of 2007 on the Chevrolet Tahoe Hybrid. The system was also featured on the GMC Graphite SUV concept vehicle at the 2005 North American International Auto Show in Detroit.
The Two-Mode Hybrid name is intended to emphasize the drivetrain's ability to operate in all-electric (Mode 1) as well as hybrid (Mode 2) modes. The design, however, allows for operation in more than two modes; two power-split modes are available a
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