Types of hybrid vehicle
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Hybrid vehicles make use of an on-board rechargeable energy storage system (RESS) and a fuelled power source for vehicle propulsion. These can be arranged in different ways however, according to:
- the structure of the drivetrain
- the degree of hybridization
- the nature of the RESS and the fuelled power source
Contents |
[edit] Types by drivetrain structure
[edit] Series hybrid
In a series hybrid design, the internal combustion engine is not directly connected to the drivetrain, but powers an electrical generator instead. This is similar to the operation of diesel-electric train locomotives, except that as of 2006, the overwhelming majority of diesel-electric locomotives do not store auxiliary power in batteries for use in propulsion, and thus can not be called hybrid vehicles. This may change if capacitators (Super or Ultracaps) are used to act as short term storage which is the case for shunting locomotives in the US by Rail Power Technologies [1] and motorized units at JR-East. A series hybrid is similar to an electric car which is recharged by electricity from a stationary fossil fuel power plant, except that the power plant is carried on board.
Electricity from the generator is fed to the motor or motors that actually move the car, and excess energy can be used to charge batteries. When large amounts of power are required, electricity comes from both the battery pack and the engine-generator section. Because electrical motors can operate quite efficiently over a wide range of speeds, this design removes or reduces the need for a complex transmission. The internal combustion engine can also be finely tuned to operate at its most efficient speed whenever it is running, for a great gain in efficiency. Separate small electric motors installed at each wheel are featured in some prototypes and concept cars; this allows the possibility of easily controlling the power delivered to each wheel, and therefore simplifies traction control, all wheel drive, and similar features.
The advantage of this type of hybrid is the flexibility afforded by the lack of a mechanical link between the internal combustion engine and the wheels. A weakness of a series hybrid system, however, is that in a series hybrid, the power from the combustion engine has to run through both the generator and electric motor; the combined losses of the motor and generator may lead in certain operation modes (such as long-distance high speed driving) to a lower energy efficiency than that of a conventional transmission. Additionally, the power delivered to the wheels by a series hybrid is limited by the electric motor(s) (which can be overloaded for a limited time however), whereas in a parallel hybrid both the combustion engine and the electric motor can provide power to the wheels.
The use of one motor per wheel eliminates the conventional mechanical transmission elements (gearbox, transmission shafts, differential). However, when the motor is integrated into the wheel, it increases the unsprung masses and for better ride characteristics the motors may be fixed to the vehicle body, which requires the use of flexible couplings to the wheels. Also, mechanical brakes need to be fitted to the wheels for safety reasons. The use of wheel motors is particularly interesting in vehicles such as urban buses, where it may facilitate the adoption of an all-low-floor design, as well as in all-wheel drive vehicles such as military vehicles (up to 8x8) where it simplifies mechanical design.
Series hybrids are the most efficient in driving cycles that incorporate many stops and starts, such as for delivery vehicles, urban buses or stop and go city driving. In such vehicle use, the combustion engine can deliver power at a constant and more efficient rate. For long-distance highway driving however, the addition of losses in the electric transmission comes forward and a parallel hybrid may be more advisable.
Series hybrids can be also be fitted with an additional RESS for peak power purposes such as a supercapacitor or a flywheel. Such configuration may improve system efficiency minimizing the losses in the battery.
More info seen here: Serial Hybrids Are Here - Ecoworld.com
[edit] Parallel hybrid
Parallel hybrid systems, which are most commonly produced at present, connect both the electrical and internal combustion systems to the mechanical transmission. They can be subcategorized depending upon how balanced the different portions are at providing motive power. In some cases, the internal combustion engine is the dominant portion and is used for primary power, with the motor turning on only when a boost is needed. Others can run with just the electric system operating alone. Most designs combine a large electrical generator and a motor into one unit, often situated between the internal combustion engine and the transmission, in the location of the flywheel, replacing both the conventional starter motor and the generator or alternator. A large battery pack is required, providing a higher voltage than the normal automotive 12 volts. Accessories such as power steering and air conditioning are powered by electric motors, so that they continue to function when the internal combustion engine is stopped; this offers the possibility of further efficiency gains, by modulating the electrical power delivered to these systems, rather than having them run directly from the engine at a speed which depends on engine speed.
[edit] Combined hybrid
Combined hybrid systems have features of both series and 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 Toyota Hybrid System THS / Hybrid Synergy Drive mode of operation with only a single power split device (incorporated as a single 3 shaft planetary gearset) is a typical example which can also be called Input-Split Hybrid, due to the fact that a fixed amount of torque is transferred via the electrical path from the engine to the wheels. 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. 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.
The main principle behind this system is the more-or-less complete decoupling of the power supplied by the engine (or other primary source) from the power demanded by the driver. Thus a smaller, less flexible engine may be used, which is designed for maximum efficiency (often using variations of the conventional Otto cycle, such as the Miller or Atkinson cycle). This contributes significantly to the higher overall efficiency of the vehicle, with regenerative braking playing a much smaller role.
The differing torque vs. rpm characteristics of the internal combustion and electrical motors operate synergistically; an internal combustion engine's torque is minimal at lower RPMs, since the engine must be its own air pump. Thus, the need for reasonably rapid acceleration from a standing start results in an engine which is much larger than required for steady speed cruising. On the other hand, an electrical motor exhibits maximum torque at stall; therefore this engine is well suited to complement the internal combustion engine's torque deficiency at low RPMs, allowing the use of a much smaller and therefore more fuel efficient engine.
Interesting variations of that simple theme, as very well known (implemented in the 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), ie. higher power density
- addition of a ravigneux-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.
General Motors, BMW, and DaimlerChrysler are working together on a so-called Two-Mode Hybrid system which is a full hybrid plus additional efficiency improvements. The technology will be released in 2008 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.[1]
The main difference to the Input-Split Hybrid is the addition of a second planetary gearset, and the addition of two clutches (which can actually operate as one). This enables the switching (two-modes) of the percentage of mechanically vs. electrically transmitted power, and in order to cope both with low- and high-speed regimes, only smaller electrical motors with much less power and torque can be used. However, no diagrams could be obtained so far explaining the 4 gear-ratios (and why that would impose limits on the concurrent use of both electric motors or not). Most likely, an additional, 3rd ravigneux-like planetary gear with additional clutches is used for shifting between distinct final gear ratios.
[edit] Types by degree of hybridization
[edit] Full hybrid
A full hybrid, sometimes also called a strong hybrid, is a vehicle that can run on just the engine, just the batteries, or a combination of both. The Prius and Escape Hybrids are examples of this, as both cars can be moved forward on battery power alone. A large, high-capacity battery pack is needed for battery-only operation. These vehicles have a split power path that allows more flexibility in the drivetrain by interconverting mechanical and electrical power, at some cost in complexity. To balance the forces from each portion, the vehicles use a differential-style linkage between the engine and motor connected to the head end of the transmission.
The Toyota brand name for this technology is Hybrid Synergy Drive, which is being used in the Prius, Highlander sport-utility vehicle (SUV), and Camry. A computer oversees operation of the entire system, determining which half should be running, or if both should be in use, shutting off the internal combustion engine when the electric motor is sufficient to provide the power. The operation of the Prius can be divided into five (5) distinct regimes:
(1) Electric vehicle mode (2) Cruise mode (3) Battery charge mode (4) Power boost mode (5) Negative split mode
In (1), the engine is off, and the battery provides electrical energy to power the motor (or the reverse when regenerative breaking is engaged). Used for idling as well when the battery SOC is high. In (2), the vehicle is cruising (i.e. not accelerating), and the engine can meet the road load demand. The power from the engine is split between the mechanical path and the generator. The latter provides electrical energy to power the motor, whose power is summed mechanically with the engine. If the battery SOC is low, part of the power from the generator is directed towards charging the battery. Regime (3) is used for idling as well, except that in this case the battery SOC is low and requires charging, which is provided by the engine and generator. Regime (4) consists of situations where the engine cannot meet the road load demand, and the battery is then used to power the motor to provide a boost to the engine power. Finally, in (5) the vehicle is cruising and the battery SOC is high. The battery provides power to both the motor (to provide mechanical power) and to the generator. The generator converts this to mechanical energy that it directs towards the engine shaft, slowing it down (although not altering its torque output). The purpose of this engine "lugging" is to increase the fuel economy of the vehicle.
The hybrid drivetrain of the Prius, in combination with aerodynamics and optimizations in the engine itself to reduce drag, results in 80%–100% gains in fuel economy compared to four-door conventional cars of similar weight and size.
[edit] Power assist hybrid
Power assist hybrids use the engine for primary power, with a torque-boosting electric motor also connected to a largely conventional powertrain. The electric motor, mounted between the engine and transmission, is essentially a very large starter motor, which operates not only when the engine needs to be turned over, but also when the driver "steps on the gas" and requires extra power. Honda's hybrids including the Insight use this design, leveraging their reputation for design of small, efficient gasoline engines; their system is dubbed Integrated Motor Assist (IMA). Assist hybrids differ fundamentally from full hybrids in that they cannot run on electric power alone. However, since the amount of electrical power needed is much smaller, the size of the battery systems is reduced. Starting with the 2006 Civic Hybrid, the IMA system now can propel the vehicle solely on electric power during medium speed cruising.
A variation on this type of hybrid is the Saturn VUE Green Line hybrid system that uses a smaller electric motor (mounted to the side of the engine), and battery pack than the Honda IMA, but functions similarly.
Another variation on this type is Mazda's e-4WD system, offered on the Mazda Demio sold in Japan. This front-wheel drive vehicle has an electric motor which can drive the rear wheels when extra traction is needed. The system is entirely disengaged in all other driving conditions, so it does not enhance performance or economy.
Ford has dubbed Honda's hybrids "mild" in their advertising for the Escape Hybrid, arguing that the Escape's full hybrid design is more efficient. However, assist hybrids should not be confused with actual mild hybrids like the Chevrolet Silverado Hybrid. The term mild hybrid is not standardized, and its use is often more inspired by marketing than by technical considerations.
[edit] Mild hybrid
Mild hybrids are essentially conventional vehicles with oversized starter motors, allowing the engine to be turned off whenever the car is coasting, braking, or stopped, yet restart quickly and cleanly. Accessories can continue to run on electrical power while the engine is off, and as in other hybrid designs, the motor is used for regenerative braking to recapture energy. The larger motor is used to spin up the engine to operating rpm speeds before injecting any fuel.
Many people do not consider these to be hybrids at all, and these vehicles do not achieve the fuel economy of full hybrid models. A major example is the 2005 Chevrolet Silverado Hybrid, a full-size pickup truck. Chevrolet was able to get a 10% improvement on the Silverado's fuel efficiency by shutting down and restarting the engine on demand. Mild hybrids often use 42 volt systems to supply the power needed for the startup motor, as well as to compensate for the increasing number of electronic accessories on modern vehicles.
General Motors followed the pickup truck hybrid with their Belt alternator starter (BAS) hybrid system, used in the 2007 Saturn VUE Green Line. For its "start-stop" functionality, it operates similarly to the system in the Silverado. But the GM BAS has broader hybrid functionality as the electric motor can also provide modest assist under acceleration and during steady driving, and captures regenerative braking, resulting in a 20% improvement in fuel efficiency; thus, the BAS can also be considered an Assist hybrid.
[edit] Plug-in hybrid
A plug-in hybrid electric vehicle (PHEV) is a full hybrid, able to run in electric-only mode, with larger batteries and the ability to recharge from the electric power grid. They are also called gas-optional, or griddable hybrids. Their main benefit is that they can be gasoline-independent for daily commuting, but also have the extended range of a hybrid for long trips. They can also be multi-fuel, with the electric power supplemented by diesel, biodiesel, or hydrogen. The Electric Power Research Institute's research indicates a lower total cost of ownership for PHEVs due to reduced service costs and gradually improving batteries. The "well-to-wheel" efficiency and emissions of PHEVs compared to gasoline hybrids depends on the energy sources of the grid (the US grid is 50% coal; California's grid is primarily natural gas, hydroelectric power, and wind power). Particular interest in PHEVs is in California where a "million solar homes" initiative is under way, and global warming legislation has been enacted.
Prototypes of PHEVs, with larger battery packs that can be recharged from the power grid, have been built in the U.S., notably at Prof. Andy Frank's Hybrid Center[2] at UC Davis and one production PHEV, the Renault Kangoo, went on sale in France in 2003. DaimlerChrysler is currently building PHEVs based on the Mercedes-Benz Sprinter van. Light Trucks are also offered by Micro-Vett SPA[3] the so called Daily Bimodale.
The California Cars Initiative has converted the '04 and newer Toyota Prius to become a prototype of what it calls the PRIUS+. With the addition of 300 lb of lead-acid batteries, the PRIUS+ achieves roughly double the gasoline mileage of a standard Prius and can make trips of up to 10 miles using only electric power.[4] Plug -in Hybrid are like Series Hybrids.
Joseph J. Romm and Prof. Frank co-authored an article, "Hybrid Vehicles Gain Traction", published in the April 2006 issue of Scientific American, in which they argue that PHEVs will soon become standard in the automobile industry.
See also: vehicle to grid
[edit] Types by nature of the power source
[edit] Electric-Internal Combustion Hybrid
There are many ways to create an electric-internal combustion hybrid. The variety of electric-ICE designs can be differentiated by how the electric and combustion portions of the powertrain connect, at what times each portion is in operation, and what percent of the power is provided by each hybrid component. Two major categories are series hybrids and parallel hybrids, though parallel designs are most common today.
Most hybrids, no matter the specific type, use regenerative braking to recover energy when slowing down the vehicle. This simply involves driving a motor so it acts as a generator.
Many designs also shut off the internal combustion engine when it is not needed in order to save energy. That concept is not unique to hybrids; Subaru pioneered this feature in the early 1980s, and the Volkswagen Lupo 3L is one example of a conventional vehicle that shuts off its engine when at a stop. Some provision must be made, however, for accessories such as air conditioning which are normally driven by the engine. Furthermore, the lubrication systems of internal combustion engines are inherently least effective immediately after the engine starts; since it is upon startup that the majority of engine wear occurs, the frequent starting and stopping of such systems reduce the lifespan of the engine considerably. Also, start and stop cycles may reduce the engine's ability to operate at its optimum temperature, thus reducing the engine's efficiency.
[edit] Fuel cell hybrid
Fuel cell vehicles are often fitted with a battery or supercapacitor to deliver peak acceleration power and to reduce the size and power constraints on the fuel cell (and thus its cost); this is effectively also a series hybrid configuration.
[edit] Hydraulic hybrid
A hydraulic hybrid vehicle uses hydraulic and mechanical components instead of electrical ones. A variable displacement pump replaces the motor/generator, and a hydraulic accumulator (which stores energy as highly compressed nitrogen gas) replaces the batteries. The hydraulic accumulator, which is essentially a pressure tank, is potentially cheaper and more durable than batteries. Hydraulic hybrid technology was originally developed by Volvo Flygmotor and was used experimentally in buses from the early 1980s and is still an active area.
Initial concept involved a giant flywheel for storage connected to a hydrostatic transmission, but it was later changed to a simpler system using a hydraulic accumulator connected to a hydraulic pump/motor. It is also being actively developed by Eaton and several other companies, primarily in heavy vehicles like buses, trucks and military vehicles. An example is the Ford F-350 Mighty Tonka concept truck shown in 2002. It features an Eaton system that can accelerate the truck up to highway speeds.
[edit] Pneumatic hybrid
Compressed air can also power a hybrid car with a gasoline compressor to provide the power. MDI in France produces such air cars (See video). An Australian company invented a highly efficient air engine which may make pneumatic hybrid vehicle more competitive. A team led by Tsu-Chin Tsao, a UCLA mechanical and aerospace engineering professor, is collaborating with engineers from Ford to get Pneumatic hybrid technology up and running. The system is similar to that of a hybrid-electric vehicle in that braking energy is harnessed and stored to assist the engine as needed during acceleration.
[edit] Hybrid vehicle operation modes
Hybrid vehicles can be used in different modes. The figure shows some typical modes for a parallel hybrid configuration.
