Types by degree of hybridization.
1. Full Hybrid is also called a strong hybrid, is a vehicle that can run on just the engine, just the batteries, or a combination of both. The Toyota Prius, Toyota Camry Hybrid, Ford Escape Hybrid/Mercury Mariner Hybrid, Ford Fusion Hybrid/Mercury Milan Hybrid, as well as the General Motors 2-mode hybrid trucks and SUVs, are examples of this type of hybridization as they are able to be propelled 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 inter-converting 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, the Highlander Hybrid SUV, and the Camry Hybrid. A computer oversees operation of the entire system, determining which half should be running, or if both should be in use. The operation of the Prius can be divided into six distinct regimes.
Ø Electric vehicle mode: The engine is off, and the battery provides electrical energy to power the motor (or the reverse when regenerative braking is engaged). Used for idling as well when the battery State Of Charge (SOC) is high.
Ø Cruise mode: 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 state-of-charge is low, part of the power from the generator is directed towards charging the battery.
Ø Overdrive mode: A portion of the rotational energy is siphoned off by the main electric motor, operating as a generator, to produce electricity. This electrical energy is used to drive the sun gear in the direction opposite its usual rotation. The end result has the ring gear rotating faster than the engine, albeit at lower torque.
Ø Battery charge mode: Also used for idling, except that in this case the battery state-of-charge is low and requires charging, which is provided by the engine and generator.
Ø Power boost mode: Employed in situations where the engine cannot meet the road load demand. The battery is then used to power the motor to provide a boost to the engine power.
Ø Negative split mode: The vehicle is cruising and the battery state-of-charge 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.
2. Mild hybrids are essentially conventional vehicles with some degree of hybrid hardware, but with limited hybrid feature utilization. Typically they are a parallel system with start-stop only or possibly in combination with modest levels of engine assist or regenerative braking features. Unlike full hybrids, Mild hybrids generally cannot provide ICE-OFF all-electric (EV) propulsion.
Another way to provide for shutting off a car's engine when it is stopped, then immediately restarting it when it's time to go, is by employing a static start engine. Such an engine requires no starter motor, but employs sensors to determine the exact position of each piston, then precisely timing the injection and ignition of fuel to turn over the engine.
3. Plug-in Hybrid Electric Vehicle (PHEV) has two defining characteristics:
b. It has some range that can be traveled on the energy it stored while plugged in.
They are full hybrid, able to run in electric-only mode, with larger batteries and the ability to recharge from the electric power grid. And can be parallel or series hybrid designs. 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.
Engine compartment of a BYD F3DM plug-in hybrid.
Types of by nature of the power source.
1. Electric-internal combustion engine hybrid
- There are many ways to create an electric-Internal Combustion Engine (ICE) 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.
2. Electric-fuel cell hybrid
- Fuel cell vehicles are often fitted with a battery or super capacitor 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.
Structure of a fuel cell hybrid electric vehicle
3. Internal Combustion Engine-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 (see Gyrobus) 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.
- The energy recovery rate is higher and therefore the system is more efficient than battery charged hybrids, demonstrating a 60% to 70% increase in economy in EPA testing. Under tests done by the EPA, a hydraulic hybrid Ford Expedition returned 32 mpg-US (7.4 L/100 km) in urban driving, and 22 mpg-US (11 L/100 km) on the highway. UPS currently has two trucks in service with this technology. While the system has faster and more efficient charge/discharge cycling, the accumulator size and pressure dictates total energy capacity, and requires more space than a battery.
4. Internal Combustion Engine-Pneumatic Hybrid
- Compressed air can also power a hybrid car with a gasoline compressor to provide the power. Motor Development International in France is developing such air-powered cars. 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.
5. Solar Photovoltaics (PV)
To date it has not been mentioned or done, the integration of photovoltaics with a Hybrid car as a way to extend the distance a car can travel on electric power alone or as a way to help keep batteries charged or to recharge them when a car is parked, or for that matter, standing at a traffic light. With the advent of thin-film photovoltaics the cost of integrating them could become negligible in mass production.
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