Hybrid vehicle drivetrain
Hybrid vehicle drivetrains transmit power to the driving wheels for hybrid vehicles. A hybrid vehicle has multiple forms of motive power.
Hybrids come in many configurations. For example, a hybrid may receive its energy by burning gasoline, but switch between an electric motor and a combustion engine.
Electrical vehicles have a long history combining internal combustion and electrical transmission – as in a
The
Among the different types of hybrid vehicles, only the electric/ICE type was commercially available as of 2017. One variety operated
Other combinations offer efficiency gains from superior energy management and regeneration that are offset by expense, complexity and the battery limitations. Combustion-electric (CE) hybrids have battery packs with far larger capacity than a combustion-only vehicle. A combustion-electric hybrid has batteries that are light that offer higher energy density that are far more costly. ICEs require only a battery large enough to operate the electrical system and ignite the engine.[1]
Types by design
Parallel hybrid
Parallel hybrid systems have both an internal combustion engine and an electric motor that can both individually drive the car or both coupled up jointly giving drive. This is the most common hybrid system as of 2016.
If they are joined at an axis (in parallel), the speeds at this axis must be identical and the supplied
With cars the two sources may be applied to the same shaft (for example with the electric motor connected between the engine and transmission), turning at equal speeds and the torques adding up with the electric motor adding or subtracting torque to the system as necessary. (The first two generations of Honda Insight use this system.)
Parallel hybrids rely more on
hybrids also employ a parallel hybrid architecture.Through the Road (TTR) hybrid
An alternative parallel hybrid is the "through the road" type.
Series hybrid
Series hybrids are also referred to as
Electric transmission has been available as an alternative to conventional mechanical transmissions since 1903. Typically mechanical transmissions impose many penalties, including weight, bulk, noise, cost, complexity and a drain on engine power with every gear-change, whether accomplished manually or automatically. Unlike ICEs, electric motors do not require a transmission.
In effect the entire mechanical transmission between the ICE and the wheels is removed and replaced by an electric generator, some cable and controls, and electric traction motors, with the benefit that the ICE is no longer directly connected to the demand.
This is a series-hybrid arrangement and is common in
The arguments of greater flexibility, higher efficiency and less emissions at the point of use are achieved in a series-hybrid system for road vehicles when an intermediate electric battery, acting as an energy buffer, sits between the electric generator and the electric traction motors.
The ICE turns a generator and is not mechanically connected to the driving wheels. This isolates the engine from demand, allowing it to consistently operate at its most efficient speed. Since the primary motive power is generated by the battery, a smaller generator/engine can be fitted as compared to a conventional direct drive engine. Electric traction motors can receive electricity from the battery, or directly from the engine/generator or both. Traction motors frequently are powered only by the electric battery, which can be charged from external sources such as the electricity grid.
This allows a vehicle with an engine/generator that only operates when needed, such as when the battery is depleted, or to charge the batteries. Vehicles of this type include Nissan's e-Power line (Note,[7] Serena,[8] Kicks,[9] X-Trail, [10] and Qashqai)[11] using a petrol engine to drive a generator and the EM57 traction motor;[12] Mazda's MX-30, when equipped with a rotary engine/generator range extender;[13] ThunderVolt hybrid transit buses integrated by ISE Corporation;[14] and transit buses fitted with BAE Systems (formerly Lockheed Martin) HybriDrive powertrains.[15][16]
Electric traction motors
Electric motors are more efficient than ICEs, with high power-to-weight ratios providing torque over a wide speed range. ICEs are most efficient when turning at a constant speed.
ICEs can run optimally when turning a generator. Series-hybrid systems offer smoother acceleration by avoiding gear changes. Series-hybrids incorporate:
- Electric traction only – using only electric motors to turn the wheels.
- ICE – turns only a generator.
- Generator – turned by the ICE to generate electricity and start the engine.
- Battery – energy buffer.
- Regenerative braking – The drive motor becomes a generator and recovers energy by converting kinetic to electrical energy, also slowing the vehicle and preventing thermal losses.
In addition:
- May be plugged into the grid to recharge the battery.
- Supercapacitors assist the battery and recover most energy from braking.
In detail
The electric motor may be entirely fed by electricity from the battery or via the generator turned by the ICE, or both. Such a vehicle conceptually resembles a
When the vehicle is stopped the ICE is switched off without idling, while the battery provides whatever power is needed at rest. Vehicles at traffic lights, or in slow moving stop-start traffic need not burn fuel when stationary or moving slowly, reducing emissions.
Series-hybrids can be fitted with a supercapacitor or a flywheel to store regenerative braking energy, which can improve efficiency by recovering energy otherwise lost as heat through the braking system. Because a series-hybrid has no mechanical link between the ICE and the wheels, the engine can run at a constant and efficient rate regardless of vehicle speed, achieving higher efficiency (37%, rather than the ICE average of 20%[17]) and at low or mixed speeds this could result in ~50% increase in overall efficiency (19% vs 29%).
Lotus offered an engine/generator set design that runs at two speeds, giving 15 kW of electrical power at 1,500 rpm and 35 kW at 3,500 rpm via the integrated electrical generator,[18] used in the Nissan concept Infiniti Emerg-e.
This operating profile allows greater scope for alternative engine designs, such as a
The ICE is matched to the electric engine by comparing the output rates at
The use of an electric motor driving a wheel directly eliminates the conventional mechanical transmission elements: gearbox, transmission shafts and differential, and can sometimes eliminate flexible couplings.
In 1997, Toyota released the first series-hybrid bus sold in Japan.
Well known automotive series hybrid models include the variant of the BMW i3 that is equipped with a range extender. Another example of a series hybrid automobile is the Fisker Karma. The Chevrolet Volt is almost a series hybrid, but also includes a mechanical link from the engine to the wheels above 70 mph.[24][25]
Series-hybrids have been taken up by the aircraft industry. The DA36 E-Star, an aircraft designed by
The Wankel was chosen because of its small size, low weight and great power to weight ratio. (Wankel engines also run efficiently at a constant speed of approximately 2,000 RPM which is suited to generator operation. Keeping to a constant/narrow band offsets many of the perceived disadvantages of the Wankel engine in automotive applications.[26])
The electric propeller motor uses electricity stored in batteries, with the engines not operating, to take off and climb reducing sound emissions. The powertrain reduces the weight of the plane by 100 kilos relative to its predecessor. The DA36 E-Star first flew in June 2013, making this the first ever flight of a series hybrid powertrain. Diamond Aircraft state that the technology is scalable to a 100-seat aircraft.[27][28]
In-wheel motors
If the motors are attached to the vehicle body,
Advantages of individual wheel motors include simplified
Other measures include lightweight aluminium wheels to reduce the unsprung mass of the wheel assembly; vehicle designs may be optimized to lower the centre of gravity by locating heavier elements (including battery) at floor level; In a typical road vehicle the power-transmission setup may be smaller and lighter than the equivalent conventional mechanical power-transmission setup, liberating space; the combustion generator set only requires cables to the driving electric motors, increasing flexibility in major component layout spread across a vehicle giving superior weight distribution and maximizing vehicle cabin space and opening up the possibility of superior vehicle designs exploiting this flexibility.
Power-split or series-parallel hybrid
Power-split hybrid or series-parallel hybrid are parallel hybrids that incorporate power-split devices, allowing for power paths from the ICE to the wheels that can be either mechanical or electrical. The main principle is to decouple the power supplied by the primary source from the power demanded by the driver.
ICE torque output is minimal at lower RPMs and conventional vehicles increase engine size to meet market requirements for acceptable initial acceleration. The larger engine has more power than needed for cruising. Electric motors produce full torque at standstill and are well-suited to complement ICE torque deficiency at low RPMs. In a power-split hybrid, a smaller, less flexible, and more efficient engine can be used. The conventional
Interesting variations of the simple design (pictured at right) found, for example, in the well-known Toyota Prius are the:
- fixed-ratio 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
- Ravigneaux[30]-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 traction 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. This is effectively accomplished in the Generation 3 Prius HSDs (Prius v, Prius Plug-in and Prius c), although the Generation 3 HSD has this second planetary gear set fixed at 2.5:1, rather than switching between 1:1 and 2.5:1 as the "carrier" is held fixed.
- Two additional planetary gear sets in combination with four clutches to create a Mercedes ML 450 hybrid.[31]
The Toyota Hybrid System THS /
The
Types by degree of hybridization
Type | Start-stop system | Regenerative braking Electric boost |
Charge-depleting mode | Rechargeable | Exhaust regeneration |
---|---|---|---|---|---|
Micro hybrid | Yes | No | No | No | No |
Mild hybrid | Yes | Yes | No | No | No |
Dual mild hybrid | Yes | Yes | No | No | Yes |
Full hybrid | Yes | Yes | Yes | No | No |
Dual full hybrid | Yes | Yes | Yes | No | Yes |
Plug-in hybrid | Yes | Yes | Yes | Yes | No |
Dual plug-in hybrid | Yes | Yes | Yes | Yes | Yes |
Dual hybrids
These contain two different energy recovery systems. This is a transversal categorization.
Micro hybrids
Micro hybrid is a general term given to vehicles that use some type of
Mild hybrids
Mild hybrids are essentially conventional vehicles with some hybrid hardware, but with limited hybrid features. Typically, they are a parallel hybrid with start-stop and modest levels of engine-assist or regenerative braking. Mild hybrids generally cannot provide all-electric propulsion.
Mild hybrids like the General Motors 2004–2007 Parallel Hybrid Truck (PHT) and the Honda Eco-Assist hybrids are equipped with a
The 2004–2007
Another way to offer start/stop 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.[40]
Mild hybrids are sometimes called power-assist hybrids as they use the ICE for primary power, with a torque-boosting electric motor connected to a (largely) conventional power train. The electric motor is mounted between the engine and transmission. It is essentially a large starter motor that operates when the engine needs to be turned over and when the driver "steps on the gas" and requires extra power. The electric motor may also restart the combustion engine and shutting down the main engine at idle, while the enhanced battery system is used to power accessories.[citation needed] GM announced Buick LaCrosse and Buick Regal mild-hybrids dubbed Eassist.
Before 2015, Honda's hybrids, including the Insight, used this design, leveraging their expertise in small, efficient gasoline engines; their system is dubbed Integrated Motor Assist (IMA). IMA hybrids cannot provide propulsion on electric power alone. However, since the amount of electrical power needed is much smaller, system size is reduced.
Another variation is the Saturn Vue Green Line BAS 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
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.
The Genesis G90 and Genesis GV80 Coupe offer mild hybrid options with an electric supercharger.[42][43]
Dual mild hybrids
These contain two different energy recovery systems.
The
Full hybrids
A full hybrid, sometimes also called a strong hybrid, is a vehicle that can run on just the engine, the batteries, or a combination. The
The Toyota brand name for this technology is
- Electric vehicle mode—The ICE is off and the battery powers the motor (or charges during regenerative braking). Used for idling when the battery state of charge (SOC) is high.
- Cruise mode—The vehicle is cruising (i.e. not accelerating), and the ICE can meet the demand. The power from the engine is split between the mechanical path and the generator. The battery also powers 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 charges the battery.
- Overdrive mode—A portion of the rotational energy produces electricity, because the ICE's full power is not needed to maintain speed. 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.
- 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.
- Power boost mode—Employed in situations where the engine cannot maintain the desired speed. The battery powers the motor to complement the engine power.
- 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.
Dual full hybrids
These contain two different energy recovery systems.
An example of dual hybrids are Formula One cars. See Formula One engines#2014–2021 and Formula One engines#2022–2025.
Another one is the Porsche 919 Hybrid.
The Infiniti Project Black S was cancelled.
Plug-in hybrid
A
- Can be plugged into an electrical outlet to be charged.
- Can travel powered only by the battery.
They are full hybrids, able to run on battery power. They offer greater battery capacity and the ability to recharge from the
The California Cars Initiative converted the 2004 and newer Toyota Prius to become a prototype of what it calls PRIUS+. With the addition of 140 kg (300 lb) of
Chinese battery manufacturer and automaker BYD Auto released the F3DM compact sedan to the Chinese fleet market on December 15, 2008,[49][50] later replaced by the BYD Qin plug-in hybrid.[51][52]
General Motors began deliveries of the
As of October 2012[update], the best selling PHEV is the Volt, with more than 33,000 units of the Volt/Ampera family sold worldwide since December 2010, led by US sales of 27,306,[55][56] followed by the Netherlands with 2,175 Amperas sold through October 2012.[57][58] The Prius Plug-in Hybrid had sold 21,600 units sold worldwide through October 2012, with US sales of 9,623 units, followed by Japan with 9,500 units.[56][59]
Dual plug-in hybrids
These contain two different energy recovery systems.
The Mercedes-AMG ONE is a plug-in dual hybrid.
The
Types by power source
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.[dubious ] Also, start and stop cycles may reduce the engine's ability to operate at its optimum temperature, thus reducing the engine's efficiency.
Electric-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.
Internal combustion engine-hydraulic hybrid
A
The initial concept involved a giant flywheel (see Gyrobus) for storage connected to a hydrostatic transmission. The system is under development 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 to highway speeds.
The system components were expensive, which precluded installation in smaller trucks and cars. A drawback was that the power motors were not efficient enough at part load. Focus switched to smaller vehicles. A British company, Artemis Intelligent Power, made a breakthrough by introducing an electronically controlled hydraulic motor/pump that is efficient at all ranges and loads, making small applications of petro-hydraulic hybrids feasible.[61] The company converted a BMW car to prove viability. The BMW 530i gave double the MPG in city driving compared to the standard car. The test used the standard 3,000 cc engine. Petro-hydraulic hybrids allows downsizing an engine to average power usage, not peak power usage. Peak power is provided by the energy stored in the accumulator.[62]
The kinetic braking energy recovery rate is higher and therefore the system is more efficient than 2013-era battery charged hybrids, demonstrating a 60% to 70% increase in economy in EPA testing.[63] In EPA tests 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.[64]
One research company's goal was to create a fresh design to improve the packaging of gasoline-hydraulic hybrid components. All bulky hydraulic components were integrated into the chassis. One design claimed to reach 130mpg in tests by using a large hydraulic accumulator that is also the structural chassis. The hydraulic driving motors are incorporated within the wheel hubs and reversing to recover braking energy. The aim is 170 mpg in average driving conditions. Energy created by shock absorbers and kinetic braking energy, that normally would be wasted, assists in charging the accumulator. An ICE sized for average power use charges the accumulator. The accumulator is sized to run the car for 15 minutes when fully charged.[65][66][67]
In January 2011, Chrysler announced a partnership with the EPA to design and develop an experimental gasoline-hydraulic hybrid powertrain suitable for use in passenger cars. Chrysler adapted an existing production minivan to the powertrain.[68][69][70][71][72]
NRG Dynamix of the U.S.A. claimed its approach reduced cost by one-third compared with electric hybrids and added only 300 lbs (136 kg) to vehicle weight vs. 1,000 lbs (454 kg) for electric hybrids. The company claimed a standard pickup vehicle powered by a 2.3-litre, 4-cylinder engine achieved 14 mpg (16.8 L/100 km) in city driving. Using the petro-hydraulic setup fuel economy reached "the mid 20s".[73]
Internal combustion engine-pneumatic
Compressed air can power a hybrid car with a gasoline compressor to provide the power.
Human power-environmental power
Many land and water vehicles use human power combined with a further power source. Common are parallel hybrids, e.g. a sailboat with oars,
Hybrid vehicle operation modes
Hybrid vehicles can be used in different modes. The figure shows some typical modes for a parallel hybrid configuration.
Aftermarket options
An
The aftermarket solution is used when the user delivers
In 2013 a University of Central Florida design team, On the Green, worked to develop a bolt-on hybrid conversion kit to transform an older model vehicle into a gas-electric hybrid.[75]
A conversion of a 1966 Mustang was demonstrated by an engineer in California. The system replaced the alternator with a 12 kW (30 kW peak) brushless electric motor. Gas mileage and power improved.[76]
There are
See also
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