Hybrid vehicle drivetrain
Hybrid vehicle drivetrains transmit power to the driving wheels for hybrid vehicles. A hybrid vehicle has multiple forms of motive power, and can 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.
A typical powertrain includes all of the components used to transform stored potential energy. Powertrains may either use chemical, solar, nuclear or kinetic energy for propulsion. The oldest example is the steam locomotive. Modern examples include electric bicycles and hybrid electric vehicles, which generally combine a battery supplemented by an internal combustion engine that can either recharge the batteries or power the vehicle. Other hybrid powertrains can use flywheels to store energy.
Among different types of hybrid vehicles, only the electric/ICE type is commercially available as of 2017. One variety operated in parallel to provide power from both motors simultaneously. Another operated in series with one source exclusively providing the power and the second providing electricity. Either source may provide the primary motive force, with the other augmenting the primary.
Other combinations offer efficiency gains from superior energy management and regeneration that are offset by cost, complexity and battery limitations. Combustion-electric 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 and are far more costly. ICEs require only a battery large enough to operate the electrical system and ignite the engine.
History
Electrical vehicles have a long history combining internal combustion and electrical transmission as in a diesel–electric power-train although they have mostly been used for rail locomotives. A diesel–electric powertrain fails the strict definition of hybrid because the electric drive transmission directly replaces the mechanical transmission rather than being a supplementary source of motive power.One of the earliest forms of hybrid land vehicle was the 'trackless' trolleybus experiment in The United States that ran from 1935 to 1948, which normally used traction current delivered by wire. The trolleybus was fitted with an internal combustion engine to power the mechanical drivetrain directly, not to generate electricity for the traction motor. This enabled the vehicle to be used for revenue service where there was no contact wire.
Since the 1990s trolleybus hybrids have been introduced with small power plants to provide a low speed capability for emergency and maintenance but not to support general revenue service.
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 , the speeds at this axis must be identical and the supplied torques will add together. When only one of the two sources is in use, the other must be connected via a one-way clutch or freewheel so it can rotate freely.
With cars the two sources may be applied to the same shaft, turning at equal speeds and the torques adding up with the electric motor adding or subtracting torque to the system as necessary.
Parallel hybrids can be further categorized by the balance between the different motors are at providing motive power: the ICE may be dominant or vice versa; while in others can run on the electric system alone but because current parallel hybrids are unable to provide electric-only or internal combustion-only modes they are often categorized as mild hybrids.
Parallel hybrids rely more on regenerative braking and the ICE can also act as a generator for supplemental recharging. This makes them more efficient in urban 'stop-and-go' conditions. They use a smaller battery pack than other hybrids. Honda's early Insight, Civic, and Accord hybrids using IMA are examples of production parallel hybrids. General Motors Parallel Hybrid Truck and BAS Hybrids such as the Saturn Vue and Aura Greenline and Chevrolet Malibu hybrids also employ a parallel hybrid architecture.
Through the Road (TTR) hybrid
An alternative parallel hybrid is the "through the road" type. In this system a conventional drivetrain powers one axle, with an electric motor or motors driving another. This arrangement was used by the earliest 'off track' trolleybuses. It in effect provides a complete backup power train. In modern motors batteries can be recharged through regenerative braking or by loading the electrically driven wheels during cruise. This allows a simpler approach to power-management. This layout also has the advantage of providing four-wheel-drive in some conditions. Vehicles of this type include the Audi 100 Duo II and Subaru VIZIV concept cars, Peugeot 3008, Peugeot 508, 508 RXH, Citroën DS5, the Volvo V60 plug-in hybrid, the BMW 2 Series Active Tourer, BMW i8 and the second generation Honda NSX.Series hybrid
Series hybrids are also referred to as extended-range electric vehicles or range-extended electric vehicles, or electric vehicle with extended range. All series hybrids are EREV, REEV or EVER, but not all EREV, REEV or EVER are series hybrids. Series hybrids with particular characteristics are classified as range-extended battery-electric vehicle by the California Air Resources Board.Electric transmissions were invented by 1903. Mechanical transmissions involve costs via their weight, bulk, noise, cost, complexity and drain on engine power with every gear-change, affecting both manual and automatic systems. Unlike ICEs, electric motors typically do not require a transmission.
Compared to parallel hybrids, the mechanical transmission between the engine and wheels is discarded. The engine instead acts as an electric generator, attached to the battery via cable. The linkage is engine to battery to electric motor to wheels. In some cases, the generator also directly links to the motor.
This serial arrangement is common in diesel–electric locomotives and ships. Ferdinand Porsche successfully used this arrangement in the early 20th century in racing cars, including the Lohner–Porsche Mixte Hybrid. Porsche named the system System Mixte, which had a wheel hub motor arrangement, with a motor in each of the two front wheels, setting speed records.
This approach isolates the engine from demand, allowing it to operate only at its most efficient speed. The engine can be much smaller, since it does not have to accommodate high speed/acceleration. Traction motors are typically powered only by the battery, which can also be charged from external sources.
Nissan's e-Power line using the engine to drive a generator and the EM57 traction motor. Mazda's MX-30, is optionally equipped with a range extender. BMW's i3 attached the generator only to the battery. ThunderVolt hybrid transit buses and transit buses fitted with BAE Systems HybriDrive powertrains are also serial hybrids.
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.
- May be plugged into the grid to recharge the battery.
- Supercapacitors assist the battery and recover most energy from braking.
In detail
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 and at low or mixed speeds this could result in ~50% increase in overall efficiency.
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, used in the Nissan concept Infiniti Emerg-e.
This operating profile allows greater scope for alternative engine designs, such as a microturbine, rotary Atkinson cycle engine or linear combustion engine.
The ICE is matched to the electric engine by comparing the output rates at cruising speed. Generally, output rates for combustion engines are provided for instantaneous output rates, but in practice these can't be used.
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. Designline International of Ashburton, New Zealand produces city buses with a microturbine powered series-hybrid system. Wrightbus produces series hybrid buses including the Gemini 2 and New Routemaster. Supercapacitors combined with a lithium ion battery bank have been used by AFS Trinity in a converted Saturn Vue SUV vehicle. Using supercapacitors they claim up to 150 mpg in a series-hybrid arrangement.
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.
Series-hybrids have been taken up by the aircraft industry. The DA36 E-Star, an aircraft designed by Siemens, Diamond Aircraft and EADS, employs a series hybrid powertrain with the propeller turned by a Siemens 70 kW electric motor. A power sapping propeller speed reduction unit is eliminated. The aim is to reduce fuel consumption and emissions by up to 25 percent. An onboard 40 hp Austro Engine Wankel rotary engine and generator provides the electricity.
The Wankel was chosen because of its small size, low weight and great power to weight ratio.
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.