Hybrid vehicle drivetrain

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Hybrid vehicles are vehicles with multiple sources (at least two) forms of motive power, defined as main components that generate power and deliver it to the road surface, water, or air. In a land vehicle this conventionally includes the engine, transmission, drive shafts, differentials, and the final drive to wheels, continuous tracks, propellers, etc.

While electrical vehicles have a long history combining internal combustion and electrical transmission - as in a diesel-electric power train - has rarely been used for any vehicular application other than railways and fails to met the definition of 'hybrid' as the electrical system simply replaces mechanical transmission rather than being a supplementary source of motive power. The earliest form of hybrid land vehicle meeting the modern definition is therefore the 'trackless' trolleybus of the 1930s, which normally used traction current delivered by wire but was commonly fitted with an internal combustion engine to either directly power the bus or to independently generate electrical power, enabling the vehicle to manoeuvre around obstacles and broken transmission wires. .

In the widest sense the power-train includes all of its components used to transform stored potential energy, be it chemical, solar, nuclear or kinetic into a form useful for propulsion purposes, the oldest example being the galley that used sails and oars, the most common on most city streets the electrically assisted pedal bicycle, the most novel the flywheel bus of the 1950s or the latest hybrid car featuring a combination of stored electrical potential in the form of battery, supplemented by a small, efficient, clean-burning internal combustion engine that can recharge the batteries on a long journey or directly power the vehicle in addition to battery power to give higher performance.

There are many different types of hybrid vehicles but only the gasoline (petrol) - electric kind is currently commercially available. As stated both sources may operate in parallel to simultaneously provide acceleration, or they may operate in series with one source exclusively providing the acceleration and the second being used to provide a power reserve. Ideally both systems are capable of being used flexibly in series and parallel as needed, but in practice it is usual to regard one source as primary, the other being used ot augment the first.

Current hybrids use both an internal combustion engine (ICE) and a battery/electric drive system (commonly using ultracapacitors) to improve fuel consumption, emission, performance and to recover energy when braking, in effect harvesting some of the energy that would otherwise be lost and storing it.

While other combinations of energy storage and conversion are possible, none are in commercial production. Efficiency gains from superior energy management and regeneration are offset by expense, complexity and the limitations of battery technology. (Combustion-electric hybrids have far larger capacity battery sets than a normal combustion-only vehicle requires and the cheapest batteries are heavy and inefficient while the lightest that ofer higher energy density are far more costly.) However battery and supercapacitor technology is advancing[1] offsetting the relatively short lifespan of conventional batteries: when they require replacement year on year it is likely newer battery sets will be technically superior and - if the pattern established in the electronics industry is replicated - superior energy storage will become increasingly affordable.

Types by drive train structure

Parallel hybrid

Structure of a parallel hybrid electric vehicle. The grey squares represent differential gears.

Parallel hybrid systems, which are most commonly produced at present, have both an internal combustion engine (ICE) and a coupled electric motor.

If they are joined at an axis in parallel, the speeds at this axis must be identical and the supplied torques add together. (Most electric bicycles are of this type.) When only one of the two sources is being used, the other must either also rotate (be idling), be connected by a one-way clutch or freewheel.

With cars the two sources may be applied to the same shaft (for example with the electric motor lying between the engine and transmission), the speeds thus being equal and the torques adding up with the electric motor adding or subtracting torque to the system as necessary. (The Honda Insight uses this system.)

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 dominant (the electric motor turns on only when a boost is needed) 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 (see below).

Because parallel hybrids rely more on regenerative braking and the internal combustion engine can also act as a generator for supplemental recharging they are more efficient in urban 'stop-and-go' conditions commonly experienced when driving in cities as compared to highway driving. They can therefore use a smaller battery pack.[2] Honda's Insight, Civic, and Accord hybrids are examples of production parallel hybrids.[2] General Motors Parallel Hybrid Truck (PHT) and BAS Hybrids such as the Saturn VUE and Aura Greenline and Chevrolet Malibu hybrids are also deemed to employ a parallel hybrid architecture.

Through The Road (TTR) Hybrid

An alternative parallel hybrid layout is the 'through the road' type.[3][4] In this system a conventional drivetrain powers one axle, with an electric motor or motors driving another. This arrangement (used by the earliest 'off track' trolleybuses) in effect provides a complete back up power train, but because in modern motors batteries can be recharged through regenerative braking or by loading the electrically driven wheels during cruise a simpler approach to power-management can be taken and this layout also has the advantage of providing four-wheel-drive in some conditions. (An example of this principle is a bicycle fitted with a front hub motor, which assists the cyclist's pedal power at the rear wheel.) Vehicles of this type include the Audi 100 Duo II and Subaru VIZIV concept cars, the Peugeot 3008 HYbrid4, the Volvo V60 plug-in hybrid and the BMW i8.

Series hybrid

Structure of a series-hybrid vehicle. The grey square represents a differential gear. An alternative arrangement (not shown) is to have electric motors at two or four wheels.

Series hybrids have also been referred to as extended-range electric vehicles (EREV)[5] or range-extended electric vehicles (REEV). (Series hybrids with particular characteristics are classified as range-extended battery-electric vehicle (BEVx) by the California Air Resources Board.[6])

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 combustion engines, with electric motors appropriate to any given vehicle performing any given role a multiple-speed transmission is not essential. T

In effect the entire mechanical transmission between the internal combustion engine and the wheels is removed and replaced by an electric generator, some cable and controls, and electric traction motors, with the benefit the internal combustion engine is no longer directly connected from the demand.

This is a series-hybrid arrangement and is common in diesel-electric locomotives and ships (the Russian river ship Vandal launched in 1903, was the world's first diesel-powered and diesel-electric powered vessel) and 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. A wheel hub motor arrangement, with a motor in each of the two front wheels, setting speed records. (Essentially the same system was used, with much less success, to power his entry for the "Tiger" tank competition, and the heaviest armoured fighting vehicle design ever built, both ventures costly failures Nazi Germany - and Porsche - could ill-afford during the late stages of World War II.)

While an old concept the arguments of greater flexibility, higher efficiencies and the new (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.

As the internal combustion engine is mechanically disconnected from the driving wheels, in effect the engine is isolated from demand, the electric traction system and generator able to operate quite independently of each other. This has many advantages: a smaller generator/engine can be fitted as compared to the size of a conventional direct drive engine; traction motors can receive electricity from the battery or generator or both, improving load balancing; and traction motors (depending on the size of the battery bank) will frequently be powered only by the electric battery which may be charged from external sources such as the electricity grid.

This allows a vehicle with a generator operating as a second stage (only when needed to drive the vehicle or charge the batteries) to be freely driven in urban areas with zero emissions requirements, the generator only cutting in for longer journeys or when the battery bank is depleted, typically on the return leg beyond the zero-emmissions zone.

In short the benefits of a series-hybrid is simplicity; the vehicle is driven only by electric traction motors with a generator set providing the electric power when needed; an electric battery acts as buffer that evens out demand and the stored energy may be used as the prime source to propel the vehicle; but when required the engine/generator can be used as a backup, to assist in acceleration, or when pulling heavy loads.

Electric Traction Motors

The choice of electric traction motors has great advantages. Unlike piston internal combustion engines, electric motors are highly efficient with exceptionally high power-to-weight ratios providing adequate torque when running over a wide speed range. Internal combustion engines run at their most efficient when turning at a constant speed.

An engine turning a generator can be designed to run at its maximum efficiency constant speed, or a series of constant speeds. Combining the two, which can operate quite independently of each other, gives maximum efficiency and performance overall. The arrangement was difficult for early production cars as synchronization of power generation and demand was inefficient, resulting in higher fuel consumption.

This is no longer an issue with modern computer engine management systems optimizing when the generator runs to match the power needed.

Since Ferdinand Porsche's series-hybrid car, electric motors have become substantially smaller, lighter and efficient over the years. One of the advantages of a series-hybrid system is the smoother progressive ride with no stepped gear ratio changes.

The electric transmission is currently viable in replacing the mechanical transmission. However, the modern series-hybrid vehicles takes the electric transmission to a higher plane adding greater value. Modern series-hybrids incorporate:

  • Electric traction only - using only one or more electric motors to turn the wheels.
  • Combustion engine - that turns only a generator.
  • A generator - turned by the combustion engine to make up a generator set that also acts as an engine starter.
  • A battery bank - which acts as an energy buffer.
  • Regenerative braking - The driving motor becomes a generator and recovers potential and kinetic (inertial) energies through its conversion to electrical energy, a process which in turn is able to slow the vehicle and thus preventing wasteful transfer of this energy as thermal losses within the friction brakes.

In addition:

  • May be plugged into the electric mains system to recharge the battery bank.
  • May have super capacitors to assist the battery bank and claw back most energy from braking - only fitted in proven prototypes currently.

In detail

The electric driving motor may run entirely fed by electricity from a large battery bank or via the generator turned by the internal combustion engine, or both; the battery bank may be charged by mains electricity reducing running costs, as the range running under the electric motors only is extended; the vehicle conceptually resembles a Diesel-electric locomotive with the addition of a large battery bank that may power the vehicle without the internal combustion engine running and acting as an energy buffer that is used to accelerate and achieve a greater top speed; the generator may simultaneously charge the battery bank and power the driving electric motor that moves the vehicle.

Another advantage is that when the vehicle is stopped the combustion engine is switched off. Vehicles at traffic lights, or in slow moving stop-start traffic need not be polluting when stationary or moving very slowly and when the vehicle moves it does so using the energy in the batteries. This reduces kerbside emissions greatly in cities and towns.

Series-hybrids can also be fitted with a supercapacitor or a flywheel to store regenerative braking energy, which can improve efficiency by clawing back energy that otherwise would be lost being dissipated via heat through the braking system.Because a series-hybrid omits 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 (37%, rather than the current average of 20%[7]) and at low or mixed speeds this could result in ~50% increase in overall efficiency (19% vs 29%).

The Lotus company has introduced 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,[8] used in the Nissan concept Infiniti Emerg-e.

As the requirements for the engine are not directly linked to vehicle speed, this gives greater scope for more efficient or alternative engine designs, such as a microturbine,[9] rotary Atkinson cycle engine or a linear combustion engine.[10]

(Note that whatever combustion engine is used, it should be matched to the electric engine by comparing the output rates at cruising speed. Generally, output rates for combustion engines are provided for instantaneous (peak) output rates,[11] but in practice these can't be used.)

General Motors in 1999 made the experimental EV1 series hybrid using a turbine generator set: the turbine weighed 220 lb (99.8 kg), measured 20 inches (50.8 cm) in diameter by 22 inches (55.9 cm) long and ran between 100,000 and 140,000 rpm. Fuel consumption was 60 mpg-US (3.9 L/100 km; 72 mpg-imp) to 100 mpg-US (2.4 L/100 km; 120 mpg-imp) in hybrid mode. Depending on the driving conditions, a highway range of more than 390 miles (627.6 km) was achieved.

The results were highly successful, and would have promised to be more successful if a smaller microturbine was used yet the EV1 project was dropped.

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. (If the motors are attached to the vehicle body, flexible couplings are required but if the traction motors are integrated into the wheels a disadvantage is that the unsprung mass increases and suspension responsiveness decreases which impacts ride performance and potentially safety, however the impact should be minimal if at all as electric motors in wheel hubs such as Hi-Pa Drive, may be very small and light having exceptionally high power-to-weight ratios and braking mechanisms can be lighter as the wheel motors brake the vehicle.)

Advantages of individual wheel motors include simplified traction control, all wheel drive if required, and lower floors, which is useful for buses and other specialised vehicles (some 8x8 all-wheel drive military vehicles use individual wheel motors). Diesel-electric locomotives have used this concept (individual motors driving axles of each pair of wheels) for 70 years.[12][full citation needed]

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 banks) at floor level; In a typical road vehicle the whole series-hybrid 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, there is greater 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.

In 1997, Toyota released the first series-hybrid bus sold in Japan.[13] 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.[14]

Well known automotive series hybrid models include variants of the Chevrolet Volt and BMW i3. Another example of a series hybrid automobile is the Fisker Karma.

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 (94 hp) 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 (30 kW) Austro Engine Wankel rotary engine and generator provides the electricity.

In this case a Wankel rotary engine was chosen because of the very small size, low weight and great power to weight ratio offered, which are assets suited to aircraft. (Wankel engines also run efficiently at a constants speed of approximately 2,000rpm which is highly suited to generator turning operation. Keeping to a near constant, or narrow band, of revolutions eliminates, or vastly reduces, many of the perceived disadvantages of the Wankel engine in automotive aplications.[15])

The electric propeller motor uses electricity stored in batteries, with the engines not operating, to take off and climb reducing sound emissions. The series hybrid powertrain using the Wankel engine reduces the weight of the plane by 100 kilos 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 using Wankel engines is scalable to a 100-seater aircraft.[16][17]

Power-split or series-parallel hybrid

Structure of a combined hybrid electric vehicle

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 output is minimal at lower RPMs and, in a conventional vehicle, a larger engine is necessary for acceptable 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 Atkinson cycle or Miller cycle (lower power density, less low-rpm torque, higher fuel efficiency; sometimes called an Atkinson-Miller cycle). The smaller engine, using a more efficient cycle and often operating in the favorable region of the brake specific fuel consumption map, 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 Ravigneaux[18]-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.
Power-splitter series-hybrid Toyota Prius.

The Toyota Hybrid System THS / Hybrid Synergy Drive has a single power-split device (incorporated as a single three-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, in the Generation 1 and Generation 2 HSDs the maximum speed is mainly limited by the speed of the smaller electric motor (usually functioning as a generator). The Generation 3 HSD separates the ICE-MG1 path from the MG2 path, each with its own, tailored, gear ratio (1.1:1 and 2.5:1, respectively, for late Priuses, including the Prius c).

Early Hybrid Synergy Drive. Generation 1/Generation 2 (chained) ICE-MG1-MG2 Power Split Device HSD is shown. MG2 ratio permanently set at 1:1.
Late Hybrid Synergy Drive. Generation 3 (chainless) ICE-MG1 Power Split Device/MG2 Motor Speed Reduction Device HSD is shown. MG2 ratio permanently set at 2.5:1.

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.[20] BYD Auto's F3DM sedan is a series-parallel plug-in hybrid automobile, which went on sale in China on December 15, 2008.[21][22][23]

The Two-Mode Hybrid name is intended to emphasize the drive-train's ability to operate in all-electric (Mode 1, or Input-Split) as well as hybrid (Mode 2, or Compound-Split) modes. The design, however, allows for operation in more than two modes; two power-split modes are available along with several fixed gear (essentially parallel hybrid) regimes. For this reason, the design can be referred to as a multi-regime design.[24] The Two-Mode Hybrid powertrain design can be classified as a compound-split design, since the addition of four clutches within the transmission allows for multiple configurations of engine power-splitting. In addition to the clutches, this transmission also has a second planetary gearset. The objective of the design is to vary the percentage of mechanically vs. electrically transmitted power to cope both with low-speed and high-speed operating conditions. This enables smaller motors to do the job of larger motors when compared to single-mode systems, because the derived electrical peak power is proportional to the width of the continuous variation range. The four fixed gears enable the Two-Mode Hybrid to function like a conventional parallel hybrid under high continuous power regions such as sustained high speed cruising or trailer towing. Full electric boost is available in fixed gear modes.[25]

Types by degree of hybridization

Type Start-stop system Regenerative braking
Electric boost
Charge-depleting mode Rechargeable
Micro hybrid Yes No No No
Mild hybrid Yes Yes No No
Full hybrid Yes Yes Yes No
Plug-in hybrid Yes Yes Yes Yes

Micro hybrids

Micro hybrid is a general term given to vehicles that use some type of start-stop system to automatically shut off the engine when idling. Strictly speaking, micro hybrids are not real hybrid vehicles, because they do not rely on two different sources of power.[26]

Mild hybrids

Engine compartment of a 2006 GMC Sierra Hybrid.

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.

Mild hybrids like the General Motors 2004-07 Parallel Hybrid Truck (PHT) and the Honda Eco-Assist hybrids are equipped with a three-phase electric motor mounted within the bell-housing between the engine and transmission, allowing the engine to be turned off whenever the truck is coasting, braking, or stopped, yet restart quickly when required. 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 large electric motor is used to spin up the engine to operating rpm speeds before injecting any fuel.

The 2004–07 Chevrolet Silverado PHT, was 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 and using regenerative braking. However the electrical motor was not used to provide propulsion or assist, rather the electrical energy was used to drive accessories like the A/C and power steering.The GM PHT used a 42 volt systems via a pack comprising three 12 V vented lead acid batteries connected in series (36V total) 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 parallel hybrid truck with their BAS Hybrid system, another mild hybrid implementation officially released on the 2007 Saturn Vue Green Line. For its "start-stop" functionality, it operates similarly to the system in the Silverado, although via a belted connection to the motor/generator unit. However the GM BAS Hybridsystem has broader hybrid functionality as the electric motor can also provide modest assist under acceleration and during steady driving, and captures energy during regenerative (blended) braking. The BAS Hybrid can result in as much as a 27% improvement in combined fuel efficiency as noted by the EPA in testing of the 2009 Saturn VUE.[27] The BAS Hybrid system can also be found on the 2008-09 Saturn Aura and the 2008-2010 Chevrolet Malibu hybrids.

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.[28]

Mild hybrids are sometimes called Power assist hybrids' as they use the engine for primary power, with a torque-boosting electric motor also connected to a largely conventional power train. 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. The electric motor may also be used to restart the combustion engine, deriving the same benefits from shutting down the main engine at idle, while the enhanced battery system is used to power accessories.[citation needed]GM is going to produce Buick LaCrosse and Buick Regal mild hybrids dubbed Eassist.

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 propulsion cannot be accomplished on electric power alone. However, since the amount of electrical power needed is much smaller, the size of the system is reduced.

A variation on this type of hybrid 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 Mazda's e-4WD system, offered on the Mazda Demio sold in Japan.[citation needed] 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 directly enhance performance or economy but allows the use of a smaller and more economical engine relative to total performance.

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.

Full hybrids

Engine compartment of a 2006 Mercury Mariner 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 Toyota Prius, Toyota Camry Hybrid, Ford Escape Hybrid/Mercury Mariner Hybrid, Ford Fusion Hybrid/Lincoln MKZ Hybrid/Mercury Milan Hybrid, Ford C-Max Hybrid, Kia Optima 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.

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.
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 battery 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.
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.
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 meet the road load demand. The battery is then used to power the motor to provide a boost to 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.

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.[citation needed]

Plug-in hybrid

Chevrolet Volt charging

A plug-in hybrid electric vehicle (PHEV) has two defining characteristics:

  1. It can be plugged into an electrical outlet to be charged.
  2. It has some range that can be traveled on the energy it stored while plugged in.

They are full hybrids, 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.

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[29] at University of California, 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[30] the so-called Daily Bimodale.

The California Cars Initiative has converted the 2004 and newer Toyota Prius to become a prototype of what it calls the PRIUS+. With the addition of 140 kg (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 16 km (10 mi) miles using only electric power.[31]

Chinese battery manufacturer and automaker BYD Auto released the F3DM compact sedan to the Chinese fleet market on December 15, 2008.[32][33] Due to low sales, BYD announced in April 2010 that the F3DM will be replaced by the BYD Qin plug-in hybrid.[34][35]

General Motors began deliveries of the Chevrolet Volt in the United States in December 2010,[5] and its sibling, the Opel Ampera, was released for retail customers in Europe by early 2012.[36][37] As of November 2012, other plug-in hybrids available in several markets are the Fisker Karma, Toyota Prius Plug-in Hybrid, and Ford C-Max Energi.

As of October 2012, the best selling plug-in hybrid is the Chevrolet Volt, with more than 33,000 units of the Volt/Ampera family sold worldwide since December 2010, with sales led by the U.S. with 27,306 Volts sold through October 2012,[38][39] followed by the Netherlands with 2,175 Amperas sold through October 2012.[40][41] The second top selling PHEV is the Prius Plug-in Hybrid, with 21,600 units sold worldwide through October 2012, with sales led by the United States with 9,623 units through October 2012, followed by Japan with 9,500 units through October 2012.[39][42]

Types by nature of the 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.

Structure of a fuel cell hybrid electric vehicle

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

Chrysler are adapting a minvan to a gasoline-hydraulic hybrid setup.

A hydraulic hybrid vehicle uses hydraulic and mechanical components instead of electrical. A variable displacement pump replaces the electric motor/generator. A hydraulic accumulator, is a vessel which stores energy. The vessel typically has a flexible bladder of pre-charged pressurized nitrogen gas inside. Pumped hydraulic fluid is compressed against the bladder storing the energy in the highly compressed nitrogen gas. Some versions have a piston in a cylinder rather than pressurized bladder of nitrogen gas. The accumulator replaces the batteries on petro-electric hybrid. The hydraulic accumulator is potentially cheaper and more durable than batteries. Hydraulic hybrid technology was originally implemented in Germany in the 1930s. Volvo Flygmotor used petro-hydraulic hybrids 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, however later changed to a simpler system using a hydraulic accumulator connected to a hydraulic pump/motor. The system 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 system components were expensive which precluded installation in smaller trucks and cars. A drawback was that the power driving motors were not efficient enough at part load. Focus has now switched to smaller vehicles. A British company has made a breakthrough by introducing an electronically controlled hydraulic motor/pump, the Digital Displacement® motor/pump, that is highly efficient at all ranges and loads making small applications of petro-hydraulic hybrids feasible.[43] The company converted a BMW car as a test bed to prove viability. The BMW 530i, gave double the MPG in city driving compared to the standard car. This was tested using the standard 3,000cc engine. Petro-hydraulic hybrids using well sized accumulators entails downsizing an engine to average power usage, not peak power usage. Peak power is provided by the energy stored in the accumulator.[44]

The kinetic braking energy recovery rate is higher and therefore the system is more efficient than current battery charged hybrids, demonstrating a 60% to 70% increase in economy in EPA testing.[45] Under tests undertaken 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.[46] UPS currently has two trucks in service with this technology.[47] 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. However, for the energy stored the accumulator is smaller in physical size than a current battery pack.

One research company's goal is to create a blank paper design new car, to maximize the packaging of gasoline-hydraulic hybrid components in the vehicle. All bulky hydraulic components are integrated into the chassis of the car. One design has claimed to return 130mpg in tests by using a large hydraulic accumulator which is also the structural chassis of the car. The small hydraulic driving motors are incorporated within the wheel hubs driving the wheels and reversing to claw-back kinetic braking energy. The aim is 170mpg in average driving conditions. Energy created by shock absorbers and kinetic braking energy that normally would be wasted assists in charging the accumulator. A small fossil fuelled piston engine sized for average power use charges the accumulator. The accumulator is sized at running the car for 15 minutes when fully charged. The aim is a fully charged accumulator with an energy storage potential of 670 HP, which will produce a 0-60 mph acceleration speed of under 5 seconds using four wheel drive.[48][49][50]

In January 2011, industry giant Chrysler announced a partnership with the U.S. Environmental Protection Agency (EPA) to design and develop an experimental gasoline-hydraulic hybrid powertrain suitable for use in large passenger cars. Chrysler adapted an existing production minvan to the new hydraulic powertrain.[51][52][53][54] Research is ongoing in this field.[55]

NRG Dynamix of the U.S.A. claims its approach reduces cost one-third compared with electric hybrids and adds only 300 lbs (136 kg) to vehicle weight vs. 1,000 lbs (454 kg) for electric hybrids. Under tests the company claim a standard pickup vehicle powered by a 2.3 litre 4 cylinder engine returns 14 mpg (16.8 l/100 km) in city driving. Using the petro-hydraulic setup fuel economy climbs to "the mid 20s".[56]

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.

Human power and environmental power hybrids

Many land and water vehicles use human power combined with a further power source. Common are parallel hybrids, e.g. a boat being rowed and also having a sail set, or motorized bicycles, or a human-electric hybrid vehicle such as the Twike. Also some series hybrids exist, see in hybrid vehicle. Such vehicles can be tribrid vehicles, combining at the same time three power sources e.g. from on-board solar cells, from grid-charged batteries, and from pedals.

Hybrid vehicle operation modes

Hybrid vehicles can be used in different modes. The figure shows some typical modes for a parallel hybrid configuration.

Hybrid modes.gif

Adding powertrains and aftermarket kits

One can install conmarket or aftermarket powertrain to a vehicle to hybridise it.

The conmarket solution is used when the user buys the glider (rolling chassis) and the hybrid (two engines) or all-electric (only an electric motor) powertrain kit to the automaker and receives it installed in the car. Also an (electric or hybrid) powertrain can be added to a glider[57] by a third party aftermarket installer.

A University of Central Florida senior design team, On the Green, is currently developing a bolt-on hybrid conversion kit to transform an older model vehicle into a gas-electric hybrid.[58]

An example of a conversion using a 1966 Mustang was demonstrated by an engineer in California. The system replaces the alternator with a 12 kW (30 kW peak) brushless electric motor. Gas mileage and power were improved.[59]

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External links