A hybrid vehicle uses two or more distinct types of power, such as internal combustion engine to drive an electric generator that powers an electric motor, e.g. in diesel-electric trains using diesel engines to drive an electric generator that powers an electric motor, and submarines that use diesels when surfaced and batteries when submerged. Other means to store energy include pressurized fluid in hydraulic hybrids.

The basic principle with hybrid vehicles is that the different motors work better at different speeds; the electric motor is more efficient at producing torque, or turning power, and the combustion engine is better for maintaining high speed (better than typical electric motor). Switching from one to the other at the proper time while speeding up yields a win-win in terms of energy efficiency, as such that translates into greater fuel efficiency, for example.

Vehicle type

Two-wheeled and cycle-type vehicles

Mopeds, electric bicycles, and even electric kick scooters are a simple form of a hybrid, powered by an internal combustion engine or electric motor and the rider's muscles. Early prototype motorcycles in the late 19th century used the same principle.

• In a parallel hybrid bicycle human and motor torques are mechanically coupled at the pedal or one of the wheels, e.g. using a hub motor, a roller pressing onto a tire, or a connection to a wheel using a transmission element. Most motorized bicycles, mopeds are of this type. ^[1]
• In a series hybrid bicycle (SHB) (a kind of chainless bicycle) the user pedals a generator, charging a battery or feeding the motor, which delivers all of the torque required. They are commercially available, being simple in theory and manufacturing. ^[2]

The first published prototype of an SHB is by Augustus Kinzel (US Patent 3'884'317) in 1975. In 1994 Bernie Macdonalds conceived the Electrilite ^[3] SHB with power electronics allowing regenerative braking and pedaling while stationary. In 1995 Thomas Muller designed and built a "Fahrrad mit elektromagnetischem Antrieb" for his 1995 diploma thesis. In 1996 Jürg Blatter and Andreas Fuchs of Berne University of Applied Sciences built an SHB and in 1998 modified a Leitra tricycle (European patent EP 1165188). Until 2005 they built several prototype SH tricycles and quadricycles. ^[4] In 1999 Harald Kutzke described an "active bicycle": the aim is to approach the ideal bicycle weighing nothing and having no drag by electronic compensation.

• A series hybrid electric-petroleum bicycle (SHEPB) is powered by pedals, batteries, a petrol generator, or plug-in charger - providing flexibility and range enhancements over electric-only bicycles.

A SHEPB prototype made by David Kitson in Australia ^[5] in 2014 used a lightweight brushless DC electric motor from an aerial drone and small hand-tool sized internal combustion engine, and a 3D printed drive system and lightweight housing, altogether weighing less than 4.5 kg. Active cooling keeps plastic parts from softening. The prototype uses a regular electric bicycle charge port.

Heavy vehicle

Hybrid power trains use diesel-electric or turbo-electric to power railway locomotives, buses, heavy goods vehicles, mobile hydraulic machinery, and ships. A diesel/ turbine engine drives an electric generator or hydraulic pump, which powers electric/hydraulic motor(s) - strictly an electric/hydraulic transmission (not a hybrid), unless it can accept power from outside. With large vehicles conversion losses decrease, and the advantages in distributing power through wires or pipes rather than mechanical elements become more prominent, especially when powering multiple drives — e.g. driven wheels or propellers. Until recently most heavy vehicles had little secondary energy storage, e.g. batteries/ hydraulic accumulators — excepting non-nuclear submarines, one of the oldest production hybrids, running on diesels while surfaced and batteries when submerged. Both series and parallel setups were used in WW2 submarines.

Engine type

Hybrid electric-petroleum vehicles

When the term hybrid vehicle is used, it most often refers to a Hybrid electric vehicle. These encompass such vehicles as the Saturn Vue, Toyota Prius, Toyota Yaris, Toyota Camry Hybrid, Ford Escape Hybrid, Ford Fusion Hybrid, Toyota Highlander Hybrid, Honda Insight, Honda Civic Hybrid, Lexus RX 400h and 450h, Hyundai Ioniq and others. A petroleum-electric hybrid most commonly uses internal combustion engines (using a variety of fuels, generally gasoline or Diesel engines) and electric motors to power the vehicle. The energy is stored in the fuel of the internal combustion engine and an electric battery set. There are many types of petroleum-electric hybrid drivetrains, from Full hybrid to Mild hybrid, which offer varying advantages and disadvantages. ^[7]

William H. Patton filed a patent application for a gasoline-electric hybrid rail-car propulsion system in early 1889, and for a similar hybrid boat propulsion system in mid 1889. ^{[8] [9]} There is no evidence that his hybrid boat met with any success, but he built a prototype hybrid tram and sold a small hybrid locomotive. ^{[10] [11]}

In 1899, Henri Pieper developed the world's first petro-electric hybrid automobile. In 1900, Ferdinand Porsche developed a series-hybrid using two motor-in-wheel-hub arrangements with an internal combustion generator set providing the electric power; Porsche's hybrid set two speed records.^{[ citation needed]} While liquid fuel/electric hybrids date back to the late 19th century, the braking regenerative hybrid was invented by David Arthurs, an electrical engineer from Springdale, Arkansas in 1978–79. His home-converted Opel GT was reported to return as much as 75 mpg with plans still sold to this original design, and the "Mother Earth News" modified version on their website. ^[12]

The plug-in-electric-vehicle (PEV) is becoming more and more common. It has the range needed in locations where there are wide gaps with no services. The batteries can be plugged into house (mains) electricity for charging, as well being charged while the engine is running.

Continuously outboard recharged electric vehicle (COREV)

Some battery electric vehicles (BEVs) can be recharged while the user drives. Such a vehicle establishes contact with an electrified rail, plate or overhead wires on the highway via an attached conducting wheel or other similar mechanism (see Conduit current collection). The BEV's batteries are recharged by this process—on the highway—and can then be used normally on other roads until the battery is discharged. For example, some of the battery-electric locomotives used for maintenance trains on the London Underground are capable of this mode of operation.

Developing a BEV infrastructure would provide the advantage of virtually unrestricted highway range. Since many destinations are within 100 km of a major highway, BEV technology could reduce the need for expensive battery systems. Unfortunately, private use of the existing electrical system is almost universally prohibited. Besides, the technology for such electrical infrastructure is largely outdated and, outside some cities, not widely distributed (see Conduit current collection, trams, electric rail, trolleys, third rail). Updating the required electrical and infrastructure costs could perhaps be funded by toll revenue or by dedicated transportation taxes.

Hybrid fuel (dual mode)

In addition to vehicles that use two or more different devices for propulsion, some also consider vehicles that use distinct energy sources or input types (" fuels") using the same engine to be hybrids, although to avoid confusion with hybrids as described above and to use correctly the terms, these are perhaps more correctly described as dual mode vehicles:

• Some electric trolleybuses can switch between an on-board diesel engine and overhead electrical power depending on conditions (see dual mode bus). In principle, this could be combined with a battery subsystem to create a true plug-in hybrid trolleybus, although as of 2006 ^[update], no such design seems to have been announced.
• Flexible-fuel vehicles can use a mixture of input fuels mixed in one tank — typically gasoline and ethanol, methanol, or biobutanol.
• Bi-fuel vehicle: Liquified petroleum gas and natural gas are very different from petroleum or diesel and cannot be used in the same tanks, so it would be impossible to build an (LPG or NG) flexible fuel system. Instead vehicles are built with two, parallel, fuel systems feeding one engine. For example, some Chevrolet Silverado 2500 HDs can effortlessly switch between petroleum and natural gas, offering a range of over 1000 km (650 miles). ^[13] While the duplicated tanks cost space in some applications, the increased range, decreased cost of fuel, and flexibility where LPG or CNG infrastructure is incomplete may be a significant incentive to purchase. While the US Natural gas infrastructure is partially incomplete, it is increasing at a fast pace, and already has 2600 CNG stations in place. ^[14] With a growing fueling station infrastructure, a large scale adoption of these bi-fuel vehicles could be seen in the near future. Rising gas prices may also push consumers to purchase these vehicles. When gas prices trade around US$4.00, the price of gasoline is US$28.00 per million British thermal units ($95.5/ MWh), compared to natural gas's $4.00 per million British thermal units ($13.6/MWh). ^[15] On a per unit of energy comparative basis, this makes natural gas much cheaper than gasoline. All of these factors are making CNG-Gasoline bi-fuel vehicles very attractive.
• Some vehicles have been modified to use another fuel source if it is available, such as cars modified to run on autogas (LPG) and diesels modified to run on waste vegetable oil that has not been processed into biodiesel.
• Power-assist mechanisms for bicycles and other human-powered vehicles are also included (see Motorized bicycle).

Electric-human power hybrid vehicle

Another form of hybrid vehicle are human power-electric vehicles. These include such vehicles as the Sinclair C5, Twike, electric bicycles, and electric skateboards.

Hybrid vehicle power train configurations

Parallel hybrid

In a parallel hybrid vehicle an electric motor and an internal combustion engine are coupled such that they can power the vehicle either individually or together. Most commonly the internal combustion engine, the electric motor and gear box are coupled by automatically controlled clutches. For electric driving the clutch between the internal combustion engine is open while the clutch to the gear box is engaged. While in combustion mode the engine and motor run at the same speed.

The first mass production parallel hybrid sold outside Japan was the 1st generation Honda Insight.

Mild parallel hybrid

These types use a generally compact electric motor (usually <20 kW) to provide auto-stop/start features and to provide extra power assist ^[16] during the acceleration, and to generate on the deceleration phase (aka regenerative braking).

On-road examples include Honda Civic Hybrid, Honda Insight 2nd generation, Honda CR-Z, Honda Accord Hybrid, Mercedes Benz S400 BlueHYBRID, BMW 7 Series hybrids, General Motors BAS Hybrids, Suzuki S-Cross, Suzuki Wagon R and Smart fortwo with micro hybrid drive.

Power-split or series-parallel hybrid

In a power-split hybrid electric drive train there are two motors: a traction electric motor and an internal combustion engine. The power from these two motors can be shared to drive the wheels via a power split device, which is a simple planetary gear set. The ratio can be from 100% for the combustion engine to 100% for the traction electric motor, or anything in between. The combustion engine can act as a generator charging the batteries.

Modern versions such as the Toyota Hybrid Synergy Drive have a second electric motor/generator connected to the planetary gear. In cooperation with the traction motor/generator and the power-split device this provides a continuously variable transmission.

On the open road, the primary power source is the internal combustion engine. When maximum power is required, for example to overtake, the traction electric motor is used to assist. This increases the available power for a short period, giving the effect of having a larger engine than actually installed. In most applications, the combustion engine is switched off when the car is slow or stationary thereby reducing curbside emissions.

Passenger car installations include Toyota Prius, Ford Escape and Fusion, as well as Lexus RX400h, RX450h, GS450h, LS600h, and CT200h.

Series hybrid

A series- or serial-hybrid vehicle is driven by an electric motor, functioning as an electric vehicle while the battery pack energy supply is sufficient, with an engine tuned for running as a generator when the battery pack is insufficient. There is typically no mechanical connection between the engine and the wheels, and the primary purpose of the range extender is to charge the battery. Series-hybrids have also been referred to as extended range electric vehicle, range-extended electric vehicle, or electric vehicle-extended range (EREV/REEV/EVER).

The BMW i3 with Range Extender is a production series-hybrid. It operates as an electric vehicle until the battery charge is low, and then activates an engine-powered generator to maintain power, and is also available without the range extender. The Fisker Karma was the first series-hybrid production vehicle.

When describing cars, the battery of a series-hybrid is usually charged by being plugged in - but a series-hybrid may also allow for a battery to only act as a buffer (and for regeneration purposes), and for the electric motor's power to be supplied constantly by a supporting engine. Series arrangements have been common in diesel-electric locomotives and ships. Ferdinand Porsche effectively invented this arrangement in speed-record-setting racing cars in the early 20th century, such as the Lohner-Porsche Mixte Hybrid. Porsche named his arrangement "System Mixt" and it was a wheel hub motor design, where each of the two front wheels was powered by a separate motor. This arrangement was sometimes referred to as an electric transmission, as the electric generator and driving motor replaced a mechanical transmission. The vehicle could not move unless the internal combustion engine was running.

In 1997 Toyota released the first series-hybrid bus sold in Japan. ^[17] GM introduced the Chevy Volt series plug-in hybrid in 2010, aiming for an all-electric range of 40 mi (64 km), ^[18] though this car also has a mechanical connection between the engine and drivetrain. ^[19] 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. ^[20]

Plug-in hybrid electric vehicle (PHEV)

Another subtype of hybrid vehicles is the plug-in hybrid electric vehicle (PHEV). The plug-in hybrid is usually a general fuel-electric (parallel or serial) hybrid with increased energy storage capacity, usually through a lithium-ion battery, which allows the vehicle to drive on all-electric mode a distance that depends on the battery size and its mechanical layout (series or parallel). It may be connected to mains electricity supply at the end of the journey to avoid charging using the on-board internal combustion engine. ^{[21] [22]}

This concept is attractive to those seeking to minimize on-road emissions by avoiding – or at least minimizing – the use of ICE during daily driving. As with pure electric vehicles, the total emissions saving, for example in CO₂ terms, is dependent upon the energy source of the electricity generating company.

For some users, this type of vehicle may also be financially attractive so long as the electrical energy being used is cheaper than the petrol/diesel that they would have otherwise used. Current tax systems in many European countries use mineral oil taxation as a major income source. This is generally not the case for electricity, which is taxed uniformly for the domestic customer, however that person uses it. Some electricity suppliers also offer price benefits for off-peak night users, which may further increase the attractiveness of the plug-in option for commuters and urban motorists.

Environmental issues

Fuel consumption and emissions reductions

The hybrid vehicle typically achieves greater fuel economy and lower emissions than conventional internal combustion engine vehicles (ICEVs), resulting in fewer emissions being generated. These savings are primarily achieved by three elements of a typical hybrid design:

1. Relying on both the engine and the electric motors for peak power needs, resulting in a smaller engine size more for average usage rather than peak power usage. A smaller engine can have less internal losses and lower weight.
2. Having significant battery storage capacity to store and reuse recaptured energy, especially in stop-and-go traffic typical of the city driving cycle.
3. Recapturing significant amounts of energy during braking that are normally wasted as heat. This regenerative braking reduces vehicle speed by converting some of its kinetic energy into electricity, depending upon the power rating of the motor/generator;

Other techniques that are not necessarily 'hybrid' features, but that are frequently found on hybrid vehicles include:

1. Using Atkinson cycle engines instead of Otto cycle engines for improved fuel economy.
2. Shutting down the engine during traffic stops or while coasting or during other idle periods.
3. Improving aerodynamics; (part of the reason that SUVs get such bad fuel economy is the drag on the car. A box shaped car or truck has to exert more force to move through the air causing more stress on the engine making it work harder). Improving the shape and aerodynamics of a car is a good way to help better the fuel economy and also improve vehicle handling at the same time.
4. Using low rolling resistance tires (tires were often made to give a quiet, smooth ride, high grip, etc., but efficiency was a lower priority). Tires cause mechanical drag, once again making the engine work harder, consuming more fuel. Hybrid cars may use special tires that are more inflated than regular tires and stiffer or by choice of carcass structure and rubber compound have lower rolling resistance while retaining acceptable grip, and so improving fuel economy whatever the power source.
5. Powering the a/c, power steering, and other auxiliary pumps electrically as and when needed; this reduces mechanical losses when compared with driving them continuously with traditional engine belts.

These features make a hybrid vehicle particularly efficient for city traffic where there are frequent stops, coasting and idling periods. In addition noise emissions are reduced, particularly at idling and low operating speeds, in comparison to conventional engine vehicles. For continuous high speed highway use these features are much less useful in reducing emissions.

Hybrid vehicle emissions

Hybrid vehicle emissions today are getting close to or even lower than the recommended level set by the EPA (Environmental Protection Agency). The recommended levels they suggest for a typical passenger vehicle should be equated to 5.5 metric tons of CO₂. The three most popular hybrid vehicles, Honda Civic, Honda Insight and Toyota Prius, set the standards even higher by producing 4.1, 3.5, and 3.5 tons showing a major improvement in carbon dioxide emissions. Hybrid vehicles can reduce air emissions of smog-forming pollutants by up to 90% and cut carbon dioxide emissions in half. ^[23]

More fossil fuel is needed to build hybrid vehicles than conventional cars but, reduces emissions when the vehicle is running. ^[24] Although this does impact the emission of fossil fuels, the underlying effect is the reduction of fossil fuels being emitted into the atmosphere when cars are on the road driving or idling.

Environmental impact of hybrid car battery

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Though hybrid cars consume less fuel than conventional cars, there is still an issue regarding the environmental damage of the hybrid car battery. Today most hybrid car batteries are one of two types: 1) nickel metal hydride, or 2) lithium ion; both are regarded as more environmentally friendly than lead-based batteries which constitute the bulk of petrol car starter batteries today. ^[25] There are many types of batteries. Some are far more toxic than others. Lithium ion is the least toxic of the two mentioned above. ^[26]

The toxicity levels and environmental impact of nickel metal hydride batteries—the type currently used in hybrids—are much lower than batteries like lead acid or nickel cadmium according to one source. ^[27] Another source claims nickel metal hydride batteries are much more toxic than lead batteries, also that recycling them and disposing of them safely is difficult. ^[28] In general various soluble and insoluble nickel compounds, such as nickel chloride and nickel oxide, have known carcinogenic effects in chick embryos and rats. ^{[29] [30] [31]} The main nickel compound in NiMH batteries is nickel oxyhydroxide (NiOOH), which is used as the positive electrode. ^{[32] [33]}

The lithium-ion battery has attracted attention due to its potential for use in hybrid electric vehicles. Hitachi is a leader in its development. In addition to its smaller size and lighter weight, lithium-ion batteries deliver performance that helps to protect the environment with features such as improved charge efficiency without memory effect. ^{[34] [35]}The lithium-ion batteries are appealing because they have the highest energy density of any rechargeable batteries and can produce a voltage more than three times that of nickel–metal hydride battery cell while simultaneously storing large quantities of electricity as well. ^[36] The batteries also produce higher output (boosting vehicle power), higher efficiency (avoiding wasteful use of electricity), and provides excellent durability, compared with the life of the battery being roughly equivalent to the life of the vehicle. ^{[37] [38]} Additionally, use of lithium-ion batteries reduces the overall weight of the vehicle and also achieves improved fuel economy of 30% better than petro-powered vehicles with a consequent reduction in CO₂ emissions helping to prevent global warming. ^{[39] [40] [41]}

Charging

There are two different levels of charging. Level one charging is the slower method as it uses a 120 V/15 a single-phase grounded outlet. Level two is a faster method; existing Level 2 equipment offers charging from 208 V or 240 V (at up to 80 A, 19.2 kW). ^{[42] [43]}It may require dedicated equipment and a connection installation for home or public units, although vehicles such as the Tesla have the power electronics on board and need only the outlet. ^[44] The optimum charging window for Lithium ion batteries is 3-4.2 V. Recharging with a 120 volt household outlet takes several hours, a 240 volt charger takes 1–4 hours, and a quick charge takes approximately 30 minutes to achieve 80% charge. ^[45] Three important factors—distance on charge, cost of charging, and time to charge ^[46] In order for the hybrid to run on electrical power, the car must perform the action of braking in order to generate some electricity. ^{[47] [48]} The electricity then gets discharged, most effectively when the car accelerates or climbs up an incline, as more battery power needs to be used. In 2014, hybrid electric car batteries can run on solely electricity for 70–130 miles (110–210 km) on a single charge. That has since changed due to new technology. Hybrid battery capacity currently ranges from 4.4 kWh to 85 kWh on a fully electric car. On a hybrid car, the battery packs currently range from 0.6 kWh to 2.4 kWh representing a large difference in use of electricity in hybrid cars. ^[49]

Raw materials increasing costs

There is an impending increase in the costs of many rare materials used in the manufacture of hybrid cars. ^[50] For example, the rare earth element dysprosium is required to fabricate many of the advanced electric motors and battery systems in hybrid propulsion systems. ^{[50] [51]} Neodymium is another rare earth metal which is a crucial ingredient in high-strength magnets that are found in permanent magnet electric motors. ^[52]

Nearly all the rare earth elements in the world come from China, ^[53] and many analysts believe that an overall increase in Chinese electronics manufacturing will consume this entire supply by 2012. ^[50] In addition, export quotas on Chinese rare earth elements have resulted in an unknown amount of supply. ^{[51] [54]}

A few non-Chinese sources such as the advanced Hoidas Lake project in northern Canada as well as Mount Weld in Australia are currently under development; ^[54] however, the barriers to entry are high ^[55] and require years to go online.

How hybrid-electric vehicles work

Hybrids-Electric vehicles (HEVs) combine the advantage of gasoline engines and electric motors. The key areas for efficiency or performance gains are regenerative braking, dual power sources, and less idling. ^[56]

• Regenerate braking. ^{[ further explanation needed]}The drivetrain can be used to convert kinetic energy (from the moving car) into stored electrical energy (batteries). The same electric motor that powers the drivetrain is used to resist the motion of the drivetrain. This applied resistance from the electric motor causes the wheel to slow down and simultaneously recharge the batteries. ^{[57] [58]}
• Dual power. Power can come from either the engine, motor or both depending on driving circumstances. Additional power to assist the engine in accelerating or climbing might be provided by the electric motor. Or more commonly, a smaller electric motor provides all of the power for low-speed driving conditions and is augmented by the engine at higher speeds. ^{[59] [60]}
• Automatic start/shutoff. It automatically shuts off the engine when the vehicle comes to a stop and restarts it when the accelerator is pressed down. This automation is much simpler with an electric motor. Also see dual power above. ^{[61] [62]}

Alternative green vehicles

Other types of green vehicles include other vehicles that go fully or partly on alternative energy sources than fossil fuel. Another option is to use alternative fuel composition (i.e. biofuels) in conventional fossil fuel-based vehicles, making them go partly on renewable energy sources. Green vehicles focus mostly on energy as it's main source of energy, straying from fossil fuels.

Other approaches include personal rapid transit, a public transportation concept that offers automated on-demand non-stop transportation, on a network of specially built guideways.

Marketing

Automakers spend around $US8 million in marketing Hybrid vehicles each year. With combined effort from many car companies, the Hybrid industry has sold millions of Hybrids.^{[ citation needed]} Hybrid car companies like Toyota, Honda, Ford and BMW have pulled together to create a movement of Hybrid vehicle sales pushed by Washington lobbyist to lower the world's emissions and become less reliant on our petroleum consumption.^{[ citation needed]} In 2005, sales went beyond 200,000 Hybrids, but in retrospect that only reduced the global use for gasoline consumption by 200,000 gallons per day — a tiny fraction of the 360 million gallons used per day.^{[ citation needed]} According to Bradley Berman author of Driving Change—One Hybrid at a time, "Cold economics shows that in real dollars, except for a brief spike in the 1970s, gas prices have remained remarkably steady and cheap. Fuel continues to represent a small part of the overall cost of owning and operating a personal vehicle". ^[63] Other marketing tactics include greenwashing which is the "unjustified appropriation of environmental virtue." ^[64] Temma Ehrenfeld explained in an article by Newsweek. Hybrids may be more efficient than many other gasoline motors as far as gasoline consumption is concerned but as far as being green and good for the environment is completely inaccurate. Hybrid car companies have a long time to go if they expect to really go green. According to Harvard business professor Theodore Levitt states "managing products" and "meeting customers' needs", "you must adapt to consumer expectations and anticipation of future desires." ^[65] This means people buy what they want, if they want a fuel efficient car they buy a Hybrid without thinking about the actual efficiency of the product. This "green myopia" as Ottman calls it, fails because marketers focus on the greenness of the product and not on the actual effectiveness. Researchers and analysts say people are drawn to the new technology, as well as the convenience of fewer fill ups. Secondly, people find it rewarding to own the better, newer, flashier, and so called greener car. In the beginning of the hybrid movement car companies reached out to the young people, by using top celebrities, astronauts, and popular TV shows to market hybrids. This made the new technology of hybrids a status to obtain for many people and a must to be cool or even the practical choice for the time. With the many benefits and status of owning a hybrid it is easy to think it's the right thing to do, but in fact may not be as green as it appears.^{[ citation needed]}

In 2019 the term "self-charging hybrid" became popular in advertising, though cars referred to by this name do not offer any different functionality than a standard hybrid vehicle provides. The only self-charging effect is in energy recovery via regenerative braking, which is also true of plug-in hybrids, fuel cell electric vehicles and battery electric vehicles. ^[66]

Adoption rate

While the adoption rate for hybrids in the US is small today (2.2% of new car sales in 2011), ^[67] this compares with a 17.1% share of new car sales in Japan in 2011, ^[68] and it has the potential to be very large over time as more models are offered and incremental costs decline due to learning and scale benefits. However, forecasts vary widely. For instance, Bob Lutz, a long-time skeptic of hybrids, indicated he expects hybrids "will never comprise more than 10% of the US auto market." ^[69] Other sources also expect hybrid penetration rates in the US will remain under 10% for many years. ^{[70] [71] [72]}

More optimistic views as of 2006 include predictions that hybrids would dominate new car sales in the US and elsewhere over the next 10 to 20 years. ^[73] Another approach, taken by Saurin Shah, examines the penetration rates (or S-curves) of four analogs (historical and current) to hybrid and electrical vehicles in an attempt to gauge how quickly the vehicle stock could be hybridized and/or electrified in the United States. The analogs are (1) the electric motors in US factories in the early 20th century, (2) diesel electric locomotives on US railways in the 1920–1945 period, (3) a range of new automotive features/technologies introduced in the US over the past fifty years, and 4) e-bike purchases in China over the past few years. These analogs collectively suggest it would take at least 30 years for hybrid and electric vehicles to capture 80% of the US passenger vehicle stock. ^[74]

See also