A hybrid vehicle is a vehicle that uses two or more distinct power sources to move the vehicle. The term most commonly refers to hybrid electric vehicles (HEVs), which combine an internal combustion engine and one or more electric motors. However other mechanisms to capture and utilize energy are included.
Power sources for hybrid vehicles include:
Two-wheeled and cycle-type vehicles
Mopeds, electric bicycles, and even electric kick scooters are a simple form of a hybrid, as power is delivered both via an internal combustion engine or electric motor and the rider's muscles. Early prototypes of motorcycles in the late 19th century used the same principles to power it up.
- In a parallel hybrid bicycle human and motor power are mechanically coupled at the pedal drive train or at the rear or the front wheel, e.g. using a hub motor, a roller pressing onto a tire, or a connection to a wheel using a transmission element. Human and motor torques are added together. Almost all manufactured Motorized bicycles, Mopeds are of this type.
- In a series hybrid bicycle (SH) the user powers a generator using the pedals. This is converted into electricity and can be fed directly to the motor giving a chainless bicycle but also to charge a battery. The motor draws power from the battery and must be able to deliver the full mechanical torque required because none is available from the pedals. SH bicycles are commercially available, because they are very simple in theory and manufacturing.
The first known prototype and publication of an SH bicycle is by Augustus Kinzel (US Patent 3'884'317) in 1975. In 1994 Bernie Macdonalds conceived the Electrilite SH lightweight vehicle which used power electronics allowing regenerative braking and pedaling while stationary. In 1995 Thomas Muller designed a "Fahrrad mit elektromagnetischem Antrieb" in his 1995 diploma thesis and built a functional vehicle. In 1996 Jürg Blatter and Andreas Fuchs of Berne University of Applied Sciences built an SH bicycle and in 1998 mounted the system onto a Leitra tricycle (European patent EP 1165188). In 1999 Harald Kutzke described his concept of the "active bicycle": the aim is to approach the ideal bicycle weighing nothing and having no drag by electronic compensation. Until 2005 Fuchs and colleagues built several prototype SH tricycles and quadricycles.
Hybrid power trains use diesel-electric or turbo-electric to power railway locomotives, buses, heavy goods vehicles, mobile hydraulic machinery, and ships. Typically some form of heat engine (usually diesel) drives an electric generator or hydraulic pump which powers one or more electric or hydraulic motors. There are advantages in distributing power through wires or pipes rather than mechanical elements especially when multiple drives—e.g. driven wheels or propellers—are required. There is power lost in the double conversion from typically diesel fuel to electricity to power an electric or hydraulic motor. With large vehicles the advantages often outweigh the disadvantages especially as the conversion losses typically decrease with size. With the exception of non-nuclear submarines, presently there is no or relatively little secondary energy storage capacity on most heavy vehicles, e.g. auxiliary batteries and hydraulic accumulators—although this is now changing. Submarines are one of the oldest widespread applications of hybrid technology, running on diesel engines while surfaced and switching to battery power when submerged. Both series-hybrid and parallel hybrid drivetrains were used in the Second World War.
The new Autorail à grande capacité (AGC or high-capacity railcar) built by the Canadian company Bombardier for service in France. This has dual mode (diesel and electric motors) and dual voltage capabilities (1500 and 25000 V) allowing it to be used on many different rail systems. The locomotive has been on trials in Rotterdam, the Netherlands with Railfeeding, a Genesse and Wyoming company.
The First Hybrid Evaluating prototype locomotive was designed and contracted by rail research center MATRAI in 1999 and the sample was ready in 2000. It was a G12 locomotive that was converted to hybrid by using a 200KW diesel generator and batteries and also was equipped with 4 AC traction motors (out of 4) retrofited in the cover of the DC traction motors.
The first operational prototype of a hybrid train engine with significant energy storage and energy regeneration capability was introduced in Japan as the KiHa E200. It utilizes battery packs of lithium ion batteries mounted on the roof to store recovered energy.
In the US, General Electric introduced a prototype railroad engine with their "Ecomagination" technology in 2007. They store energy in a large set of sodium nickel chloride (Na-NiCl2) batteries to capture and store energy normally dissipated in dynamic braking or coasting downhill. They expect at least a 10% reduction in fuel use with this system and are now spending about $2 billion/yr on hybrid research.
Variants of the typical diesel electric locomotive include the Green Goat (GG) and Green Kid (GK) switching/yard engines built by Canada's Railpower Technologies. They utilize a large set of heavy duty long life (~10 yr) rechargeable lead acid (Pba) batteries and 1000 to 2000 HP electric motors as the primary motive sources and a new clean burning diesel generator (~160 Hp) for recharging the batteries that is used only as needed. No power or fuel are wasted for idling—typically 60–85% of the time for these type locomotives. It is unclear if dynamic braking (regenerative) power is recaptured for reuse; but in principle it should be easily utilized.
Since these engines typical need extra weight for traction purposes anyway the battery pack's weight is a negligible penalty. In addition the diesel generator and battery package are normally built on an existing "retired" "yard" locomotive's frame for significant additional cost savings. The existing motors and running gear are all rebuilt and reused. Diesel fuel savings of 40–60% and up to 80% pollution reductions are claimed over that of a "typical" older switching/yard engine. The same advantages that existing hybrid cars have for use with frequent starts and stops and idle periods apply to typical switching yard use. "Green Goat" locomotives have been purchased by Canadian Pacific Railway, BNSF Railway, Kansas City Southern Railway, and Union Pacific Railroad among others.
Railpower Technologies engineers working with TSI Terminal Systems are testing a hybrid diesel electric power unit with battery storage for use in Rubber Tyred Gantry (RTG) cranes. RTG cranes are typically used for loading and unloading shipping containers onto trains or trucks in ports and container storage yards. The energy used to lift the containers can be partially regained when they are lowered. Diesel fuel and emission reductions of 50–70% are predicted by Railpower engineers. First systems are expected to be operational in 2007.
Road transport, commercial vehicles
Early hybrid systems are being investigated for trucks and other heavy highway vehicles with some operational trucks and buses starting to come into use. The main obstacles seem to be smaller fleet sizes and the extra costs of a hybrid system are yet compensated for by fuel savings, but with the price of oil set to continue on its upward trend, the tipping point may be reached by the end of 2015.[dated info] Advances in technology and lowered battery cost and higher capacity etc. developed in the hybrid car industry are already filtering into truck use as Toyota, Ford, GM and others introduce hybrid pickups and SUVs. Kenworth Truck Company recently introduced a hybrid-electric truck, called the Kenworth T270 Class 6 that for city usage seems to be competitive. FedEx and others are starting to invest in hybrid delivery type vehicles—particularly for city use where hybrid technology may pay off first.
Military off-road vehicles
Since 1985, the U.S. military has been testing serial hybrid Humvees and have found them to deliver faster acceleration, a stealth mode with low thermal signature/ near silent operation, and greater fuel economy.
Ships with both mast-mounted sails and steam engines were an early form of hybrid vehicle. Another example is the diesel-electric submarine. This runs on batteries when submerged and the batteries can be re-charged by the diesel engine when the craft is on the surface.
Newer hybrid ship-propulsion schemes include large towing kites manufactured by companies such as SkySails. Towing kites can fly at heights several times higher than the tallest ship masts, capturing stronger and steadier winds.
Taxiing and other ground operations of Boeing 737NGs will soon be done using hybrid electric drives as WheelTug ground propulsion systems become available. By using the APU (powered by a turbine) to energize a Chorus electric motor mounted in the landing gear for ground movement, aircraft will be operating in a hybrid configuration where the main engines are used only for take off, landing, and flight.
The Boeing Fuel Cell Demonstrator Airplane has a Proton Exchange Membrane (PEM) fuel cell/lithium-ion battery hybrid system to power an electric motor, which is coupled to a conventional propeller. The fuel cell provides all power for the cruise phase of flight. During takeoff and climb, the flight segment that requires the most power, the system draws on lightweight lithium-ion batteries.
The demonstrator aircraft is a Dimona motor glider, built by Diamond Aircraft Industries of Austria, which also carried out structural modifications to the aircraft. With a wing span of 16.3 meters (53.5 feet), the airplane will be able to cruise at about Template:Convert/kim/h on power from the fuel cell.
Hybrid FanWings have been designed. A FanWing is created by two engines with the capability to autorotate and landing like a helicopter.
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 Camry Hybrid, Ford Escape Hybrid, Toyota Highlander Hybrid, Honda Insight, Honda Civic Hybrid, Lexus RX 400h and 450h and others. A petroleum-electric hybrid most commonly uses internal combustion engines (generally gasoline or Diesel engines, powered by a variety of fuels) and electric batteries to power the vehicle. There are many types of petroleum-electric hybrid drivetrains, from Full hybrid to Mild hybrid, which offer varying advantages and disadvantages.[not in citation given]
Henri Pieper in 1899 developed the first petro-electric hybrid automobile in the world. In 1900, Ferdinand Porsche developed a series-hybrid using two motor-in-wheel-hub arrangements with a combustion generator set providing the electric power, setting two speed records. 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 75MPG with plans still sold to this original design, and the "Mother Earth News" modified version on their website.
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)
Given suitable infrastructure, permissions and vehicles, BEVs can be recharged while the user drives. The BEV 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. Some of battery-electric locomotives used for maintenance trains on the London Underground are capable of this mode of operation. Power is picked up from the electtrified rails where possible, switching to battery power where the electricity supply is disconnected.
This provides the advantage, in principle, of virtually unrestricted highway range as long as you stay where you have BEV infrastructure access. Since many destinations are within 100 km of a major highway, this may reduce the need for expensive battery systems. Unfortunately private use of the existing electrical system is nearly universally prohibited.
The technology for such electrical infrastructure is old and, outside of some cities, is not widely distributed (see Conduit current collection, trams, electric rail, trolleys, third rail). Updating the required electrical and infrastructure costs can be funded, in principle, by toll revenue, gasoline or other 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, or 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 Chevys Silverado 2500 HD, which is now on the road, can effortlessly switch between petroleum and natural gas, and offers a range of over 650 miles. While the duplicated tanks cost space in some applications, the increased range, decreased cost of fuel and flexibility where (LPG or NG) infrastructure is incomplete may be a significant incentive to purchase. While the U.S. Natural gas infrastructure is partially incomplete, it is increasing at a fast pace, and already has 2600 CNG stations in place. 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 $4.00, the price per MMBTU of gasoline is $28.00, compared to natural gas's $4.00 per MMBTU. 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).
Fluid power hybrid
Hydraulic and pneumatic hybrid vehicles use an engine to charge a pressure accumulator to drive the wheels via hydraulic or pneumatic (i.e. compressed air) drive units. In most cases the engine is detached from the drivetrain merely only to change the energy accumulator. The transmission is seamless.
A French company, MDI, has designed and has running models of a petro-air hybrid engine car. The system does not use air motors to drive the vehicle, being directly driven by a hybrid engine. The engine uses a mixture of compressed air and gasoline injected into the cylinders. A key aspect of the hybrid engine is the "active chamber", which is a compartment heating air via fuel doubling the energy output. Tata Motors of India assessed the design phase towards full production for the Indian market and moved into "completing detailed development of the compressed air engine into specific vehicle and stationary applications".
Petro-hydraulic configurations have been common in trains and heavy vehicles for decades. The auto industry recently focused on this hybrid configuration as it now shows promise for introduction into smaller vehicles.
In petro-hydraulic hybrids, the energy recovery rate is high and therefore the system is more efficient than battery charged hybrids using the current battery technology, demonstrating a 60% to 70% increase in energy economy in U.S. Environmental Protection Agency (EPA) testing. The charging engine needs only to be sized for average usage with acceleration bursts using the stored energy in the hydraulic accumulator, which is charged when in low energy demanding vehicle operation. The charging engine runs at optimum speed and load for efficiency and longevity. Under tests undertaken by the U.S. Environmental Protection Agency (EPA), a hydraulic hybrid Ford Expedition returned 32 miles per US gallon (7.4 L/100 km; 38 mpg-imp) City, and 22 miles per US gallon (11 L/100 km; 26 mpg-imp) highway. UPS currently has two trucks in service with this technology.
Although petro-hybrid technology has been known for decades, and used in trains and very large construction vehicles, heavy costs of the equipment precluded the systems from lighter trucks and cars. In the modern sense an experiment proved the viability of small petro-hybrid road vehicles in 1978. A group of students at Minneapolis, Minnesota's Hennepin Vocational Technical Center, converted a Volkswagen Beetle car to run as a petro-hydraulic hybrid using off-the shelf components. A car rated at 32mpg was returning 75mpg with the 60HP engine replaced by 16HP engine. The experimental car reached 70 mph.
In the 1990s, a team of engineers working at EPA’s National Vehicle and Fuel Emissions Laboratory succeeded in developing a revolutionary type of petro-hydraulic hybrid powertrain that would propel a typical American sedan car. The test car achieved over 80 mpg on combined EPA city/highway driving cycles. Acceleration was 0-60 mph in 8 seconds, using a 1.9 liter diesel engine. No lightweight materials were used.The EPA estimated that produced in high volumes the hydraulic components would add only $700 to the base cost of the vehicle.
While the petro-hydraulic system has faster and more efficient charge/discharge cycling and is cheaper than petro-electric hybrids, the accumulator size dictates total energy storage capacity and may require more space than a battery set.
Research is underway in large corporations and small companies. Focus has now switched to smaller vehicles. 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. 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 speed ranges and loads making small applications of petro-hydraulic hybrids feasible. 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 test was 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. A smaller more efficient constant speed engine reduces weight and liberates space for a larger accumulator.
Current vehicle bodies are designed around the mechanicals of existing engine/transmission setups. It is restrictive and far from ideal to install petro-hydraulic mechanicals into existing bodies not designed for hydraulic setups. One research project's goal is to create a blank paper design new car, to maximize the packaging of petro-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 hub motors eliminates the need for friction brakes, mechanical transmissions, drive shafts and U joints, reducing costs and weight. Hydrostatic drive with no friction brakes are used in industrial vehicles. 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.
In January 2011 industry giant Chrysler announced a partnership with the U.S. Environmental Protection Agency (EPA) to design and develop an experimental petro-hydraulic hybrid powertrain suitable for use in large passenger cars. In 2012 an existing production minvan will be adapted to the new hydraulic powertrain.
PSA Peugeot Citroën exhibited an experimental "Hybrid Air" engine at the 2013 Geneva Motor Show. The vehicle uses nitrogen gas compressed by energy harvested from braking or deceleration to power an hydraulic drive which supplements power from its conventional gasoline engine. The hydraulic and electronic components were supplied by Robert Bosch GmbH. Production versions priced at about $25,000, £17,000, are scheduled for 2015 or 2016. Mileage was estimated to be about 80 miles per gallon for city driving if installed in a Citroën C3.
Electric-human power hybrid vehicle
Hybrid vehicle power train configurations
In a parallel hybrid vehicle, the single electric motor and the internal combustion engine are installed such that they can power the vehicle either individually or together. In contrast to the power split configuration typically only one electric motor is installed. 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 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, 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: an electric motor and an internal combustion engine. The power from these two motors can be shared to drive the wheels via a power splitter, which is a simple planetary gear set. The ratio can be from 0–100% for the combustion engine, or 0–100% for the electric motor, or anything in between, such as 40% for the electric motor and 60% for the combustion engine. The electric motor can act as a generator charging the batteries.
Modern versions such as the Toyota Hybrid Synergy Drive have a second electric motor/generator on the output shaft (connected to the wheels). In cooperation with the "primary" motor/generator and the mechanical power-split 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 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 engine is switched off when the car is slow or stationary reducing curbside emissions.
A series- or serial-hybrid vehicle has also been referred to as an Extended Range Electric Vehicle or Range-Extended Electric Vehicle (EREV/REEV); however, range extension can be accomplished with either series or parallel hybrid layouts.
Series-hybrid vehicles are driven by the electric motor with no mechanical connection to the engine. Instead there is an engine tuned for running a generator when the battery pack energy supply isn't sufficient for demands.
This arrangement is not new, being common in diesel-electric locomotives and ships. Ferdinand Porsche used this setup in the early 20th century in racing cars, effectively inventing the series-hybrid arrangement. Porsche named the arrangement "System Mixt". A wheel hub motor arrangement, with a motor in each of the two front wheels was used, setting speed records. 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.
The setup has never proved to be suitable for production cars, however it is currently being revisited by several manufacturers.
In 1997 Toyota released the first series-hybrid bus sold in Japan. GM introduced the Chevy Volt EREV in 2010, aiming for an all-electric range of 40 miles, and a price tag of around $40,000. 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.
Plug-in hybrid electric vehicle (PHEV)
Another subtype added to the hybrid market is the Plug-in Hybrid Electric Vehicle (PHEV). The PHEV is usually a general fuel-electric (parallel or serial) hybrid with increased energy storage capacity (usually Li-ion batteries). It may be connected to mains electricity supply at the end of the journey to avoid charging using the on-board internal combustion engine.
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.
Fuel cell, electric hybrid
The fuel cell hybrid is generally an electric vehicle equipped with a fuel cell. The fuel cell as well as the electric battery are both power sources, making the vehicle a hybrid. Fuel cells use hydrogen as a fuel and power the electric battery when it is depleted. The Chevrolet Equinox FCEV, Ford Edge Hyseries Drive and Honda FCX are examples of a fuel cell/electric hybrid.
Road safety for cyclists, pedestrians
A 2009 National Highway Traffic Safety Administration report examined HEV accidents that involved pedestrians and cyclists and compared them to accidents involving combustion-engine vehicles. The findings showed that, in certain road situations, HEVs are more dangerous for those on foot or bicycle. For accidents where a vehicle was slowing or stopping, backing up, entering or leaving a parking space (when the sound difference between HEVs and CEVs is most pronounced), HEVs were twice as likely to be involved in a pedestrian crash than CEVs. For crashes involving cyclists or pedestrians, there was a higher incident rate for HEVs than CEVs when a vehicle was turning a corner. But there was no statistically significant difference between the types of vehicles when they were driving straight.
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:
- relying on both the engine and the electric motors for peak power needs, resulting in a smaller engine sized more for average usage rather than peak power usage. A smaller engine can have less internal losses and lower weight.
- having significant battery storage capacity to store and reuse recaptured energy, especially in stop-and-go traffic typical of the city driving cycle.
- 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:
- using Atkinson cycle engines instead of Otto cycle engines for improved fuel economy.
- shutting down the engine during traffic stops or while coasting or during other idle periods.
- 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 handling at the same time.
- 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.
- 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 carbon dioxide. 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.
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. There are many types of batteries. Some are far more toxic than others. Lithium ion is the least toxic of the two mentioned above.
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. In general various soluble and insoluble nickel compounds, such as nickel chloride and nickel oxide, have known carcinogenic effects in chick embryos and rats. The main nickel compound in NiMH batteries is nickel oxyhydroxide (NiOOH), which is used as the positive electrode.
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. 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. 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. 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. 
Raw materials increasing costs
There is an impending increase in the costs of many rare materials used in the manufacture of hybrid cars. For example, the rare earth element dysprosium is required to fabricate many of the advanced electric motors and battery systems in hybrid propulsion systems. Neodymium is another rare earth metal which is a crucial ingredient in high-strength magnets that are found in permanent magnet electric motors.
Nearly all the rare earth elements in the world come from China, and many analysts believe that an overall increase in Chinese electronics manufacturing will consume this entire supply by 2012. In addition, export quotas on Chinese rare earth elements have resulted in an unknown amount of supply.
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; however, the barriers to entry are high and require years to go online.
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.
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While the adoption rate for hybrids in the U.S. is small today (2.2% of new car sales in 2011), this compares with a 17.1% share of new car sales in Japan in 2011, and it has to 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 U.S. auto market." Other sources also expect hybrid penetration rates in the U.S. will remain under 10% for many years.
More optimistic views include predictions that hybrids would dominate new car sales in the U.S. and elsewhere over the next 10 to 20 years. Another approach 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 U.S. factories in the early 20th century, (2) diesel electric locomotives on U.S. railways in the 1920–1945 period, (3) a range of new automotive features/technologies introduced in the U.S. 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 U.S. passenger vehicle stock. 
- Alternative propulsion
- Bivalent (engine)
- Efficient energy use
- Electric vehicle
- Global Hybrid Cooperation
- Global warming
- Human-electric hybrid vehicle
- Hybrid electric vehicle
- Hybrid locomotive
- Hybrid vehicle drivetrain
- List of hybrid vehicles
- Multifuel stove
- Plug-in hybrid
- Solid oxide fuel cell
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