Hybrid Fuel Cars  |  Discussion forums Glossary of motoring terms


Hybrid vehicle is a vehicle using an on-board rechargeable energy storage system (RESS) and a fueled propulsion power source for vehicle propulsion. They are low-polluting and low-petroleum consuming cars. The different propulsion power systems may have common subsystems or components.

The term commonly refers to petroleum-electric hybrid vehicles, also called Hybrid-electric vehicle (HEV) which use gasoline (petrol) or diesel to power internal-combustion engines, and electric batteries to power electric motors. Modern mass-produced hybrids  recharge their batteries by capturing kinetic energy via regenerative braking. As well, when cruising or in other situations where just light thrust is needed, "full" hybrids can use the combustion engine to generate electricity by spinning a generator to either recharge the battery or directly feed power to an electric motor that drives the vehicle. This contrasts with all-electric cars which use batteries charged by an external source such as the grid, or a range extending trailer. Nearly all hybrids still require gasoline and diesel as their sole fuel source though other fuels such as ethanol or plant based oils have also seen occasional use. Hybrid cars also use Hydrogen gas.

The term hybrid when used in relation with cars also has other uses. Prior to its modern meaning of hybrid propulsion, the word hybrid was used in the United States to mean a vehicle of mixed national origin; generally, a European car fitted with American mechanical components. This meaning has fallen out of use. In the import scene, hybrid was often used to describe an engine swap, such as the common Honda B16 engine into a Honda Civic. Some have also referred to flexible-fuel vehicles as hybrids because they can use a mixture of different fuels ó typically gasoline and ethanol alcohol fuel. However, There are no diesel flexible-fuel vehicles, because diesel cars can already use petroleum and biodiesel at the same time.


In 1898 Ferdinand Porsche designed the Lohner-Porsche carriage, a series-hybrid vehicle that used a one-cylinder gasoline internal combustion engine that spun a generator which powered four wheel-mounted electric motors. The car was presented at the 1900 World Exhibition in Paris. The up to 56 km/h (35 mph) fast carriage broke several Austrian speed records, and also won the Exelberg Rally in 1901 with Porsche himself driving. Over 300 of the Lohner-Porsche carriages were sold to the public.

In 1959 the development of the first transistor-based electric car - the Henney Kilowatt - heralded the development of the electronic speed control that paved the way for modern hybrid electric cars. The Henney Kilowatt was the first modern production electric vehicle and was developed by a cooperative effort between National Union Electric Company, Henney Coachworks, Renault, and the Eureka Williams Company. Although sales of the Kilowatt were dismal, the development of the Kilowatt served was a historical "who's who" of electric propulsion technology.

A more recent working prototype of the electric-hybrid vehicle was built by Victor Wouk (one of the scientists involved with the Henney Kilowatt and also brother of author Herman Wouk ). Wouk's work with electric hybrid vehicles in the 1960s and 1970s earned the title as the "Godfather of the Hybrid". Wouk installed a prototype electric-hybrid drivetrain into a 1972 Buick Skylark provided by GM for the 1970 Federal Clean Car Incentive Program, but the program was killed by the EPA in 1976. Since then, hobbyists have continued to build hybrids but none was put into mass production by a major manufacturer until the waning years of the twentieth century.

The regenerative-braking hybrid, the core design concept of most production hybrids, was developed by Electrical Engineer David Arthurs around 1978 using off-the shelf components and an Opel GT. However the voltage controller to link the batteries, motor (a jet-engine starter motor), and DC generator was Mr. Arthurs'. The vehicle exhibited ~75 mpg fuel efficiency and plans for it (as well as somewhat updated versions) are still available through the Mother Earth News web site. The Mother Earth News' own 1980 version claimed nearly 84 mpg.

In the intervening period, the widest use of hybrid technology was actually in diesel-electric locomotives. It is also used in diesel-electric submarines, which operate in essentially the same manner as hybrid electric cars. However, in this case the goal was to allow operation underwater without consuming large amounts of oxygen, rather than economizing on fuel. Since then, many submarines have moved to nuclear power, which can operate underwater indefinitely, though a number of nations continue to rely on diesel-electric fleets.

Automotive hybrid technology became successful in the 1990s when the Honda Insight and Toyota Prius became available. These vehicles have a direct linkage from the internal combustion engine to the driven wheels, so the engine can provide acceleration power. The 2000s saw development of plug-in hybrid electric vehicles (PHEVs), which can be recharged from the electrical power grid and do not require conventional fuel for short trips. The Renault Kangoo was the first production model of this design, released in France in 2003.

However, the environmental benefits of plug-in hybrids depend somewhat on the source of the electrical power. In particular, electricity generated with wind would be cleaner than electricity generated with coal, the most polluting source. On the other hand, electricity generated with coal in a central power plant is still much cleaner than pure gasoline propulsion, due to the much greater efficiencies of a central plant. Furthermore, coal is only one source of centrally generated power, and in some places such as California is only a minor contributor, overshadowed by natural gas and other cleaner sources.

The Prius has been in high demand since its introduction. Newer designs have more conventional appearance and are less expensive, often appearing and performing identically to their non-hybrid counterparts while delivering 50% better fuel efficiency. The Honda Civic Hybrid appears identical to the non-hybrid version, for instance, but delivers about 50 US mpg (4.7 L/100km). The redesigned 2004 Toyota Prius improved passenger room, cargo area, and power output, while increasing energy efficiency and reducing emissions. The Honda Insight, while not matching the demand of the Prius, is still being produced and has a devoted base of owners. Honda has also released a hybrid version of the Accord.

2005 saw the first hybrid SUV released, Ford Motor Company's Escape. Toyota and Ford entered into a licensing agreement in 2004 allowing Ford to use 20 patents from Toyota related to hybrid technology, although Ford's engine was independently designed and built. In exchange for the hybrid licenses, Ford licensed patents involving their European diesel engines to Toyota. Toyota announced model year 2005 hybrid versions of the Toyota Highlander and Lexus RX 400h with 4WD-i which uses a rear electric motor to power the rear wheels negating the need for a differential. Toyota also plans to add hybrid drivetrains to every model it sells in the coming decade.

For 2007 Lexus is offering a hybrid version of their GS sport sedan dubbed the GS450h with "well in excess of 300hp". The 2007 Camry Hybrid has been announced and is slated to launch in late Spring as a 2007 model. It will be built in Kentucky, USA. Also, Nissan announced the release of the Altima hybrid around 2007.

An R.L. Polk survey of 2003 model year cars showed that hybrid car registrations in the United States rose to 43,435 cars, a 25.8% increase from 2002 numbers. California, the nation's most populous state at one-eighth of the total population, had the most hybrid cars registered: 11,425. The proportionally high number may be partially due to the state's higher gasoline prices and stricter emissions rules, which hybrids generally have little trouble passing.

Honda, which offers Insight, Civic and Accord hybrids, sold 26,773 hybrids in the first 11 months of 2004. Toyota has sold a cumulative 306,862 hybrids between 1997 and November 2004, and Honda has sold a total of 81,867 hybrids between 1999 and November 2004.

Trains, trucks and buses

In May 2003 JR East started test runs with the so called NE train and validated the system's operability (series hybrid with lithium ion battery) in cold regions. In 2004, RailPower Technologies had been running pilots in the US with the so called Green Goats which led to orders by the Union Pacific and Canadian Pacific Railways starting in early 2005.

Also in 2005 GE introduced its hybrid shifters on the market. Toyota claims to have started with the Coaster Hybrid Bus in 1997 on the Japanese market. In May 2003 GM started to tour with hybrid buses developed together with Allison. Several hundreds of those buses have entered into daily operation in the US. The Blue Ribbon City Hybrid bus was presented by Hino, a Toyota affiliate, in January 2005.

In 2003 GM introduced a diesel hybrid military (light) truck, equipped with a diesel electric and a fuel cell auxiliary power unit. Hybrid light trucks were introduced 2004 by Mercedes (Hybrid Sprinter) and Micro-Vett SPA (Daily Bimodale). International Truck and Engine Corp. and Eaton Corp. have been selected to manufacture diesel-electric hybrid trucks for a US pilot program serving the utility industry in 2004. In mid 2005 Isuzu introduced the Elf Diesel Hybrid Truck on the Japanese Market. They claim that approximately 300 vehicles, mostly route buses are using Hinos HIMR (Hybrid Inverter Controlled Motor & Retarder) system.

A promising but as-yet unseen application for hybrid vehicle technology would be in garbage trucks, since these vehicles do stop-start driving and often stand idling.


There are many ways to create an electric-internal combustion 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. Also, start and stop cycles may reduce the engine's ability to operate at its optimum temperature, thus reducing the engine's efficiency.


In a series design, the internal combustion engine is not directly connected to the drivetrain at all, but powers an electrical generator instead. This is similar to the operation of diesel-electric train locomotives, except that as of 2006, the overwhelming majority of diesel-electric locomotives do not store auxiliary power in batteries for use in propulsion. A series hybrid is similar to an electric car which is recharged by electricity from a stationary fossil fuel power plant, except that the power plant is carried on board.

Electricity from the generator is fed to the motor or motors that actually move the car, and excess energy can be used to charge batteries. When large amounts of power are required, electricity comes from both the battery pack and the engine-generator section. Because electrical motors can operate quite efficiently over a wide range of speeds, this design removes or reduces the need for a complex transmission. The internal combustion engine can also be finely tuned to operate at its most efficient speed whenever it is running, for a great gain in efficiency. Separate small electric motors installed at each wheel are featured in some prototypes and concept cars; this allows the possibility of easily controlling the power delivered to each wheel, and therefore simplifies traction control, all wheel drive, and similar features.

The advantage of this type of hybrid is the flexibility afforded by the lack of a mechanical link between the internal combustion engine and the wheels. A weakness of a series hybrid system, however, is that series hybrids require separate motor and generator portions, which can be combined in some parallel hybrid designs; the combined efficiency of the motor and generator will be lower than that of a conventional transmission, offsetting the efficiency gains that might otherwise be realized.

Additionally, the power delivered to the wheels by a series hybrid is limited by the electric motor(s) (whereas in a parallel hybrid the combustion engine can provide additional power to the wheels). Still, series hybrids are useful in driving cycles that incorporate many stops and starts, such as for delivery vehicles, or stop and go city driving. It is likely that some fuel cell cars will use a series-style setup, with the fuel cells replacing the engine-generator section; this would eliminate the loss of efficiency inherent in converting the mechanical output of an internal combustion engine to electrical power.


Parallel systems, which are most commonly produced at present, connect both the electrical and internal combustion systems to the mechanical transmission. They can be subcategorized depending upon how balanced the different portions are at providing motive power. In some cases, the internal combustion engine is the dominant portion and is used for primary power, with the motor turning on only when a boost is needed. Others can run with just the electric system operating alone. Most designs combine a large electrical generator and a motor into one unit, often situated between the internal combustion engine and the transmission, in the location of the flywheel, replacing both the conventional starter motor and the generator or alternator.

A large battery pack is required, providing a higher voltage than the normal automotive 12 volts. Accessories such as power steering and air conditioning are powered by electric motors, so that they continue to function when the internal combustion engine is stopped; this offers the possibility of further efficiency gains, by modulating the electrical power delivered to these systems, rather than having them run directly from the engine at a speed which depends on engine speed.

Full 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 Prius and Escape Hybrids are examples of this, as both cars can be moved forward 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 interconverting 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, Highlander sport-utility vehicle (SUV), and Camry. A computer oversees operation of the entire system, determining which half should be running, or if both should be in use, shutting off the internal combustion engine when the electric motor is sufficient to provide the power. The normal mode of operation is on electrical power alone, with the gasoline engine running only in cases where the extra power is required, or where the batteries are discharged. The hybrid drivetrain of the Prius, in combination with aerodynamics and optimizations in the engine itself to reduce drag, results in 80%Ė100% gains in fuel economy compared to four-door conventional cars of similar weight and size.

Input-split Hybrid

The Toyota Hybrid System THS / Hybrid Synergy Drive mode of operation with only a single power split device (incorporated as a single 3 shaft planetary gearset) is more generically called Input-Split Hybrid, due to the fact that a fixed amount of torque is transferred via the electrical path from the engine to the wheels. This in turn makes this setup very simple in mechanical terms, but does have some drawbacks of its own. For example the maximum speed is mainly limited by the speed of the smaller electric motor. Also, the efficiency of the transmission is heavily dependent on the amount of power being transmitted over the electrical path, as multiple conversions, each with their own, less than perfect efficiency, lead to a low efficiency of that path (~0.7) compared with the purely mechanical path (~0.98). Especially in higher speed regimes the efficiency (of the transmission alone) therefore drops below that of a generic automatic transmission with hydrodynamic coupler.

The main principle behind this system is the more-or-less complete decoupling of the power supplied by the engine from the power demanded by the driver. Thus a smaller, less flexible engine may be used, which is designed for maximum efficiency (often using variations of the conventional Otto cycle, such as the Miller or Atkinson cycle). This contributes significantly to the higher overall efficiency of the vehicle, with regenerative braking playing a much smaller role.

The differing torque vs. rpm characteristics of the internal combustion and electrical motors operate synergistically; an internal combustion engine's torque is minimal at lower RPMs, since the engine must be its own air pump. Thus, the need for reasonably rapid acceleration from a standing start results in an engine which is much larger than required for steady speed cruising. On the other hand, an electrical motor exhibits maximum torque at stall; therefore this engine is well suited to complement the internal combustion engine's torque deficiency at low RPMs, allowing the use of a much smaller and therefore more fuel efficient engine.

Interesting variations of that simple theme, as very well known (implemented in the 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), ie. higher power density addition of a ravigneux-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 "drive" motor) to the wheel shaft is switched, either for higher torque or higher speed while sustaining better transmission efficiency.

Combined-Split Hybrid

General Motors, BMW, and DaimlerChrysler are working together on a so-called Two-Mode Hybrid system which is a full hybrid plus additional efficiency improvements. The technology will be released in 2008 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.

The main difference to the Input-Split Hybrid is the addition of a second planetary gearset, and the addition of two clutches. This enables the switching (two-modes) of the percentage of mechanically vs. electrically transmitted power, and in order to cope both with low- and high-speed regimes, only smaller electrical motors with much less power and torque can be used. However, no diagrams could be obtained so far explaining the 4 gear-ratios (and why that would impose limits on the concurrent use of both electric motors or not). Most likely, an additional, 3rd ravigneux-like planetary gear with additional clutches is used for shifting between distinct final gear ratios.

Assist hybrid

Assist hybrids use the engine for primary power, with a torque-boosting electric motor also connected to a largely conventional powertrain. The electric motor 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. Honda's hybrids 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 they cannot run on electric power alone. However, since the amount of electrical power needed is much smaller, the size of the battery systems is reduced.

A variation on this type is Mazda's e-4WD system, offered on the Mazda Demio sold in Japan. 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 enhance performance or economy.

Mild hybrid

Mild hybrids are essentially conventional vehicles with oversized starter motors, allowing the engine to be turned off whenever the car is coasting, braking, or stopped, yet restart quickly and cleanly. 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 larger motor is used to spin up the engine to operating rpm speeds before injecting any fuel.

Many people do not consider these to be hybrids at all, and these vehicles do not achieve the fuel economy of full hybrid models. A major example is the 2005 Chevrolet Silverado Hybrid, 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. Mild hybrids often use 48 volt systems 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 pickup truck hybrid with their Belt alternator starter (BAS) hybrid system, used in the 2006 Saturn VUE Green Line. It operates in much the same manner as the "start-stop" system in the Silverado, but the electric motor can also provide modest assist under acceleration.

Plug-in hybrid

A plug-in hybrid electric vehicle (PHEV) is a full hybrid, able to run in electric-only mode, with larger batteries and the ability to recharge from the electric power grid. 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. Particular interest in PHEVs is in California where a "million solar homes" initiative is under way, and global warming legislation has been enacted.

Prototypes of plug-in hybrid cars, 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 at UC 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 the so called Daily Bimodale.

The California Cars Initiative has converted the '04 and newer Toyota Prius to become a prototype of what it calls the PRIUS+. With the addition of 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 10 miles using only electric power.

Hydraulic hybrid

A hydraulic hybrid vehicle uses hydraulic and mechanical components instead of electrical ones. A variable displacement pump replaces the motor/generator, and a hydraulic accumulator replaces the batteries. The hydraulic accumulator, which is essentially a pressure tank, is potentially cheaper and more durable than batteries. Hydraulic hybrid technology was originally developed by Volvo Flygmotor and was used experimentally in buses from the early 1980s and is still an active area.

Initial concept involved a giant flywheel for storage connected to a hydrostatic transmission, but it was later changed to a simpler system using a hydraulic accumulator connected to a hydraulic pump/motor. It 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.

Pneumatic hybrid

Compressed air can also power a hybrid car with a gasoline compressor to provide the power. MDI in France produces such air cars. An Australian company invented a highly efficient air engine which may make pneumatic hybrid vehicle more competitive. 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.

High-Power Biodiesel Hybrid

The newest hybrid still yet to reach the market is the High-Power Biodiesel Hybrid (HPBH). These cars run on a hybrid engine with a mix of fuels and have excellent fuel efficiency and power. In 2006, Students from Philadelphia created a HPBH car which used soybean fuel that could go from 0-60 mph in 4.0 seconds and still achieve 50 MPG.

Engines and fuel sources

Gasoline engines are used in most hybrid designs, and will likely remain dominant for the foreseeable future. While petroleum-derived gasoline is the primary fuel, it is possible to mix in varying levels of ethanol created from renewable energy sources. Like most modern ICE-powered vehicles, hybrids can typically use up to about 15% bioethanol. Manufacturers may move to flexible fuel engines, which would increase allowable ratios, but no plans are in place at present.

Nowadays petroleum gasoline engines can use directly biobutanol.

One particularly interesting hybrid vehicle combination uses a diesel engine for power. Diesels are excellent at delivering constant power for long periods of time, suffering less wear while operating at higher efficiency. The Diesel engine's high torque, combined with hybrid technology, may offer performance in a car of over 100 mpg US (2.35 L per 100 km).

Nowadays most diesel vehicles, and therefore the diesel part of hybrids, have the advantage they can use 100% pure biofuels (biodiesel), so they can use but do not need petroleum at all.

Diesels are not widely used for passenger cars in the United States, as US diesel fuel has long been considered very "dirty", with relatively high levels of sulfur and other contaminants in comparison to the Eurodiesel fuel in Europe, where greater restrictions have been in place for many years. Despite the "legally allowed" dirtier fuel, the US has tough restrictions on exhaust, and it has been difficult for car manufacturers to meet emissions levels given what is put into the engine. However, ultra-low sulfur diesel is set to be mandated in the United States in October 2006.

PSA Peugeot CitroŽn offers PSA Peugeot CitroŽn has unveiled two demonstrator vehicles featuring a diesel-electric hybrid powertrain: the Peugeot 307 and CitroŽn C4 Hybride HDi (PDF).

VW made a prototype diesel-electric hybrid car that achieved 118 mpg US fuel economy (2 liters per 100 km), but has yet to sell a hybrid vehicle.

So far, production diesel-electric engines have mostly just appeared in mass transit buses. Current manufacturers of diesel-electric hybrid buses include New Flyer Industries, Gillig, Orion Bus Industries, and North American Bus Industries. In 2008, NovaBus will add a diesel-electric hybrid option as well.

The ECD company is reported to have converted a Toyota Prius to run on hydrogen fuel. BMW plans to offer a 7 Series car that runs on both petrol and hydrogen (see bivalent). In 2005, New Flyer Industries introduced a concept for a hydrogen-electric powered mass transit bus known as the HE40LF.

Fuel cells

Some fuel cell-powered vehicles currently in development use some hybrid-like technology to store auxiliary energy. Like diesels above and steam power outlined below, fuel cells are best at delivering a fairly constant flow of electricity, so having a secondary system is helpful. In some cases, batteries have been replaced with ultra capacitors, which can store and retrieve energy quickly, but are inappropriate for long-term electrical storage.

Turbines and steam engines

From the 1950s to the 1970s Chrysler created several turbine-powered vehicles, though only small numbers were produced; they had complex drivetrains and achieved relatively slow starting speeds, with effects reminiscent of "turbo lag", but demonstrated that turbines could be used for automobiles. Also, in late 1990s General Motors made a gas turbine series hybrid prototype based on now-cancelled EV1 electric vehicle. At present, no current or announced mass-market car is driven by a gas turbine or a steam engine, but hybrid technology could bring back gas turbines and the steam-powered car.

Both gas turbines and steam turbines (see below) are lighter than reciprocating steam and internal combustion engines, respectively, and more efficient than the corresponding reciprocating types when operating at their optimum power output. On the other hand, they have very limited optimum power output ranges, and must be used with electric drive or some other sort of transmission. Operation of turbines outside of their optimum power output ranges drastically reduces their efficiency. This is not prohibitive for a ship or aircraft that is mostly operated at very constant power output, or for a power plant containing many turbines that can be put on-line or off-line as needed to match load, but has resulted in near-eradication of turbine engines from land vehicles.

In the early 20th century, cars made by the Stanley Steamer Company with reciprocating steam engines did compete successfully with the internal combustion engine. Reciprocating steam engines have a much larger range of operating speeds than do internal combustion engines, including the ability to produce full torque at stall, thus eliminating the need for a transmission; however, they have not been able to compete with internal combustion for land vehicles for several reasons:

Lower thermal efficiency possible with today's materials ó a heat engine such as an internal combustion engine or steam has efficiency limited by its Carnot cycle temperature differential. A steam engine must transfer combustion heat through the material of the boiler, which therefore must be able to withstand the heat, while an internal combustion engine can bypass this limitation by having the piston and cylinder materials always remain at much less than the combustion temperature (at the cost of some loss of efficiency due to unwanted cooling of the combustion gas).

Longer warm-up time and slow throttle response ó this is no great problem for trains and ships which are restricted from quick acceleration by their huge mass and which also generally have predictable demand for power, but is a challenging issue for automobiles, trucks, and buses
More complex controls ó the driver of a Stanley Steamer had to keep a close eye on several pressure and temperature gauges while driving (on the other hand, with modern computers, much of this could be handled automatically)

Gas turbine (or other internal combustion engine), steam turbine, and hybrid technology could be combined to alleviate the disadvantages of gas turbines and steam engines while retaining most of their advantages. In combined cycle power plants, gas turbines drive generators, and their exhaust is used to generate steam for steam turbines, thus recovering some of the energy from the heat of the exhaust that would otherwise be wasted. This principle can be used in vehicles, and is currently in use in ships as COGAS or COGES, although the only public proposal for such technology in an automobile uses a conventional internal combustion engine for this purpose instead of a gas turbine (a configuration that has also seen use on ships).

A combined cycle gas turbine/steam turbine (or internal combustion engine/steam turbine) set could be combined with hybrid technology to allow the combined cycle system to operate at its most efficient power output. The energy storage system would store energy from the combined cycle system when its output exceeds propulsion requirements and provide energy to the propulsion system when propulsion requirements exceed the combined cycle system output, including combined cycle system startup. The energy storage system would need to have an especially high capacity to work well with a combined cycle system, since the combined cycle system would operate inefficiently during startup and shutdown; therefore, the energy storage system would need to support long intervals between combined cycle startup and shutdown.

Hybrid fuel

In addition to vehicles that use two or more different devices for creating motive power, some also consider vehicles that use distinct energy input types (fuels) to be hybrids, although to avoid confusion with hybrids as described above, these are better described as dual mode vehicles:

Some electric trolleybuses can switch between an onboard diesel engine and overhead electrical power depending on conditions. In principle, this could be combined with a battery subsystem to create a true plug-in hybrid trolleybus, although as of 2006, no such design seems to have been announced.
Flexible-fuel vehicles can use a mixture of input fuel ó typically gasoline and ethanol, though diesel-biodiesel and liquid petroleum gas-natural gas (LPG-NG) vehicles would also qualify.
Some vehicles have been modified to use another fuel source if it is available, such as cars modified to run on propane and diesels modified to run on waste vegetable oil.
Power-assist mechanisms for bicycles and other human-powered vehicles are also included.

Human power

Motorized bicycles use human pedal power and an attached motor. Some bicycle conversion kits aided popularization of "hybrid" vehicle bicycles that used electric hub motors (such as Bionx and Wilderness Energy), internal combustion engines (such as the 1940s "Pixie" bicycle motor), and pedal power. Such machines include electric bicycles and mopeds, which may often be simultaneously propelled by human and engine power. More sophisticated constructions are three wheeled and provide at least a windscreen (ZAP EPOD, TWIKE).

Benefits of the hybrid design include:

The internal-combustion engine in a hybrid vehicle is much smaller, lighter, and more efficient than the one in a conventional vehicle, because the engine can be sized for slightly above average power demand rather than peak power demand. A standard combustion engine is required to operate over a range of speed and power, yet its highest efficiency is in a narrow range of operation - in a hybrid vehicle, the engine operates within its range of highest efficiency. The power curve of electric motors is better suited to variable speeds and can provide substantially greater torque at low speeds compared with internal-combustion engines.

Like many electric cars, but in contrast to conventional vehicles, braking in a hybrid is controlled in part by the electric motor which can recapture part of the kinetic energy of the car to partially recharge the batteries. This is called regenerative braking and one of the reasons for the high efficiency of hybrid cars. In a conventional vehicle, braking is done by mechanical brakes, and the kinetic energy of the car is wasted as heat.
Hybrids are much more energy efficient than traditional internal combustion engine vehicles because they generally provide greater fuel economy. This statistic has a major implication for the reducing petroleum consumption and vehicle air pollution emissions worldwide
Reduced wear and tear on the gasoline engine.
Reduced wear on brakes from the regenerative braking system use.
Reduced noise emissions resulting from substantial use of electric engine at low speeds, leading to roadway noise reduction and beneficial noise health effects. Note, however, that this is not always an advantage; for example, people who are blind or visually-impaired, and who rely on vehicle-noise while crossing streets, find it much more difficult to do safely.
Reduced air pollution emissions due to less fuel consumed per travel mile, leading to improved human health with regard to respiratory and other illness. In fact composite driving tests indicate total air pollution of carbon monoxide and reactive hydrocarbons are 80 to 90 percent cleaner for hybrid versus conventional vehicles.
Increased driving range without refueling or recharging, compared with electric vehicles and perhaps even compared with internal-combustion vehicles. Limitations in range have been a problem for traditional electric vehicles.


In some cases, manufacturers are producing hybrid vehicles that use the added energy provided by the hybrid systems to give vehicles a power boost, rather than significantly improved fuel efficiency compared to their traditional counterparts. The trade-off between added performance and improved fuel efficiency is mainly something controlled by the software within the hybrid system. In the future, manufacturers may provide hybrid-owners with the ability to set this balance (fuel efficiency vs. added performance) as they wish, through a user-controlled setting. Toyota announced in January, 2006 that it was considering a "high-efficiency" button.

It has been observed that the success of the hybrid systems comes despite the need to carry two complete power systems. In a poorly designed car this might increase the weight and size and therefore greater losses in acceleration and aerodynamic drag, although the Prius is lighter and more aerodynamic than many other cars. In fact, the relative desirability of this concept rests on the deficiencies of the two underlying systems; the unfavorable torque curve of the internal combustion engine, referred to above, and the lack of a system of storing and delivering electrical power with anything near the energy density of combustible liquid fuels, so that a fuel tank, internal combustion engine, and generator together still represent a better source of electrical power than the equivalent weight and volume of batteries.

In the event of relatively large leaps forward in battery or fuel cell technology, the internal combustion portion of the hybrid will become superfluous. Somewhat less likely is the possibility of a change in the general popular mode of automobile use largely supplanting short trips by use of mass transportation, so that the majority of automotive operation becomes steady speed cruising rather than stopping and starting; this would eliminate the advantage gained from regenerative braking and the low rpm torque boost of the electrical portion of the hybrid, and allow very small forced induction internal combustion engines to become viable competitors of the heavier hybrid systems.

Skeptics claim that mechanics are not fond of working on hybrid vehicles due to added complexity, but the Toyota mechanics in Atlanta and other U.S. cities say they are delighted by the cars, and hundreds of enthusiastic engineer-owners gather on the Internet and in clubs. The complexity may result in greater repair costs, although hybrid manufacturers typically encourage buyers with generous warrantees so this has not yet affected end users. These vehicles have been available for ten years and the lifespan and resale values are good. Hundreds of thousands are in use, but Toyota reports very few problems with battery packs. One additional problem is the lack of towing hook, the hybrid cars have limited power resources so often they can't be used for high power applications like towing boats.

Disposal is an additional issue. By its nature, a battery must be made of reactive chemicals; the more power density the battery offers, the more reactive the chemicals it contains. However, all discarded hybrid vehicles will be returned for proper recycling and disposal; dealers and mechanics are trained for this, and rigorous regulations are in effect. Virtually all automobile batteries in the U.S. are recycled, and the environmental effects of leachates from the small number of hybrid battery packs that are not recycled will be no worse than they are from ordinary automobile batteries.

Finally, the typical hybrid vehicle is more expensive than corresponding non-hybrids. Although the variables involved are many, those more concerned about economics than the environment might steer away from hybrids in favor of traditional economy vehicles, as they would result in a lower cost in most cases.

Hybrids vs. electric vehicles

Battery powered all-electric cars (BEVs) are more popular in Europe than in the U.S. (Most European electric vehicles are purchased from manufactures, while due to unavailability of manufactured vehicles, U.S. vehicles are owner produced versions of  conventional vehicles.) The major U.S. automobile manufacturers argue that customer demand for pure electric cars is small. In addition, the long suburban commutes common in the U.S. make range an important criterion for electric vehicle design. However, if advances in battery technology allow increased range at comparable cost to gasoline-powered vehicles, manufacturers will likely mass-market electric vehicles. The relative cost of gasoline to an equivalent amount of electrical energy will also be a critical factor in the electric vehicle market.

For now, car manufacturers are focusing on fuel cell-based cars and hybrids. Fuel cell vehicles are being developed in a long-term research environment, rather than with expectations of production at any definite time. Toyota intends all of its vehicles to have a hybrid option by 2012

Above text originally from  This text is available under GPL.

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