Can't tell an EV from an FCV? Wondering why that sedan is only a "partial" zero-emissions vehicle? Struggling to figure the difference between a series hybrid and a parallel hybrid?

You're not alone. With the greening of the automobile, we're being inundated with an entirely new vocabulary, but one most motorists are going to have to learn in the coming years. is here to give you a head start. Here's the complete glossary of green machine terms and technologies,
from A to Z:

Alternative fuels

Gasoline and to a lesser extent diesel, currently make up 98 percent of the fuels used to power the U.S. car and truck fleet. The situation is similar in most other countries but Brazil , which has made a major switch to sugar cane-derived ethanol. The term "alternative fuels" generally refers to any non-petroleum fuels, including ethanol, hydrogen, synthetic diesel compressed natural gas, or CNG, and hydrogen.


A device using a chemical interaction to produce direct current electricity. There are a wide range of single-use batteries, such as lead-acid and alkaline, and a growing assortment of rechargeable batteries, including nickel-cadmium, nickel-metal hydride and lithium-ion. Each has distinct characteristics affecting their ability to store, discharge and recharge. Current batteries are heavy, expensive and relatively inefficient, so super-high-efficiency batteries are perhaps the single most holy of Holy Grails among advanced powertrain researchers.


A synthetic alternative to petroleum-based diesel fuel, it can be derived from sources such as used cooking oil - hence the distinct smell of French fries in the exhaust of some vehicles running on bio-diesel. A populist movement has sprung up to produce this alternative and renewable fuel, but there is growing interest by government regulators, putting pressure on "Big Oil" to provide biodiesel. Typically, it requires some modification to a conventional diesel powertrain, and may be mixed with petroleum-based diesel fuel. B85, for example, is a mix of 85 percent biodiesel and 15 percent petro-diesel.


A patented diesel emissions control system developed by Mercedes-Benz that is designed to sharply reduce emissions of oxides of nitrogen and particulates. It cuts down on those emissions by treating exhaust with a spray of urea that reduces the oxides of nitrogen passing out of the vehicle.

Carbon dioxide (CO2)

Made up of one part carbon and two parts oxygen, this common molecule was long considered a harmless result of the combustion process - unlike highly poisonous carbon monoxide, or CO. In recent years, it has been theorized that a build-up of CO2, largely from burning fossil fuels, is contributing to a warming of Earth's atmosphere (see global warming).


A potential method of producing ethanol that would no longer require sugar-rich vegetable matter, such as sugar cane or corn, to be processed. Cellulose compounds, such as wheat chaff and even scrap newspaper could be used instead, increasing the potential supply of biomass available to produce this alcohol fuel. Currently in prototype production, one cellulosic process relies on an enzyme initially discovered in the belly of a beetle.

Cylinder deactivation

Known by a variety of names, including Displacement-on-Demand, the concept is to simple shut down one or more of an internal combustion engine's cylinders when power demands are low - usually while cruising at a steady highway speed. Increasingly common in today's automobiles, experts believe the technology can reduce fuel consumption by as much as 10 percent.


More formally known as the continuously variable transmission, the CVT is being billed as an alternative to conventional automatic and manual gearboxes. A transmission is used to reduce to take rotational power from the engine and link it to the driven wheels. Under most conditions, that rotational speed must be altered to be most efficient. A conventional transmission uses fixed-speed "step" gears that are inherently a compromise. CVT technology uses a system of belts and pulleys to constantly select the most effective gear ratio for road conditions and a motorist's driving style. CVTs generally deliver at least a 5 percent improvement in fuel economy and can have far fewer pieces than an automatic transmission.


This increasingly popular alternative to the gasoline engine was developed by German inventor Rudolph Diesel, in 1892, about 16 years after the invention of the gasoline-powered internal combustion engine. In many ways, the two technologies have much in common. They both require the injection of a mixture of air and fuel into a cylinder, and when it is burned, expanding gases move a piston which, in turn, sends energy out to the wheels. But in a diesel, the air/fuel mixture is compressed to the point where there is spontaneous combustion. Diesel fuel itself is heavier than gasoline and contains more energy. In years past, diesels were derided as noisy, smelly and slow, but new technologies, such as direct injection, have overcome most of those problems. Manufacturers are still struggling to meet tough new U.S. regulations for particulates and nitrogen oxides, which are limiting sales, but in Europe, high-mileage, diesel-powered vehicles are now outselling gasoline vehicles.


A blend of 85 percent ethanol and 15 percent gasoline, this is the most common blend of the alcohol-based fuel, and is slowly increasing in distribution around the United States . Adding that small amount of petroleum offers a number of advantages, including the ability to start in extremely cold climates.

Electric Vehicle

A car, truck or crossover, such as General Motors' EV1 and the golf cart-like Gem, which runs solely on electric power. EVs are the only true zero-emissions vehicles, as they produce absolutely no emissions while running on battery power. Proponents argue that EVs meet the needs of most American drivers, but motorists tend to buy their vehicles based on worst-case scenarios, such as the long trips they might take once or twice a year. The trade-off in terms of EV range, charging times, performance and features doomed the generation of electrics mandated by California in the 1990s. The rules were quickly repealed, though proponents still believe there is an EV future.


An electrical process in which high current levels are used to break water molecules down into their most basic, atomic components, two parts of hydrogen and one part water. There are other means of producing hydrogen, but they usually require starting from a fossil fuel base, such as coal or natural gas. Electrolysis is extremely energy intensive, thus inefficient, but the energy needed can be provided by nuclear energy or renewable sources, such as wind, solar or geothermal power.


An alcohol fuel that is viewed by many as a viable alternative to gasoline and other petroleum-based fuels. Until now, ethanol had to be produced from sugar-rich sources - corn is most common in the U.S., sugar cane in Brazil - but a new cellulosic process (see above) could expand the number of feed stocks. Ethanol may actually increase the performance of an internal combustion engine. But compared to gasoline, it also contains less energy, measured as BTUs, per liter, and thus typically results in lower mileage.


A process developed in the 1920s in which carbon monoxide and hydrogen react to a catalyst to form a synthetic form of petroleum, typically used for either lubricants or fuels. Sometimes known as "designer fuels," or "designer diesel," they may closely resemble conventional petroleum products, but offer improved combustion or lower emissions. A synthetic diesel fuel is used to power the Audi R10 race car.

Flexible-fuel vehicles

Vehicles designed to run on more than one type of fuel, most commonly either gasoline or ethanol, or a blend of the two. BMW and Ford have produced gasoline/hydrogen FFVs, and a new prototype "Multi-Fuel Vehicle," from Volvo, would be able to run on a wide range of both liquid and gaseous fuels, including gasoline, ethanol and compressed natural gas.

Fuel cell

First conceived in the mid-19th century, the fuel cell was first put to serious application as the energy source for America's Apollo moon program. At its simplest, the technology feeds hydrogen into one side of a "stack," and oxygen (or regular air) into the other. In the most common fuel cell design, the hydrogen passes through a catalyst covered proton exchange membrane (see below), which causes it to combine with the oxygen. The resulting reaction produces both water vapor and a small amount of current that can be used to power a spacecraft - or drive an electric motor. Fuel cells are sometimes referred to as "refillable batteries."

Fuel-cell vehicle

Also known as an FCV, this is, in function, an electric vehicle which relies on a fuel cell, rather than a battery, to provide the power for its motor(s). The first FCV was developed by General Motors in the 1960s, and in recent years, virtually every major automaker has developed its own, running prototype. But while several companies, including Honda, claim to be "selling" small numbers of fuel-cell vehicles, they are really just field-testing the technology. The technology has been improving dramatically in recent years, becoming smaller, more efficient and more durable, but cost and manufacturability are still serious issues and most experts believe truly competitive FCVs won't be ready for market before the middle of the next decade - at the earliest.

Global warming

Many scientists theorize that the use of fossil fuels is responsible for a steady increase in atmospheric levels of carbon dioxide. This and other so-called greenhouse gases (see below) are thought to trap heat in the atmosphere. Those who attribute the warming of the planet-at least 1 degree Fahrenheit in the past century-to use of fossil fuels believe that the process is to blame for the melting of many glaciers and parts of the polar ice caps. Global warming theorists say temperatures could rise perhaps five degrees or more this century, leading to greater melting - and resulting in flooding of low-lying coastal regions. A vocal minority of scientists believe the earth's warming trend is caused by natural variations in surface temperatures.

Greenhouse gases

Any of numerous gases that absorb infrared radiation when they are released into the atmosphere. This leads to the so-called greenhouse effect, which is one source of global warming. Greenhouse gases include not only CO2, but also methane, oxides of nitrogen and various fluorocarbons.


These cars, trucks and crossovers combine at least two different forms of power in order to improve fuel economy and reduce emissions. The most common approach is to use both gasoline and battery/electric power, though several manufacturers, including Peugeot, are developing diesel-electric hybrids. The typical hybrid recaptures energy lost during braking and coasting, storing it in an onboard battery pack. When a burst of energy is needed, power is drawn from the batteries to run the onboard motors as well as the gas engine. Not all hybrids are alike. Some only feature this "electric supercharger" mode, others can automatically shut down their gas engine when idling. So-called "full" hybrids are able to run on electric power only, usually for short distances and at low speeds. There is extensive debate over the amount of fuel saved by hybrids and whether added vehicle costs are justified, but sales have been steadily growing, especially in the U.S.


The lightest element on the periodic table - its most common form consists of just a single proton and an orbiting electron - hydrogen is also the most abundant element in the universe, the fuel for the sun's thermonuclear furnace. Many experts are encouraging the development of a "hydrogen economy," replacing petroleum fuels with the lightweight gas. But unlike oil, hydrogen is best considered an "energy carrier." On earth, it exists only as part of various chemical compounds, such as water or petrochemicals. So adopting a hydrogen economy will require not only the development of devices like the fuel cell, but also an efficient means to produce, transport and store the gas. In a fuel-cell, hydrogen and oxygen are passed through a membrane, producing electricity and water vapor.

Lead-acid battery

One of the most common types of battery, this technology combines lead plates and an acid, such as hydrochloric, to produce direct current. The interaction of the acid on the lead creates an electrical current. It is heavy, but relatively cheap and durable.


The central ingredient in two of the newer and more promising forms of batteries, lithium-ion and lithium-polymer. Already common in rechargeable portable devices such as MP3 players, laptop computers and cellphones, lithium-based batteries store lots of current (high energy density), and can both store and discharge quickly. But they are also quite expensive, which limits their use in devices, such as hybrid and electric vehicles, which need extremely large battery packs.

Low-sulfur diesel

Sulfur is an every-present part of petroleum, even in so-called "sweet oil." The chemical is largely responsible for the foul smell in traditional diesels, and it "poisons," or renders inactive, many of the most advanced pollution control devices. In order to clean up diesel exhaust by making possible the use of new technology, refiners are being required to sharply reduce sulfur content, from about 500 parts per million, to just 15 ppm.


A blend of 85 percent methanol and 15 percent gasoline, it was once considered a strong contender to replace or supplement gasoline in the U.S.


Methyl tertiary-butyl ether is a common fuel additive which oxygenates gasoline, improving octane. MTBE has been used widely as a means to reduce emissions, but it is now being phased out because of the discovery that this potentially carcinogenic compound has seeped into groundwater in much of the U.S.


An alcohol fuel commonly known as gasohol. It has a number of advantages, including a boost in horsepower and a reduction in ozone-forming emissions. Methanol is frequently used in race cars, such as top-fuel dragsters and the Indy Racing League. But methyl alcohol has a number of disadvantages, as well. It burns invisibly, so it is difficult to see if there is a fire. This particular alcohol is extremely toxic and even a small leak can poison a region's water supply.


"Mild" hybrids, or mybrids, make more limited use of their battery-generated power than a full hybrid. Though a mybrid may be able to stop and start its gasoline engine at a light, it cannot run in electric-only mode. Mybrids are generally less expensive than a full hybrid, but may not see as much of an increase in fuel economy.


The first, popular form of rechargeable battery, but one slowly losing popularity. The NiCad has several disadvantages. For one thing, it uses an extremely toxic chemical, cadmium, which is especially difficult to dispose of. And NiCads tend to lose power over time, especially if they are improperly charged and discharged - the "memory effect."

Nickel-metal hydride

This alternative to the NiCad is rapidly becoming the most popular rechargeable battery, both for consumer appliances, such as digital cameras, and for hybrid-electric vehicles. The NiMH battery poses fewer environmental problems, and doesn't have many of the drawbacks of NiCad cells, such as the memory effect.

Parallel hybrid

Most hybrids use two sources of power in a parallel arrangement. In other words, the gasoline engine and electric motor components are each directly connected to the vehicle's drivetrain, where they can operate independent of one another.

Particulate filter/trap

Particulates are microscopic particles of soot formed by the diesel combustion process. There is great scientific debate over the dangers posed by particulates, though some reports show them as being carcinogenic when drawn deep into the lungs. New U.S. regulations - even stricter in California - sharply reduce the amount of particulates than can be emitted by diesels. The new regulations have caused manufacturers such as Volkswagen to pull existing diesels from the market; Mercedes-Benz has developed Bluetec technology specifically to meet the new regulations for particulate emissions.

Plug-in hybrid

A type of hybrid vehicle that allows for extended use in electric-only mode. It can be thought of as an electric vehicle with an onboard engine. Proponents argue that plug-ins can meet most of the needs of the typical American commuter, who typical drives only 30 to 60 miles daily. Unlike a pure EV, range is not limited by batter size. When the pack discharges, on longer trips, the vehicle can run on its gasoline engine, recharging the batteries when power demand is low. Critics content there is too much of a cost penalty, and the added weight of the oversize battery pack can reduce mileage.

Proton exchange membrane

A membrane usually coated with exotic metals, such as platinum, used in a fuel cell to catalyze a reaction between hydrogen and oxygen. The result is water vapor and electric current.


A Partial-Zero-Emission Vehicle may sound like an oxymoron, but it is a class of vehicles created by the Environmental Protection Agency to include some of the cleanest vehicles on the road. The current crop of PZEVs run about 90 percent cleaner than the typical car on the road, and produce effectively no evaporative emissions. That's extremely significant, as a car of the 1970s could produce more hydrocarbon emissions while parked than a modern car would give off while driving across the country.

Renewable fuels

The most common form of fuels today come from fossilized deposits found deep in the earth - including petroleum, natural gas and coal. Some geologists believe fossil fuels are running out, and some environmentalists believe their use is contributing to global warming. Renewable fuels come from resources that can be re-grown, or remade from other materials, or harvested by other means. They fall into several categories. There are bio-fuels, such as corn-based ethanol, which come from renewable stocks, and which may be burned in the same engines and furnaces as petro-fuels or coal. Other renewables, including wind, solar, wave and geothermal power, can be tapped to generate electricity. Costs are generally higher than traditional fuels, but have been steadily declining as they go into more widespread usage.

Rolling resistance

The typical tire is responsible for using about 20 percent of the energy consumed by an automobile. A key determinant of a tire's contribution to total energy use is its rolling resistance. This is the friction caused by the flexing of the tire as it turns and deforms under load. Size, tread pattern and inflation all directly impact rolling resistance, as do the compounds used in the tire. New materials, such as silica, can significantly reduce rolling resistance and improve mileage.

Series hybrid

In this form of hybrid, the wheels are driven by only the electric motors. The onboard gasoline engine is used to either charge the hybrid's batteries, or to send electric current to the motors. Though not currently used by any manufacturer, the series approach has some potential advantages. Among other things, the gasoline engine can be run at a more steady, optimized speed, reducing both fuel consumption and emissions.


A capacity is, at its simplest, an electronic component designed to hold an electrical charge. Unlike a battery, it does not require a chemical process to charge or discharge. Small capacitors are found in almost every electronic device, from primitive tube radios to laptop computers, but researchers are working to develop oversized ultracapacitors that can store as much or more energy than a conventional battery. These devices can charge and discharge quite rapidly, a particular advantage in automotive applications. Honda is using an ultracapacitor as an alternative to batteries in its FCX fuel cell vehicle.


This water-soluble compound is the primary end product of metabolizing protein, and is found in the urine of mammals and other organisms. Urea has numerous applications, and is part of the approach being taken by Mercedes-Benz, among others to eliminate oxides of nitrogen from diesel emissions. By filtering exhaust vapors through a special urea system, dubbed Bluetec, nitrogen oxides are broken down into nitrogen and water vapor.


Every fuel or powertrain technology offers a potential advantage. But where one approach may boost mileage, it may also increase emissions. To properly compare future technologies, one must look at every element of its design and operation. Hydrogen, for example, is considered a "clean" fuel, because the only emission from a vehicle is water vapor. But it is really just an energy carrier. There are no underground deposits of the gas, which must be produced from some other source. Making hydrogen from electrolysis is extremely clean if the energy comes from wind or geothermal power. If it comes from a coal plant, or if the hydrogen is generated by cracking coal, there may be more CO2 and other emissions produced, wells-to-wheels, than in a conventional, gasoline-powered vehicle.

Zero-emission vehicle (ZEV)

A ZEV is a vehicle that produces absolutely no emissions when it is running. The only true zero-emission vehicles are EVs, or electric vehicles. FCVs, or fuel cell vehicles, emit a small amount of water vapor. Even burning hydrogen in an internal combustion engine results in the production of a small amount of oxides of nitrogen.

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