Home - Background: Glossary

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AFVs running on CNG, ZEVs, PZEVs, AT-PZEVs--there's a lot of alphabet soup and jargon in the field of alternative transportation, so I'll provide concise definitions for some of these terms here, in an alphabetical list. In several cases, I'll just give a link to a different page dedicated to the topic. If you have a more specific question in mind, this site has a Frequently Asked Questions (FAQ) page, and for further information on alternative fuels and drivetrains generally or on a specific fuel, you might try some of the links on the More Information page.

AFV: Alternative-fuel vehicle, a vehicle that runs on something else besides gasoline or petroleum-based diesel fuel.

AT-PZEV: An advanced-technology partial zero-emission vehicle, a vehicle that meets the PZEV standards and also contains some "ZEV-enabling" technology, like the electric drivetrain components of a hybrid or the high-pressure gaseous fuel tank of a CNG vehicle (which will be needed to store hydrogen for fuel-cell vehicles).

B20: A blend of 20% biodiesel with 80% petroleum diesel fuel, which can be used in diesel vehicles with no modifications to their engines, fuel systems, or refueling infrastructure.

BEV: A battery electric vehicle, as opposed to a hybrid-electric vehicle or a fuel-cell electric vehicle; usually just called an EV.

Bi-fuel vehicle: A vehicle that can run either on an alternative fuel or on gasoline, switching back and forth between them as needed (see diagram on my page about alternative drivetrains).

Blended hybrid: A plug-in hybrid that can't go any distance on utility electricity alone. While most people's idea of a plug-in hybrid is one that can run with the gasoline engine off, using just wall-plug electricity (that is, a vehicle that is a full hybrid to begin with), a mild hybrid that has to use gasoline to move can also have its "boost" electrical power supplemented by plugging in.

Cellulosic ethanol: Ethanol that, instead of being made from the sugar or starch in food crops like corn, sugar cane, or sugar beets, is made from the cellulose in waste materials like rice straw or sawdust, or low-value crops like switchgrass.

CNG: Compressed natural gas.

Dedicated alternative-fuel vehicle: A vehicle that runs only on an alternative fuel, with no gasoline or diesel capability (see diagram on my page about alternative drivetrains).

Depletion hybrid: Short for "charge-depletion hybrid"; another name for a plug-in hybrid, since its controller will be programmed to use up the charge in the battery pack (down to a certain level) to make maximum use of the electricity from external charging. A non-plug-in hybrid, by contrast, is a "charge-maintenance hybrid," since its controller keeps the battery at a fairly constant level, just using it as a buffer to provide "surge" power or regenerative braking when needed.

Dual-fuel vehicle: A vehicle that runs on both an alternative fuel and a petroleum fuel in a fixed ratio, typically natural gas and diesel (see diagram on my page about alternative drivetrains).

Dual-mode hybrid: A variant of the full hybrid, developed by GM, DaimlerChrysler, and BMW for future vehicles, that uses two electric motors and both fixed and variable gear ratios to boost fuel economy both in stop-and-go crawling and on the freeway. The three automakers contrast it with most hybrids that are focused more on improving fuel economy in one or the other type of driving.

E85: A blend of 85% ethanol and 15% gasoline.

EV: An electric vehicle, a vehicle that runs on electricity stored onboard in a battery pack.

FCEV: A fuel-cell electric vehicle, a vehicle with electric motors powered by energy from a fuel cell rather than from a battery pack, though most FCEVs have small battery packs as a buffer for extra acceleration and to absorb energy during regenerative braking.

FFV: A flex-fuel vehicle, a vehicle that runs on an alternative fuel like ethanol or methanol, which can be mixed with gasoline in the same fuel tank (see diagram on my page about alternative drivetrains).

Full hybrid: A hybrid whose electric motor can turn the drive wheels by itself under light load, without assistance from the combustion engine, in contrast to a mild hybrid.

HEV: A hybrid-electric vehicle, a vehicle that combines an internal-combustion engine with an electric motor. There are many sub-types of HEV; see other entries on this page with "HEV" or the word "hybrid" in them, or the diagrams on my page about alternative drivetrains.

Hydraulic hybrid: A vehicle that, instead of storing energy in a battery and tapping it through an electric motor like a hybrid-electric vehicle, stores energy in a pressurized hydraulic accumulator and taps it through mechanical pumps and turbines. Such a drivetrain provides some of the same benefits ("surge" power and regenerative braking) as a HEV drivetrain; it's most popular in heavy-duty vehicles like garbage trucks that already have high-pressure, high-volume hydraulic systems onboard.

ILEV: Inherently low-emission vehicle, a Federal emission standard that includes limits on both exhaust pollution and the fuel-cycle (fuel manufacture, distribution, and dispensing) emissions. Unlike other (non-zero) emission standards like LEV, this one can't be met by gasoline vehicles because of the fuel-cycle emission limits.

LEV: Low-emission vehicle, the basic pollution level defined in the California Low-Emission Vehicle regulations. Some new vehicles were required to meet this standard in the 1997 model year, and all new vehicles had to be at least this clean as of the 2000 model year. My 1993 Dodge natural-gas van was the first vehicle certified to meet this standard, four years early.

LNG: Liquefied natural gas.

LPG: Liquefied petroleum gas, also known as propane.

M85: A blend of 85% methanol and 15% gasoline.

Micro hybrid: A hybrid whose electric motor doesn't add thrust to the combustion engine, but instead acts as a starter/generator to allow the combustion engine to stop and restart instantly to avoid idling, and to enable regenerative braking. Also called a "stop/start hybrid" or a "hollow hybrid" (this last is pejorative, because a micro hybrid drivetrain doesn't improve fuel economy as much as a mild or full hybrid drivetrain). I have read that BMW is introducing such a modified drivetrain for vehicles sold in Europe, and not even bothering to call it a hybrid...

Mild hybrid: A hybrid whose electric motor cannot turn the drive wheels by itself, but instead only assists the combustion engine, in contrast to a full hybrid.

Muscle hybrid: A hybrid whose drivetrain is set up to provide more power than a standard drivetrain with somewhat better fuel economy, in contrast to many hybrids that provide about the same power as a standard drivetrain but with much better fuel economy.

NEV: A "neighborhood electric vehicle," an electric vehicle with top speed limited to 25 MPH that is legal in many states on streets with a posted speed limit of 35 MPH or less. Also called a "low-speed vehicle" (LSV); since the safety standards these must meet are less stringent than those for freeway-legal vehicles, they tend to be a lot cheaper (if you don't have to drive on any 40 MPH streets).

Parallel hybrid: A hybrid whose combustion engine can turn the drive wheels directly, either alone or in conjunction with the electric motor, in contrast to a series hybrid (see diagram on my page about alternative drivetrains).

PHEV: A plug-in hybrid, a hybrid that can be plugged in to recharge from wall-plug (utility) electricity, instead of generating all its electrical power onboard from gasoline. Such a vehicle can run using less gasoline than a non-plug-in hybrid, or even none at all for much of a day's driving; this reduces petroleum use and pollution, at the cost of requiring provision for external charging and a larger battery pack than an ordinary hybrid.

PZEV: Partial zero-emission vehicle, an awkward name (part of zero?) that probably should be called "partial-credit" ZEV. This is a vehicle that meets the SULEV tailpipe emission limits, has no evaporative emissions (gasoline escaping after the vehicle is warmed up and parked), and has a 15-year warranty against failure of emissions-related components. Because of automaker lawsuits, lobbying, and foot-dragging, California modified its vehicle emission regulations so that automakers don't actually have to build any substantial number of zero-emission vehicles; rather, they can instead fill their ZEV quota by making PZEVs and a small number of fuel-cell vehicle prototypes. Sometimes confused with plug-in hybrids that can run in zero-emission mode part of the time, but these are not the same thing.

Regenerative braking: Most electric motors can also function as generators, so that in addition to converting electrical energy into energy of motion to speed up a vehicle, they can also slow the vehicle by converting energy of motion back into electricity. This means the energy can be stored in a battery or ultracapacitor and reused for later acceleration instead of being wasted as heat in friction brakes, increasing the vehicle's efficiency.

Series hybrid: A hybrid whose combustion engine doesn't turn the drive wheels directly, even part of the time, but instead turns a generator to send electricity to an electric motor (or motors) that turns the wheels, in contrast to a parallel hybrid (see diagram on my page about alternative drivetrains).

SULEV: Super-ultra-low-emission vehicle (I'm not making this up); an emissions level cleaner than the ULEV standard defined in California's Low-Emission Vehicle regulations, and the cleanest one short of a zero-emission vehicle.

SVO: "Straight vegetable oil," which can be burned in diesel engines with some modifications; contrasted with biodiesel, which is a refined fuel (often based on SVO) with properties similar to ordinary petroleum diesel.

TLEV: Transitional low-emission vehicle, the dirtiest emission level defined in the California Low-Emission Vehicle regulations. Superseded by LEV as of the 1997 model year.

ULEV: Ultra-low-emission vehicle, a pollution level cleaner than the LEV level defined in the California Low-Emission Vehicle regulations. Phased in starting with the 1997 model year; the 1994 Chrysler CNG minivan was the first vehicle certified to meet this standard.

Ultracapacitor: An ordinary battery stores electrical energy in the form of chemical energy, changing a mixture of chemicals from one form to another as electrical energy moves into and out of the battery; an ultracapacitor stores electrical energy as a charge of "static electricity" on insulated pieces of conductive material, and can be used instead of a battery as the buffer in a hybrid-electric or fuel-cell vehicle. The "ultra" part of the name refers to the fact that such a device holds far more charge than the little components you buy at Radio Shack!

ZEV: Zero-emission vehicle, a vehicle that emits no tailpipe pollutants, like an electric or fuel-cell vehicle. In the early 1990s California required that 2% of 1998 model year vehicles sold in the state be ZEVs, with the percentage ramping up to 10% by the 2003 model year; this has been repeatedly watered down, so that now automakers can fulfill their ZEV quotas by by making PZEVs to amass partial credits, and building a few fuel-cell prototypes.

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new 24 September 2006