Glossary

anaerobic digestion

The decomposition of biomass in the absence of oxygen, producing a mixture of methane and carbon dioxide.

battery electric vehicle (BEV)

A vehicle that runs on electricity drawn from a battery (as opposed to a fuel cell).

BEV

See battery electric vehicle.

biodiesel

A biomass-derived alternative to DERV. The term can refer to a pure biofuel, or to a blend of DERV and biofuel.

bioethanol

An alcohol that can be produced from virtually any fermentable source of sugar.

biofuels

Types of fuel derived from biomass.

biogas

The mixture of methane and CO₂ produced by the anaerobic digestion of animal waste and wet food waste in a closed container.

carbon dioxide (CO₂)

A gas produced from the combustion of hydrocarbons and by respiration in plants and animals. Strictly speaking it is not toxic in the same sense as carbon monoxide, but it does not support respiration or burning and is a greenhouse gas.

carbon monoxide (CO)

A highly toxic gas that is produced by the combustion of any carbon-based fuel that takes place with insufficient oxygen. A powerful reducing agent used in the smelting of metals such as iron.

Carnot efficiency

The theoretical maximum efficiency of a heat engine or heat pump, which depends upon the difference between the starting and final temperatures of the process.

catalyst

A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.

catalytic converter

See three-way catalytic converter.

compressed natural gas

A gaseous fuel that can be used as an alternative to petrol or diesel for transportation, typically stored in high pressure tanks.

compression ratio

The reduction in volume during the compression phase of a four-stroke engine cycle. It is expressed as a ratio of the original volume to the reduced volume.

Delivered (or final) energy

Another term for final energy. The energy actually received by a consumer, for example that measured by a consumer’s electricity meter after the losses in generation and distribution. Contrast with primary energy.

DERV

Diesel fuel. The acronym stands for Diesel Engines for Road Vehicles.

diesel engine

A type of internal combustion engine. It has a higher compression ratio than a petrol engine. The diesel fuel ignites under pressure, rather than being ignited by an electric spark in a petrol engine.

Diesel particulate filters (DPFs)

A device for reducing the particulate matter emissions of diesel road vehicles.

E10

A blend of 10% ethanol and 90% petrol.

energy service

The ultimate aims for which energy systems are built: warm homes, cooked food, illumination, mobility and manufactured articles.

final energy

see delivered energy

fuel cell

A device for producing an electric current by means of what is essentially the reverse process to electrolysis – combining two gases (typically hydrogen and oxygen) to produce electricity.

fuel cell vehicle (FCV)

An electric car or bus that uses a fuel cell (and hence may need to carry hydrogen as a fuel) rather than a battery.

internal combustion engine (ICE)

A type of engine in which the fuel is burned inside the engine, such as the petrol and diesel engine.

‘joule (J)’

SI unit of energy

kilowatt-hour (kWh)

A unit of energy. If a device operates with a power of one kilowatt for one hour, the total cumulative energy that has flowed in that hour is one kilowatt-hour. The standard ‘unit’ of electricity bills.

lead-acid battery

A rechargeable battery based on plates of lead and lead dioxide with a sulfuric acid electrolyte, commonly used for car starting and lighting batteries.

‘light-off’ temperature

The engine temperature at which a three-way catalytic converter starts to work.

Liquefied petroleum gas

Butane and propane, alkanes which are gases at room temperature and can be liquefied under mild pressure. Sold for use in road vehicles, domestic heating and camping stoves and lanterns. Not to be confused with liquefied natural gas (LNG) which is mainly methane.

lithium ion batteries

A rechargeable battery based on lithium. It has a high energy density and is being widely used for computer batteries and electric cars.

low heating value (LHV)

The energy content of a hydrocarbon that is burned in a system where the water given off by the combustion is not condensed. Also known as the lower calorific value (LCV) or the net calorific value (NCV).

methane (CH₄)

The lightest of the alkanes, with the chemical formula CH₄. The major component of natural gas.

methanol

An alcohol, often used as a vehicle fuel. Largely produced from natural gas, but can also be produced from biomass.

modal shift

Moving journeys away from energy-intensive modes of travel and on to more energy-frugal modes. For example moving from car use to walking and cycling.

nickel metal-hydride (NiMh)

A rechargeable battery based on nickel and hydrogen. It may include a ‘rare earth’ such as lanthanum.

Nitrogen oxides (NO_x)

NO_x consists of two oxides of nitrogen: the acidic gases NO, nitric oxide, and NO₂, nitrogen dioxide, both produced in high-temperature combustion. NO₂ is the more powerful acid gas. N₂O, nitrous oxide, is not normally classified as part of NO_x although it is an oxide of nitrogen. It is a powerful greenhouse gas.

octane rating

A rating of the capacity of petrol to resist spontaneous ignition (‘knocking’) prior to the ignition phase of the four-stroke engine cycle.

Otto cycle

Another term for the four-stroke petrol engine cycle.

oxidation catalytic converters

A device for reducing the particulate matter emissions of heavy-duty road vehicles. Also known as a one-way catalyst.

particulate matter (PM)

Solid particles that are small enough to float in air (a few microns in size). Also known as particulates.

particulates

Another term for particulate matter.

petrol

Confusingly, not the same as petroleum, but one of its products – a blend of light distillate hydrocarbons. Alternative names for petrol are motor spirit and gasoline.

petrol–electric hybrid

General term for a car that has both a petrol engine and a rechargeable battery used to drive an electric motor.

petrol engine

A type of internal combustion engine. Also known as the spark ignition engine because it uses a spark plug to ignite a petrol–air mixture in the cylinders. Most petrol engines use the four-stroke engine cycle.

petroleum

Alternative term for crude oil. (Not the same as petrol.)

plug-in hybrid

A car with a petrol engine and a battery that can be recharged by the engine but can also be charged by mains electricity.

PM

See particulate matter.

PM10

Particulate matter less than 10 microns in size.

PM2.5

Particulate matter less than 2.5 microns in size.

primary energy

The total energy content of an energy resource before that energy is extracted/transformed/processed. Primary energy is the starting point for an energy transformation system: the incoming ‘amount we have to work with’ before it is acted upon by power stations. See also notional primary energy input and final and delivered energy.

regenerative braking

The use of an electric motor during the braking of a vehicle to convert some of the kinetic energy of the vehicle into electrical energy to recharge the battery.

reserve/production (R/P) ratio

The number of years a given reserve will last if it continues to be produced/extracted at the current rate.

selective catalytic reduction (SCR)

A technique for reducing NO_x emissions using injected ammonia to reduce it to nitrogen. Variants of this method can be used in both coal-fired power stations and diesel engines.

sulfur dioxide (SO₂)

A gaseous by-product of the combustion of any fuel that contains sulfur. It contributes to acid rain and is a respiratory irritant.

three-way catalytic converter

A device for reducing the emissions of CO, NO_x and unburned hydrocarbons from the exhausts of petrol engines.

tetraethyl lead

A poisonous compound that used to be added to petrol in order to improve its octane rating.

volatile organic compounds (VOCs)

A range of hydrocarbons, many carcinogenic. They may be produced by combustion or are evaporated chemical solvents.