Idling refers to running a vehicle's engine and the vehicle is not in motion, or when the vehicle drops to its resting point of RPMs. This commonly occurs when drivers are stopped at a red light, waiting while parked outside a business or residence, or otherwise stationary with the engine running. When idling, the engine runs without any loads except the engine accessories, and without the additional fuel via the gas pedal. If the vehicle moves while in gear and idling, the "idle speed" mechanically should be adjusted.

Idle speed

Idle speed, sometimes simply called "idle", is the

rotational speed Rotational frequency, also known as rotational speed or rate of rotation (symbols ''ν'', lowercase Greek nu, and also ''n''), is the frequency of rotation of an object around an axis. Its SI unit is the reciprocal seconds (s−1); other com ...

an engine runs at when the engine is idling, that is when the engine is uncoupled from the

drivetrain A drivetrain (also frequently spelled as drive train or sometimes drive-train) or transmission system, is the group of components that deliver mechanical power from the prime mover to the driven components. In automotive engineering, the driv ...

and the throttle pedal is not depressed. In

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...

engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ge ...

s, idle speed is generally measured in

revolutions per minute Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines. One revolution per minute is equivalent to hertz. Standards ISO 80000-3:2019 de ...

(rpm) of the

crankshaft A crankshaft is a mechanical component used in a reciprocating engine, piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating Shaft (mechanical engineering), shaft containing one or more crankpins, ...

. At idle speed, the engine generates enough

power Power may refer to: Common meanings * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power, a type of energy * Power (social and political), the ability to influence people or events Math ...

to run reasonably smoothly and operate its ancillaries (

water pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of applications such ...

alternator An alternator (or synchronous generator) is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field wit ...

, and, if equipped, other accessories such as

power steering Power steering is a system for reducing a driver's effort to turn a steering wheel of a motor vehicle, by using a power source to assist steering. Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver can ...

), but usually not enough to perform useful work, such as moving an

automobile A car, or an automobile, is a motor vehicle with wheels. Most definitions of cars state that they run primarily on roads, Car seat, seat one to eight people, have four wheels, and mainly transport private transport#Personal transport, peopl ...

unless it is set too high. The opposite of idle speed is

redline The redline is the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine. The redline of an eng ...

, the maximum rotational speed the engine can be ran at without risking serious engine damage.

Car, truck, and motorcycle engines

For a passenger

car A car, or an automobile, is a motor vehicle with wheels. Most definitions of cars state that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people rather than cargo. There are around one billio ...

engine, idle speed is customarily between 600 and 1000 rpm. For medium and heavy duty trucks, it is approximately 600 rpm. For many single-cylinder

motorcycle engine A motorcycle engine is an engine that powers a motorcycle. Motorcycle engines are typically two-stroke or four-stroke internal combustion engines, but other engine types, such as Wankel engine, Wankels and electric motors, have been used. Altho ...

s, idle speed is set between 900 and 1100 rpm. Two-cylinder motorcycle engines are often set around 1000 rpm. If the engine is operating a large number of accessories, particularly

air conditioning Air conditioning, often abbreviated as A/C (US) or air con (UK), is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature, and in some cases, also controlling the humidity of internal air. Air c ...

, the idle speed must be raised to ensure that the engine generates enough power to run smoothly and operate the accessories. Most air conditioning-equipped engines have an automatic adjustment feature in the

carburetor A carburetor (also spelled carburettor or carburetter) is a device used by a gasoline internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the Ventu ...

fuel injection Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of a fuel injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines. All c ...

system that raises the idle when the air conditioning is running. Engines modified for power at high engine speeds, such as

auto racing Auto racing (also known as car racing, motor racing, or automobile racing) is a motorsport involving the racing of automobiles for competition. In North America, the term is commonly used to describe all forms of automobile sport including non ...

engines, tend to have very rough (unstable) idle unless the idle speed is raised significantly. Idle speed may refer to the idle creep of a vehicle with an automatic transmission.

Aircraft engines

Commercial aircraft descend with a minimum thrust, that is, the engines are operating at idle speed. This situation happens when an aircraft is gliding and during the landing flare, for approach the engines are usually not operated at idle power.

Vehicle emissions

Both running an engine and idling an engine produce several pollutants that are monitored in the United States by the Environmental Protection Agency (EPA): It is often believed that stopping and restarting the engine uses more fuel than idling. According to the

Environmental Defense Fund Environmental Defense Fund or EDF (formerly known as Environmental Defense) is a United States–based nonprofit environmental advocacy group. The group is known for its work on issues including global warming, ecosystem restoration, oceans, an ...

citing an engine studies report from 2000 by

Environment and Climate Change Canada Environment and Climate Change Canada (ECCC; )Environment and Climate Change Canada is the applied title under the Federal Identity Program; the legal title is Department of the Environment (). is the department of the Government of Canada res ...

, an engine restart uses fuel approximately equal to 10 seconds of idling. Consequently, recommendations have been made to shut off a car's engine after ten seconds of idling to reduce emissions.

Winter conditions

Assuming a temperature of -1 °C (30 °F), and with a gasoline

Reid vapor pressure {{Short description, Measure of the volatility of gasoline and other petroleum products Reid vapor pressure (RVP) is a common measure of the Gasoline#Volatility, volatility of gasoline and other petroleum products. It is defined as the absolute va ...

(RVP) of 896 hPa (13.0) psi. Legend: * LDGVLight duty gasoline vehicle * LDGTLight duty gasoline truck * HDGVHeavy duty gasoline vehicle * LDDVLight duty diesel vehicle * LDDTLight duty diesel truck * HDDVHeavy duty diesel vehicle * MCMotorcycle

Summer conditions

Assuming a temperature of 24 °C (75 °F), and with a gasoline Reid vapor pressure of 620 hPa (9.0 psi). Source:

Health effects of idling pollutants

Health effects of idling are related to engine exhaust, and include acute effects such as eye, throat, and bronchial irritation; nausea; cough, phlegm congestion; allergic or asthma-like respiratory response; increased risk for cardiac events; cancer, and chronic effects, such as bronchitis, decreased lung function, damage to reproductive function (low birth weight and damage to sperm chromatin and DNA). Prepared by the National Center for Environmental Assessment, Washington, DC, for the Office of Transportation and Air Quality; EPA/600/8-90/057F. Available from: National Technical Information Service, Springfield, VA; PB2002-107661, and www.epa.gov/ncea.HEI Panel on the Health Effects of Traffic-Related Air Pollution. (2010) Traffic-Related Air Pollution: A Critical Review of the Literature on Emissions, Exposure, and Health Effects. HEI Special Report 17. Health Effects Institute, Boston, MA. These health effects are more damaging in those with preexisting heart disease, asthma, or other lung problems. Children are also more susceptible, due to their faster breathing rate and the fact that their respiratory system is still developing. Idling pollutants also disproportionately affect the elderly, who have limited physiological reserve to compensate for the adverse effects of the pollutants.

Strategies to reduce idling in traffic

Effort has been made to reduce the amount of time engines spend idling, chiefly due to fuel economy and emissions concerns, although some engines can also be damaged if kept idling for extended periods. In the United States, about a billion gallons (3.8 billion liters) of fuel is consumed by idling heavy-duty truck and locomotive engines each year. Many newer semi-trucks have small

auxiliary power unit An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft and naval ships as well as some large land vehicles. Aircraft APUs generally produce 115&n ...

s (APUs) to run accessories more efficiently while the truck is parked.

Hybrid vehicle A hybrid vehicle is one that uses two or more distinct types of power, such as submarines that use diesel when surfaced and batteries when submerged. Other means to store energy include pressurized fluid in hydraulic hybrids. Hybrid powertrai ...

s typically shut down their

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

s while stopped, although some conventional vehicles are also including

start-stop system A start-stop system (also referred to as idling stop or micro hybrid) is a technology that automatically shuts down and restarts a vehicle's internal combustion engine to reduce idle time, with the aim of lowering fuel consumption and emissio ...

s to shut off the engine when it would otherwise idle. At the macro level, governments can implement strategies to reduce reliance on motorised transport, including investing in public transport and implementing transit-oriented development.

Anti-idling legislation

Belgium

Idling is forbidden unless there is a specific reason to do so (version 1975-2026, art. 8.6 ), to be superseded by version 2026-09, art. 8.7

Canada

The city of

Toronto Toronto ( , locally pronounced or ) is the List of the largest municipalities in Canada by population, most populous city in Canada. It is the capital city of the Provinces and territories of Canada, Canadian province of Ontario. With a p ...

enacted the first idling bylaw (No. 673-1998 Chapter 517 in the Municipal Code) in

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

in 1996 to reduce idle time to 3 minutes for vehicles and marine vessels. There are plans by the health department to ask for the bylaw to be amended to a limit of one minute and no exemptions to be made for the city's fleet, including the

Toronto Transit Commission The Toronto Transit Commission (TTC) is the primary public transport agency in Toronto, Ontario, Canada, operating the majority of the city's transit bus, bus and rail services. It is the oldest and largest of the urban transit service providers ...

buses. Other Canadian municipalities have followed Toronto's lead: *

Mississauga, Ontario Mississauga is a Canadian city in the province of Ontario. Situated on the north-western shore of Lake Ontario in the Regional Municipality of Peel, it borders Toronto (Etobicoke) to the east, Brampton to the north, Milton to the northwest, ...

Idling Control By-law, 194-2009 *

Oakville, Ontario Oakville is a town and List of municipalities in Ontario#Lower-tier municipalities, lower-tier municipality in Regional Municipality of Halton, Halton Region, Ontario, Canada. Generally seen as a commuter suburb of Toronto, it is located on Lake ...

Anti-Idling Bylaw 2002-153 *

Waterloo, Ontario Waterloo is a city in the Canadian province of Ontario. It is one of three cities in the Regional Municipality of Waterloo (formerly Waterloo County, Ontario, Waterloo County). Waterloo is situated about west-southwest of Toronto, but it is n ...

By-law 2009-077 *

Red Deer, Alberta Red Deer is a city in Alberta, Canada, located midway on the Calgary–Edmonton Corridor. Red Deer serves central Alberta, and its key industries include health care, retail trade, construction, oil and gas, hospitality, manufacturing and educati ...

(2009) *

Sudbury, Ontario Sudbury, officially the City of Greater Sudbury, is the largest city in Northern Ontario by population, with a population of 166,004 at the 2021 Canadian Census. By land area, it is the largest in Ontario and the List of the largest cities and t ...

(Campaign only2010) *

Dawson Creek, British Columbia Dawson Creek is a city in northeastern British Columbia, Canada. The municipality of had a population of 12,978 in 2016. Dawson Creek derives its name from the creek of the same name that runs through the community. The creek was named after G ...

(Dawson Creek's Energy Planmunicipal fleet only) *

Vancouver, British Columbia Vancouver is a major city in Western Canada, located in the Lower Mainland region of British Columbia. As the List of cities in British Columbia, most populous city in the province, the 2021 Canadian census recorded 662,248 people in the cit ...

(2006) *

Hamilton, Ontario Hamilton is a port city in the Canadian Provinces and territories of Canada, province of Ontario. Hamilton has a 2021 Canadian census, population of 569,353 (2021), and its Census Metropolitan Area, census metropolitan area, which encompasses ...

(2007) *

Ajax, Ontario Ajax (; 2021 Canadian census, 2021 population: 126,666) is a waterfront town in Regional Municipality of Durham, Durham Region in Southern Ontario, Canada, located in the eastern part of the Greater Toronto Area. The town is named for , a Royal N ...

(2009) *

Ottawa, Ontario Ottawa is the capital city of Canada. It is located in the southern Ontario, southern portion of the province of Ontario, at the confluence of the Ottawa River and the Rideau River. Ottawa borders Gatineau, Gatineau, Quebec, and forms the cor ...

Vaughan, Ontario Vaughan ( ) (2022 population 344,412) is a city in Ontario, Canada. It is located in the Regional Municipality of York, just north of Toronto. Vaughan was the fastest-growing municipality in Canada between 1996 and 2006 with its population increa ...

Hong Kong

In a bid to reduce air pollution, the Hong Kong Government enacted the Motor Vehicle Idling (Fixed Penalty) Ordinance from December 2011. The law prohibits drivers from idling for more than three minutes in any 60-minute period. Both police traffic wardens and inspectors of the

Environmental Protection Department Environmental Protection Department (EPD) is a department of Hong Kong Government concerning the issues of environmental protection in Hong Kong. The EPD is responsible for developing policies covering environmental protection, nature conser ...

can fine offenders HK$320.

United States

Both the Department of Energy and the Environmental Protection Agency have programs in place to reduce idling. The DOE is funding research and development for alternative and advanced vehicles, which includes the gathering of quantitative data on medium-duty trucks, examining idling reduction alternatives, and the CoolCab project for semi-truck curtains and installation. The EPA's programs include the Environmental Technology Verification Program, the Smart Way Transport Partnership (freight incentives), the Model State Idling Law (diesel) and Clean School Bus USA. All but 11 states have at least one incentive or law in place to reduce idling, while 7 states have at least four. The state of Colorado has in place a tax credit for alternative fuel and qualified idle reduction technologies, as well as the Green Truck Grant Program which allows the Governor's Energy Office to provide reimbursement of up to 25% of costs to owners of commercial trucks used in interstate commerce to reduce emissions. There are many local ordinances and programs to discourage idling, such as ordinances limiting the minutes per hour in which a vehicle can idle. One example of a local program is Denver, Colorado's "Engines Off!" citywide anti-idling campaign, which aims to improve air quality and reduce greenhouse gas emissions by promoting voluntary behavior change in idling behavior.

References

{{reflist Engines Environmentalism Green politics Environmental law