Automobile emissions control
covers all the technologies that are employed to reduce the air pollution
-causing emissions produced by automobiles
. Exhaust emissions control systems were first required on 1966 model year vehicles produced for sale in the state of California
, followed by the United States
as a whole in model year 1968, although the overall reduction in pollution has been much slower.
The emissions produced by a vehicle fall into these basic categories:
- Tailpipe emissions: This is what most people think of when they think of vehicle air pollution; the products of burning fuel in the vehicle's engine, emitted from the vehicle's exhaust system. The major pollutants emitted include:
- Hydrocarbons: this class is made up of unburned or partially burned fuel, and is a major contributor to urban smog, as well as being toxic. They can cause liver damage and even cancer.
- Nitrogen oxides (NOx): These are generated when nitrogen in the air reacts with oxygen under the high temperature and pressure conditions inside the engine. NOx emissions contribute to both smog and acid rain.
- Carbon monoxide (CO): a product of incomplete combustion, carbon monoxide reduces the blood's ability to carry oxygen and is dangerous to people with heart disease.
- Carbon dioxide (CO2): Emissions of carbon dioxide are an increasing concern as its role in global warming as a greenhouse gas has become more apparent.
- ''Particulates. Particle of micrometre size.
- Sulphur oxide (SOx) General term for oxides of sulphur, mostly sulfur dioxide and some sulfur trioxide, from coal or unrefined oil.
Tailpipe emissions control
Tailpipe emissions control can be categorised into three parts:
- Increasing engine efficiency
- Increasing vehicle efficiency
- Cleaning up the emissions
Increasing engine efficiency
Engine efficiency has been gradually improved with progress in following technologies:
Increasing vehicle efficiency
Contributions to the goal of reducing fuel consumption and related emissions come from
- lightweight vehicle design
- minimized air resistance
- reduced rolling resistance
- improved powertrain efficiency
- increasing spark to the spark plug (this topic should be under the ignition system)
- regenerative braking
Each of these items breaks down into a number of factors.
Increasing driving efficiency
Significant reduction of emissions come from
- driving technique (some 10-30% reduction)
- unobstructed traffic conditions
- cruising at an optimum speed for the vehicle
- reducing the number of cold starts
Cleaning up the emissions
Advances in engine and vehicle technology continually reduce the amount of pollutants generated, but this is generally considered insufficient to meet emissions goals. Therefore, technologies to react with and clean up the remaining emissions have long been an essential part of emissions control.
A very early emissions control system, the Air injection reactor
(AIR) reduces the products of incomplete combustion (hydrocarbons and carbon monoxide) by injecting fresh air into the exhaust manifolds
of the engine. In the presence of this oxygen-laden air, further combustion occurs in the manifold and exhaust pipe
. Generally the air is delivered through an engine-driven 'smog pump' and air tubing to the manifolds.
Exhaust Gas Recirculation
Many engines produced after the 1973 model year have an exhaust gas recirculation
(EGR) valve between the exhaust and intake manifolds; its sole purpose is to reduce NOx emissions by introducing a metered, and quite small amount of inert gas into the air/fuel mixture, lowering peak combustion temperatures. In the case of EGR, the exhaust gasses are inert enough to serve this purpose.
The catalytic converter
is a device, placed in the exhaust pipe, which converts various emissions into less harmful ones using, generally, a combination of platinum, palladium and rhodium as catalysts. They make for a significant, and easily applied, method for reducing tailpipe emissions. Catalytic converters are damaged when used on engines that burn leaded fuels. Unleaded fuels were marketed in 1973 and by 1996 leaded fuels were banned completely for on-road use in the USA.
Evaporative emissions control
Efforts at the reduction of evaporative emissions include the capturing of vented vapors from within the vehicle, and the reduction of refuelling emissions.
Capturing vented vapors
Within the vehicle, vapors from the fuel tank are channelled through canisters containing activated carbon
instead of being vented to the atmosphere. These are known as carbon canisters. The vapors are adsorbed
within the canister, which feeds into the inlet manifold
of the engine.
In 1966, the first emission test cycle
was enacted in the State of California measuring tailpipe emissions in PPM (parts per million).
The Environmental Working Group
used California ASM emissions data to create an Auto Asthma Index that rates vehicle models based on emissions of hydrocarbons and nitrogen oxides, the chemicals that create smog.
Some cities are also using a technology developed by Dr. Stedman,of University of Denver which uses lasers to detect emissions while vehicles pass by on public roads, thus eliminating the need for owners to go to a test center. Stedman's laser detection of exhaust gases is commonly used in metropolitan areas.