Automobile emissions control

From Academic Kids

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. Their use became widespread in the following decades and now they are ubiquitous in industrialised nations and common in most others.

Emissions controls have been highly successful in reducing the emissions produced by motor vehicles in terms of quantity per distance travelled. However, substantial increases in the distance travelled by each vehicle, and equally substantial increases in the number of vehicles in use, have meant that the overall reduction in pollution has been much slower.

The emissions produced by a vehicle fall into two basic categories:

  1. 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:
    1. 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.
    2. 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.
    3. Carbon monoxide (CO): a product of incomplete combustion, carbon monoxide reduces the blood's ability to carry oxygen and are dangerous to people with heart disease.
    4. Carbon dioxide (CO2): although this is a product of the complete combustion of hydrocarbons, is plentiful in the atmosphere, has no immediate harmful effects to humans and is essential to plant life, emissions of carbon dioxide are considered a pollutant because it is a significant greenhouse gas and increasing its levels in the atmosphere is thought by many to be a contributor to global warming.
  2. Evaporative emissions: These are produced from the evaporation of fuel, and are a large contributor to urban smog, since these heavier molecules stay closer to ground level. Fuel tends to evaporate in these ways:
    1. Gas tank venting: the heating of the vehicle as the temperature rises from the night-time temperature to the hottest temperatures of the day mean that gasoline in the tank evaporates, increasing the pressure inside the tank above atmospheric pressure. This pressure must be relieved, and before emissions control it was simply vented into the atmosphere.
    2. Running losses: the escape of gasoline vapors from the hot engine.
    3. Refuelling losses: these can cause a lot of hydrocarbon vapor emission. The empty space inside a vehicle's tank is filled with hydrocarbon gases, and as the tank is filled, these gases are forced out into the atmosphere. In addition, there is loss from further evaporation and fuel spillage.
Contents

Tailpipe emissions control

Tailpipe emissions control can be categorised into three parts:

  1. Increasing engine efficiency
  2. Increasing vehicle efficiency
  3. Cleaning up the emissions

Increasing engine efficiency

Engine efficiency have been improved with the addition of the following:

Increasing vehicle efficiency

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.

Air injection

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. This technology was introduced in 1966 in California, and was in use for the next several decades. It is not generally in use any longer, having been supplanted by cleaner burning engines and better catalytic converters.

Catalytic converters

The catalytic converter is a device, placed in the exhaust pipe, which converts various emissions into less harmful ones using platinum as a catalyst. Catalytic converters have been steadily improved over the years. They make for a significant, and easily applied, method for reducing tailpipe emissions. Their other significant effect on pollution was that they were incompatible with the use of tetraethyl lead as an octane booster in gasoline, prompting the phasing-out of that additive as converter-fitted cars became more prevalent. The lead emissions were highly damaging to human health, and its virtual elimination has been one of the most successful reductions in air pollution.

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, vented vapors -- from the fuel tank or other sources -- are channelled through canisters containing activated carbon instead of being vented to the atmosphere. The vapors are absorbed within the canister, which feeds into the inlet manifold of the engine. When the vehicle is running, the vapors desorb from the carbon, are drawn into the engine and burned.

Reducing refuelling losses

All modern vehicles have tank filler necks that instead of just being a tube into the tank, as in earlier vehicles, now have a small-diameter hinged and spring-loaded door only large enough for the tip of the filler nozzle. This prevents vapor leakage when the filler cap is removed, and also prevents a catalytic converter-fitted vehicle being refuelled with leaded fuel (since the leaded fuel nozzle is too large to fit).

This is accompanied by modifications to the filling station pumps. They are now equipped to suck the vapors back into the pump as they are displaced by fuel. Some have intakes around the head of the filler nozzle, while others have a rubber 'boot' that presses securely around the end of the filler neck to prevent vapors escaping.

See also: emission standards.

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