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Understanding Automotive Emissions and Controls

Understanding car emissions and controls

In the 1950s and 60’s there were numerous federal, state and local government studies about the effects of air pollution, and as it was popularly referred to, smog. The result of these studies was not good and people from the more populated urban areas, where you could see the air, to the formerly pristine wilderness, where effects were just beginning to appear, began to wonder what could be done about it.

California had always been a leader in environmental issues and automobile emissions and in 1961 mandated that all cars sold there vent crankcase gasses via a PCV valve, to reduce unburned hydrocarbons. Then, in 1967, they created the California Air Resources Board (CARB) to oversee all vehicle emissions and other air pollution. In 1963 the federal Clean Air Act was passed to investigate and regulate air pollution, and in 1970 the United States Environmental Protection Agency (EPA) was established to further that goal.

Both the California and federal governmental agency now create and enforce emission regulations for automobiles in the United States.

The first effort at controlling air pollution was the PCV (positive crankcase ventilation) system, which rerouted unburned blow-by gasses from the crankcase back into the intake manifold. The PCV system uses engine vacuum to draw these hydrocarbon fumes, a precursor of photochemical smog, back into the combustion chamber where they are burned. A year after it was required by California, New York required it as well, and by 1964 most new cars sold in the United States used a PCV valve instead of a road draft tube.

In 1966, California set another piece of groundbreaking anti-pollution legislation - exhaust tailpipe emission standards for cars sold in that state.  The first emission test cycle was created by the state to measure tailpipe emissions in PPM (parts per million) or hydrocarbons (HC) and carbon monoxide (CO). These standards were progressively tightened year by year, as mandated by the CARB making it more and more difficult for cars to pass. Unfortunately, the decrease in HC and CO came with an increase in nitrogen oxides (NOx) another air pollutant.

The federal laws lagged California by a few years, but made similar demands of the auto industry to clean up what was coming out of cars.

As the 1970s progressed, open the hood of a car and you would be greeted by a mass of hoses. In order to control NOx pollution catalytic converters were needed, and those required unleaded gasoline with lower octane, so engines were detuned. But, by 1974, it was becoming obvious that detuning automobile engines to pass the pollution standards was not viable because they used more gasoline, they were way down on power.

It took quite a few years to catch on, but by the late 1970s computer controlled fuel injection (EFI) and ignition timing began reducing emissions with more efficient combustion inside the motor. By the end of the 1990s a new generation of computer designed engines incorporated new ideas about emissions control within the combustion chamber itself, and power levels and fuel economy returned.

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The problem with emissions

The emissions given off by the burning of gasoline have shown to be toxic to people and animals when breathed. But they also contribute to the ugly haze called smog, which hangs around the atmosphere causing problems long after the the car has moved on. Here are the major pollutants:

Hydrocarbons (HC)
Hydrocarbons are toxins and a major contributor to smog. Prolonged exposure to hydrocarbons contribute to asthma, liver disease, lung disease, and cancer. Methane, which is an HC, is not directly toxic, but is a powerful greenhouse gas. 

HC emissions come not only from a vehicle's engine but also from fuel evaporating from the tank 24 hours a day, even when the engine is turned off. This is why all cars now have charcoal evaporative canisters to collect unburned vapors, and burn them while driving. Just spilling a splash of fuel while filling your tank can contribute more HC pollution in seconds than a new car emits in miles of driving.

Carbon monoxide (CO) 
A product of incomplete combustion, carbon monoxide reduces the blood's ability to carry oxygen if inhaled. Overexposure can be fatal, which is why you never run a car in en enclosed space like a garage.

Nitrogen Oxide (NOx)
Generated when nitrogen in the air reacts with oxygen in the high temperature and pressure conditions that occurs inside the engine. NOx is a precursor to smog and acid rain. 

Volatile organic compounds (VOCs) 
These are carbon compounds that evaporate very easily into the air, and include things such as the solvents used in paints, CFCs used in the air conditioning system, and fuel additives like MBTE. They have various long term heath effects, and many have been shown to be carcinogenic. 

Particulate matter
Mostly a problem for diesel vehicles, it is soot made up of particles in the micrometer size range and has negative health effects, including respiratory disease and cancer. Very fine particulate matter has also been linked to cardiovascular disease.

Sulfur Oxide (SOx)
Another concern mostly with diesels, SOx is a general term for oxides of sulfur, which are emitted from automobiles burning fuel containing sulfur. Reducing the level of fuel sulfur reduces the level of Sulfur oxide emitted from the tailpipe.

Emissions controls explained

In spite of unleaded gasoline, computerized engine management, direct injection, and modern combustion chambers and ports, still more is needed to make emission systems work.

Air Injection
Air injection was one of the first emission control systems. This system injects more oxygen into the exhaust system immediately after the exhaust valve to help burn unburned hydrocarbons . Air injection is now used to support the catalytic converter's oxidation reaction as the air injection system speeds up catalytic warm-up. 

Exhaust Gas Recirculation
This system originated in 1972 and routes a metered amount of exhaust gas into the intake tract. The exhaust gas doesn’t burn, but instead dilutes the air/fuel charge to reduces peak combustion temperatures, and thus reduces formation of  NOx.

Catalytic Converter
The catalytic converter is a unit placed in the exhaust system which reduces hydrocarbons, carbon monoxide, and NOx by converting them into less harmful gases using a combination of platinum, palladium and rhodium as catalysts for a chemical reaction. The catalytic converter does not begin working until it gets very hot from the exhaust gas.