Meghan Hodges
Gas engines are a major source of air pollution, releasing a range of harmful pollutants into the atmosphere. These pollutants are released directly or as a result of chemical reactions with other elements in the atmosphere. The pollutants include nitrogen dioxide, carbon monoxide, hydrocarbons, ground-level ozone, particulate matter, and lead. The Clean Air Act, first passed in 1970, has helped to reduce air pollution by requiring engines and fuels to produce less pollution. Despite this, air pollution from burning fossil fuels is estimated to kill over 5 million people annually, with vehicle electrification being a proposed solution.
| Characteristics | Values |
|---|---|
| Carbon dioxide (CO2) emissions from a gallon of gasoline | 8,887 grams CO2/ gallon |
| Carbon dioxide (CO2) emissions from a gallon of diesel | 10,180 grams CO2/ gallon |
| Carbon monoxide | Colourless, odourless gas that prevents the body from taking up oxygen, causing dizziness, headaches, fatigue, visual impairment, reduced work capacity, reduced manual dexterity, and poor learning ability |
| Particulate matter | Combustion in vehicles' engines releases tiny solid particles and drops of liquid measuring less than 2.5 microns in diameter, which travel through the airways into lungs and cause increased respiratory problems and disease, decreased lung function, alterations of the body’s defence systems, and premature mortality |
| Nitrogen oxides (NOx) | Produced from incomplete combustion |
| Hydrocarbons (CxHy) | Produced from unburnt fuel |
| Nitric acid vapour | Produced from the combustion of nitromethane, which is corrosive and causes muscular reaction, making it impossible to breathe |
| Ground-level ozone | Secondary pollutant produced by the reaction of hydrocarbons and nitrogen oxides in the presence of sunlight |
| Sulphur dioxide | Removed by scrubbers in ships |
| Lead | Phased out of U.S. fuel systems by 1996 |
| Methyl tertiary butyl ether (MTBE) | Toxic chemical added to gasoline to help it burn cleaner, banned in several U.S. states in the late 1990s |
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What You'll Learn
Carbon monoxide
The danger of carbon monoxide lies in its ability to prevent red blood cells from transporting oxygen to the body's tissues. As a colorless, odorless, and tasteless gas, carbon monoxide can go undetected and quickly reach lethal levels, especially in enclosed or partially enclosed spaces. Hundreds of people die each year in the United States alone from carbon monoxide poisoning, often from recreational or commercial vessels, and engine-powered equipment.
Gasoline engines, including those in cars, trucks, buses, and generators, are a common source of carbon monoxide emissions. The amount of carbon monoxide produced by these engines can vary depending on various factors, such as engine type, fuel type, and the presence of emissions-control devices like catalytic converters. Diesel engines, for example, produce far less carbon monoxide than gasoline engines, but they can still generate lethal amounts in enclosed spaces over time.
To reduce carbon monoxide emissions and protect public health, governments and organizations have implemented various measures. The Clean Air Act in the United States, first passed in 1970, aims to reduce air pollution by requiring engines and fuels to produce fewer emissions. This led to the installation of catalytic converters in new vehicles from 1976 onwards, which help break down exhaust gas pollutants. Additionally, the Environmental Protection Agency (EPA) has established emissions standards for different types of vehicles and engines, promoting the use of cleaner-burning fuels and technologies.
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Nitrogen oxides
NOx emissions have been regulated since the 1960s, and the Clean Air Act, first passed in 1970, has played a crucial role in reducing air pollution from engines and fuels. The Act led to the development and mandatory use of emissions-control devices, such as catalytic converters, which break down exhaust gases and reduce NOx emissions.
The amount of NOx formed during combustion is directly related to the peak combustion temperature. As the temperature rises, the rate of NOx formation increases. This is why combustion temperatures are generally lower in diesel engines compared to gasoline engines, as diesel combustion has a leaner air-fuel mixture with a higher proportion of air.
To minimize NOx emissions, two primary approaches are employed. The first is to lower the combustion temperature, which reduces the formation of NOx. The second approach involves using aftertreatment devices, such as catalytic converters, to induce a chemical reaction that converts NOx in the exhaust into nitrogen, water, and/or carbon dioxide. While these methods help reduce NOx emissions, the challenge of completely eliminating them from gasoline engines persists.
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Particulate matter
The formation of particulate matter in gasoline engines is influenced by factors such as the injection pressure, fuel injection technology, and the homogeneity of the fuel-air mixture. Gasoline direct injection (GDI) engines, which inject fuel directly into the cylinder, have been associated with increased particulate emissions. This is due to the limited time for mixture formation, which can lead to rich pockets of fuel in the cylinder and subsequent particulate formation.
The harmful effects of diesel engine PM are also assumed to be associated with gasoline PM. Regulations limiting PM emissions from gasoline engines, especially GDI engines, have been implemented in several countries, including the USA, Europe, Japan, China, and India. These regulations aim to reduce the health and environmental impacts of particulate matter, which is a major contributor to air pollution.
The measurement of particulate matter is determined by gravimetric analysis of a PM sample obtained by filtering diluted engine exhaust at a specific temperature. This procedure simulates the conditions under which exhaust particulates are released into the atmosphere. Devices called dilution tunnels are used in laboratories to produce the mixture of air and engine exhaust gas. The high-efficiency sample collection filters used in this process capture solid particles, liquid droplets, and mist that condense from exhaust gases.
While gasoline engines have traditionally been associated with negligible particulate emissions due to their homogeneous fuel-air mixture, the shift towards GDI technology has increased PM emissions from gasoline vehicles. This shift has resulted in particulate emissions comparable to or even higher than those from diesel engines, especially modern diesel engines equipped with diesel particulate filters (DPF).
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Hydrocarbons
The combustion of hydrocarbon fuels releases carbon dioxide (CO2), which is a greenhouse gas. Carbon dioxide is an unavoidable byproduct of hydrocarbon combustion, and its emissions contribute to climate change. The energy density and carbon footprint of a fuel depend on the hydrocarbon chain length and the complexity of its hydrocarbon molecules.
Unburnt hydrocarbons are even more harmful to the environment. They contain toxic and carcinogenic molecules that are released into the atmosphere through engine exhaust, evaporating petroleum, and gas. Heavier hydrocarbons can contaminate soil and groundwater. Methane, a potent greenhouse gas, is a common hydrocarbon that contributes significantly to climate change when leaked into the atmosphere in its unburnt form.
To reduce hydrocarbon emissions, the U.S. Environmental Protection Agency (EPA) has implemented regulations and standards for vehicle emissions. These include the phase-out of leaded gasoline, the use of catalytic converters, and the promotion of alternative fuels such as natural gas and electric vehicles. Natural gas, for instance, can reduce hydrocarbon emissions when used in place of gasoline in smaller applications or older conventional vehicles. Electric vehicles (EVs) offer a more sustainable option, as they produce zero tailpipe emissions.
Additionally, the Clean Air Act and its amendments have played a crucial role in reducing air pollution by requiring engines and fuels to produce fewer emissions. These initiatives aim to minimize the environmental impact of hydrocarbon combustion and mitigate climate change.
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Lead
The move away from leaded gasoline began in 1976 with the introduction of catalytic converters in new vehicles. These converters break down the pollution of exhaust gases using a catalyst. However, the presence of lead in fuel damages catalytic converters, rendering them ineffective.
Sources of lead emissions vary across regions. At the national level, major sources include ore and metals processing, and piston-engine aircraft operating on leaded aviation fuel. Other sources are waste incinerators, utilities, and lead-acid battery manufacturers. The highest air concentrations of lead are usually found near lead smelters.
Once lead is introduced into the body, it distributes throughout the blood and accumulates in the bones. Depending on the level of exposure, lead can adversely affect the nervous system, kidney function, immune system, reproductive and developmental systems, and the cardiovascular system. It also affects the oxygen-carrying capacity of the blood.
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Frequently asked questions
Pollutants produced by gas engines include carbon monoxide, hydrocarbons, nitrogen oxides, ground-level ozone, particulate matter, lead, sulfur dioxide, and nitrogen dioxide.
These pollutants have been linked to a range of health issues, including increased respiratory problems and disease, decreased lung function, alterations to the body's defence systems, premature mortality, dizziness, headaches, fatigue, visual impairment, reduced work capacity, and poor learning ability.
Catalytic converters in cars help break down the pollution of exhaust gases using a catalyst. They also prevent vehicles from operating on leaded gasoline, which was phased out in the US by 1996 due to public health concerns.
Electric vehicles do not emit tailpipe emissions and produce less carbon dioxide than gasoline-powered cars, contributing to reduced air pollution and improved public health.
