Greta Jimenez
The evaporative emission control system in vehicles is an essential mechanism for reducing pollutants and mitigating harmful emissions. Without this system, vehicles would release higher levels of volatile organic compounds (VOCs), specifically hydrocarbons (HCs) or unburned gasoline vapors, into the atmosphere. These vapors contribute to the formation of photochemical smog and can lead to respiratory ailments under prolonged exposure. Additionally, the absence of an effective evaporative emission control system can result in increased fuel consumption and higher levels of harmful exhaust gases, such as nitrogen oxides and carbon monoxide, being released into the environment.
| Characteristics | Values |
|---|---|
| Pollutant | Hydrocarbons (HCs) or unburned gasoline vapours |
| Percentage of HC emissions from vehicles that originate from evaporative sources | 20% |
| Sources of HC production | Diurnal evaporation, running losses, "hot soak", refueling |
| Impact of increased HC emissions | Formation of photochemical smog, respiratory ailments under prolonged exposure, accelerated wear on engine components |
| Role of evaporative emission control system | Captures and stores fuel vapours, prevents their release into the atmosphere, redirects them back to the engine to be burnt |
| Other benefits of the system | Reduces fuel consumption and overall pollution, maintains stable fuel tank pressure, ensures smooth engine operation |
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What You'll Learn
Hydrocarbons (HCs)
HC vapours in the atmosphere contribute to the formation of photochemical smog, which has been linked to a variety of respiratory ailments under certain conditions of prolonged exposure. To address this issue, automobile manufacturers have implemented Evaporative (EVAP) Emissions Systems, which aim to reduce or eliminate the release of HC vapours into the atmosphere. These systems trap, store, and release HC vapours into the engine to be burned and prevent them from escaping into the environment.
The primary function of the evaporative emissions system is to prevent HCs from being released into the atmosphere and store them until they can be reintroduced into the intake air stream at a later time. The vapours are stored in a charcoal canister, which provides a surface area for the fuel vapours to be adsorbed and stored. These stored vapours can then be released back into the incoming air charge during vehicle operation.
There are several ways in which HC vapours escape and contribute to evaporative emissions. One way is through diurnal evaporation, which occurs during the day when the fuel is heated by an increase in ambient temperature. Another way is through running losses, which are caused by heat in the engine compartment from the exhaust system and engine operation, leading to fuel vaporisation. Additionally, hot soak occurs when the engine remains hot after the vehicle has been turned off, causing extended periods of gasoline vaporisation. Refuelling can also displace fuel vapours in the fuel tank, venting them into the atmosphere.
The introduction of positive crankcase ventilation (PCV) in the 1960s marked the beginning of evaporative emission control systems. The PCV system captures crankcase vapours heavy in unburned HCs and draws them into the engine's intake tract to be burned, preventing them from being released unburned into the atmosphere. This was an important step in reducing the formation of photochemical smog caused by HC vapours.
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Volatile organic compounds (VOCs)
The European Union defines a VOC as "any organic compound as well as the fraction of creosote, having a vapour pressure of 0.01 kPa or more at a temperature of 293.15 K, or having a corresponding volatility under specific conditions of use." The volatility of a VOC is generally higher when its boiling point temperature is lower. The European Union's definition of a VOC is based on its boiling point rather than its volatility.
In the context of automotive evaporative emission systems, the primary concern regarding VOCs is hydrocarbons (HCs) or unburned gasoline vapors. Gasoline evaporates at temperatures as low as -40 degrees Fahrenheit, which means fuel is constantly evaporating and creating harmful emissions. Without an evaporative emission control system, these VOCs would be released into the atmosphere.
The purpose of automotive evaporative emission systems is to reduce or eliminate the release of these VOCs. These systems capture fuel vapors in a charcoal canister and redirect them back to the engine to be burned, preventing their release into the environment. By managing these emissions, the system helps to reduce pollution and fuel consumption. Additionally, it helps prevent the vehicle from wasting fuel and causing unnecessary harm to the environment.
It is important to note that VOCs are not only a concern in automotive emissions but also in indoor environments. Concentrations of VOCs are often higher indoors compared to outdoors, and they can be emitted from various products used in buildings, such as building materials, furnishings, and cleaning supplies. Reducing VOC concentrations is crucial for both indoor and outdoor air quality, as well as for human health and environmental well-being.
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Carbon monoxide
An evaporative emission system helps to reduce carbon monoxide emissions by capturing and storing fuel vapors that would otherwise be released directly into the atmosphere. These vapors are redirected back into the engine to be burned and generate power, preventing them from becoming harmful pollutants.
The system is designed to work intelligently, using a combination of technologies to capture and manage fuel vapors. This includes the use of specialized gas caps, charcoal canisters, valves, hoses, and the fuel tank gas gap. By trapping and redirecting the vapors, the evaporative emission system plays a crucial role in reducing carbon monoxide emissions and mitigating their harmful effects.
Without an effective evaporative emission system in place, carbon monoxide levels in vehicle emissions would be significantly higher. This would not only impact air quality but also pose serious health risks to individuals exposed to these emissions. Therefore, the presence of an efficient evaporative emission system is vital in controlling and reducing carbon monoxide pollution.
Additionally, regular maintenance and inspections of the evaporative emission system are essential to ensure its optimal functioning. This includes monitoring for any warning signs, such as engine warning lights, and addressing any issues promptly to maintain the system's effectiveness in reducing carbon monoxide emissions.
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Nitrogen oxides
NOx emissions are formed during combustion when nitrogen bound in the fuel is released as a free radical and ultimately forms free N2 or NO. Fuel can contribute up to 50% of total NOx emissions through combusting oil and up to 80% through combusting coal. While the exact mechanism is not fully understood, there are two main pathways of formation. The first involves the oxidation of volatile nitrogen species during the early stages of combustion, leading to the formation of NO. The second pathway involves the combustion of nitrogen in the char matrix during the combustion of the char portion of the fuels.
To control NOx emissions, modern vehicles are equipped with evaporative emission control systems, which capture and redirect fuel vapours back to the engine to be burnt and generate power. This prevents the release of harmful vapours into the environment and reduces fuel consumption and pollution. The system includes a specialised gas cap that traps fuel vapours, directing them into a charcoal canister. These vapours are then siphoned into the engine using a vacuum to mix with oxygen and undergo combustion.
Additionally, aftertreatment methods are used to minimise NOx emissions from vehicles. One method is the use of a lean NOx trap (LNT), which temporarily stores NOx from the exhaust. By periodically increasing the proportion of fuel in the air-fuel mixture, the stored NOx reacts with hydrocarbons in the exhaust to produce nitrogen, water, and/or CO2. Another method is selective catalytic reduction (SCR), which uses ammonia as a reductant to reduce NOx over a catalyst.
While the evaporative emission control system helps mitigate NOx emissions, it is important to recognise that NOx emissions can still occur through fuel evaporation, especially at low temperatures, and from other sources such as lightning and ship traffic. Therefore, a comprehensive approach is necessary to effectively manage and reduce NOx emissions.
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Unburned fuel
UHCs are caused by fuel "avoiding" the flame zones. For example, in piston engines, some of the fuel-air mixture may "hide" from the flame in the crevices provided by the piston ring grooves. Additionally, some regions of the combustion chamber may have a very weak flame, causing the emission of intermediate species such as formaldehyde and alkenes.
The evaporative emission control system helps to mitigate the amount of pollution released by capturing and redirecting fuel vapors back to the engine to be burned and generate power. This system is comprised of many different components working together to prevent the vehicle from wasting fuel and causing unnecessary harm to the environment.
Without an evaporative emission system, UHCs would be released into the atmosphere, contributing to air pollution. These vapors have been linked to a variety of respiratory ailments under conditions of prolonged exposure. Additionally, UHCs contribute to the formation of smog and acid rain, which have negative impacts on the environment and human health.
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Frequently asked questions
Hydrocarbons (HCs) or unburned gasoline vapours will be increased without an evaporative emission system. These HC vapours contribute to the formation of photochemical smog.
Without an evaporative emission system, there will be higher emissions from the vehicle, which can cause the vehicle to fail an emissions test. Other symptoms of a failed emissions test may include poor acceleration or decreased overall vehicle performance.
To minimize fuel vapour evaporation, one should take steps to keep their fuel as cool as possible. This includes refuelling early in the morning or late at night when the sun is down, and parking your vehicle in the shade whenever possible.
A typical evaporative emission system consists of the fuel tank, the fuel tank cap with a vacuum check valve, a charcoal canister with vacuum and pressure check valves, a thermo vacuum valve, and a ported vacuum purge port.
