March 6, 2020 No Comments Mechanical Engineering FAHADH V HASSAN

The purpose of evaporative emissions systems is to reduce or eliminate the release of vaporized HC into the atmosphere. The HC vapors, such as volatile organic compounds (VOCs), react in the atmosphere with nitrogen oxides (NOx) and contribute to the formation of ground-level ozone and photochemical smog. Ground-level ozone is an air pollutant with harmful effects on plants, human respiratory systems, and an irritant to our eyes.

The Evaporative Emission Control (EVAP) System seals the fuel system of the vehicle in order to prevent fuel vapors from the fuel tank and fuel system from escaping into the atmosphere. This is important because fuel vapors contain a variety of hydrocarbons. Hydrocarbons from smog when they react with air and sunlight.

Gasoline evaporates very quickly, so if the fuel system is open to the atmosphere a vehicle can pollute 24 hours per day without even being turned on. These uncontrolled evaporative emissions account for as much as 20% of the pollution produced by a vehicle!


Types of evaporative emissions are classified into five categories:

• Diurnal: This represents gasoline that evaporates due to the rise in ambient temperature.

• Running losses: Represent gasoline that vaporizes due to the heat of the engine and exhaust system during normal operation.

• Resting losses: Natural permeation that occurs from the fuel delivery system while not operating under ambient conditions.

• Hot Soak: Vaporization of fuel due to the retained heat of the engine after the engine is turned off.

• Refueling: Represents the fuel vapors that escape from the tank by the displacement of liquid fuel.


The major components of the EVAP system are:

• Fuel Tank:

The fuel tank obviously stores the gasoline when you fill-up. But, you know when people tell you not to continue filling the tank after the pump automatically stops? That is because the tank has some expansion space at the top so the fuel can expand without overflowing or forcing the EVAP system to leak.

• Gas Cap:

Tighten until click. The gas cap seals off the filler neck of the gas tank from the outside atmosphere. Damaged or missing gas caps are the most common cause of EVAP system failure codes that trigger the check engine light.

• Liquid-Vapor Separator:

This prevents liquid gasoline from entering the EVAP canister, which would overload its ability to store fuel vapors.

• EVAP Canister:

This canister is connected to the fuel tank by the tank vent line. The EVAP canister houses 1-2 pounds of activated charcoal that acts like a sponge by absorbing and storing fuel vapors until the purge valve opens and allows the vacuum of the engine intake to siphon the fuel vapors from the charcoal into the engine intake manifold.

• Vent Control Valve:

This allows the flow of the fuel vapors from the fuel tank into the EVAP canister.

• Purge Valve/Sensor:

Allows the engine intake vacuum to siphon the precise amount of fuel vapors from the EVAP canister into the engine intake manifold.

• Vent Hoses:

The means by which the fuel vapors flow to different components of the EVAP system.

• Fuel Tank Pressure Sensor:

Monitors the pressure within the fuel tank for leaks and to make sure too much pressure does not build.

• Fuel Level Sensor:

Monitors the level of fuel in the tank.


The evaporative emission control (EVAP) system captures gasoline fumes and other emissions.

When the fuel evaporates inside the gas tank, the excess vapors are transferred to the charcoal canister. They’re stored there until they can safely be transferred back to the engine to be burned with the normal air-fuel mixture.

When that’s ready to happen, a valve creates a vacuum that draws the vapors into the engine. Fresh air is also drawn in through the vents and valves to mix with the vapors for better combustion. These systems can be controlled mechanically, or like on most on newer cars, through the engine’s computer.

If the fuel tank was sealed tight, the fuel pump could create enough negative pressure to collapse it.

So, On older EVAP systems, the tank is vented by a spring-loaded valve inside the gas cap. While, On newer vehicles, it is vented through the EVAP canister.

About The Author
FAHADH V HASSAN Hi, I'm Fahadh V Hassan an aspiring blogger with an obsession for all things tech. this blog is dedicated to helping people learn about technology.

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