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 to plants, human respiratory systems, and an irritant to our eyes.
The major source of evaporative emissions is from the fuel system and, therefore, it is not surprising that fuel injection technology provides significant evaporative emissions benefits. The second major source of evaporative emissions comes from the positive crankcase ventilation (PCV) system. PCV systems collect and consume fuel that is pushed by the piston rings during the compression and power strokes in a 4-stroke (4-cycle) engine. These systems have been used on automobiles since the 1960s in the form of PCV valves. Evaporative emissions regulations have been in effect for automobiles since 1970 in California. Fuel injection systems effectively eliminated the vaporization of fuel from open carburetors. Evaporative emission control systems on cars have increased in complexity over the years to achieve the extremely low evaporative emissions (0.054g/day) from the fuel system as required by California's Partial Zero Emission Vehicle (PZEV) "zero" evaporative emission limits. Other significant sources of evaporative emissions from the fuel system include permeation of the fuel tank and fuel delivery hoses.
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.
Evaporative emissions are measured using a sealed housing for evaporative determination (SHED) apparatus over the course of a multi-day Federal Test Procedure (FTP) to quantify all of the various forms of evaporative emissions. This testing is generally most effective in determining diurnal and hot soak emissions.
In addition to low permeation hoses, fuel tanks, and seals, evaporative emission controls consists of carbon canisters connected to the fuel system to capture and recycle HC vapors back to the intake of the engine to be consumed as fuel. The carbon is a high-surface area, pelletized material that adsorbs fuel vapors via loose chemical bonds and releases them in a controlled fashion via a purge solenoid. The purge solenoid is activated by the on-board control module when the engine's control system is operating in "closed" loop fuel control. Carbon materials or other materials that adsorb vapor phase hydrocarbons such as zeolites can also be displayed in the engine's air intake system to control evaporative emissions associated with fuel leakage from fuel injectors when the engine is not running (see Figure below).
Metal fuel tanks exhibit no permeation; however, the need for lighter weight and complex shapes has led to the development of low permeation plastic tanks. In this case, gas tanks are made of layered polymers and blends that reduce tank permeation by 95% compared to traditional plastic fuel tanks. Similar types of thermo-polymers are molded as thin (0.1 mm) layers in the inside of fuel lines to achieve similar fuel permeation reductions. A vented fuel cap serves to allow air to enter as fuel is depleted while venting expanded vapors in the fuel tank into the carbon canister.
The combination of carbon canisters, the use of low permeable polymers, and fuel injection systems has been demonstrated as a very effective evaporative emissions control strategy. Regulatory agencies around the world are continuing to apply evaporative emission limits to a broad range of on-road and off-road vehicles and engines including motorcycles, small garden equipment, and marine applications.
In the figure below, carbon canisters come in many shapes and sizes, from advanced multi-stage PZEV units (A), to canisters designed for marine (B), to motorcycle and small engine applications (C). (D) is an integrated device to be added to a LEV II evap system/canister vent port to make it a PZEV evap system/canister.