Gasoline fumes contain a variety of hydrocarbons that constantly evaporate from the engine and fuel tank into the atmosphere. If the fuel is in the atmosphere, it can cause pollution, even if the vehicle's engine is off.
Evaporative emission control systems (EVAP) are used in cars' fuel systems to prevent gasoline fumes from leaking into the atmosphere. The EVAP system runs diagnostics for possible fuel vapor leaks and will trigger a fault code if they’re present. It will also activate the check engine light on the dashboard.
The EVAP system comprises the fuel tank, a vapor storage canister, valves, hoses, and the fuel tank gas gap. To prevent the gasoline vapors from escaping directly into the atmosphere, vent lines from the fuel tank pass the fumes or vapor to the vapor storage canister, which is confined and stored until the vehicle starts. Once the vehicle is running, and the engine is warmer, the storage canister releases the vapors through a purge valve opened by the vehicle's powertrain control module (PCM) into the intake manifold. The vapors are then burnt together with the air-fuel mixture inside the engine.
EVAP systems are active systems and require no maintenance. However, faults can develop, and the system may not function as designed. Hence the vehicle's PCM monitor runs self-checks to verify if there is airflow from the canister to the engine and whether there are no leaks in the fuel tank, the canister, fuel lines, and vents. The system will trigger a fault code and illuminate the check engine light even if a leak smaller than the size of a pinprick is detected.
The PCM is responsible for assessing the integrity of the EVAP system by running an EVAP diagnostic monitor during certain driving conditions to detect evaporative leaks. It will set a diagnostic fault code (DTC) and the accompanying check engine light if leaks occur. The PCM is also responsible for adequately metering the stored gasoline vapor back into the engine by monitoring engine and vehicle operating conditions, fuel level, cycle times, and ambient temperature.
The fuel tank stores fuel until the engine requires it. It also stores some fuel vapor before passing them to the EVAP storage canister. The fuel tank is air-tight, with seals at access points where fuel and vapor pass to the engine and canister. Any breakages or ruptures below the fuel tank lead to fuel leakages, whiles those above the tank results in gasoline vapors escaping into the atmosphere. The EVAP system will identify and trigger a fault code when such problems occur.
The gas cap prevents fuel vapors or actual fuel from getting into the atmosphere due to the gasoline evaporation in the fuel tank. It also allows fresh air into the fuel tank to replace the fuel transferred into the engine. It, therefore, is an essential component of the fuel system and must be correctly designed, calibrated, and installed to provide an air-tight seal. Gas caps come As vented or unvented and must be replaced with the same type if missing. Otherwise, the EVAP system will trigger a fault code.
The fuel tank pressure sensor measures positive and negative pressure in the fuel tank. During EVAP monitoring, the PCM uses this pressure to detect vapor leakages in the fuel tank and will trigger a fault code and illuminate the check engine light when a leak is detected or if the sensor fails.
The EVAP canister is usually a plastic container filled with activated charcoal and connected to the fuel tank by a vent line. It absorbs and confines the gasoline vapors until the vehicle's engine starts. As the vehicle drive, the canister purge valve opens to allow the intake vacuum to draw vehicles into the engine for combustion.
The canister purge valve is usually electrically operated and allows the engine vacuum to draw fuel vapors. The valve, however, can get clogged or be held open by charcoal particles or debris drawn from the canister. The vehicle's PCM assesses the valve condition and activates the check engine light if an issue is detected.
The leak detection pump provides pressure for conducting positive pressure testing of the EVAP system by pumping air into the fuel tank and charcoal canister. The PCM measures the EVAP system's pressure detail once the system is pressurized.
Vehicle pollution encompasses more than just tailpipe emissions. Gasoline fumes escaping from the engine and fuel tank are also a significant source of hydrocarbon pollution, whether the engine is running or not, and can continue to pollute the atmosphere until there's no fuel left in the engine or fuel tank. EVAP systems prevent these hydrocarbons from escaping and prompt the driver whenever a leak occurs.
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