Evaporative emissions (EVAP) occur when fuel vapors escape from the fuel system into the atmosphere, contributing to air pollution (hydrocarbons, VOCs). The primary sources are:
Fuel Tank (Most significant source)
Carburetor (In older engines)
Fuel Lines & Connections
1. Sources of Evaporative Emissions
A. Fuel Tank Emissions
Diurnal Emissions: Fuel vapors expand and escape as the tank heats up (e.g., sunlight, engine heat).
"Breathing" Losses: Vapors escape when the tank is filled or due to temperature/pressure changes.
Permeation: Fuel vapors seep through plastic/rubber materials over time.
B. Carburetor Emissions (Older Engines)
Hot Soak Vapors: After engine shutdown, residual heat boils fuel in the carburetor float bowl, releasing vapors.
Fuel Evaporation: Venting through the carburetor bowl vent.
C. Modern Fuel Systems (EFI) vs. Carbureted Systems
| Feature | Carbureted Engines | Modern EFI Engines |
|---|---|---|
| Evap Control | Often no EVAP system | Sealed system + EVAP canister |
| Main Source | Carb bowl + fuel tank | Mostly fuel tank only |
| Regulations | Less strict (older) | Strict (EPA/CARB compliant) |
2. How Are Evaporative Emissions Controlled?
A. Charcoal Canister (EVAP System)
Activated carbon traps fuel vapors from the tank & carburetor.
Purge Valve allows vapors to be drawn into the engine and burned.
B. Sealed Fuel System (Modern Vehicles)
No vented gas cap (pressure-vacuum relief instead).
Leak Detection System (OBD-II monitors EVAP integrity).
C. Carburetor Modifications (Older Engines)
Closed-vent float bowls (prevents direct vapor escape).
Vapor recovery systems (aftermarket EVAP kits for classics).
3. Testing & Diagnosing EVAP Leaks
Smoke Test – Detects leaks in hoses, seals, or the fuel tank.
OBD-II Scan – Checks for EVAP system faults (P0440-P0457).
Pressure Test – Measures system integrity.
Conclusion
Older engines (carbureted) emit more evaporative fumes due to lack of sealing.
Modern engines use sealed EVAP systems to meet emissions standards.
Fixes: Adding a charcoal canister or upgrading to a sealed fuel system can reduce emissions.
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