ASE Fuel Systems & Emissions Control 1 — Questions and Answers
Question 1: What is the stoichiometric air-fuel ratio for a gasoline engine?
- 14.7:1 (Correct answer)
- 12.5:1
- 16.1:1
- 10.5:1
Correct answer: 14.7:1
The stoichiometric ratio of 14.7:1 represents the ideal mixture for complete combustion, producing the fewest harmful emissions while maximizing efficiency.
Question 2: What is the primary purpose of the EVAP (Evaporative Emission Control) system?
- Prevent fuel vapors from escaping into the atmosphere (Correct answer)
- Recirculate exhaust gases back into the engine intake
- Control fuel pressure at the injector rail
- Filter contaminants from the fuel before delivery
Correct answer: Prevent fuel vapors from escaping into the atmosphere
The EVAP system captures fuel vapors from the fuel tank, stores them in a charcoal canister, and routes them into the engine to be burned rather than venting to the atmosphere.
Question 3: Which sensor provides the primary feedback signal to the ECM for closed-loop fuel control?
- Oxygen (O2) sensor (Correct answer)
- Mass Airflow (MAF) sensor
- Throttle Position Sensor (TPS)
- Engine Coolant Temperature (ECT) sensor
Correct answer: Oxygen (O2) sensor
The upstream oxygen sensor measures residual oxygen in the exhaust, giving the ECM real-time feedback to adjust injector pulse width and maintain the stoichiometric 14.7:1 ratio.
Question 4: What does a MAF (Mass Airflow) sensor measure?
- The mass of air entering the engine per unit of time (Correct answer)
- The temperature of intake air only
- The pressure differential across the air filter
- The volume of exhaust gases leaving the engine
Correct answer: The mass of air entering the engine per unit of time
The MAF sensor measures the actual mass of air entering the intake manifold, enabling the ECM to calculate precise fuel delivery amounts for any operating condition.
Question 5: What is the primary purpose of the EGR (Exhaust Gas Recirculation) system?
- Reduce NOx emissions by lowering peak combustion temperatures (Correct answer)
- Increase fuel efficiency by re-burning exhaust gases completely
- Prevent carbon buildup on intake valves in port-injected engines
- Filter particulate matter from exhaust gases before emission
Correct answer: Reduce NOx emissions by lowering peak combustion temperatures
The EGR system dilutes the intake charge with inert exhaust gases, lowering peak combustion temperatures below the threshold at which NOx forms (approximately 2,500°F).
Question 6: What component stores fuel vapors in the EVAP system between purge events?
- Activated charcoal canister (Correct answer)
- Fuel pressure regulator
- PCV valve and crankcase assembly
- Catalytic converter substrate
Correct answer: Activated charcoal canister
The activated charcoal (carbon) canister adsorbs and holds fuel vapors from the fuel tank until the ECM opens the purge valve to draw the vapors into the intake for combustion.
Question 7: A port fuel injector is best described as a:
- Electronically controlled solenoid valve that meters fuel (Correct answer)
- Mechanically operated diaphragm pump driven by cam lobe
- Vacuum-controlled needle valve regulated by manifold pressure
- Spring-loaded check valve opened by fuel pressure alone
Correct answer: Electronically controlled solenoid valve that meters fuel
A fuel injector is an ECM-controlled solenoid valve; when energized, a magnetic coil lifts a pintle or ball off its seat, spraying atomized fuel for the duration of the pulse width signal.
ASE Fuel Systems & Emissions Control 1
What is the stoichiometric air-fuel ratio for a gasoline engine?