ABA Engine Performance & Tuning 2 — Questions and Answers
Question 1: What is the stoichiometric air-fuel ratio for gasoline engines?
- 12.5:1
- 14.7:1 (Correct answer)
- 16.1:1
- 10.3:1
Correct answer: 14.7:1
The stoichiometric air-fuel ratio for gasoline is 14.7:1, meaning 14.7 parts air to 1 part fuel for complete combustion.
Question 2: Which sensor directly measures the mass of air entering the engine?
- MAP sensor
- TPS sensor
- MAF sensor (Correct answer)
- IAT sensor
Correct answer: MAF sensor
The Mass Air Flow (MAF) sensor directly measures the mass of air entering the intake, allowing the ECU to calculate fuel delivery.
Question 3: What does advancing ignition timing beyond the optimal point typically cause?
- Increased fuel economy
- Engine knock/detonation (Correct answer)
- Higher torque output
- Reduced exhaust temperatures
Correct answer: Engine knock/detonation
Over-advancing ignition timing causes the fuel-air mixture to ignite too early, resulting in knock or detonation that can damage pistons and bearings.
Question 4: A lambda value of 0.85 indicates the engine is running:
- Lean of stoichiometric
- At stoichiometric
- Rich of stoichiometric (Correct answer)
- In open-loop mode
Correct answer: Rich of stoichiometric
Lambda below 1.0 indicates a rich mixture (excess fuel), while above 1.0 indicates a lean mixture; 0.85 means 15% more fuel than stoichiometric.
Question 5: What is the primary purpose of a blow-off valve (BOV) in a turbocharged engine?
- Increase boost pressure
- Release excess boost when throttle closes (Correct answer)
- Cool intake air temperature
- Control wastegate actuation
Correct answer: Release excess boost when throttle closes
A blow-off valve vents pressurized air from the intake tract when the throttle closes, preventing compressor surge and protecting the turbocharger.
Question 6: Which compression ratio range is most common for naturally aspirated performance gasoline engines?
- 6:1 to 7:1
- 8:1 to 9:1
- 10:1 to 12:1 (Correct answer)
- 14:1 to 16:1
Correct answer: 10:1 to 12:1
High-performance naturally aspirated gasoline engines typically use 10:1 to 12:1 compression ratios to maximize thermal efficiency without requiring extremely high octane fuel.
Question 7: What does a negative fuel trim value indicate about engine operation?
- The ECU is adding fuel to compensate for a lean condition
- The ECU is removing fuel to compensate for a rich condition (Correct answer)
- The oxygen sensor has failed
- Fuel pressure is below specification
Correct answer: The ECU is removing fuel to compensate for a rich condition
Negative fuel trim means the ECU is subtracting fuel from the base map, indicating the engine is running richer than expected and the ECU is compensating.
ABA Engine Performance & Tuning 2
What is the stoichiometric air-fuel ratio for gasoline engines?