NCED Engine Performance & Tuning 2 — Questions and Answers
Question 1: What does AFR stand for in the context of engine tuning?
- Air Flow Restriction
- Air-Fuel Ratio (Correct answer)
- Alternator Frequency Rating
- Automated Fuel Relay
Correct answer: Air-Fuel Ratio
AFR (Air-Fuel Ratio) describes the mass ratio of air to fuel in the combustion mixture, which directly affects power, efficiency, and emissions.
Question 2: What is the stoichiometric air-fuel ratio for a gasoline engine?
- 12.5:1
- 14.7:1 (Correct answer)
- 16.1:1
- 10.3:1
Correct answer: 14.7:1
The stoichiometric ratio for gasoline is 14.7:1, meaning 14.7 parts air to 1 part fuel, which achieves complete combustion with no excess oxygen or unburned fuel.
Question 3: What is engine knock (detonation) caused by?
- Oil pressure that is too high
- Uncontrolled auto-ignition of the air-fuel mixture before the spark plug fires (Correct answer)
- Exhaust gases re-entering the intake manifold
- A faulty crankshaft position sensor
Correct answer: Uncontrolled auto-ignition of the air-fuel mixture before the spark plug fires
Knock occurs when portions of the air-fuel mixture ignite spontaneously from heat and pressure rather than from the spark plug, causing pressure waves that can damage pistons and bearings.
Question 4: Advancing ignition timing means the spark plug fires:
- Later in the compression stroke
- Earlier in the compression stroke (Correct answer)
- During the exhaust stroke
- After top dead center
Correct answer: Earlier in the compression stroke
Advancing timing fires the spark plug earlier in the compression stroke, giving the mixture more time to fully combust by the time the piston reaches TDC, which can increase power but risks knock.
Question 5: What does a Mass Air Flow (MAF) sensor measure?
- The pressure of air in the intake manifold
- The mass of air entering the engine per unit time (Correct answer)
- The temperature of exhaust gases
- The velocity of air at the throttle body
Correct answer: The mass of air entering the engine per unit time
The MAF sensor measures the mass flow rate of air entering the engine, allowing the ECU to calculate the correct fuel quantity for the desired air-fuel ratio.
Question 6: In engine management, 'fuel trim' refers to:
- The physical trimming of fuel injector nozzles
- The ECU's real-time adjustment of fuel delivery to maintain target AFR (Correct answer)
- The fuel filter replacement interval
- The maximum fuel pressure setting
Correct answer: The ECU's real-time adjustment of fuel delivery to maintain target AFR
Fuel trim is the ECU's correction factor (short-term and long-term) applied to base fuel maps to compensate for sensor drift, wear, or changes in fuel quality.
Question 7: Volumetric efficiency (VE) in an engine refers to:
- The efficiency of the oil pump in circulating lubricant
- How well the engine fills its cylinders with air-fuel mixture compared to theoretical maximum (Correct answer)
- The ratio of fuel burned to heat produced
- The alternator's electrical output efficiency
Correct answer: How well the engine fills its cylinders with air-fuel mixture compared to theoretical maximum
VE compares the actual volume of air-fuel mixture drawn into a cylinder to the cylinder's displacement, with 100% VE meaning the cylinder is completely filled; forced induction can exceed 100%.
NCED Engine Performance & Tuning 2
What does AFR stand for in the context of engine tuning?