AME Reciprocating Engine Theory and Operation 2 — Questions and Answers
Question 1: What is the purpose of valve overlap in a 4-stroke reciprocating engine?
- To allow intake and exhaust valves to cool between opening events
- To improve scavenging of exhaust gases and increase volumetric efficiency (Correct answer)
- To prevent valve float at high RPM power settings
- To synchronize ignition timing precisely with valve events
Correct answer: To improve scavenging of exhaust gases and increase volumetric efficiency
Valve overlap — both intake and exhaust valves briefly open simultaneously near TDC — allows outgoing exhaust gases to help draw in fresh charge, improving cylinder filling.
Question 2: What does BMEP (Brake Mean Effective Pressure) represent in engine performance analysis?
- The peak cylinder pressure measured during the power stroke
- An average effective pressure that would produce the measured brake torque throughout the full stroke (Correct answer)
- The pressure differential between intake manifold and exhaust back-pressure
- The minimum oil pressure required at maximum rated power output
Correct answer: An average effective pressure that would produce the measured brake torque throughout the full stroke
BMEP is a calculated mean pressure that, if acting constantly throughout the entire power stroke, would produce the same torque measured at the crankshaft, enabling comparison across engines of different displacements.
Question 3: How is compression ratio defined for a reciprocating engine cylinder?
- The ratio of cylinder bore diameter to piston stroke length
- The ratio of total cylinder volume at BDC to clearance volume at TDC (Correct answer)
- The ratio of intake manifold pressure to exhaust back-pressure
- The ratio of fuel injection pressure to induction air pressure
Correct answer: The ratio of total cylinder volume at BDC to clearance volume at TDC
Compression ratio equals the total cylinder volume when the piston is at BDC (swept volume plus clearance volume) divided by the remaining clearance volume at TDC.
Question 4: What does a low manifold absolute pressure (MAP) reading at full throttle indicate on a normally aspirated reciprocating engine?
- Normal operation at cruise altitude
- A possible induction system air leak or restriction (Correct answer)
- Excessive fuel enrichment causing back-pressure
- High outside air temperature reducing induction air density
Correct answer: A possible induction system air leak or restriction
On a normally aspirated engine at full throttle, MAP should approach ambient pressure; a significantly lower reading indicates an induction system leak or obstruction preventing full atmospheric pressure from reaching the intake.
Question 5: How does preignition differ from detonation in a reciprocating engine?
- Preignition occurs after TDC while detonation always occurs before TDC
- Preignition is ignition before the spark plug fires; detonation is explosive auto-ignition of the end charge after normal ignition (Correct answer)
- Preignition is harmless at normal power while detonation causes immediate structural damage
- Preignition and detonation are interchangeable terms for the same abnormal combustion event
Correct answer: Preignition is ignition before the spark plug fires; detonation is explosive auto-ignition of the end charge after normal ignition
Preignition is caused by hot carbon deposits or incandescent spots igniting the mixture before the spark plug fires, while detonation is the explosive auto-ignition of the unburned end charge after normal spark ignition has occurred.
Question 6: What is the primary purpose of cylinder cooling fins and baffles on an air-cooled reciprocating engine?
- To reduce the overall installed weight of the engine
- To direct airflow over cylinder surfaces and maximize heat dissipation to the passing air (Correct answer)
- To reduce intake air turbulence for more uniform combustion
- To dampen mechanical vibration during high-power operation
Correct answer: To direct airflow over cylinder surfaces and maximize heat dissipation to the passing air
Baffles channel ram air tightly around the cylinders while fins dramatically increase the heat-transfer surface area, together maximizing the rate at which combustion heat is removed.
Question 7: What does a sudden, significant drop in oil pressure during flight most likely indicate?
- The engine has reached its normal operating temperature
- A serious oil system malfunction such as a major leak or bearing failure requiring immediate action (Correct answer)
- Normal pressure variation during a power reduction
- The need to add oil at the next scheduled maintenance stop
Correct answer: A serious oil system malfunction such as a major leak or bearing failure requiring immediate action
A sudden oil pressure drop can indicate catastrophic oil loss, pump failure, or bearing failure — any of which can cause rapid engine destruction and require immediate emergency action.
AME Reciprocating Engine Theory and Operation 2
What is the purpose of valve overlap in a 4-stroke reciprocating engine?