NYC Auto Mechanic (Diesel) Civil Service actual Exam |
Complete Practice Questions & Answers 2026/2027
SECTION 1 — DIESEL ENGINE CONSTRUCTION & OPERATION
1.
What is the primary purpose of the flywheel on a diesel engine?
A. Increase fuel pressure
B. Store rotational energy and smooth crankshaft speed
C. Cool the pistons
D. Control injector timing
Correct Answer: B
Explanation: The flywheel stores rotational energy and helps maintain
smoother crankshaft rotation between power strokes.
2.
Which component converts the piston's reciprocating motion into
rotary motion?
A. Camshaft
B. Crankshaft
C. Connecting rod bearing
D. Rocker arm
Correct Answer: B
Explanation: The crankshaft converts the pistons' up-and-down motion
into rotary motion used to drive the vehicle.
,3.
What is the function of a connecting rod?
A. Connect the injector to the fuel rail
B. Connect the piston to the crankshaft
C. Connect the alternator to the starter
D. Connect the valve to the camshaft
Correct Answer: B
Explanation: The connecting rod transfers combustion force from the
piston to the crankshaft.
4.
A diesel engine's compression ratio is generally higher than that of a
gasoline engine primarily because diesel engines:
A. Use spark plugs
B. Rely on compression-generated heat for ignition
C. Operate without valves
D. Use lower cylinder pressure
Correct Answer: B
Explanation: Diesel fuel ignites when injected into highly compressed,
heated air, requiring substantial compression.
5.
Which engine component opens and closes the intake and exhaust
valves?
A. Camshaft
B. Flywheel
,C. Oil pump
D. Water pump
Correct Answer: A
Explanation: Camshaft lobes actuate the valve train according to the
engine's timing sequence.
6.
What is the purpose of the timing gear or timing chain?
A. Regulate coolant pressure
B. Synchronize crankshaft and camshaft rotation
C. Increase oil viscosity
D. Control battery voltage
Correct Answer: B
Explanation: Proper synchronization allows valves to operate at the
correct point in relation to piston movement.
7.
What occurs during the compression stroke of a four-stroke diesel
engine?
A. Exhaust gases leave the cylinder
B. Intake valve opens and air enters
C. Air is compressed while both valves are closed
D. Fuel burns and pushes the piston downward
Correct Answer: C
Explanation: During compression, the piston moves upward with both
valves closed, raising air temperature and pressure.
, 8.
During which stroke is fuel normally injected into a conventional four-
stroke diesel engine?
A. Near the end of compression
B. Near the beginning of intake
C. During exhaust
D. After the power stroke
Correct Answer: A
Explanation: Fuel is injected near the end of compression so that it
encounters highly compressed, hot air.
9.
What is valve overlap?
A. The period when two cylinders fire simultaneously
B. The period when intake and exhaust valves are briefly open together
C. The amount of valve clearance
D. The distance between valves
Correct Answer: B
Explanation: Valve overlap occurs around the transition between
exhaust and intake strokes and assists cylinder gas exchange.
10.
Excessive valve lash can cause:
A. Improved compression
B. Valve-train noise and incorrect valve timing
Complete Practice Questions & Answers 2026/2027
SECTION 1 — DIESEL ENGINE CONSTRUCTION & OPERATION
1.
What is the primary purpose of the flywheel on a diesel engine?
A. Increase fuel pressure
B. Store rotational energy and smooth crankshaft speed
C. Cool the pistons
D. Control injector timing
Correct Answer: B
Explanation: The flywheel stores rotational energy and helps maintain
smoother crankshaft rotation between power strokes.
2.
Which component converts the piston's reciprocating motion into
rotary motion?
A. Camshaft
B. Crankshaft
C. Connecting rod bearing
D. Rocker arm
Correct Answer: B
Explanation: The crankshaft converts the pistons' up-and-down motion
into rotary motion used to drive the vehicle.
,3.
What is the function of a connecting rod?
A. Connect the injector to the fuel rail
B. Connect the piston to the crankshaft
C. Connect the alternator to the starter
D. Connect the valve to the camshaft
Correct Answer: B
Explanation: The connecting rod transfers combustion force from the
piston to the crankshaft.
4.
A diesel engine's compression ratio is generally higher than that of a
gasoline engine primarily because diesel engines:
A. Use spark plugs
B. Rely on compression-generated heat for ignition
C. Operate without valves
D. Use lower cylinder pressure
Correct Answer: B
Explanation: Diesel fuel ignites when injected into highly compressed,
heated air, requiring substantial compression.
5.
Which engine component opens and closes the intake and exhaust
valves?
A. Camshaft
B. Flywheel
,C. Oil pump
D. Water pump
Correct Answer: A
Explanation: Camshaft lobes actuate the valve train according to the
engine's timing sequence.
6.
What is the purpose of the timing gear or timing chain?
A. Regulate coolant pressure
B. Synchronize crankshaft and camshaft rotation
C. Increase oil viscosity
D. Control battery voltage
Correct Answer: B
Explanation: Proper synchronization allows valves to operate at the
correct point in relation to piston movement.
7.
What occurs during the compression stroke of a four-stroke diesel
engine?
A. Exhaust gases leave the cylinder
B. Intake valve opens and air enters
C. Air is compressed while both valves are closed
D. Fuel burns and pushes the piston downward
Correct Answer: C
Explanation: During compression, the piston moves upward with both
valves closed, raising air temperature and pressure.
, 8.
During which stroke is fuel normally injected into a conventional four-
stroke diesel engine?
A. Near the end of compression
B. Near the beginning of intake
C. During exhaust
D. After the power stroke
Correct Answer: A
Explanation: Fuel is injected near the end of compression so that it
encounters highly compressed, hot air.
9.
What is valve overlap?
A. The period when two cylinders fire simultaneously
B. The period when intake and exhaust valves are briefly open together
C. The amount of valve clearance
D. The distance between valves
Correct Answer: B
Explanation: Valve overlap occurs around the transition between
exhaust and intake strokes and assists cylinder gas exchange.
10.
Excessive valve lash can cause:
A. Improved compression
B. Valve-train noise and incorrect valve timing
