NYC Auto Mechanic (Diesel) DCAS actual Certification Exam |
Complete Questions & Answers 2026/2027
SECTION 1 — DIESEL ENGINE FUNDAMENTALS
1. Which stroke of a four-stroke diesel engine compresses the air
before fuel injection?
A. Intake
B. Compression
C. Power
D. Exhaust
Correct Answer: B
Explanation: During the compression stroke, the piston moves upward
with the valves closed and compresses the intake air to a high
temperature. Fuel is then injected near the end of compression.
2. What primarily causes diesel fuel to ignite inside the cylinder?
A. Spark from a spark plug
B. Heat produced by compressed air
C. Battery voltage
D. Exhaust-gas pressure
Correct Answer: B
Explanation: Diesel engines normally use compression ignition.
Compressing air raises its temperature sufficiently for injected diesel
fuel to ignite.
,3. During the intake stroke of a four-stroke diesel engine, the intake
valve is generally:
A. Open
B. Closed
C. Locked mechanically
D. Electrically grounded
Correct Answer: A
Explanation: The intake valve opens as the piston moves downward,
allowing fresh air to enter the cylinder.
4. What is the primary purpose of piston rings?
A. Increase fuel pressure
B. Seal combustion gases and control oil
C. Operate the injectors
D. Cool the radiator
Correct Answer: B
Explanation: Piston rings maintain compression, limit combustion-gas
leakage into the crankcase, and help control lubricating oil on the
cylinder wall.
5. Excessive blow-by is commonly associated with:
A. Worn piston rings or cylinders
B. A new air filter
,C. High battery voltage
D. A clean fuel filter
Correct Answer: A
Explanation: Blow-by occurs when combustion gases pass the piston
rings into the crankcase. Excessive blow-by can indicate worn rings,
pistons, or cylinder walls.
6. What component connects the piston to the crankshaft?
A. Pushrod
B. Connecting rod
C. Rocker arm
D. Cam gear
Correct Answer: B
Explanation: The connecting rod transfers the piston's reciprocating
motion to the crankshaft.
7. The crankshaft converts:
A. Rotary motion into electrical energy
B. Reciprocating motion into rotary motion
C. Hydraulic motion into electrical energy
D. Heat into fuel pressure
Correct Answer: B
Explanation: Pistons move up and down while the crankshaft rotates.
Connecting rods transfer this motion to the crankshaft.
, 8. What is the function of the camshaft?
A. Store electrical energy
B. Operate engine valves at the proper time
C. Pressurize engine oil only
D. Cool the fuel
Correct Answer: B
Explanation: The camshaft controls valve opening and closing and,
depending on the engine design, may also drive or control other
components.
9. A timing mark is primarily used to establish:
A. Tire pressure
B. Correct relationship between engine components
C. Battery electrolyte level
D. Brake-fluid level
Correct Answer: B
Explanation: Timing marks help establish the correct relationship
between crankshaft, camshaft, and other timed components.
10. What is the main purpose of engine compression?
A. Reduce cylinder temperature
B. Raise air temperature and pressure
Complete Questions & Answers 2026/2027
SECTION 1 — DIESEL ENGINE FUNDAMENTALS
1. Which stroke of a four-stroke diesel engine compresses the air
before fuel injection?
A. Intake
B. Compression
C. Power
D. Exhaust
Correct Answer: B
Explanation: During the compression stroke, the piston moves upward
with the valves closed and compresses the intake air to a high
temperature. Fuel is then injected near the end of compression.
2. What primarily causes diesel fuel to ignite inside the cylinder?
A. Spark from a spark plug
B. Heat produced by compressed air
C. Battery voltage
D. Exhaust-gas pressure
Correct Answer: B
Explanation: Diesel engines normally use compression ignition.
Compressing air raises its temperature sufficiently for injected diesel
fuel to ignite.
,3. During the intake stroke of a four-stroke diesel engine, the intake
valve is generally:
A. Open
B. Closed
C. Locked mechanically
D. Electrically grounded
Correct Answer: A
Explanation: The intake valve opens as the piston moves downward,
allowing fresh air to enter the cylinder.
4. What is the primary purpose of piston rings?
A. Increase fuel pressure
B. Seal combustion gases and control oil
C. Operate the injectors
D. Cool the radiator
Correct Answer: B
Explanation: Piston rings maintain compression, limit combustion-gas
leakage into the crankcase, and help control lubricating oil on the
cylinder wall.
5. Excessive blow-by is commonly associated with:
A. Worn piston rings or cylinders
B. A new air filter
,C. High battery voltage
D. A clean fuel filter
Correct Answer: A
Explanation: Blow-by occurs when combustion gases pass the piston
rings into the crankcase. Excessive blow-by can indicate worn rings,
pistons, or cylinder walls.
6. What component connects the piston to the crankshaft?
A. Pushrod
B. Connecting rod
C. Rocker arm
D. Cam gear
Correct Answer: B
Explanation: The connecting rod transfers the piston's reciprocating
motion to the crankshaft.
7. The crankshaft converts:
A. Rotary motion into electrical energy
B. Reciprocating motion into rotary motion
C. Hydraulic motion into electrical energy
D. Heat into fuel pressure
Correct Answer: B
Explanation: Pistons move up and down while the crankshaft rotates.
Connecting rods transfer this motion to the crankshaft.
, 8. What is the function of the camshaft?
A. Store electrical energy
B. Operate engine valves at the proper time
C. Pressurize engine oil only
D. Cool the fuel
Correct Answer: B
Explanation: The camshaft controls valve opening and closing and,
depending on the engine design, may also drive or control other
components.
9. A timing mark is primarily used to establish:
A. Tire pressure
B. Correct relationship between engine components
C. Battery electrolyte level
D. Brake-fluid level
Correct Answer: B
Explanation: Timing marks help establish the correct relationship
between crankshaft, camshaft, and other timed components.
10. What is the main purpose of engine compression?
A. Reduce cylinder temperature
B. Raise air temperature and pressure
