NYC Auto Mechanic (Diesel) DCAS Exam 2026/2027 |
Complete Practice Questions, Correct Answers & Detailed
Rationales
Section 1: Diesel Engine Fundamentals
1. What is the primary difference between a diesel engine and a
gasoline engine?
A. Diesel engines use spark plugs for ignition
B. Diesel engines ignite fuel primarily through compression heat
C. Diesel engines do not use fuel injectors
D. Diesel engines cannot use turbochargers
Correct Answer: B
Rationale: Diesel engines normally compress air to a high temperature
and then inject fuel, causing ignition from the heat of compression.
2. During the intake stroke of a conventional four-stroke diesel engine,
the intake valve is generally:
A. Closed
B. Open
C. Partially burned
D. Electrically grounded
Correct Answer: B
Rationale: The intake valve opens to allow fresh air into the cylinder
during the intake stroke.
3. The correct order of the four strokes is:
A. Compression, intake, power, exhaust
B. Intake, compression, power, exhaust
,C. Power, exhaust, intake, compression
D. Exhaust, power, compression, intake
Correct Answer: B
Rationale: A four-stroke cycle proceeds through intake, compression,
power, and exhaust.
4. What occurs during the compression stroke?
A. Air is compressed and its temperature rises
B. Exhaust gases enter the cylinder
C. Fuel is completely removed
D. The intake valve remains fully open
Correct Answer: A
Rationale: Compressing air increases its pressure and temperature,
preparing it for fuel ignition.
5. The power stroke begins when:
A. The intake valve opens
B. Combustion produces expanding gases that force the piston
downward
C. The exhaust valve opens
D. The starter disengages
Correct Answer: B
Rationale: Combustion raises cylinder pressure and pushes the piston
downward during the power stroke.
6. The exhaust stroke removes:
A. Fresh intake air
B. Lubricating oil
C. Combustion gases
D. Coolant
,Correct Answer: C
Rationale: The exhaust valve opens so combustion gases can leave the
cylinder.
7. The crankshaft converts:
A. Rotary motion into fuel pressure only
B. Reciprocating piston motion into rotary motion
C. Electrical energy into coolant flow
D. Air pressure into lubrication
Correct Answer: B
Rationale: Connecting rods transfer piston movement to the crankshaft,
producing rotary motion.
8. The camshaft primarily controls:
A. Fuel tank capacity
B. Valve timing
C. Battery voltage
D. Tire pressure
Correct Answer: B
Rationale: Cam lobes operate the valve train to open and close valves at
precise times.
9. A cylinder's displacement is determined primarily by its:
A. Tire size and axle ratio
B. Bore, stroke, and number of cylinders
C. Battery size
D. Radiator capacity
Correct Answer: B
Rationale: Engine displacement is calculated from cylinder bore, stroke,
and cylinder count.
, 10. Increasing compression ratio generally increases:
A. Compression temperature
B. Tire pressure
C. Coolant volume
D. Battery capacity
Correct Answer: A
Rationale: Greater compression raises the temperature of the
compressed air.
11. Diesel engine combustion depends heavily on:
A. Correct fuel injection timing and atomization
B. Spark plug gap
C. Distributor points
D. Carburetor float height
Correct Answer: A
Rationale: Diesel combustion depends on properly timed injection and
effective fuel atomization.
12. What is the purpose of a glow plug in many diesel engines?
A. Increase oil pressure
B. Assist cold starting
C. Control alternator output
D. Reduce tire wear
Correct Answer: B
Rationale: Glow plugs provide heat in the combustion chamber to
improve cold starting.
13. A diesel engine that cranks normally but is difficult to start when
cold may have a problem with:
A. Glow plugs
Complete Practice Questions, Correct Answers & Detailed
Rationales
Section 1: Diesel Engine Fundamentals
1. What is the primary difference between a diesel engine and a
gasoline engine?
A. Diesel engines use spark plugs for ignition
B. Diesel engines ignite fuel primarily through compression heat
C. Diesel engines do not use fuel injectors
D. Diesel engines cannot use turbochargers
Correct Answer: B
Rationale: Diesel engines normally compress air to a high temperature
and then inject fuel, causing ignition from the heat of compression.
2. During the intake stroke of a conventional four-stroke diesel engine,
the intake valve is generally:
A. Closed
B. Open
C. Partially burned
D. Electrically grounded
Correct Answer: B
Rationale: The intake valve opens to allow fresh air into the cylinder
during the intake stroke.
3. The correct order of the four strokes is:
A. Compression, intake, power, exhaust
B. Intake, compression, power, exhaust
,C. Power, exhaust, intake, compression
D. Exhaust, power, compression, intake
Correct Answer: B
Rationale: A four-stroke cycle proceeds through intake, compression,
power, and exhaust.
4. What occurs during the compression stroke?
A. Air is compressed and its temperature rises
B. Exhaust gases enter the cylinder
C. Fuel is completely removed
D. The intake valve remains fully open
Correct Answer: A
Rationale: Compressing air increases its pressure and temperature,
preparing it for fuel ignition.
5. The power stroke begins when:
A. The intake valve opens
B. Combustion produces expanding gases that force the piston
downward
C. The exhaust valve opens
D. The starter disengages
Correct Answer: B
Rationale: Combustion raises cylinder pressure and pushes the piston
downward during the power stroke.
6. The exhaust stroke removes:
A. Fresh intake air
B. Lubricating oil
C. Combustion gases
D. Coolant
,Correct Answer: C
Rationale: The exhaust valve opens so combustion gases can leave the
cylinder.
7. The crankshaft converts:
A. Rotary motion into fuel pressure only
B. Reciprocating piston motion into rotary motion
C. Electrical energy into coolant flow
D. Air pressure into lubrication
Correct Answer: B
Rationale: Connecting rods transfer piston movement to the crankshaft,
producing rotary motion.
8. The camshaft primarily controls:
A. Fuel tank capacity
B. Valve timing
C. Battery voltage
D. Tire pressure
Correct Answer: B
Rationale: Cam lobes operate the valve train to open and close valves at
precise times.
9. A cylinder's displacement is determined primarily by its:
A. Tire size and axle ratio
B. Bore, stroke, and number of cylinders
C. Battery size
D. Radiator capacity
Correct Answer: B
Rationale: Engine displacement is calculated from cylinder bore, stroke,
and cylinder count.
, 10. Increasing compression ratio generally increases:
A. Compression temperature
B. Tire pressure
C. Coolant volume
D. Battery capacity
Correct Answer: A
Rationale: Greater compression raises the temperature of the
compressed air.
11. Diesel engine combustion depends heavily on:
A. Correct fuel injection timing and atomization
B. Spark plug gap
C. Distributor points
D. Carburetor float height
Correct Answer: A
Rationale: Diesel combustion depends on properly timed injection and
effective fuel atomization.
12. What is the purpose of a glow plug in many diesel engines?
A. Increase oil pressure
B. Assist cold starting
C. Control alternator output
D. Reduce tire wear
Correct Answer: B
Rationale: Glow plugs provide heat in the combustion chamber to
improve cold starting.
13. A diesel engine that cranks normally but is difficult to start when
cold may have a problem with:
A. Glow plugs
