NYC Auto Mechanic (Diesel) Exam Prep
2026/2027 | DCAS Exam No. 2105 | 300 Practice
Questions with Correct Answers & Detailed
Rationales
1. A diesel engine relies on which principle for ignition?
A) Spark ignition
B) Compression ignition
C) Electrical ignition
D) Vacuum ignition
Answer B: Compression ignition
Rationale: Diesel engines ignite fuel using the heat generated by high compression of air in the cylinder,
rather than spark plugs .
2. What are the four strokes of a four-stroke diesel engine in correct order?
A) Intake, compression, power, exhaust
B) Intake, power, compression, exhaust
C) Compression, intake, power, exhaust
D) Intake, exhaust, compression, power
Answer A: Intake, compression, power, exhaust
Rationale: The piston moves down for intake, up for compression, down for power, and up for exhaust .
3. During the compression stroke of a diesel engine, what occurs?
A) Air and fuel mixture is compressed and ignited by a spark plug
B) Only air is compressed, raising its temperature above the autoignition point of diesel fuel
C) Exhaust gases are forced out through the open exhaust valve
D) The intake valve opens to allow fresh air into the cylinder
,Answer B: Only air is compressed, raising its temperature above the autoignition point of diesel
fuel
Rationale: Diesel engines compress air alone; diesel fuel is injected and ignites spontaneously due to the
heat, with no spark plug .
4. The crankshaft converts the reciprocating motion of the pistons into what type of motion?
A) Oscillating motion
B) Rotary motion
C) Linear thrust
D) Hydraulic pressure
Answer B: Rotary motion
Rationale: The crankshaft transforms the up-and-down motion of the pistons into rotational motion to
drive the transmission and accessories .
5. A camshaft in a diesel engine is responsible for:
A) Driving the oil pump
B) Opening and closing the intake and exhaust valves
C) Delivering fuel to the injectors
D) Converting linear motion to rotary motion
Answer B: Opening and closing the intake and exhaust valves
Rationale: The camshaft controls the opening and closing of the intake and exhaust valves, ensuring
proper timing of the engine's breathing cycle .
6. The component that seals the combustion chamber and transfers force to the connecting rod is
the:
A) Cylinder head
B) Piston ring
C) Piston
,D) Valve
Answer C: Piston
Rationale: The piston seals the combustion chamber via piston rings and transfers the force of
combustion to the connecting rod .
7. Which of the following is a function of the piston rings?
A) Transferring torque to the crankshaft
B) Sealing the combustion chamber, controlling oil, and transferring heat
C) Opening valves at correct timing
D) Injecting fuel into the cylinder
Answer B: Sealing the combustion chamber, controlling oil, and transferring heat
Rationale: Compression rings seal combustion gases, oil control rings scrape excess oil, and rings
transfer heat from the piston to the cylinder walls .
8. A warped cylinder head is most likely to cause which problem?
A) Low oil pressure
B) Coolant leaking into the combustion chamber or combustion gas entering the coolant
C) Improper fuel injection timing
D) Starter motor failure
Answer B: Coolant leaking into the combustion chamber or combustion gas entering the coolant
Rationale: Warpage breaks the head gasket seal, allowing coolant and combustion gases to mix, leading
to overheating and white exhaust smoke .
9. What does the term "valve lash" refer to?
A) The clearance between the valve stem and rocker arm when the valve is closed
B) The tension on the timing chain
C) The angle of the valve seat
D) The gap in piston rings
, Answer A: The clearance between the valve stem and rocker arm when the valve is closed
Rationale: Valve lash is the mechanical clearance in the valve train when the valve is fully seated,
compensating for thermal expansion and ensuring proper sealing .
10. What is the purpose of a vibration damper (harmonic balancer) mounted on the crankshaft?
A) To balance the flywheel
B) To absorb torsional vibrations and prevent crankshaft failure
C) To adjust ignition timing
D) To increase compression ratio
Answer B: To absorb torsional vibrations and prevent crankshaft failure
Rationale: The harmonic balancer reduces torsional twisting vibrations in the crankshaft caused by
power impulses, protecting the crankshaft from fatigue .
11. Piston deck clearance refers to:
A) The clearance between the piston skirt and cylinder wall
B) The distance from the piston crown to the block deck at top dead center
C) The gap between piston rings and cylinder wall
D) The clearance between the connecting rod and crankshaft
Answer B: The distance from the piston crown to the block deck at top dead center
Rationale: Deck clearance is the measurement of how far the piston crown is above or below the block
deck at TDC, influencing compression ratio .
12. What is the primary advantage of a diesel engine over a gasoline engine?
A) Lower initial cost
B) Higher thermal efficiency and fuel economy
C) Simpler emissions control
D) Quieter operation
Answer B: Higher thermal efficiency and fuel economy
2026/2027 | DCAS Exam No. 2105 | 300 Practice
Questions with Correct Answers & Detailed
Rationales
1. A diesel engine relies on which principle for ignition?
A) Spark ignition
B) Compression ignition
C) Electrical ignition
D) Vacuum ignition
Answer B: Compression ignition
Rationale: Diesel engines ignite fuel using the heat generated by high compression of air in the cylinder,
rather than spark plugs .
2. What are the four strokes of a four-stroke diesel engine in correct order?
A) Intake, compression, power, exhaust
B) Intake, power, compression, exhaust
C) Compression, intake, power, exhaust
D) Intake, exhaust, compression, power
Answer A: Intake, compression, power, exhaust
Rationale: The piston moves down for intake, up for compression, down for power, and up for exhaust .
3. During the compression stroke of a diesel engine, what occurs?
A) Air and fuel mixture is compressed and ignited by a spark plug
B) Only air is compressed, raising its temperature above the autoignition point of diesel fuel
C) Exhaust gases are forced out through the open exhaust valve
D) The intake valve opens to allow fresh air into the cylinder
,Answer B: Only air is compressed, raising its temperature above the autoignition point of diesel
fuel
Rationale: Diesel engines compress air alone; diesel fuel is injected and ignites spontaneously due to the
heat, with no spark plug .
4. The crankshaft converts the reciprocating motion of the pistons into what type of motion?
A) Oscillating motion
B) Rotary motion
C) Linear thrust
D) Hydraulic pressure
Answer B: Rotary motion
Rationale: The crankshaft transforms the up-and-down motion of the pistons into rotational motion to
drive the transmission and accessories .
5. A camshaft in a diesel engine is responsible for:
A) Driving the oil pump
B) Opening and closing the intake and exhaust valves
C) Delivering fuel to the injectors
D) Converting linear motion to rotary motion
Answer B: Opening and closing the intake and exhaust valves
Rationale: The camshaft controls the opening and closing of the intake and exhaust valves, ensuring
proper timing of the engine's breathing cycle .
6. The component that seals the combustion chamber and transfers force to the connecting rod is
the:
A) Cylinder head
B) Piston ring
C) Piston
,D) Valve
Answer C: Piston
Rationale: The piston seals the combustion chamber via piston rings and transfers the force of
combustion to the connecting rod .
7. Which of the following is a function of the piston rings?
A) Transferring torque to the crankshaft
B) Sealing the combustion chamber, controlling oil, and transferring heat
C) Opening valves at correct timing
D) Injecting fuel into the cylinder
Answer B: Sealing the combustion chamber, controlling oil, and transferring heat
Rationale: Compression rings seal combustion gases, oil control rings scrape excess oil, and rings
transfer heat from the piston to the cylinder walls .
8. A warped cylinder head is most likely to cause which problem?
A) Low oil pressure
B) Coolant leaking into the combustion chamber or combustion gas entering the coolant
C) Improper fuel injection timing
D) Starter motor failure
Answer B: Coolant leaking into the combustion chamber or combustion gas entering the coolant
Rationale: Warpage breaks the head gasket seal, allowing coolant and combustion gases to mix, leading
to overheating and white exhaust smoke .
9. What does the term "valve lash" refer to?
A) The clearance between the valve stem and rocker arm when the valve is closed
B) The tension on the timing chain
C) The angle of the valve seat
D) The gap in piston rings
, Answer A: The clearance between the valve stem and rocker arm when the valve is closed
Rationale: Valve lash is the mechanical clearance in the valve train when the valve is fully seated,
compensating for thermal expansion and ensuring proper sealing .
10. What is the purpose of a vibration damper (harmonic balancer) mounted on the crankshaft?
A) To balance the flywheel
B) To absorb torsional vibrations and prevent crankshaft failure
C) To adjust ignition timing
D) To increase compression ratio
Answer B: To absorb torsional vibrations and prevent crankshaft failure
Rationale: The harmonic balancer reduces torsional twisting vibrations in the crankshaft caused by
power impulses, protecting the crankshaft from fatigue .
11. Piston deck clearance refers to:
A) The clearance between the piston skirt and cylinder wall
B) The distance from the piston crown to the block deck at top dead center
C) The gap between piston rings and cylinder wall
D) The clearance between the connecting rod and crankshaft
Answer B: The distance from the piston crown to the block deck at top dead center
Rationale: Deck clearance is the measurement of how far the piston crown is above or below the block
deck at TDC, influencing compression ratio .
12. What is the primary advantage of a diesel engine over a gasoline engine?
A) Lower initial cost
B) Higher thermal efficiency and fuel economy
C) Simpler emissions control
D) Quieter operation
Answer B: Higher thermal efficiency and fuel economy
