NYC AUTO MECHANIC (DIESEL) DCAS
CERTIFICATION EXAM 2026 WITH VERIFIED
QUESTIONS AND ANSWERS
SECTION 1: DIESEL ENGINE FUNDAMENTALS (Questions 1–50)
1. A diesel engine relies on which principle for ignition?
A) Spark ignition
B) Compression ignition
C) Electrical ignition
D) Vacuum ignition
Answer: B
Rationale: Diesel engines ignite fuel using the heat generated by high
compression of air in the cylinder, rather than spark plugs. The air is
compressed to a ratio of 14:1 to 25:1, raising its temperature above the
autoignition point of diesel fuel.
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
Rationale: In a four-stroke diesel engine, the piston moves down for the intake
of air, up for compression, down during the power stroke after fuel injection
ignites, 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
Rationale: Diesel engines compress air alone to high pressures, creating
extreme temperatures that cause the fuel to auto-ignite when injected.
,4. What is the typical compression ratio range for a heavy-duty diesel
engine?
A) 6:1 to 8:1
B) 8:1 to 10:1
C) 14:1 to 25:1
D) 30:1 to 40:1
Answer: C
Rationale: Diesel engines operate with compression ratios typically between
14:1 and 25:1, significantly higher than gasoline engines.
5. What temperature range is typically reached in a diesel engine cylinder
during compression?
A) 200-400°F
B) 400-600°F
C) 800-1000°F
D) 1200-1500°F
Answer: C
Rationale: Air is compressed to high pressures (300-500 psi), generating
temperatures of 800-1000°F, sufficient for diesel fuel autoignition.
6. What is the primary function of a glow plug in a diesel engine?
A) Primary ignition source during normal operation
B) Cold starting assistance only
C) Fuel atomization
D) Exhaust gas recirculation
Answer: B
Rationale: Glow plugs assist cold starting but are not the primary ignition
source in diesel engines.
7. In a V-type diesel engine, the cylinders are arranged:
A) In a straight line
B) In two banks at an angle (V configuration)
C) Horizontally opposed
D) Radially
Answer: B
Rationale: V-type engines have cylinders arranged in two banks that form a "V"
shape, typically with a V angle of 60-90 degrees.
8. Intake and exhaust valves in a diesel engine:
A) Open and close at specific times to allow air in and exhaust out
B) Remain open at all times
C) Are controlled by the fuel system
,D) Are not used in modern diesel engines
Answer: A
Rationale: Valves open and close at precise times during the engine cycle,
controlled by the camshaft and timing mechanism.
9. What component converts the reciprocating motion of the piston into
rotational motion?
A) Camshaft
B) Crankshaft
C) Connecting rod
D) Flywheel
Answer: B
Rationale: The crankshaft converts the linear (reciprocating) motion of the
pistons into rotational motion to drive the vehicle.
10. What is the purpose of the flywheel on a diesel engine?
A) To increase engine power
B) To store rotational energy and smooth out power pulses
C) To control valve timing
D) To filter fuel
Answer: B
Rationale: The flywheel stores rotational energy and helps smooth out the
power pulses between combustion events.
11. Which type of diesel engine cycle describes the ideal thermodynamic
process?
A) Otto cycle
B) Diesel cycle
C) Atkinson cycle
D) Miller cycle
Answer: B
12. What is the primary difference between a two-stroke and four-stroke
diesel engine?
A) Two-stroke engines have no valves
B) Two-stroke engines complete a power cycle in one crankshaft revolution
C) Two-stroke engines use spark plugs
D) Two-stroke engines are less powerful
Answer: B
Rationale: Two-stroke engines complete the power cycle in one revolution of
the crankshaft, while four-stroke engines require two revolutions.
, 13. What is the purpose of piston rings in a diesel engine?
A) To seal the combustion chamber and control oil consumption
B) To connect the piston to the crankshaft
C) To open and close valves
D) To cool the piston
Answer: A
14. Which component regulates engine timing by opening and closing
valves?
A) Crankshaft
B) Camshaft
C) Connecting rod
D) Piston
Answer: B
15. What is the function of the cylinder head in a diesel engine?
A) To house the crankshaft
B) To contain the combustion chambers and valves
C) To hold the piston rings
D) To store engine oil
Answer: B
16. What material is commonly used for diesel engine cylinder blocks?
A) Aluminum
B) Cast iron
C) Plastic composite
D) Titanium
Answer: B
17. What is the purpose of the oil pan in a diesel engine?
A) To cool the engine
B) To store engine oil
C) To filter fuel
D) To house the camshaft
Answer: B
18. What does "BTDC" stand for in engine timing?
A) Before Top Dead Center
B) Below Top Dead Center
C) Beyond Top Dead Center
D) Between Top Dead Center
Answer: A
CERTIFICATION EXAM 2026 WITH VERIFIED
QUESTIONS AND ANSWERS
SECTION 1: DIESEL ENGINE FUNDAMENTALS (Questions 1–50)
1. A diesel engine relies on which principle for ignition?
A) Spark ignition
B) Compression ignition
C) Electrical ignition
D) Vacuum ignition
Answer: B
Rationale: Diesel engines ignite fuel using the heat generated by high
compression of air in the cylinder, rather than spark plugs. The air is
compressed to a ratio of 14:1 to 25:1, raising its temperature above the
autoignition point of diesel fuel.
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
Rationale: In a four-stroke diesel engine, the piston moves down for the intake
of air, up for compression, down during the power stroke after fuel injection
ignites, 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
Rationale: Diesel engines compress air alone to high pressures, creating
extreme temperatures that cause the fuel to auto-ignite when injected.
,4. What is the typical compression ratio range for a heavy-duty diesel
engine?
A) 6:1 to 8:1
B) 8:1 to 10:1
C) 14:1 to 25:1
D) 30:1 to 40:1
Answer: C
Rationale: Diesel engines operate with compression ratios typically between
14:1 and 25:1, significantly higher than gasoline engines.
5. What temperature range is typically reached in a diesel engine cylinder
during compression?
A) 200-400°F
B) 400-600°F
C) 800-1000°F
D) 1200-1500°F
Answer: C
Rationale: Air is compressed to high pressures (300-500 psi), generating
temperatures of 800-1000°F, sufficient for diesel fuel autoignition.
6. What is the primary function of a glow plug in a diesel engine?
A) Primary ignition source during normal operation
B) Cold starting assistance only
C) Fuel atomization
D) Exhaust gas recirculation
Answer: B
Rationale: Glow plugs assist cold starting but are not the primary ignition
source in diesel engines.
7. In a V-type diesel engine, the cylinders are arranged:
A) In a straight line
B) In two banks at an angle (V configuration)
C) Horizontally opposed
D) Radially
Answer: B
Rationale: V-type engines have cylinders arranged in two banks that form a "V"
shape, typically with a V angle of 60-90 degrees.
8. Intake and exhaust valves in a diesel engine:
A) Open and close at specific times to allow air in and exhaust out
B) Remain open at all times
C) Are controlled by the fuel system
,D) Are not used in modern diesel engines
Answer: A
Rationale: Valves open and close at precise times during the engine cycle,
controlled by the camshaft and timing mechanism.
9. What component converts the reciprocating motion of the piston into
rotational motion?
A) Camshaft
B) Crankshaft
C) Connecting rod
D) Flywheel
Answer: B
Rationale: The crankshaft converts the linear (reciprocating) motion of the
pistons into rotational motion to drive the vehicle.
10. What is the purpose of the flywheel on a diesel engine?
A) To increase engine power
B) To store rotational energy and smooth out power pulses
C) To control valve timing
D) To filter fuel
Answer: B
Rationale: The flywheel stores rotational energy and helps smooth out the
power pulses between combustion events.
11. Which type of diesel engine cycle describes the ideal thermodynamic
process?
A) Otto cycle
B) Diesel cycle
C) Atkinson cycle
D) Miller cycle
Answer: B
12. What is the primary difference between a two-stroke and four-stroke
diesel engine?
A) Two-stroke engines have no valves
B) Two-stroke engines complete a power cycle in one crankshaft revolution
C) Two-stroke engines use spark plugs
D) Two-stroke engines are less powerful
Answer: B
Rationale: Two-stroke engines complete the power cycle in one revolution of
the crankshaft, while four-stroke engines require two revolutions.
, 13. What is the purpose of piston rings in a diesel engine?
A) To seal the combustion chamber and control oil consumption
B) To connect the piston to the crankshaft
C) To open and close valves
D) To cool the piston
Answer: A
14. Which component regulates engine timing by opening and closing
valves?
A) Crankshaft
B) Camshaft
C) Connecting rod
D) Piston
Answer: B
15. What is the function of the cylinder head in a diesel engine?
A) To house the crankshaft
B) To contain the combustion chambers and valves
C) To hold the piston rings
D) To store engine oil
Answer: B
16. What material is commonly used for diesel engine cylinder blocks?
A) Aluminum
B) Cast iron
C) Plastic composite
D) Titanium
Answer: B
17. What is the purpose of the oil pan in a diesel engine?
A) To cool the engine
B) To store engine oil
C) To filter fuel
D) To house the camshaft
Answer: B
18. What does "BTDC" stand for in engine timing?
A) Before Top Dead Center
B) Below Top Dead Center
C) Beyond Top Dead Center
D) Between Top Dead Center
Answer: A
