NYC Auto Mechanic (Diesel) DCAS Certification Practice Examination 2026
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; diesel fuel is
then 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
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
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
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 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
Rationale: Compression rings seal combustion gases, oil control rings scrape excess oil from cylinder walls,
and rings transfer heat from the piston to the cylinder walls.
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,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
Rationale: A warped cylinder head can break the seal provided by the head gasket, allowing coolant and
combustion gases to cross-contaminate.
9. What is the primary difference between a diesel engine and a gasoline engine?
A) Diesel engines use spark plugs for ignition
B) Diesel engines use compression ignition; gasoline engines use spark ignition
C) Diesel engines use carburetors
D) Gasoline engines use higher compression ratios
Answer: B
Rationale: Diesel engines rely on compression ignition (heat of compressed air ignites fuel), while gasoline
engines use spark plugs to ignite the air-fuel mixture.
10. What is the typical compression ratio of a diesel engine?
A) 8:1 to 10:1
B) 10:1 to 14:1
C) 14:1 to 25:1
D) 25:1 to 35:1
Answer: C
Rationale: Diesel engines have higher compression ratios (typically 14:1 to 25:1) than gasoline engines,
which is necessary to generate the heat required for compression ignition.
11. Which component is responsible for delivering fuel to the combustion chamber at high pressure?
A) Fuel filter
B) Fuel injection pump
C) Fuel transfer pump
3
, D) Fuel tank
Answer: B
Rationale: The fuel injection pump delivers fuel to the injectors at high pressure, which is essential for
proper atomization and combustion in a diesel engine.
12. What is the function of a turbocharger in a diesel engine?
A) Reduces exhaust emissions
B) Increases engine displacement
C) Forces more air into the cylinders, increasing power output
D) Cools the engine coolant
Answer: C
Rationale: A turbocharger uses exhaust gases to spin a turbine that compresses intake air, allowing more air
(and fuel) into the cylinders, increasing power output.
13. What is the purpose of the intercooler (charge air cooler) in a turbocharged diesel engine?
A) To heat the intake air
B) To cool the compressed intake air, increasing its density
C) To cool the exhaust gases
D) To cool the engine oil
Answer: B
Rationale: The intercooler cools the compressed intake air from the turbocharger, increasing its density,
which improves combustion efficiency and reduces emissions.
14. Which of the following is a symptom of low engine compression in one cylinder?
A) Increased fuel economy
B) Misfiring, rough idle, and loss of power
C) Increased engine speed
D) Cooler exhaust temperature
Answer: B
Rationale: Low compression in a cylinder prevents proper combustion, leading to misfiring, rough idle, and
a noticeable loss of power.
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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; diesel fuel is
then 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
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
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
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 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
Rationale: Compression rings seal combustion gases, oil control rings scrape excess oil from cylinder walls,
and rings transfer heat from the piston to the cylinder walls.
2
,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
Rationale: A warped cylinder head can break the seal provided by the head gasket, allowing coolant and
combustion gases to cross-contaminate.
9. What is the primary difference between a diesel engine and a gasoline engine?
A) Diesel engines use spark plugs for ignition
B) Diesel engines use compression ignition; gasoline engines use spark ignition
C) Diesel engines use carburetors
D) Gasoline engines use higher compression ratios
Answer: B
Rationale: Diesel engines rely on compression ignition (heat of compressed air ignites fuel), while gasoline
engines use spark plugs to ignite the air-fuel mixture.
10. What is the typical compression ratio of a diesel engine?
A) 8:1 to 10:1
B) 10:1 to 14:1
C) 14:1 to 25:1
D) 25:1 to 35:1
Answer: C
Rationale: Diesel engines have higher compression ratios (typically 14:1 to 25:1) than gasoline engines,
which is necessary to generate the heat required for compression ignition.
11. Which component is responsible for delivering fuel to the combustion chamber at high pressure?
A) Fuel filter
B) Fuel injection pump
C) Fuel transfer pump
3
, D) Fuel tank
Answer: B
Rationale: The fuel injection pump delivers fuel to the injectors at high pressure, which is essential for
proper atomization and combustion in a diesel engine.
12. What is the function of a turbocharger in a diesel engine?
A) Reduces exhaust emissions
B) Increases engine displacement
C) Forces more air into the cylinders, increasing power output
D) Cools the engine coolant
Answer: C
Rationale: A turbocharger uses exhaust gases to spin a turbine that compresses intake air, allowing more air
(and fuel) into the cylinders, increasing power output.
13. What is the purpose of the intercooler (charge air cooler) in a turbocharged diesel engine?
A) To heat the intake air
B) To cool the compressed intake air, increasing its density
C) To cool the exhaust gases
D) To cool the engine oil
Answer: B
Rationale: The intercooler cools the compressed intake air from the turbocharger, increasing its density,
which improves combustion efficiency and reduces emissions.
14. Which of the following is a symptom of low engine compression in one cylinder?
A) Increased fuel economy
B) Misfiring, rough idle, and loss of power
C) Increased engine speed
D) Cooler exhaust temperature
Answer: B
Rationale: Low compression in a cylinder prevents proper combustion, leading to misfiring, rough idle, and
a noticeable loss of power.
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