NYC Auto Mechanic (Diesel) DCAS ACTUAL Exam No. 2105 |
250+ Practice Questions, Correct Answers & Detailed
Rationales
SECTION 1 — DIESEL ENGINE FUNDAMENTALS
1. What is the primary difference between a conventional diesel
engine and a gasoline engine?
A. Diesel engines use spark plugs for normal combustion
B. Diesel engines normally ignite fuel through compression heat
C. Diesel engines do not use fuel injectors
D. Diesel engines cannot use turbochargers
Correct Answer: B
Explanation: Diesel engines normally use compression ignition. Air is
compressed to a high temperature, and fuel injected into the hot
compressed air ignites without requiring a conventional spark plug.
2. During which stroke is the air compressed in a four-stroke diesel
engine?
A. Intake
B. Compression
C. Power
D. Exhaust
Correct Answer: B
,Explanation: During the compression stroke, the piston moves upward
while both valves are closed, compressing the intake air and
substantially increasing its temperature.
3. Which sequence represents the normal four-stroke engine cycle?
A. Intake, compression, power, exhaust
B. Compression, intake, exhaust, power
C. Intake, power, compression, exhaust
D. Exhaust, power, intake, compression
Correct Answer: A
Explanation: A four-stroke engine completes intake, compression,
power, and exhaust strokes in that order.
4. What is the primary purpose of the piston rings?
A. Increase fuel pressure
B. Seal combustion gases and control oil
C. Operate the fuel injector
D. Control coolant flow
Correct Answer: B
Explanation: Piston rings help seal combustion gases, transfer heat
from the piston to the cylinder wall, and regulate lubricating oil on the
cylinder wall.
5. Excessive blow-by is most commonly associated with:
,A. Worn piston rings or cylinder wear
B. A new air filter
C. A properly sealed combustion chamber
D. Low windshield-washer fluid
Correct Answer: A
Explanation: Blow-by occurs when combustion gases pass the piston
rings into the crankcase. Excessive blow-by commonly indicates worn
rings, cylinders, or related components.
6. What is the main purpose of a connecting rod?
A. Connect the piston to the crankshaft
B. Connect the radiator to the thermostat
C. Connect the alternator to the battery
D. Connect the injector to the fuel tank
Correct Answer: A
Explanation: The connecting rod transfers the piston's reciprocating
motion to the crankshaft, which converts it into rotary motion.
7. What does the crankshaft primarily do?
A. Converts rotary motion into electrical energy
B. Converts reciprocating piston motion into rotary motion
C. Controls coolant temperature
D. Filters diesel fuel
Correct Answer: B
, Explanation: The crankshaft receives force from the connecting rods
and converts piston movement into rotary motion used to drive the
drivetrain and accessories.
8. What is the purpose of the engine flywheel?
A. Store rotational energy and help smooth engine operation
B. Filter engine oil
C. Increase coolant pressure
D. Regulate fuel temperature
Correct Answer: A
Explanation: The flywheel stores rotational energy, smooths crankshaft
speed fluctuations, and provides a mounting surface for components
such as the clutch in manual-transmission applications.
9. A diesel engine with low compression in one cylinder may
experience:
A. Improved combustion
B. Hard starting and misfire
C. Increased battery voltage
D. Higher tire pressure
Correct Answer: B
Explanation: Adequate compression is necessary to generate the
temperature required for diesel ignition. Low compression can cause
hard starting, poor combustion, smoke, and misfire.
250+ Practice Questions, Correct Answers & Detailed
Rationales
SECTION 1 — DIESEL ENGINE FUNDAMENTALS
1. What is the primary difference between a conventional diesel
engine and a gasoline engine?
A. Diesel engines use spark plugs for normal combustion
B. Diesel engines normally ignite fuel through compression heat
C. Diesel engines do not use fuel injectors
D. Diesel engines cannot use turbochargers
Correct Answer: B
Explanation: Diesel engines normally use compression ignition. Air is
compressed to a high temperature, and fuel injected into the hot
compressed air ignites without requiring a conventional spark plug.
2. During which stroke is the air compressed in a four-stroke diesel
engine?
A. Intake
B. Compression
C. Power
D. Exhaust
Correct Answer: B
,Explanation: During the compression stroke, the piston moves upward
while both valves are closed, compressing the intake air and
substantially increasing its temperature.
3. Which sequence represents the normal four-stroke engine cycle?
A. Intake, compression, power, exhaust
B. Compression, intake, exhaust, power
C. Intake, power, compression, exhaust
D. Exhaust, power, intake, compression
Correct Answer: A
Explanation: A four-stroke engine completes intake, compression,
power, and exhaust strokes in that order.
4. What is the primary purpose of the piston rings?
A. Increase fuel pressure
B. Seal combustion gases and control oil
C. Operate the fuel injector
D. Control coolant flow
Correct Answer: B
Explanation: Piston rings help seal combustion gases, transfer heat
from the piston to the cylinder wall, and regulate lubricating oil on the
cylinder wall.
5. Excessive blow-by is most commonly associated with:
,A. Worn piston rings or cylinder wear
B. A new air filter
C. A properly sealed combustion chamber
D. Low windshield-washer fluid
Correct Answer: A
Explanation: Blow-by occurs when combustion gases pass the piston
rings into the crankcase. Excessive blow-by commonly indicates worn
rings, cylinders, or related components.
6. What is the main purpose of a connecting rod?
A. Connect the piston to the crankshaft
B. Connect the radiator to the thermostat
C. Connect the alternator to the battery
D. Connect the injector to the fuel tank
Correct Answer: A
Explanation: The connecting rod transfers the piston's reciprocating
motion to the crankshaft, which converts it into rotary motion.
7. What does the crankshaft primarily do?
A. Converts rotary motion into electrical energy
B. Converts reciprocating piston motion into rotary motion
C. Controls coolant temperature
D. Filters diesel fuel
Correct Answer: B
, Explanation: The crankshaft receives force from the connecting rods
and converts piston movement into rotary motion used to drive the
drivetrain and accessories.
8. What is the purpose of the engine flywheel?
A. Store rotational energy and help smooth engine operation
B. Filter engine oil
C. Increase coolant pressure
D. Regulate fuel temperature
Correct Answer: A
Explanation: The flywheel stores rotational energy, smooths crankshaft
speed fluctuations, and provides a mounting surface for components
such as the clutch in manual-transmission applications.
9. A diesel engine with low compression in one cylinder may
experience:
A. Improved combustion
B. Hard starting and misfire
C. Increased battery voltage
D. Higher tire pressure
Correct Answer: B
Explanation: Adequate compression is necessary to generate the
temperature required for diesel ignition. Low compression can cause
hard starting, poor combustion, smoke, and misfire.
