NYC Auto Mechanic (Diesel) DCAS actual Exam Prep | Diesel
Engine, Fuel, Electrical, Brakes & Diagnostics Questions and
answers
DIESEL ENGINE CONSTRUCTION & OPERATION
1. Which engine component converts the reciprocating motion of the
piston into rotary motion?
A. Camshaft
B. Crankshaft
C. Flywheel housing
D. Rocker arm
Correct Answer: B
Explanation: The crankshaft converts piston movement into rotary
motion that can be transmitted to the drivetrain.
2. What is the primary purpose of piston rings?
A. Control combustion chamber sealing and oil movement
B. Increase injector pressure
C. Operate the valves
D. Drive the water pump
Correct Answer: A
Explanation: Piston rings seal combustion gases, regulate lubricating oil,
and transfer heat from the piston to the cylinder wall.
,3. A diesel engine's compression ratio is primarily important because
it:
A. Determines alternator output
B. Produces the temperature needed for compression ignition
C. Controls coolant pressure
D. Determines transmission shift timing
Correct Answer: B
Explanation: Diesel engines rely on compression-generated heat to
ignite injected fuel.
4. What is the main function of a flywheel on a diesel engine?
A. Filter lubricating oil
B. Store rotational energy and smooth engine speed fluctuations
C. Regulate fuel pressure
D. Cool the cylinder head
Correct Answer: B
Explanation: The flywheel stores rotational energy and helps maintain
smoother crankshaft rotation between power events.
5. Excessive clearance between a piston and cylinder wall can
commonly cause:
A. Piston slap and increased oil consumption
B. Higher coolant concentration
C. Reduced alternator voltage only
D. Increased tire pressure
,Correct Answer: A
Explanation: Excessive piston-to-cylinder clearance can permit piston
movement, noise, blow-by, and increased oil consumption.
6. What is the purpose of a connecting rod?
A. Connect the piston to the crankshaft
B. Connect the alternator to the starter
C. Connect the injector to the fuel tank
D. Connect the radiator to the thermostat
Correct Answer: A
Explanation: The connecting rod transfers combustion force from the
piston to the crankshaft.
7. Which component controls the opening and closing of engine
valves?
A. Camshaft
B. Oil pump
C. Flywheel
D. Fuel tank
Correct Answer: A
Explanation: Camshaft lobes operate the valve train according to the
engine's valve timing.
8. What would excessive valve lash most likely produce?
A. Valve-train noise and incorrect valve timing
, B. Higher tire pressure
C. Increased battery capacity
D. Lower transmission fluid level
Correct Answer: A
Explanation: Excessive clearance can produce valve-train noise and
prevent valves from operating at their intended timing and lift.
9. Why are cylinder liners used in many diesel engines?
A. To provide a replaceable wear surface
B. To increase battery voltage
C. To control injector timing electronically
D. To filter exhaust gases
Correct Answer: A
Explanation: Liners provide a durable cylinder surface that can be
replaced or serviced without necessarily replacing the entire engine
block.
10. What does engine displacement represent?
A. Total swept volume of the cylinders
B. Oil capacity only
C. Cooling-system pressure
D. Maximum alternator amperage
Correct Answer: A
Explanation: Engine displacement is based on the combined swept
volume of the pistons.
Engine, Fuel, Electrical, Brakes & Diagnostics Questions and
answers
DIESEL ENGINE CONSTRUCTION & OPERATION
1. Which engine component converts the reciprocating motion of the
piston into rotary motion?
A. Camshaft
B. Crankshaft
C. Flywheel housing
D. Rocker arm
Correct Answer: B
Explanation: The crankshaft converts piston movement into rotary
motion that can be transmitted to the drivetrain.
2. What is the primary purpose of piston rings?
A. Control combustion chamber sealing and oil movement
B. Increase injector pressure
C. Operate the valves
D. Drive the water pump
Correct Answer: A
Explanation: Piston rings seal combustion gases, regulate lubricating oil,
and transfer heat from the piston to the cylinder wall.
,3. A diesel engine's compression ratio is primarily important because
it:
A. Determines alternator output
B. Produces the temperature needed for compression ignition
C. Controls coolant pressure
D. Determines transmission shift timing
Correct Answer: B
Explanation: Diesel engines rely on compression-generated heat to
ignite injected fuel.
4. What is the main function of a flywheel on a diesel engine?
A. Filter lubricating oil
B. Store rotational energy and smooth engine speed fluctuations
C. Regulate fuel pressure
D. Cool the cylinder head
Correct Answer: B
Explanation: The flywheel stores rotational energy and helps maintain
smoother crankshaft rotation between power events.
5. Excessive clearance between a piston and cylinder wall can
commonly cause:
A. Piston slap and increased oil consumption
B. Higher coolant concentration
C. Reduced alternator voltage only
D. Increased tire pressure
,Correct Answer: A
Explanation: Excessive piston-to-cylinder clearance can permit piston
movement, noise, blow-by, and increased oil consumption.
6. What is the purpose of a connecting rod?
A. Connect the piston to the crankshaft
B. Connect the alternator to the starter
C. Connect the injector to the fuel tank
D. Connect the radiator to the thermostat
Correct Answer: A
Explanation: The connecting rod transfers combustion force from the
piston to the crankshaft.
7. Which component controls the opening and closing of engine
valves?
A. Camshaft
B. Oil pump
C. Flywheel
D. Fuel tank
Correct Answer: A
Explanation: Camshaft lobes operate the valve train according to the
engine's valve timing.
8. What would excessive valve lash most likely produce?
A. Valve-train noise and incorrect valve timing
, B. Higher tire pressure
C. Increased battery capacity
D. Lower transmission fluid level
Correct Answer: A
Explanation: Excessive clearance can produce valve-train noise and
prevent valves from operating at their intended timing and lift.
9. Why are cylinder liners used in many diesel engines?
A. To provide a replaceable wear surface
B. To increase battery voltage
C. To control injector timing electronically
D. To filter exhaust gases
Correct Answer: A
Explanation: Liners provide a durable cylinder surface that can be
replaced or serviced without necessarily replacing the entire engine
block.
10. What does engine displacement represent?
A. Total swept volume of the cylinders
B. Oil capacity only
C. Cooling-system pressure
D. Maximum alternator amperage
Correct Answer: A
Explanation: Engine displacement is based on the combined swept
volume of the pistons.
