NYC AUTO MECHANIC (DIESEL) DCAS EXAM NO. 2105
EXAMINATION COMPLETE QUESTIONS AND DETAILED
SOLUTIONS LATEST UPDATE THIS YEAR JUST RELEASED
Coverage Areas
1. Diesel engine fundamentals and operation
2. Fuel injection and fuel delivery systems
3. Air intake, turbocharging, and exhaust systems
4. Cooling and lubrication systems
5. Electrical and electronic systems
6. Emissions and aftertreatment systems
7. Brakes and air-brake systems
8. Steering, suspension, drivetrain, and transmission
9. Preventive maintenance and troubleshooting
10. Shop safety, diagnostics, and practical mechanical judgment
1. Which operating principle allows a diesel engine to ignite injected
fuel without using conventional spark plugs?
A. Vacuum ignition
B. Compression ignition
C. Magneto ignition
D. Carburetor ignition
Answer: B. Compression ignition
Rationale: A diesel engine compresses air to a sufficiently high
temperature before injecting fuel, causing the fuel to ignite from
compression heat.
2. During which stroke of a conventional four-stroke diesel engine is
only air normally drawn into the cylinder?
,A. Intake stroke
B. Compression stroke
C. Power stroke
D. Exhaust stroke
Answer: A. Intake stroke
Rationale: During the intake stroke, the intake valve opens while the
piston moves downward, drawing fresh air into the cylinder.
3. What is the primary purpose of the compression stroke in a diesel
engine before fuel injection occurs?
A. Cool the piston
B. Increase air temperature and pressure
C. Remove exhaust gases
D. Reduce cylinder pressure
Answer: B. Increase air temperature and pressure
Rationale: Compressing the intake air substantially raises its pressure
and temperature, creating conditions necessary for compression
ignition.
4. Which component converts the reciprocating movement of the
pistons into rotary movement used to turn the drivetrain?
A. Camshaft
B. Crankshaft
C. Rocker arm
D. Flywheel housing
,Answer: B. Crankshaft
Rationale: The connecting rods transfer piston movement to the
crankshaft, which converts reciprocating motion into continuous rotary
motion.
5. What is the principal function of the camshaft in a four-stroke
diesel engine?
A. Pressurize engine oil
B. Control valve opening and closing
C. Charge the batteries
D. Regulate tire pressure
Answer: B. Control valve opening and closing
Rationale: Camshaft lobes operate the valve train so intake and
exhaust valves open and close at precisely timed intervals.
6. Why are diesel engines generally designed with higher compression
ratios than gasoline engines?
A. Diesel fuel requires spark plugs
B. Higher compression produces ignition temperature
C. Diesel engines do not require lubrication
D. Higher compression eliminates exhaust valves
Answer: B. Higher compression produces ignition temperature
, Rationale: Diesel engines depend on heat generated through
compression to ignite fuel, requiring substantially higher compression
than spark-ignition engines.
7. What is the primary function of piston rings installed around the
piston in a diesel engine?
A. Operate the fuel injector
B. Seal combustion gases and control lubricating oil
C. Open the intake valve
D. Drive the alternator
Answer: B. Seal combustion gases and control lubricating oil
Rationale: Compression rings prevent combustion gases from escaping,
while oil-control rings regulate lubricant remaining on cylinder walls.
8. Which component connects the piston to the crankshaft and
transfers combustion force between these two assemblies?
A. Connecting rod
B. Valve stem
C. Timing gear
D. Pushrod
Answer: A. Connecting rod
Rationale: The connecting rod transfers piston force to the crankshaft
while allowing the piston and crankshaft to move differently.
EXAMINATION COMPLETE QUESTIONS AND DETAILED
SOLUTIONS LATEST UPDATE THIS YEAR JUST RELEASED
Coverage Areas
1. Diesel engine fundamentals and operation
2. Fuel injection and fuel delivery systems
3. Air intake, turbocharging, and exhaust systems
4. Cooling and lubrication systems
5. Electrical and electronic systems
6. Emissions and aftertreatment systems
7. Brakes and air-brake systems
8. Steering, suspension, drivetrain, and transmission
9. Preventive maintenance and troubleshooting
10. Shop safety, diagnostics, and practical mechanical judgment
1. Which operating principle allows a diesel engine to ignite injected
fuel without using conventional spark plugs?
A. Vacuum ignition
B. Compression ignition
C. Magneto ignition
D. Carburetor ignition
Answer: B. Compression ignition
Rationale: A diesel engine compresses air to a sufficiently high
temperature before injecting fuel, causing the fuel to ignite from
compression heat.
2. During which stroke of a conventional four-stroke diesel engine is
only air normally drawn into the cylinder?
,A. Intake stroke
B. Compression stroke
C. Power stroke
D. Exhaust stroke
Answer: A. Intake stroke
Rationale: During the intake stroke, the intake valve opens while the
piston moves downward, drawing fresh air into the cylinder.
3. What is the primary purpose of the compression stroke in a diesel
engine before fuel injection occurs?
A. Cool the piston
B. Increase air temperature and pressure
C. Remove exhaust gases
D. Reduce cylinder pressure
Answer: B. Increase air temperature and pressure
Rationale: Compressing the intake air substantially raises its pressure
and temperature, creating conditions necessary for compression
ignition.
4. Which component converts the reciprocating movement of the
pistons into rotary movement used to turn the drivetrain?
A. Camshaft
B. Crankshaft
C. Rocker arm
D. Flywheel housing
,Answer: B. Crankshaft
Rationale: The connecting rods transfer piston movement to the
crankshaft, which converts reciprocating motion into continuous rotary
motion.
5. What is the principal function of the camshaft in a four-stroke
diesel engine?
A. Pressurize engine oil
B. Control valve opening and closing
C. Charge the batteries
D. Regulate tire pressure
Answer: B. Control valve opening and closing
Rationale: Camshaft lobes operate the valve train so intake and
exhaust valves open and close at precisely timed intervals.
6. Why are diesel engines generally designed with higher compression
ratios than gasoline engines?
A. Diesel fuel requires spark plugs
B. Higher compression produces ignition temperature
C. Diesel engines do not require lubrication
D. Higher compression eliminates exhaust valves
Answer: B. Higher compression produces ignition temperature
, Rationale: Diesel engines depend on heat generated through
compression to ignite fuel, requiring substantially higher compression
than spark-ignition engines.
7. What is the primary function of piston rings installed around the
piston in a diesel engine?
A. Operate the fuel injector
B. Seal combustion gases and control lubricating oil
C. Open the intake valve
D. Drive the alternator
Answer: B. Seal combustion gases and control lubricating oil
Rationale: Compression rings prevent combustion gases from escaping,
while oil-control rings regulate lubricant remaining on cylinder walls.
8. Which component connects the piston to the crankshaft and
transfers combustion force between these two assemblies?
A. Connecting rod
B. Valve stem
C. Timing gear
D. Pushrod
Answer: A. Connecting rod
Rationale: The connecting rod transfers piston force to the crankshaft
while allowing the piston and crankshaft to move differently.
