NYC Auto Mechanic (Diesel) (DCAS)
PRACTICE EXAM 2 Question with
Answers and Rationales Latest Version
Question 1
What is the main advantage of direct fuel injection in a diesel engine?
A. Fuel is injected directly into the combustion chamber
B. Fuel is mixed with coolant before entering the cylinder
C. Fuel is injected into the exhaust manifold
D. Fuel is injected into the crankcase
Answer: A. Fuel is injected directly into the combustion chamber
Rationale: In a direct-injection diesel engine, fuel is injected directly
into the combustion chamber at precisely controlled timing and pressure.
This promotes efficient combustion and contributes to good power and
fuel economy.
Question 2
What happens during the compression stroke of a diesel engine?
A. Only air is compressed
B. Only fuel is compressed
C. Coolant is compressed
D. Exhaust gas is compressed in the fuel tank
Answer: A. Only air is compressed
,Rationale: In a conventional diesel engine, air is drawn into the cylinder
and compressed to a high pressure and temperature. Fuel is injected near
the end of the compression stroke, where the hot compressed air allows
the fuel to ignite.
Question 3
Which factor is especially important for diesel fuel ignition?
A. Adequate compressed-air temperature
B. Low engine compression
C. Low battery voltage
D. High coolant level
Answer: A. Adequate compressed-air temperature
Rationale: Diesel combustion relies primarily on the heat generated by
compression rather than a spark plug. If compression temperature is
inadequate, starting and combustion can be poor.
Question 4
A diesel engine cranks at normal speed but produces no smoke from the
exhaust during cranking. What may this suggest?
A. Fuel may not be reaching the cylinders
B. The battery is necessarily overcharged
C. The cooling fan is operating normally
D. The brakes are properly adjusted
Answer: A. Fuel may not be reaching the cylinders
Rationale: Under many circumstances, a complete absence of smoke
during cranking can indicate that fuel is not being injected. The
,technician should verify fuel supply, rail pressure, injector command,
engine-speed signals, and control-system operation.
Question 5
What is injection timing?
A. The timing of fuel delivery relative to engine position
B. The time required to change engine oil
C. The time required to inflate a tire
D. The timing of radiator-fan operation only
Answer: A. The timing of fuel delivery relative to engine position
Rationale: Injection timing determines when fuel enters the cylinder in
relation to piston and crankshaft position. Incorrect timing can cause
hard starting, poor power, excessive smoke, noise, and increased
emissions.
Question 6
What could occur if diesel injection timing is significantly incorrect?
A. Poor combustion and reduced engine performance
B. Improved combustion under every condition
C. Increased brake friction
D. Increased tire pressure
Answer: A. Poor combustion and reduced engine performance
Rationale: Diesel engines depend on precisely controlled injection
timing. Incorrect timing can alter combustion pressure and temperature,
resulting in poor performance, smoke, increased fuel consumption, and
possible engine damage.
, Question 7
What is pilot injection in a modern diesel engine?
A. A small fuel injection event occurring before the main injection
B. Fuel injected into the radiator
C. Fuel returned directly to the crankcase
D. Coolant injected into the intake
Answer: A. A small fuel injection event occurring before the main
injection
Rationale: Pilot injection can help control combustion noise, pressure
rise, emissions, and combustion characteristics. The exact injection
strategy depends on the engine's control system.
Question 8
What is post-injection?
A. Fuel injection occurring after the main injection event
B. Fuel injection before the engine starts
C. Coolant injection into the fuel rail
D. Oil injection into the radiator
Answer: A. Fuel injection occurring after the main injection event
Rationale: Post-injection may be used for emissions-control purposes,
including helping increase exhaust temperature during certain DPF
regeneration strategies.
Question 9
PRACTICE EXAM 2 Question with
Answers and Rationales Latest Version
Question 1
What is the main advantage of direct fuel injection in a diesel engine?
A. Fuel is injected directly into the combustion chamber
B. Fuel is mixed with coolant before entering the cylinder
C. Fuel is injected into the exhaust manifold
D. Fuel is injected into the crankcase
Answer: A. Fuel is injected directly into the combustion chamber
Rationale: In a direct-injection diesel engine, fuel is injected directly
into the combustion chamber at precisely controlled timing and pressure.
This promotes efficient combustion and contributes to good power and
fuel economy.
Question 2
What happens during the compression stroke of a diesel engine?
A. Only air is compressed
B. Only fuel is compressed
C. Coolant is compressed
D. Exhaust gas is compressed in the fuel tank
Answer: A. Only air is compressed
,Rationale: In a conventional diesel engine, air is drawn into the cylinder
and compressed to a high pressure and temperature. Fuel is injected near
the end of the compression stroke, where the hot compressed air allows
the fuel to ignite.
Question 3
Which factor is especially important for diesel fuel ignition?
A. Adequate compressed-air temperature
B. Low engine compression
C. Low battery voltage
D. High coolant level
Answer: A. Adequate compressed-air temperature
Rationale: Diesel combustion relies primarily on the heat generated by
compression rather than a spark plug. If compression temperature is
inadequate, starting and combustion can be poor.
Question 4
A diesel engine cranks at normal speed but produces no smoke from the
exhaust during cranking. What may this suggest?
A. Fuel may not be reaching the cylinders
B. The battery is necessarily overcharged
C. The cooling fan is operating normally
D. The brakes are properly adjusted
Answer: A. Fuel may not be reaching the cylinders
Rationale: Under many circumstances, a complete absence of smoke
during cranking can indicate that fuel is not being injected. The
,technician should verify fuel supply, rail pressure, injector command,
engine-speed signals, and control-system operation.
Question 5
What is injection timing?
A. The timing of fuel delivery relative to engine position
B. The time required to change engine oil
C. The time required to inflate a tire
D. The timing of radiator-fan operation only
Answer: A. The timing of fuel delivery relative to engine position
Rationale: Injection timing determines when fuel enters the cylinder in
relation to piston and crankshaft position. Incorrect timing can cause
hard starting, poor power, excessive smoke, noise, and increased
emissions.
Question 6
What could occur if diesel injection timing is significantly incorrect?
A. Poor combustion and reduced engine performance
B. Improved combustion under every condition
C. Increased brake friction
D. Increased tire pressure
Answer: A. Poor combustion and reduced engine performance
Rationale: Diesel engines depend on precisely controlled injection
timing. Incorrect timing can alter combustion pressure and temperature,
resulting in poor performance, smoke, increased fuel consumption, and
possible engine damage.
, Question 7
What is pilot injection in a modern diesel engine?
A. A small fuel injection event occurring before the main injection
B. Fuel injected into the radiator
C. Fuel returned directly to the crankcase
D. Coolant injected into the intake
Answer: A. A small fuel injection event occurring before the main
injection
Rationale: Pilot injection can help control combustion noise, pressure
rise, emissions, and combustion characteristics. The exact injection
strategy depends on the engine's control system.
Question 8
What is post-injection?
A. Fuel injection occurring after the main injection event
B. Fuel injection before the engine starts
C. Coolant injection into the fuel rail
D. Oil injection into the radiator
Answer: A. Fuel injection occurring after the main injection event
Rationale: Post-injection may be used for emissions-control purposes,
including helping increase exhaust temperature during certain DPF
regeneration strategies.
Question 9
