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NYC Auto Mechanic (Diesel) Exam Prep 2026 | 2027 DCAS Exam No. 2105 |
Original Practice Questions with Answers & Rationales
Exam Overview
Category Description
Exam Title Auto Mechanic (Diesel)
Exam Number DCAS Exam No. 2105
Administering NYC Department of Citywide Administrative Servic
Agency (DCAS)
Exam Blueprint
SET 1: Diesel Engine Fundamentals & Operation (Questions 1–
25)
1. In a four-stroke diesel engine, what is the correct sequence
of strokes?
A. Intake, Compression, Power, Exhaust
B. Intake, Power, Compression, Exhaust
C. Compression, Intake, Power, Exhaust
D. Power, Intake, Compression, Exhaust
,https://www.stuvia.com/user/performance
Answer: A
Rationale: The four-stroke cycle is Intake (air enters),
Compression (air is compressed and heated), Power (fuel is
injected and ignited), and Exhaust (gases expelled).
2. 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.
3. 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
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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.
4. 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
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.
5. 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
, https://www.stuvia.com/user/performance
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.
6. 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.
7. 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
NYC Auto Mechanic (Diesel) Exam Prep 2026 | 2027 DCAS Exam No. 2105 |
Original Practice Questions with Answers & Rationales
Exam Overview
Category Description
Exam Title Auto Mechanic (Diesel)
Exam Number DCAS Exam No. 2105
Administering NYC Department of Citywide Administrative Servic
Agency (DCAS)
Exam Blueprint
SET 1: Diesel Engine Fundamentals & Operation (Questions 1–
25)
1. In a four-stroke diesel engine, what is the correct sequence
of strokes?
A. Intake, Compression, Power, Exhaust
B. Intake, Power, Compression, Exhaust
C. Compression, Intake, Power, Exhaust
D. Power, Intake, Compression, Exhaust
,https://www.stuvia.com/user/performance
Answer: A
Rationale: The four-stroke cycle is Intake (air enters),
Compression (air is compressed and heated), Power (fuel is
injected and ignited), and Exhaust (gases expelled).
2. 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.
3. 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
,https://www.stuvia.com/user/performance
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.
4. 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
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.
5. 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
, https://www.stuvia.com/user/performance
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.
6. 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.
7. 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
