NYC Auto Mechanic (Diesel) Exam – 250 Practice Questions with
Answers & Detailed Rationales – DCAS Exam No. 2105 – Pass on
Your First Attempt
SECTION 1: DIESEL ENGINE FUNDAMENTALS & OPERATION (Questions 1–50)
1. A diesel engine relies on which principle for ignition?
A. Spark ignition
B. Compression ignition
C. Electrical ignition
D. Glow plug ignition
Explanation: Diesel engines operate on compression ignition. Air is drawn into the cylinder
and compressed to a high ratio (typically 14:1 to 25:1), raising the air temperature above the
autoignition point of diesel fuel. When fuel is injected, it ignites spontaneously without a
spark plug .
2. What are the four strokes of a four-stroke diesel engine in correct order?
A. Intake, compression, power, exhaust
B. Intake, power, compression, exhaust
C. Compression, intake, power, exhaust
D. Intake, exhaust, compression, power
,Explanation: The four-stroke cycle follows: Intake (piston down, drawing in air),
Compression (piston up, compressing air and raising temperature), Power (fuel injected and
ignited, forcing piston down), and Exhaust (piston up, pushing out combustion gases) .
3. During the compression stroke of a diesel engine, what occurs?
A. Air and fuel mixture is compressed and ignited by a spark plug
B. Only air is compressed, raising its temperature above the autoignition point of
diesel fuel
C. Exhaust gases are forced out through the open exhaust valve
D. The intake valve opens to allow fresh air into the cylinder
Explanation: Diesel engines compress air alone to high pressures, creating extreme
temperatures; diesel fuel is then injected and ignites spontaneously due to the heat, with no
spark plug .
4. The crankshaft converts the reciprocating motion of the pistons into what type of
motion?
A. Oscillating motion
B. Rotary motion
C. Linear thrust
D. Hydraulic pressure
,Explanation: The crankshaft transforms the up-and-down motion of the pistons into
rotational motion to drive the transmission and accessories .
5. A camshaft in a diesel engine is responsible for:
A. Driving the oil pump
B. Opening and closing the intake and exhaust valves
C. Delivering fuel to the injectors
D. Converting linear motion to rotary motion
Explanation: The camshaft controls the opening and closing of the intake and exhaust
valves, ensuring proper timing of the engine's breathing cycle .
6. The component that seals the combustion chamber and transfers force to the
connecting rod is the:
A. Cylinder head
B. Piston ring
C. Piston
D. Valve
Explanation: The piston seals the combustion chamber via piston rings and transfers the
force of combustion to the connecting rod .
7. Which of the following is a function of piston rings?
A. Transferring torque to the crankshaft
, B. Sealing the combustion chamber, controlling oil, and transferring heat
C. Opening valves at correct timing
D. Injecting fuel into the cylinder
Explanation: Compression rings seal combustion gases, oil control rings scrape excess oil
from cylinder walls, and rings transfer heat from the piston to the cylinder walls .
8. 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
Explanation: 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
.
9. 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
Answers & Detailed Rationales – DCAS Exam No. 2105 – Pass on
Your First Attempt
SECTION 1: DIESEL ENGINE FUNDAMENTALS & OPERATION (Questions 1–50)
1. A diesel engine relies on which principle for ignition?
A. Spark ignition
B. Compression ignition
C. Electrical ignition
D. Glow plug ignition
Explanation: Diesel engines operate on compression ignition. Air is drawn into the cylinder
and compressed to a high ratio (typically 14:1 to 25:1), raising the air temperature above the
autoignition point of diesel fuel. When fuel is injected, it ignites spontaneously without a
spark plug .
2. What are the four strokes of a four-stroke diesel engine in correct order?
A. Intake, compression, power, exhaust
B. Intake, power, compression, exhaust
C. Compression, intake, power, exhaust
D. Intake, exhaust, compression, power
,Explanation: The four-stroke cycle follows: Intake (piston down, drawing in air),
Compression (piston up, compressing air and raising temperature), Power (fuel injected and
ignited, forcing piston down), and Exhaust (piston up, pushing out combustion gases) .
3. During the compression stroke of a diesel engine, what occurs?
A. Air and fuel mixture is compressed and ignited by a spark plug
B. Only air is compressed, raising its temperature above the autoignition point of
diesel fuel
C. Exhaust gases are forced out through the open exhaust valve
D. The intake valve opens to allow fresh air into the cylinder
Explanation: Diesel engines compress air alone to high pressures, creating extreme
temperatures; diesel fuel is then injected and ignites spontaneously due to the heat, with no
spark plug .
4. The crankshaft converts the reciprocating motion of the pistons into what type of
motion?
A. Oscillating motion
B. Rotary motion
C. Linear thrust
D. Hydraulic pressure
,Explanation: The crankshaft transforms the up-and-down motion of the pistons into
rotational motion to drive the transmission and accessories .
5. A camshaft in a diesel engine is responsible for:
A. Driving the oil pump
B. Opening and closing the intake and exhaust valves
C. Delivering fuel to the injectors
D. Converting linear motion to rotary motion
Explanation: The camshaft controls the opening and closing of the intake and exhaust
valves, ensuring proper timing of the engine's breathing cycle .
6. The component that seals the combustion chamber and transfers force to the
connecting rod is the:
A. Cylinder head
B. Piston ring
C. Piston
D. Valve
Explanation: The piston seals the combustion chamber via piston rings and transfers the
force of combustion to the connecting rod .
7. Which of the following is a function of piston rings?
A. Transferring torque to the crankshaft
, B. Sealing the combustion chamber, controlling oil, and transferring heat
C. Opening valves at correct timing
D. Injecting fuel into the cylinder
Explanation: Compression rings seal combustion gases, oil control rings scrape excess oil
from cylinder walls, and rings transfer heat from the piston to the cylinder walls .
8. 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
Explanation: 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
.
9. 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
