1|Page
NYC Auto Mechanic (Diesel) Exam 2026 |
DCAS Exam No. 2105 | Complete 300+
Q&A Practice Test with Detailed
Rationales | Pass First Time Guaranteed
| Updated for 2026 Curriculum | Includes
Diesel Engine, Fuel Systems, Emissions,
Electrical, Brakes, Suspension & Safety
Pass the NYC Auto Mechanic (Diesel) Exam 2026 – DCAS
No. 2105 with confidence!
This comprehensive Q&A guide includes 300+ expertly
crafted multiple-choice questions covering diesel engine
fundamentals, fuel injection systems (common rail, HEUI,
mechanical), electrical & electronic diagnostics, turbochargers
& boost systems, emission controls (DPF, EGR, SCR, DEF),
starting/charging, cooling, lubrication, air brakes, steering,
suspension, driveline, safety procedures, and preventive
maintenance. Every question includes correct answers +
detailed rationales explaining the "why" behind each answer.
Written by industry professionals, this guide mirrors the real
DCAS computer-based test format. Perfect for NYC
Sanitation, MTA, DEP mechanics, and civil service
candidates. Updated for 2026! Boost your score, reduce test
anxiety, and land your city job faster. Download now and start
passing!
,2|Page
1. The component that converts the reciprocating motion of the piston into
rotary motion is the:
A) Camshaft
B) Crankshaft
C) Flywheel
D) Connecting rod
Answer: B) Crankshaft
Rationale: The crankshaft is specifically designed to convert the linear (up-and-down)
motion of the pistons, transmitted via the connecting rods, into rotational motion that
powers the drivetrain.
2. In a four-stroke diesel engine, the correct order of strokes is:
A) Intake, power, compression, exhaust
B) Compression, intake, power, exhaust
C) Intake, compression, power, exhaust
D) Power, intake, compression, exhaust
Answer: C) Intake, compression, power, exhaust
Rationale: The four-stroke cycle must occur in this specific sequence: intake (air enters),
compression (air is heated), power (fuel ignites and forces piston down), and exhaust
(gases are expelled).
3. Diesel engines ignite fuel using:
A) A spark plug
B) Compression heat
C) A glow plug only
D) An electric arc
Answer: B) Compression heat
Rationale: Diesel engines are compression-ignition engines. Air is compressed to a high
ratio, raising its temperature above the autoignition point of diesel fuel, which then
ignites spontaneously when injected.
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4. 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
Answer: C) Piston
Rationale: The piston itself forms the moving seal within the cylinder, transmits the
force of combustion through the wrist pin to the connecting rod, and ultimately drives
the crankshaft.
5. What is the primary function of piston rings?
A) To lubricate the cylinder wall
B) To seal the combustion chamber and control oil
C) To cool the piston
D) To connect the piston to the rod
Answer: B) To seal the combustion chamber and control oil
Rationale: Piston rings provide a gas-tight seal between the piston and cylinder wall
(compression rings), control oil consumption (oil rings), and assist in heat transfer from
the piston to the cylinder wall.
6. The camshaft is responsible for:
A) Delivering fuel to the injectors
B) Opening and closing the valves at the correct time
C) Converting rotary to reciprocating motion
D) Storing rotational energy
Answer: B) Opening and closing the valves at the correct time
Rationale: The camshaft has lobes (cams) that actuate the valve train components to
open and close intake and exhaust valves precisely timed with piston position.
, 4|Page
7. The cylinder head in a diesel engine typically contains all of the following
EXCEPT:
A) Intake and exhaust valves
B) Fuel injectors
C) The crankshaft
D) Glow plugs (in some engines)
Answer: C) The crankshaft
Rationale: The crankshaft is housed in the engine block (crankcase), not the cylinder
head. The cylinder head contains valves, injectors, and sometimes glow plugs.
8. A flywheel's primary function is to:
A) Start the engine
B) Smooth out power pulses between cylinders
C) Drive the camshaft
D) Cool the engine
Answer: B) Smooth out power pulses between cylinders
Rationale: The flywheel stores rotational kinetic energy during the power stroke and
releases it during the other strokes, providing smooth, continuous rotation and
reducing vibration.
9. Which component is the main structural housing for the cylinders and is
typically made of cast iron?
A) Cylinder head
B) Engine block
C) Oil pan
D) Intake manifold
Answer: B) Engine block
Rationale: The engine block is the foundational casting that contains the cylinders,
provides mounting points for other components, and is typically made from cast iron or
aluminum for durability and strength.
NYC Auto Mechanic (Diesel) Exam 2026 |
DCAS Exam No. 2105 | Complete 300+
Q&A Practice Test with Detailed
Rationales | Pass First Time Guaranteed
| Updated for 2026 Curriculum | Includes
Diesel Engine, Fuel Systems, Emissions,
Electrical, Brakes, Suspension & Safety
Pass the NYC Auto Mechanic (Diesel) Exam 2026 – DCAS
No. 2105 with confidence!
This comprehensive Q&A guide includes 300+ expertly
crafted multiple-choice questions covering diesel engine
fundamentals, fuel injection systems (common rail, HEUI,
mechanical), electrical & electronic diagnostics, turbochargers
& boost systems, emission controls (DPF, EGR, SCR, DEF),
starting/charging, cooling, lubrication, air brakes, steering,
suspension, driveline, safety procedures, and preventive
maintenance. Every question includes correct answers +
detailed rationales explaining the "why" behind each answer.
Written by industry professionals, this guide mirrors the real
DCAS computer-based test format. Perfect for NYC
Sanitation, MTA, DEP mechanics, and civil service
candidates. Updated for 2026! Boost your score, reduce test
anxiety, and land your city job faster. Download now and start
passing!
,2|Page
1. The component that converts the reciprocating motion of the piston into
rotary motion is the:
A) Camshaft
B) Crankshaft
C) Flywheel
D) Connecting rod
Answer: B) Crankshaft
Rationale: The crankshaft is specifically designed to convert the linear (up-and-down)
motion of the pistons, transmitted via the connecting rods, into rotational motion that
powers the drivetrain.
2. In a four-stroke diesel engine, the correct order of strokes is:
A) Intake, power, compression, exhaust
B) Compression, intake, power, exhaust
C) Intake, compression, power, exhaust
D) Power, intake, compression, exhaust
Answer: C) Intake, compression, power, exhaust
Rationale: The four-stroke cycle must occur in this specific sequence: intake (air enters),
compression (air is heated), power (fuel ignites and forces piston down), and exhaust
(gases are expelled).
3. Diesel engines ignite fuel using:
A) A spark plug
B) Compression heat
C) A glow plug only
D) An electric arc
Answer: B) Compression heat
Rationale: Diesel engines are compression-ignition engines. Air is compressed to a high
ratio, raising its temperature above the autoignition point of diesel fuel, which then
ignites spontaneously when injected.
,3|Page
4. 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
Answer: C) Piston
Rationale: The piston itself forms the moving seal within the cylinder, transmits the
force of combustion through the wrist pin to the connecting rod, and ultimately drives
the crankshaft.
5. What is the primary function of piston rings?
A) To lubricate the cylinder wall
B) To seal the combustion chamber and control oil
C) To cool the piston
D) To connect the piston to the rod
Answer: B) To seal the combustion chamber and control oil
Rationale: Piston rings provide a gas-tight seal between the piston and cylinder wall
(compression rings), control oil consumption (oil rings), and assist in heat transfer from
the piston to the cylinder wall.
6. The camshaft is responsible for:
A) Delivering fuel to the injectors
B) Opening and closing the valves at the correct time
C) Converting rotary to reciprocating motion
D) Storing rotational energy
Answer: B) Opening and closing the valves at the correct time
Rationale: The camshaft has lobes (cams) that actuate the valve train components to
open and close intake and exhaust valves precisely timed with piston position.
, 4|Page
7. The cylinder head in a diesel engine typically contains all of the following
EXCEPT:
A) Intake and exhaust valves
B) Fuel injectors
C) The crankshaft
D) Glow plugs (in some engines)
Answer: C) The crankshaft
Rationale: The crankshaft is housed in the engine block (crankcase), not the cylinder
head. The cylinder head contains valves, injectors, and sometimes glow plugs.
8. A flywheel's primary function is to:
A) Start the engine
B) Smooth out power pulses between cylinders
C) Drive the camshaft
D) Cool the engine
Answer: B) Smooth out power pulses between cylinders
Rationale: The flywheel stores rotational kinetic energy during the power stroke and
releases it during the other strokes, providing smooth, continuous rotation and
reducing vibration.
9. Which component is the main structural housing for the cylinders and is
typically made of cast iron?
A) Cylinder head
B) Engine block
C) Oil pan
D) Intake manifold
Answer: B) Engine block
Rationale: The engine block is the foundational casting that contains the cylinders,
provides mounting points for other components, and is typically made from cast iron or
aluminum for durability and strength.
