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PAKISTAN NAVY ENGINEERING COLLEGE – NUST
FIRST ONE-HOURLY EXAMINATIONS SPRING 2025
Subject: Internal Combustion Engines
Credit Hours: 3-0
Course Code: ME 439
Instructor: Khurram Jamal Hashmi
Semester: VIII (ME) B1, B2 & C
Discipline: Mechanical Engineering
Time Allowed: 01 hour
Date of Exam: March 3, 2025
Maximum Marks: 100
Marks Obtained:
Q. No.
CLO
PLO
BT
1.
CLO-2
PLO-3
C3
Marks
Q. No.
CLO
PLO
BT
2.
CLO-1
PLO-1
C2
Marks
____________________ DO NOT WRITE ABOVE THIS LINE _____________________
Name: _________________________
NUST Registration No.: _________________________
Note:
a) This question paper carries two questions in all spanned over two pages.
b) Students are to attempt all questions. Answers are to be given on space provided below
each question. If more space is required use back of question paper or a ‘B’ copy. In these
cases, clearly mark the question number for which answer is given.
c) It is a closed notes / closed book test. Formula sheets are not allowed.
d) All answers must be backed with reasons and/or calculations as required. Answers without
reasons/calculations or completely incorrect reasons/calculations will be awarded zero
marks.
e) Relevant CLO - PLO are indicated with questions.
1.
A four-cylinder automotive spark ignition engine is being designed to provide a
maximum brake torque of 180 N m at 3200 rev/min. Estimate required engine
displacement, bore and stroke, and maximum brake power the engine will deliver.
Assume brake mean effective pressure at maximum brake torque is 1000 kPa, brake
mean effective pressure at maximum brake power is 800 kPa, bore is equal to 0.8
times the stroke and maximum mean piston speed is 15 m/s.
[CLO 2 – PLO 3]
Solution
For engine displacement:
VE = [Ẇb / (N/n)] / bmep = [2πNτ / (N/n)] / bmep = 2 π n τ / bmep
V = 2π (2) 180 / (1000 x 103) = 2.26 x 10-3m3 or 2.26 L
E
For bore and stroke:
VE = NC Vd = NC (0.25πB2S)
(50)
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Name: _____________________________________
NUST Registration Number: ____________________
S = [(2.26 x 10-3) / {(4) (0.25π) (0.8)2}]1/3 = 0.104 m or 104 mm
B = 0.8 (0.104) = 0.0832 m or 83.2 mm
For maximum brake power:
Ẇb,max = VE (N@Ẇb,max /n) bmep@Ẇb,max
N@Pbmax can be estimated from maximum piston speed:
N@Pbmax = ŨP / (2 S) = 15 / (2 x 0.104) = 72.1 m/s
Ẇ , = (2.26 x 10-3) (72.1/2) (800 x 103) = 65.2 kW
b max
(50)
2.
[CLO 1 – PLO 1]
a. In the engine above label: piston, connecting rod, crankshaft, cam, camshaft,
combustion chamber (cylinder), cylinder block, cylinder head, intake valve, exhaust
valve, valve spring and crank case.
b. Is this a spark-ignition engine or a compression ignition engine? Is this a
four-stroke engine or a two-stroke engine? Give reasons for your answers.
Answer:
This is a four-stroke engine as intake and exhaust valves along with cam
arrangement is visible in the figure of the engine. Also, spark plugs can be seen in
the cut section. Thus, it is a spark ignition engine.
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Name: _____________________________________
NUST Registration Number: ____________________
c. Describe the working cycle of this type of engine.
Answer:
This is a four-stroke spark ignition engine. The operating cycle consists of the
following processes:
Intake stroke: Starts with the piston at the top dead center and ends with the piston
at the bottom dead center with open intake valve. The movement of piston draws
fresh charge into the cylinder (combustion chamber). In the ideal cycle, intake valve
opens at top dead center and closes at bottom dead center. To increase inducted
mass of charge, in real cycles, intake valve opens shortly before piston reaches top
dead center and closes shortly after the piston moves away from the bottom dead
center. The exhaust valve closes just after the piston moves away from the top dead
center. The period when both valves are open is known as valve overlap.
Compression stroke: Starts with the piston at the bottom dead center (at end of
intake stroke) and ends with the piston at the top dead center. In case of, ideal cycle,
both valves remain close during the stroke. In real cycle intake valve closes shortly
after the start of the intake stroke. As the space of the combustion chamber
decreases, the charge (air-fuel mixture) is compressed to a fraction of original
volume resulting in increased pressure and temperature. Shortly before the piston
reaches the top dead center, spark is initiated resulting in combustion. This further
increases pressure and temperature towards the end of the compression stroke.
Expansion or Power stroke: Starts with the piston at the top dead center (at end
of compression stroke) and ends with the piston at the bottom dead center. The
combustion process (that was initiated shortly before the piston reaches the top
dead center in the compression stroke) continues for a short duration after the top
dead center. This creates high-temperature and high-pressure expanding gas in the
cylinder. The expanding gas pushes the piston, providing power for running the
engine and external load. The intake valve remains close during the stroke. Exhaust
valve opens towards the end of the stroke. Because of the high pressure of
combustion products, the gases are exhausted out through exhaust valve. This
process is known as blowdown exhaust. In the ideal cycle, exhaust valve opens at
bottom dead center.
Exhaust stroke: Starts with the piston at the bottom dead center (at end of
expansion stroke) and ends with the piston at the top dead center. The combustion
products are forced out of the system by movement of piston towards the exhaust
valve (displacement exhaust). The intake valve opens towards the end of the
stroke. The exhaust valve closes just after the piston moves away from the top dead
center. In the ideal cycle, the intake valve opens and the exhaust valve closes at the
top dead center.