ic engine

1. 1
INTERNAL COMBUSTION ENGINE AND RECIPROCATING MACHINE
SOLUTION FOR MID EXAM, December 2011
1. Describe the major functions of the following reciprocating engine components:
Connecting rods, Crankshaft, Piston ring, Glow plug, and Camshaft.
Answer:
Function of connecting rod
 Connects the piston to the crankshaft
 Converts reciprocating piston motion to rotary motion at the crankshaft
Crankshaft
 Works with connecting rod to change reciprocating motion of the piston to rotary
motion
 Transmits mechanical energy from the engine to drives camshafts, generator,
pumps, etc.
Piston rings
 Compression ring forms a seal between the piston and cylinder walls and to
restrict the high-pressure gases in the combustion chamber from leaking past the
piston into the crankcase (blowby)
 Oil ring assists in lubricating the cylinder walls and scrapes away excess oil to
reduce oil consumption
 Transfer heat from the piston to the cylinder wall
Glow plug
 It is used to preheat the chamber enough so that combustion will occur when
first starting a cold engine. The glow plug is turned off after the engine is
started.
Camshaft
 Rotating shaft used to push open valves at the proper time in the engine cycle

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2. List five important differences between the design and operating characteristics of SI
engines and CI engines.
Answer:
Description SI Engine CI Engine
Basic Cycle Otto Cycle Heat addition @
Constant volume
Diesel cycle, Heat addition @ constant
pressure
Fuel Gasoline, Highly volatile fuel,
self ignition temp is high
Diesel oil, non volatile fuel, self-ignition
temp is comparatively low
Introduction of
Fuel
Fuel-air mixture introduced
during suction stroke (Carburetor
is necessary)
Fuel directly injected to the combustion
chamber at high pressure (fuel pump and
injector is necessary)
Load control Throttle controls the quantity of
mixture introduce
The quantity of fuel is regulated in the
pump. Air quantity is not control
Thermal
Efficiency
Lower thermal Efficiency (lower
compression ratio)
Higher thermal efficiency (higher
compression ratio)
Ignition Required an ignition system with
spark plug
Ignition system & sparkplug are not
necessary
Compression
ratio
6-10, upper limit is fixed by anti
knock quantity of fuel
16-20 upper limit is limited by weight
increase of the engine
Speed They are high speed engines
(light weight & homogeneous
combustion)
They are low speed engines ( heavy weight
& heterogeneous combustion)
3 Suggest reasons why multi-cylinder engine prove more attractive than single cylinder
engines once the total engine displaced volume exceeds a hundred cubic centimeters
Answer:
 As the rated power increases the advantages of smaller cylinders in regard to
size, weight, improved engine balance and smoothness point toward
increasing the number of cylinders per engine
 The increased frequency of power strokes with a multi cylinder engine
produces much smoother torque characteristics
 Multi cylinder engines can also achieve a much better state of balance than
single cylinder engines

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4. Explain at least four important differences between design and operating characteristics of
two stroke and four stroke IC engines
Answer:
4-Stroke Engines 2-Stroke Engine
The cycle completed in 4 strokes of
piston or 2-revolution of crankshaft
The cycle is completed in two strokes of the piston or one
power strokes obtained in every revolution. of crankshaft
Turning moment is not uniform &
hence a heavier flywheel is needed
Turning moment is more uniform & hence a lighter
flywheel can be used
The power produced for the same
size engine is less (2 revolution
gives 1 power)
Power produced for the same size of engine is more
(Theoretically twice, actually 1.3 times) due to 1 Power
stroke in 1 revolution. Of Crank shaft)
Lesser cooling & lubrication
requirement, Lower rate of wear &
tear
Greater cooling & lubrication requirements
Higher rate of wear & tear
Contains valve & Valve
mechanisms to open & close valves
No valves but ports (some 2-s engines are fitted with
exhaust valves or reed valve)
The initial cost is very high (Coz of
heavy wt. & complicated valve
mechanism)
Initial cost of the engine is less (Coz of light wt. &
simplicity)
Volumetric Efficiency is more
(Coz of more time for induction)
Volumetric Efficiency is low (Coz of lesser time for
induction)
Thermal Efficiency is higher, part
load efficiency is better
Thermal Efficiency is less, part load efficiency is poor
compared 4-s engine
Used where Efficiency is important
( in Cars, Buses, aero planes, etc…
Used where low cost, compactness & Light Wt. are
important (in Scooters, Motorcycles
5. The indicated thermal efficiency for an ideal air standard Otto cycle is as large as 60%
operating with rc = 8, while for the actual cycle operating with the same compression ratio
the brake thermal efficiency is around 30%. Mention five reasons for these variations in
thermal efficiency. Also with the help of PV diagram show the indicated work losses in an
actual cycle relative to fuel air cycle.
Answer:
i. Loss due to variation of specific heat and chemical equilibrium
ii. Burning time loss
iii. Incomplete combustion loss

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iv. Direct heat loss
v. Exhaust blowdown loss
vi. Pumping loss
vii. Rubbing friction loss
6. With the help of PV diagram indicate the successive strokes in an actual petrol engine cycle.
Label approximately the starting and ending points for each stroke.
Answer:
i. For engine fitted with turbocharger or supercharger
 Engines with superchargers or turbochargers can have intake pressures greater
than the exhaust pressure, giving a positive pump work

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ii. For engine working at part throttle
iii. For engine working unthrottled
7 Briefly describe the variation of brake specific fuel consumption (bsfc) with
Answer:
i. Engine size
 Brake specific fuel consumption generally decreases with engine size, being
best (lowest) for very large engines.
 One reason for this is less heat loss due to the higher volume to surface area
ratio of the combustion chamber in large engines.
 Also large engines operate at lower speeds which reduce friction losses

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ii. Speed
 Brake specific fuel consumption decreases as engine speed increases, reaches a
minimum, and then increases at high speeds.
 Fuel consumption increases at high speeds because of greater friction losses.
 At low engine speed, the longer time per cycle allows more heat loss and fuel
consumption goes up.

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8 What is the term mean effective pressure in the analysis of IC engine performance? List
the possible thermodynamic and engine parameters that affect mean effective pressure
and explain how these parameter affect the mean effective pressures.
Answer:
 MEP is a fictitious pressure that, if acted on the piston during the entire power
stroke, would produce the same amount of net work as that produced during
the actual cycle
 Mean effective pressure (mep) is the work done per unit displacement volume.
mep = Wnet/Vd
Mep/p1 is a function of
Heat added, initial temperature, compression ratio and properties of air, namely, Cv and k
 Increasing the average heat addition increase the mean effective pressure
 Decreasing the inlet temperature increasing the mean effective pressure
9 Before an engine is getting overhaul maintenance there are at least two standard engine
tests that are recommended to be performed so as to know the type and level of engine
wearing. What are these two well known engine test? How one is differing from another?
Answer:
 Compression test
 Compression test indicates whether there are wearing or not mainly on piston
rings, pistons and cylinder liner. If there is appreciable wearing of piston rings
during compression test the peak pressure in the cylinder will drop out from the
designed maximum value
 Oil test
 Oil test is performed on the lubricating oil so as to identify whether there are
excessive wearing on many parts of internal combustion engine. The part that is
subjected to excessive wear can be identified from analysis made on the oil.
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10. What is the effect of opening the exhaust valve too early say at 7 bar or too late at BDC
during the expansion stroke? Just briefly explain how this affects the power output.
Answer:
 If the exhaust valve is opened at BDC, the piston has to do work against high
cylinder pressure during the early part of the exhaust stroke
 If the exhaust valve is opened too early, a part of the expansion stroke is lost
 The best compromise is to open the exhaust valve 400
to 700
before BDC thereby
reducing the cylinder pressure to halfway (say 3.5 bar) before the exhaust stroke
begins
11 In SI engine operating at higher speeds like sport car, valve timing for closing the intake
valve is as large as 60o
after BDC while for low speed SI engines the intake valve is
usually closed nearly 10o
after BDC. What is the main reason for this variation in valve
timing for the two cases?
Answer:
 At low engine speeds, the charge is moving into the cylinder relatively slowly,
and its inertia is relatively low. If the intake valve were to remain open much
beyond BDC, the up-coming piston on the compression stroke would tend to
force some of the charge, already in the cylinder back into the intake manifold,
with subsequent reduction in the volumetric efficiency. Hence, the intake valve is
closed relatively early after BDC for slow speed engine.
 High speed engine bring the charge in through the intake manifold at greater
speeds, and the charge has greater inertia. As the piston moves up during the
compression stroke, there is ram effect produced by the incoming mixture which
tends to pack more charge into the cylinder. In the high speed engine, therefore,
the intake valve closing is delayed for a greater period of time after BDC in order
to take advantage of this ram and induct the maximum quantity of charge.
12 Typical value of residual gas fraction is in the range of 3% to 12% in most internal
combustion engine. However, the value of the residual gas fraction in diesel engine is
smaller that of Petrol engine. What do you think the reason behind this? Make reasonable
assumption for your justification.
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P
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9. 9
Where f is the residual gas mass fraction, r is the compression ratio, Pe the exhaust
pressure, P4 is the pressure at the beginning of blowdown process and k is the specific
heat ratio.
Answer:
 For the same value pressure ratio and specific heat ratio in the above expression as
the compression ratio for diesel engine is greater than that of petrol engine
consequently the residual gas fraction decreases for the diesel engine.