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Transcript of 02-Engine Operation and Testing
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Engine Operation and Testing
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SI Engine Operation
The minimum cylinder volume is called the clearance
volume, Vc
The volume swept out by the piston, the
difference between the maximum or totalvolume, Vt and the clearance volume is
called the displaced or swept volume, Vd
The ratio of maximum volume to minimum
volume is the compression ratio, r c
Typical values of r c are 8 to 12 for SI
engines and 12 to 24 for CI engines
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SI Engine Operation (4-Stroke Cycle)
• Intake Stroke or Induction
– The piston travels from TDC to BDC with the intake valve
open and exhaust valve closed
– This creates an increasing volume in the combustion
chamber, which in turn creates a vacuum
– The resulting pressure differential through the intake
system from atmospheric pressure on the outside to the
vacuum on the inside causes air to be pushed into the
cylinder – As the air passes through the intake system, fuel is added
to it in the desired amount by means of fuel injectors or a
carburetor
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SI Engine Operation (4-Stroke Cycle)
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SI Engine Operation (4-Stroke Cycle)
• Compression Stroke
– When the piston reaches BDC, the intake valve closes and
the piston travels back to TDC with all valves closed.
– This compresses the air-fuel mixture, raising both the
pressure and temperature in the cylinder
– The finite time required to close the intake valve means
that actual compression doesn't start until sometime aBDC
– Near the end of the compression stroke, the spark plug isfired and combustion is initiated
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SI Engine Operation (4-Stroke Cycle)
• Combustion
– Combustion of the air-fuel mixture occurs in a very short
but finite length of time with the piston near TDC (i.e.,
nearly constant-volume combustion)
– It starts near the end of the compression stroke slightly
bTDC and lasts into the power stroke slightly aTDC
– Combustion changes the composition of the gas mixture to
that of exhaust products and increases the temperature in
the cylinder to a very high peak value – This, in turn, raises the pressure in the cylinder to a very
high peak value
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SI Engine Operation (4-Stroke Cycle)
• Expansion Stroke or Power Stroke
– With all valves closed, the high pressure created by the
combustion process pushes the piston away from TDC
– This is the stroke which produces the work output of the
engine cycle
– As the piston travels from TDC to BDC, cylinder volume is
increased, causing pressure and temperature to drop
• Exhaust Blowdown
– Late in the power stroke, the exhaust valve is opened andexhaust blow down occurs
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SI Engine Operation (4-Stroke Cycle)
– Pressure and temperature in the cylinder are still high
relative to the surroundings at this point, and a pressure
differential is created through the exhaust system which is
open to atmospheric pressure
– This pressure differential causes much of the hot exhaustgas to be pushed out of the cylinder and through the
exhaust system when the piston is near BDC
– This exhaust gas carries away a high amount of enthalpy,
which lowers the cycle thermal efficiency
– Opening the exhaust valve before BDC reduces the work
obtained during the power stroke but is required because
of the finite time needed for exhaust blowdown
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SI Engine Operation (4-Stroke Cycle)
• Exhaust Stroke
– By the time the piston reaches BDC, exhaust blowdown is
complete, but the cylinder is still full of exhaust gases at
approximately atmospheric pressure
– With the exhaust valve remaining open, the piston now
travels from BDC to TDC in the exhaust stroke
– This pushes most of the remaining exhaust gases out of
the cylinder into the exhaust system at about atmospheric
pressure, leaving only that trapped in the clearancevolume when the piston reaches TDC
– Near the end of the exhaust stroke bTDC, the intake valve
starts to open, so that it is fully open by TDC when the new
intake stroke starts the next cycle9
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SI Engine Operation (4-Stroke Cycle)
(a)Intake stroke
(b)Compression stroke
(c)Combustion (ignition)
(d)Power or expansion
stroke
(e)Exhaust blowdown(f) Exhaust stroke
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SI Engine Operation (2-stroke Cycle)
• Combustion
– With the piston at TDC combustion occurs very quickly,
raising the temperature and pressure to peak values,
almost at constant volume
• Expansion Stroke or Power Stroke
– Very high pressure created by the combustion process
forces the piston down in the power stroke
– The expanding volume of the combustion chamber causes
pressure and temperature to decrease as the pistontravels towards BDC
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SI Engine Operation (2-stroke Cycle)
– Fuel is added to the air with either a carburetor or fuel
injection
– This incoming mixture pushes much of the remaining
exhaust gases out the open exhaust valve and fills the
cylinder with a combustible air-fuel mixture, a processcalled scavenging
– The piston passes BDC and very quickly covers the intake
port and then the exhaust port (or the exhaust valve
closes)
– The higher pressure at which the air enters the cylinder is
established in one of two ways
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SI Engine Operation (2-stroke Cycle)
– Large two-stroke cycle engines generally have a
supercharger, while small engines will intake the air
through the crankcase
– On these engines the crankcase is designed to serve as a
compressor in addition to serving its normal function• Compression Stroke
– With all valves (or ports) closed, the piston travels towards
TDC and compresses the air-fuel mixture to a higher
pressure and temperature – Near the end of the compression stroke, the spark plug is
fired; by the time the piston gets to IDC, combustion
occurs and the next engine cycle begins
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CI Engine Operation (4-Stroke Cycle)
• Intake Stroke
– The same as the intake stroke in an SI engine with one
major difference: no fuel is added to the incoming air
• Compression Stroke
– The same as in an SI engine except that only air is
compressed and compression is to higher pressures and
temperature
– Late in the compression stroke fuel is injected directly into
the combustion chamber, where it mixes with the very hotair
– This causes the fuel to evaporate and self-ignite, causing
combustion to start
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CI Engine Operation (4-Stroke Cycle)
• Combustion
– Combustion is fully developed by TDC and continues at
about constant pressure until fuel injection is complete and
the piston has started towards BDC
• Power Stroke
– The power stroke continues as combustion ends and the
piston travels towards BDC
• Exhaust Blowdown
– Same as with an SI engine
• Exhaust Stroke
– Same as with an SI engine
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CI Engine Operation (2-Stroke Cycle)
• The two-stroke cycle for a CI engine is similar to that of
the SI engine, except for two changes
– No fuel is added to the incoming air, so that compression
is done on air only
– Instead of a spark plug, a fuel injector is located in the
cylinder
– Near the end of the compression stroke, fuel is injected
into the hot compressed air and combustion is initiated byself-ignition
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SI Engine Construction
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SI Engine Construction
• Piston and connecting rod assembly
– Pistons are made of aluminium, cast steel, or iron and may
be full-skirt, or slipper type
• Skirts bear the side thrust
– Pistons are fitted with at least three rings – the upper two
rings are compression rings and the lower ring is the oil
ring
• Compression rings contain the compression and combustion
pressures and prevent blowby• Oil ring scrapes off excess oil from the cylinder wall
– Connecting rods are forged-steel with I-beam sections –
one end connects the crankshaft and the other the piston
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SI Engine Construction
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SI Engine Construction
• Valve mechanism
– Consists of camshaft, driven by the crankshaft, tappet or
valve lifter, pushrod, rocker arm, etc.
• Camshafts are driven by toothed belt, gears, or inverted
chains – termed timing belts or gears• Many engines use overhead camshafts and multiple
camshafts
– Intake valves are made of Cr-Ni alloy steel and the
exhaust valves from silchrome alloy
• Some valves have hollow stems or are partially filled with
liquid sodium for better heat transfer
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SI Engine Construction
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Engine Testing
• Testing of ICEs is an important part of research,
development and teaching
• Engine tests are performed to –
– find out performance before mass production and fitting it
into a vehicle
– improve the design and configuration, to integrate new
materials and technology
– find out the power and fuel consumption, also to test
effectiveness of cooling, vibration and noise, lubrication,controllability, etc.
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Basic Instrumentation for Engine Test
• Power/torque measurement
• Engine speed measurement
• Air flow rate measurement• Fuel flow rate measurement
• Test Equipment and Instruments
– Emission equipment, Thermocouples, Pressuretransducers (in cylinder measurement), Turbine flow
meters, Smoke measurement, Fuel measurement, Blow-
by measurement, Air flow measurement
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Engine Testing
• The fundamental output of the engine is engine torque,
usually expressed in N-m
• Torque/power is measured by a dynamometer or an ‘in-
line’ device
• The principle is rather simple – typically the engine
flywheel has a band of friction material around its
circumference, and the torque reaction on the friction
material corresponds to the torque output of the engine
• The term Brake Horse Power (bhp) derives from the
simplest form of engine dynamometer, the friction brake
(or Prony brake)
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Dynamometer Principle
Rope-Brake Dyno
Power Measurement
P = T ω = 2π (RPM/60) Wr
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Early Dynamometers
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Torque Measurement
torque (T) = restraining force (F) × radius of moment arm (r)
power (P) = torque (T) × angular speed (w)
angular speed (w) = 2π × engine speed (N rev/s)
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Dynamometer
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Chassis Dynamometer
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Dynamometer Types
• Dynamometers can be classified by the type of
absorption unit or absorber/driver that they use
• Some of these are as follows:
– Eddy current or electromagnetic brake (absorption)
• used in modern chassis dynos
• provide the quick load change rate for rapid load settling
• air cooled, but some are designed to require external water
cooling systems
• require an electrically conductive core, shaft or disc, movingacross a magnetic field to produce resistance to movement
• use variable electromagnets to change the magnetic field
strength to control the amount of braking
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Dynamometer Types
– Magnetic Powder brake (absorption)
• similar to an eddy current dynamometer, but a fine magnetic
powder is placed in the air gap between the rotor and the coil
• The resulting flux lines create "chains" of metal particulate
that are constantly built and broken apart during rotationcreating great torque
• typically limited to lower RPM due to heat dissipation issues
– Hysteresis Brake dynamometers (absorption)
• use a steel rotor that is moved through flux lines generated
between magnetic pole pieces
• as in the usual "disc type" eddy current absorbers, allows for
full torque to be produced at zero speed, as well as at full
speed
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Dynamometer Types
• Hysteresis and "disc type" EC dynamometers are one of the
most efficient technologies in small (200 hp (150 kW) and
less) dynamometers
• A hysteresis brake is an eddy current absorber that, unlikemost "disc type" eddy current absorbers, puts the
electromagnet coils inside a vented and ribbed cylinder and
rotates the cylinder, instead of rotating a disc between
electromagnets
• The potential benefit for the hysteresis absorber is that the
diameter can be decreased and operating RPM of the
absorber may be increased
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Dynamometer Types
– Hydraulic brake (absorption)
• consists of a hydraulic pump (usually a gear type pump), a
fluid reservoir and piping between the two parts
• the fluid used was hydraulic oil, but recent synthetic multi-
grade oils may be a better choice• the engine is brought up to the desired RPM and the valve is
incrementally closed and as the pumps outlet is restricted,
the load increases and the throttle is simply opened until at
the desired throttle opening
• power is calculated by factoring flow volume (calculated frompump design specs), hydraulic pressure and RPM
• renowned for having the absolute quickest load change
ability, just slightly surpassing the eddy current absorbers
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Froude Water Brake Dynamometer
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Dynamometer Types
– Compound dyno (usually an absorption dyno in tandem
with an electric/motoring dyno)
• Torque measurement is somewhat complicated since there
are two machines in tandem; an inline torque transducer is
the preferred method of torque measurement in this case
• An eddy-current or waterbrake dynamometer with electronic
control combined with a variable frequency drive and AC
induction motor is a commonly used configuration of this type
• Disadvantages include requiring a second set of test cell
services (electrical power and cooling), and a slightly morecomplicated control system
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Fuel Consumption Measurement
• Orifice-type flow meter gives instantaneous readings, butis less accurate
• Automatic flow meter adopts both of these
• A continuous measurement system employs a hydraulic
equivalent of aWheatstone bridge
• Another system uses
Coriolis acceleration
principle to measurefuel flow rate
It uses a U-shaped tube
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Temperature and Pressure Measurements
• Mercury-in-glass thermometers and thermocouples both
provide economical means of measuring temperature,
with the potential of achieving and accuracy of about
0.1 K – same level of accuracy can be obtained from
platinum resistance thermometer • Bourdon pressure gauges and manometers provide a
cheap and accurate means of measuring steady
pressures
– Pressures in the range of 1 – 100 kN/m2 can be measuredwith an accuracy of about 1 percent
• Pressure transducers utilise a piezo-resistive effect
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In-cylinder Pressure Measurement
• The simplest form of engine indicator is the DobbieMcInnes mechanical indicator
• The area of the diagram
corresponds to the indicated
work per cylinder • imep = k hd = k (Ad/ld)
where Ad = diagram area
ld = diagram length
hd = mean height of diagram• Mechanical indicators can only be used at speeds of up
to 600 rpm
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In-cylinder Pressure Measurement
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In-cylinder Pressure Measurement
• The location of tdc is not straightforward because of
– the finite stiffness of the crank-slider mechanism
– the flexibility of the crankshaft is such that at full load there
can be about 1 twist at certain speeds
• To convert the time base to a piston displacement base
it is usual to assume constant angular velocity
throughout each revolution
• The output from an oscilloscope can be plotted in an x-yplotter
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In-cylinder Pressure Measurement
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In-cylinder Pressure Measurement
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Techniques for Estimating Indicated Power
• Very often a pressure transducer cannot be readily fittedto an engine, so alternative means of deducing imep areuseful
• The difference between indicated power and brake
power is the power absorbed by friction – often assumedto be dependent only on engine speed
• However, the friction power also depends on theindicated power since the increased gas pressurescause increases in piston friction
• If the friction power is assumed to be independent of theindicated power, then the friction power can be deducedfrom the Morse test
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Techniques for Estimating Indicated Power
– This is applicable only to multi-cylinder engines, as eachcylinder is disabled in turn
– When a cylinder is disabled the load is reduced so that the
engine returns to the test speed; the reduction in power
corresponds to the indicated power of that cylinder
For a n-cylinder engine
indicated power - friction power = (brake power)n
With one cylinder disabled
indicated power - friction power = (brake power)n – 1
Subtracting:indicated power of disabled cylinder = reduction in brake power
n
n
n 1
n
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Techniques for Estimating Indicated Power
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Techniques for Estimating Indicated Power
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Estimating AF Ratio from Exhaust Analysis
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Engine Test Conditions
• Various standards authorities (BS, DIN, SAE, ASTM) are
involved with specifying the test conditions for engines,
and how allowances can be made for variations in
ambient conditions• Corrections for datum conditions vary, and in general
they are more involved for CI engines
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Energy Balance
• Experiments with engines very often involve an energybalance on the engine
• Energy is supplied to the engine as the chemical energyof the fuel and leaves as energy in the cooling water,
exhaust, brake work and extraneous heat transfer – The heat transfer to the cooling water is found from thetemperature rise in the coolant as it passes through theengine and the mass flow rate of the coolant
– The energy leaving in the exhaust is more difficult to
determine – Heat transfer from the engine cannot be readily
determined
– Brake power should be used in the energy balance
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