02-engine operation and testing

8/9/2019 02-Engine Operation and Testing

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Engine Operation and Testing

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

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

• 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

• 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

• 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

– 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

• 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

(a)Intake stroke

(b)Compression stroke

(c)Combustion (ignition)

(d)Power or expansion

stroke

(e)Exhaust blowdown(f) Exhaust stroke

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

– 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

– 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

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

• 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

• 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

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

• 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

• 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

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.

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

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)

Dynamometer Principle

Rope-Brake Dyno

Power Measurement

P = T ω = 2π (RPM/60) Wr

Early Dynamometers

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)

Dynamometer

Chassis Dynamometer

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

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

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

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

Froude Water Brake Dynamometer

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

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

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

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

• 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

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

– 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

Estimating AF Ratio from Exhaust Analysis

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

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

02-engine operation and testing

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