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 Mechanical Engineering Department

     

Me  410      

Mechanical  Engineering  Systems  Laboratory  

       

Performance  Characteristics  of  an  Internal  Combustion  Engine  Experiment  No.:  4  

           

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1.  Purpose  of  the  Experiment    The  objective  of  this  experiment  is  to  study  the  variations  of  the  engine  performance  

characteristics,   such   as   brake   power,   torque,   brake   specific   fuel   consumption,  volumetric   efficiency   and   etc.   under   different   engine   loading   conditions   using   a  hydraulic  dynamometer  coupled  to  a  single  cylinder  gasoline  engine.    2.  Introduction    Perhaps  the  best-­‐known  engine  in  the  world  is  the  reciprocating  internal  combustion  

(IC)   engine.  Virtually   every  person  who  has  driven  an  automobile  or  pushed  a  power  lawnmower  has  used  one.  By  far  the  most  widely  used  IC  engines  are  the  spark-­‐ignition  (SI)   gasoline   engine,   used   in   everyday   passenger   cars   and   the   Diesel   engine,   the  workhorse   of   the   heavy   truck   industry  which   is  widely   used   in   industrial   power   and  marine   applications.   A   newer   type   of   IC   engine   is   called   Homogeneous   Charge  Compression  Ignition  (HCCI)  engine  which  is  basically  the  combination  of  both  SI  and  CI  engines  in  operating  principle.  A  reciprocating  IC  engine  basically  consists  of:    

• Engine  block,    • Cylinder  head,    • Piston  and  piston  pin,    • Connecting  rod,    • Crankshaft,  flywheel,    • Valves  and  valve  mechanisms  and  camshaft  

 There   are   usually   one   or  more   cylinders   in   the   engine   block.   For   water   cooled   IC  

engines  these  cylinders  are  surrounded  by  an  outer  shell.  Between  the  outer  shell  and  the  cylinders  there  are  water  passages  for  cooling  the  engine.  For  air  cooled  IC  engines  the  cylinders  are  surrounded  by   fins   for  air  cooling.  For  multiple  cylinder  engines   the  cylinders  will  be  arranged  side  by  side  in  a  row  (inline),  opposite  to  each  other,  in  a  V  or  W  form  or  even  flat.  Each  piston  is  connected  by  a  piston  pin  to  a  connecting  rod  which  in  turn  is  connected  to  the  related  crankpin  of  the  crankshaft.  The  crankshaft  which  is  placed  in  the  crankcase  of  the  engine  block  is  supported  by  journal  bearings.      The  back  end  of  the  crankshaft  is  coupled  to  a  flywheel.  The  flywheel  acts  to  absorb  

the   fluctuations   in   the   speed   of   the   crankshaft   which   is   mainly   due   to   uneven  distribution,  both  spatially  and  temporal,  of  the  cyclic  thermodynamic  events  among  the  cylinders.  The  crankshaft  of  an   IC  engine  may  then  be  coupled   to  a  gear  box  as   in   the  case  of  transport  vehicles  or  to  the  shaft  of  a  water  pump  or  to  the  shaft  of  an  electric  generator   or   to   the   shaft   of   a   ships   propeller   or   to   the   shaft   of   the   propeller   of   an  airplane  or  even   to   the  shaft  of   the  propeller  of  a  model  airplane.   It   is  evident   that   IC  engines  are  very  versatile.  They  come  in  all  sizes  producing  powers  from  40  000  kW  to  

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0.2   kW.  They   are   easily   transported   and   the  mainly   liquid   fuel   that   they  use   is   easily  available,   relatively   cheap   and   easily   transportable.   They   are   reliable.   You   can   expect  them  to  work   for   long  hours  with   the  same  performance  and  over  and  over  again   for  years   with   proper   maintenance.   They   are   easy   to   start   and   operate.   Their   transient  characteristics  (acceleration,  deceleration)  are  excellent.  All  in  all,  we  can  easily  say  that  the  IC  engine  has  been  the  greatest  mechanical  achievement  of  mankind,  both  socially  and  economically  and  it  is  rapidly  becoming  mankind's  foremost  concern,  ecologically.      

3.  Theory  of  IC  Engines    IC  engines  may  operate  on  a  4  stroke  cycle  or  a  2  stroke  cycle.  In  a  4  stroke  cycle  the  

piston   has   to   go   through   4   strokes   in   order   to   complete   cyclic   thermodynamic  processes.  In  the  2  stroke  cycle  the  piston  goes  through  only  2  strokes  to  complete  the  cycle.  This  seems  to  make  the  2  stroke  cycle  more  advantageous.  However,  if  the  engine  speed  is  high  then  the  gas  exchange  processes  are  not  as  efficient  as  in  the  4  stroke  cycle  engines   and   so   the   2   stroke   cycle   is   applied  more   to   marine   type   slow   and   large   CI  engines  and  to  light  SI  engines  used  on  motorcycles  and  lawn  mowers,  etc.  (since  there  won't  be  any  need  for  the  valves  and  valve  mechanisms).  On  the  other  hand  there  are  2  stroke  cycle  CI  engines  in  the  power  range  of  200-­‐500  kW  and  operating  at  speeds  of  up  to  approximately  2000  rpm.      In   the   two   stroke   engine,   the   inlet   and   exhaust   valves   are   eliminated   by   using   the  

piston  to  cover  and  uncover  ‘ports’  or  passages  in  the  cylinder  and  crankcase.  Beginning  the  cycle  with   the  piston  about   the  half-­‐way   through   its   compression  stroke,  all   three  ports   are   covered.  The  upward  movement  of   the  piston   compresses  a   fresh   charge  of  mixture  in  the  combustion  chamber.  At  the  same  time  the  pressure  in  the  crankcase  is  reduced  below  atmospheric  pressure.  Near  the  top  of  the  stroke  the  lower  edge  of  the  piston   uncovers   the   inlet   port,   allowing   the   pressure   of   the   atmosphere   to   fill   the  crankcase   of   the   engine   with   fresh   mixture   from   the   carburetor.   The   mixture   in   the  combustion  chamber  is  ignited  in  the  same  way  as  in  the  four  stroke  engine  near  the  top  of   the   stroke.   The   high   pressure   of   the   burned   gases   drives   the   piston   down   the  cylinder.   Just   below   TDC   the   piston   covers   the   inlet   port,   and   further   downward  movement  compresses  the  mixture  in  the  crankcase.  Near  the  bottom  of  the  stroke  the  top  edge  of  the  piston  uncovers  the  exhaust  port,  allowing  the  burned  gases  to  flow  out  of  the  cylinder  under  their  own  pressure.      3.1.  Operation  of  IC  Engines    3.1.1  SI  Engines  

 Spark   ignition  engines  are  mainly  used   in  automotive  vehicles  such  as  automobiles  

and   motorcycles.   These   engines   cannot   be   very   big   in   size   because   of   auto   ignition  

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(abnormal   combustion)   problems   of   flame   propagated   combustion   of   premixed  mixtures.  They  induce  a  mixture  of  air  and  fuel  during  the  induction  process  and  then  compress  the  induced  charge  to  a  pressure  of  approximately  12-­‐15  atmospheres  and  a  temperature  of  500-­‐600  K  during  the  compression  process  and  towards  the  end  of  the  compression  process  the  hot  and  compressed  mixture  is  ignited  by  a  spark  produced  by  the  electrical  ignition  system  of  the  engine  across  the  points  of  spark  plug  situated  in  the  cylinder   (10-­‐20   degrees   before   TDC).   Then   the   pressure   and   temperature   of   the   gas  inside  the  cylinder  rapidly  rise  to  a  maximum  of  approximately  70-­‐80  atmospheres  and  a  temperature  of  2400-­‐2600  K  during  the  combustion  process.  A  flame,  starting  at  the  spark   plug   location,   sweeps   across   the   combustion   chamber   (volume   between   the  cylinder  head  and  piston  top)  at  mean  speeds  which  may  reach  10-­‐20  m/s,  such  that  the  movement   of   the   piston   towards   TDC   and   away   from   TDC   is   negligibly   low   as   this  happens.   Therefore   for  most   practical   calculations   this   type   of   combustion   process   is  considered  to  happen  at  constant  volume.      The  products  of  combustion  then  push  the  piston  away  from  TDC  and  the  expansion  

of   these  gases  during   the  expansion  process  goes  on  until   the  piston  nearly  arrives  at  BDC.  At  about  40-­‐50  degrees  crank  angles  away  from  BDC  the  exhaust  valve  is  opened  by  the  valve  mechanism  which  is  synchronized  to  the  motion  of  the  crankshaft  through  the   camshaft.   Even   though   the   piston   continues   to   travel   towards   BDC   the   pressure  inside  the  cylinder  rapidly  decreases  from  about  4  atmospheres  when  the  exhaust  valve  opens  to  about  1.1  to  1.25  atmospheres,  as  the  gases  rush  out  of  the  exhaust  valve  into  the  exhaust  port  and  from  there  into  the  exhaust  manifold  and  exhaust  pipe.  The  piston  then   returns   towards   TDC   and   starts   pushing   out   the   remaining   gases   out   forcefully  during  the  exhaust  process.  This  motion  of  the  piston  requires  outside  work  which  will  be   supplied   by   one   of   the   other   pistons   (which   will   be   going   through   the   expansion  process)  or  in  the  case  of  a  single  cylinder  engine  it  will  be  supplied  by  the  flywheel.    Towards  the  end  of  the  exhaust  process  the  inlet  valve  opens  and  mixture  of  air  and  fuel  vapor  enters  the  cylinder  even  though  there  will  still  be  some  exhaust  gases  going  out  of  the  exhaust  valve  which  will  normally  be  closed  after  TDC.  This  overlapping  of  the  inlet  and  exhaust  valves  occurs  for  almost  all  IC  engines.  How  many  degrees  crankangle  this  overlap  should  be  depends  on  the  engine  type  and  operating  speeds.  Inertia  effects  on  the  gases  is   important  in  determining  the  valve  timing  of  IC  engines  and  this  timing  is  usually   done   by   testing   the   performance   of   the   engine   in   order   to   arrive   at   optimum  values.      3.1.2  CI  Engines  

 Compression  ignition  engines  have  a  much  broader  field  of  application.  It's  possible  

to  produce  approximately  2000  kW  per  cylinder  as  well  as  0.2  kW  per  cylinder  with  this  type  of  engine.  Since  they  can  operate  at  much  higher  powers  than  SI  engines  they  are  more   suitable   for   commercial   applications.   These   engines   induce   only   air   (except   the  dual  fuel  engines)  during  the  induction  process.  For  naturally  aspirated  engines,  the  air  

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is   compressed   to   approximately   40   atmospheres   and   900   K   during   the   compression  process.  Liquid  fuel  is  injected  into  the  cylinder  towards  the  end  of  compression  (10-­‐20  degrees   before   TDC)   and   the   fuel   spray   atomizes   into   small   droplets,   evaporates   and  mixes  with  hot  air,   forms  pockets  of   local   combustible  mixtures  and   then  auto   ignites  after  having  gone  through  a  series  of  preliminary  (slow  rate)  reactions  in  these  pockets.  Once   combustion   starts,   the   remaining   fuel   rapidly   evaporates   and   enters   the  combustion  reaction.  During  all  this  the  injection  of  fuel  is  still  continuing.      After   the   initially   fast   spontaneous  burning  of   the   fuel  which  entered   first   into   the  

combustion   chamber   the   continued   injection   of   fuel   results   in   a   diffusive   type   of  burning,   since   this   fuel  has   to  diffuse   through   the  products  of   combustion   in  order   to  meet  with   the   oxygen  molecules.   This   kind   of   combustion   of   course   takes  more   time  than  the  flame  propagation  in  SI  engines.  Therefore  CI  engines  cannot  normally  operate  as   fast   as   SI   engines.   On   the   other   hand   they   can   have   cylinder   bores   up   to  approximately  one  meter  whereas  SI  engine  cylinder  bores  are  normally  limited  to  0.15  m   The   expansion   and   exhaust   processes   of   4   stroke   cycle   CI   engines   are   exactly   the  same  as  in  4  stroke  cycle  SI  engines.      3.1.3.  HCCI  Engines  

 In   the   Homogeneous   Charge   Compression   Ignition   (HCCI)   engine,   a   homogeneous  

mixture  is  formed  in  the  combustion  chamber  and  the  mixture  is  compression-­‐ignited.  The  auto-­‐ignition   is   first   initiated  by  several  hot  auto-­‐igniting  spots  at   the  core  region  where  temperature  is  higher  than  the  other  regions.  It  can  be  said  that  HCCI  is  similar  to  SI  in  the  sense  that  both  engines  use  premixed  charge  and  similar  to  CI  as  both  rely  on  auto-­‐ignition  to  initiate  combustion.  But  unlike  traditional  SI  combustion  that  relies  on  the  flame  propagation  and  diesel  combustion  that  is  heavily  dependent  on  the  fuel/air  mixing,   HCCI   combustion   is   a   chemical   kinetic   combustion   process   controlled   by  temperature,  pressure,  and  composition  of  the  in-­‐cylinder  charge.  Compared  to  an  Otto  engine,   HCCI   allows   the   engine   to   operate   at   higher   compression   ratios,   resulting   in  greater  (Diesel-­‐like)  efficiencies.  Greater  efficiencies  is  provided  by  wide  open  throttle  operation  at  part   loads  unlike  SI  engines  and  reduced  cycle   to  cyclic  variations  due  to  absence  of  spark  ignition  and  early  developing  flame  growth.  HCCI  engine  also  produces  dramatically   lower   emissions   compared   to   SI   and   CI   engines.   Figure   1   show   general  schematics  the  operation  principles  of  SI,  CI  and  HCCI  engines.    

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 Figure  1  –  SI  vs.  CI  vs.  HCCI  Engines  

 3.3  Performance  Testing  of  IC  Engines  

 The  aspect  of  engine  performance  testing  is  to  determine  how  the  torque  and  brake  

power  vary  with  engine  speed.  In  real  life,  vehicles  always  operate  against  a  resistance.  This  resistance  may  be  made  of  rolling  friction,  slope,  and  air  and  inertia  resistance.  The  dynamometer   loading   simulates   the   total   of   these   resistances.   Therefore   the   steady  state   performance   of   IC   engines   can  be   tested   on  dynamometers   and   the   variation   of  performance  parameters  monitored  and  analyzed.  Some  of  the  important  performance  parameters  are  as  follows:    • Brake  power  and  torque  • Mechanical  efficiency  • Fuel-­‐air  ratio  • Volumetric  efficiency  • Specific  power  output  • Specific  fuel  consumption  • Thermal  efficiency    • Exhaust  smoke  and  emissions  • Effective  pressure    

 3.4.  Dynamometer  

 A  dynamometer  is  a  mechanical  device  that  measures  the  torque  of  a  given  machine  

under   test.   A   common   dynamometer   in   use   in   industry   is   the   engine   dynamometer  where  it  is  connected  to  the  crankshaft  of  the  engine.  The  dynamometer  then  applies  a  resistance,  or  load,  to  the  engine  at  different  angular  velocities.  The  load  can  be  applied  by  using  a  variety  of  brakes  including  an  electric  brake,  water  brake,  or  friction  brake.  Figure   2   demonstrates   a   simple   schematic   of   this   process.   In   this   system,   the  dynamometer  is  seated  in  bearings,  allowing  it  to  rotate.  This  rotation  is  prevented  by  a  

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torque   arm   with   an   attached   force-­‐measuring   scale,   generally   a   strain   gage.   As   the  dynamometer   loads   the   engine,   the   torque   arm   experiences   a   force.   This   force  multiplied  by   the  distance   from   its   center  of   rotation  equals   the   torque  of   the  engine.  With  the  known  torque  and  angular  velocity,  the  power  of  the  system  can  be  calculated  from   the   product   of   these   two   values.   The   purpose   of   the   engine   dynamometer   is   to  examine   the   engine’s   performance.   There   are   generally   two   types   of   dynamometers  namely  the  hydraulic  and  electric  dynamometers.  

 

 Figure  2  –  Schematics  of  a  typical  dynamometer  

 3.4.1.  Hydraulic  Dynamometers  

 Water brake dynamometers utilize water flow proportional to the applied load to create

resistance to the motor. A controlled flow of water through the inlet manifold is directed at the center of the rotor in each absorption section. This water is then expelled towards the outside of the dynamometer body by centrifugal force. As it is directed outward, the water is accelerated into pockets on the stationary stator plates where it is decelerated. This continuous acceleration/deceleration of the water creates the applied load to the motor.    3.4.2.  Electric  Dynamometers  

 Electric  dynamometer  is  essentially  an  electric  generator  used  for  loading  the  engine.  

The  output  of  the  generator  must  be  measured  by  electric  instruments  and  corrected  in  magnitude  for  generator  efficiency.  Since  the  generator  efficiencies  depend  on  loading,  speed   and   temperature,   the   results   obtained   will   not   be   very   precise.   However   the  generator  may  be  cradled  and   the   torque  exerted  by   the  stator   frame  may  directly  be  measured.   This   torque   arises   from   the  magnetic   coupling   between   the   armature   and  stator  and  is  equal  to  the  engine  brake  torque.  DC  or  AC  type  electric  generators  of  may  be  used   in   these  dynamometers.  AC   type  electric  dynamometers  have  better  dynamic  response  characteristics  and  are  used  in  cycle  simulation  tests.          

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4.  Experimental  Setup    4.1.  Engine    The   engine   under   test   is   a   single   cylinder   gasoline   engine   from  MITSUBISHI   with  

specification  given  in  table  1.  Figure  3  also  shows  a  picture  of  the  engine.      

Table  1  –  Engine  specifications  

Make     MITSUBISHI  

Type   Air-­‐cooled  4-­‐Stroke  Cycle  OHV  Gasoline  Engine  with  Slant  Cylinder  

Swept  Volume     181  cc  

Bore  x  Stroke  [mm]   68  x  50  

Maximum  Torque   11.6  Nm    

Maximum  Power       4.6  kW    

Continuous  Rated  Output     3.4  kW  

Starting  System     Recoil  Starter  

   

 Figure  3  –  The  OHV  MITSUBISHI  engine  used  for  tests  

         

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4.2.  Instrumentation  Unit    The   instrumentation   unit   for   this   experiment   is   TecQuipment   TD114   designed   to  

stand  beside  the  engine  under  test.  In  addition  to  the  housing  for  the  measuring  devices,  the  unit  also  contains  the  fuel  tank  and  delivery  systems  and  the  also  an  air-­‐box/viscous  flow  meter  which  are  used   to  damp   the   intake  air  before   induction   to   the  engine  and  also  measuring  the  consumption  of  air.    Figure  3  shows  the  front  and  back  view  of  the  unit.  The  working  dials  are  marked  in  the   figure.  The  unit   indicators  consist  of  engine  RPM  meter,  torque  meter,  fuel  pipette,  a  slant  manometer  (air-­‐flow  manometer),  tubes  and   vanes   to   and   from   the   engine.   In   the   back   of   the   unit,   fuel   tank   and   airbox   are  located  with  intake  tubes.  The  airbox  is  used  as  a  flow  damper  before  the  engine  intake,  this  is  due  to  the  fact  that  single  cylinder  engines  tend  to  induce  a  pulsating  flow  of  air.      

   Figure  3  –  The  front  and  back  view  of  the  instrumentation  

 The  engine  speed  is  measured  electronically  by  a  pulse  counting  system.  An  optical  head  mounted  on  the  dynamometer  chassis  contains  an  infrared  transmitter  and  receiver.  A  rotating  disk  with  radial  slots  is  situated  between  the  optical  source  and  sensor  and  as  the   engine   rotates,   the  beam   is   interrupted.  The   resulting  pulse   train   is   electronically  processed  to  provide  a  read  out  of  engine  speed.  The  electronic  tachometer  is  calibrated  against  a  signal  generator  at  the  factory  and  should  not  need  adjusting.    4.2.1.  Air  Flow  meter  

 Air  Flow  meter  is  a  device  that  measures  the  mass  flow  rate  of  intake  air  charge.  The  air  flow  meter  used  in  this  test  instrument  is  a  viscous  flow-­‐meter  located  in  the  intake  port  of   the   airbox   at   the   back   of   the   unit   as   can   be   seen   in   figure   3   and   schematically   on  

FUEL PIPETTE

TACHOMETER TORQUEMETER

AIRFLOW MANOMETER

FUEL TO ENGINE

AIR INTAKE VISCOUS FLOW-METER

AIRBOX AS DAMPER

FUEL TANK

10

figure   4.   As   it   can   be   seen   on   the   figure,   air   is   drawn   in   through   an   inlet   and   flows  through  the  viscous  flow-­‐meter  which  consists  of  thousands  of  small  bore  tubes  before  entering   the   damping   volume.   The   size   of   the   tubes   is   chosen   so   that   the   Reynolds  Number   (ud/υ)   is   less   than   2300.   This   ensures   that   airflow   through   the   element   is  entirely  viscous,  in  which  case  the  pressure  drop  is  given  by  Poieseuille’s  equation.    

2

32 l updµ

Δ =    

 Figure  4  –  The  schematic  of  the  airbox  for  damping  of  the  intake  air  to  the  engine  

 For  a  given  pressure  difference  reading  on  the  slant  manometer,  as  shown  in  figure  5,  the  air  mass  flow  rate  can  be  calculated  using  figure  5.    To  account  for  temperature  and  pressure  differences  of  the  test  location  with  that  of  the  calibrated  curve,  a  factor  is  used  to  correct  the  calculated  air  flow  rate  as  below.    

!mAir−Actual =PambientPcalibrated

.Tambient +114Tcalibrated +114

.(TcalibratedTambient

)52 !mcalibrated    

   

 Figure  5–  FL15  Type  manometer  for  pressure  reading  in  the  test  

   

11

 Figure  6  –  Viscous  flow  meter  calibration  curve  

 Figure   7   also   shows   the   inner   part   of   the   viscous   flow-­‐meter  which   consists   of   small  bore  tubes  to  maintain  viscous  flow  and  air  flow  rate  calculations.  

   

     Figure  7  –  The  viscous  flow-­‐meter  at  the  inlet  port  of  the  airbox    

 4.2.2.  Fuel  flow  meter  

 Figure  8  shows  the  fuel  flow  meter  of  the  unit  which  includes  a  fast  flow  pipette  and  

the  capacity  scale  of  the  pipette.  The  pipette  is  connected  to  the  fuel  tank  at  the  back  of  the   unit   from   the   bottom   through   a   vane.   The   fuel   consumption   rate   is   measured  manually.  While   the   engine   is   running   at   a   constant   desired   speed,   the   vane  which   is  controlling  the  fuel  flow  from  the  tank  to  pipette  is  closed  so  that  the  existing  fuel  in  the  pipette  is  used  for  running  the  engine.  While  the  vane  is  closed,  the  amount  of  fuel  to  the  engine  is  indicated  on  the  scale  and  a  chronometer  is  used  to  measure  the  required  time  for   consumption   of   a   specific   amount   of   fuel   i.e.   8ml,   16ml   or   32ml   as   shown   on   the  figure  below.      

12

 

 

Figure  8  –  fuel  pipette  and  the  capacity  scale    4.3.  TechQuipment  TD114  Water  Brake  Dynamometer    The  hydraulic  dynamometer  used  in  this  experiment  is  TechQuipment  TD114.  Figure  

9   shows   the   principles   and   the   layout   of   the   dynamometer.   The   flow   of   water   is  controlled   by   a   valve   (A)   near   the   engine   bed.   Water   flows   into   the   top   of   the  dynamometer  casing  (B)  and  out  through  the  bottom,  discharging  into  a  drain  through  tap   (C).   The   dynamometer   also   has   an   air   vent.   The   quantity   of   water   in   the  dynamometer,  and  hence  the  power  absorbed  from  the  engine,  depends  on  the  settings  of   the   valve   (A)   and   a   tap   (C).   The   engine   shaft   drives   a   paddle   (D)   inside   the   vaned  casing  (B)  churning  up  the  water  inside  the  dynamometer.  If  not  restrained,  the  casing  would  rotate  at  almost  the  same  speed  as  the  paddle.  Restraint  is  provided  by  a  spring  loaded  nylon  cord  €  which  passes  round  the  casing  (B)  and  is  clamped  to  the  top  of  the  casing.   Two   springs   (F)   have   equal   stiffness,   and   are   always   in   tension   as   the  dynamometer  casing  rotates.  A  damper  (G)  filled  with  lubricating  oil  is  connected  to  the  casing.    The   angular   position   taken   up   by   the   casing   (B)   depends   on   the   torque   T   and   the  

stiffness   of   the   two   springs   (F).   The   peripheral   displacement   of   the   casing   is  proportional  to  the  torque  T  and  is  measured  by  a  rotary  potentiometer  (H),  the  output  of  which  is  fed  in  to  the  input  of  the  TD114  torquemeter.    

13

 Figure  9  –  The  schematic  of  the  TD114  dynamometer  

   

5.  Experiments    5.1.  Variable  Speed  Variable  Load  Test  

 With  a  growing  demand  for  transportation  IC  engines  have  gained  lot  of  importance  

in  automobile   industry.   It   is   therefore   necessary   to   produce   efficient   and   economical  engines.  While   developing   an   IC   engine   it   is   required   to   take   in   consideration   all   the  parameters   affecting   the   engines   design   and   performance.   There   are   enormous  parameters   so   it   becomes   difficult   to   account   them   while   designing   an   engine.   So   it  becomes  necessary   to   conduct   tests   on   the   engine   and  determine   the  measures   to  be  taken  to  improve  the  engines  performance.  In  this  experiment,  the  throttle  of  the  engine  is   fully   open   during   the   whole   test.   Therefore   as   the   load   is   increased,   the   throttle  cannot  be  opened  wider   to  maintain   the   same   constant   speed   since   it   is   already   fully  open.  As  a  result  of  this,  the  engine  speed  will  gradually  drop  as  it  is  loaded.  This  case  may  also  be  visualized  in  real   life.  Consider  a  car  going  with  maximum  speed  on  a  flat  road.  Here  maximum  speed  corresponds  to  the  fully  pressed  gas  pedal,   therefore  fully  opened  throttle.  When  it  starts  to  climb  up  an  inclined  plane,  its  speed  will  begin  to  drop  since  the  driver  cannot  press  the  gas  pedal  more  which  is  already  fully  pressed.  So  this  test  will   begin   at   fully   opened   throttle   position   at   nearly   no-­‐load   condition.   Then   the  load  will  be  increased  gradually.  At  each  engine  speed,  the  required  values  such  as  air  flow  rate,  fuel  flow  rate,  torque  and  fuel  consumption  rate  will  be  recorded  to  calculate  the  performance  parameters  of  the  engine.      

 

14

5.2.  Test  Procedure    The   outline   of   the   engine   test   bed   can   be   seen   in   figure   10.   The   single   cylinder  

gasoline  engine  is  coupled  to  the  hydraulic  dynamometer  via  a  shaft.  The  water  source  for   the   dynamometer   is   a   tank   located   above   the   ground   level   to   ensure   constant  pressure   flow   into   the   dynamometer  while   operating.   The  water   flow  which   controls  the   dynamometer   and   thus   the   engine   load   and   RPM   is   adjusted   through   a   butterfly  valve.    

       Figure  10  –  View  of  the  test  bed  

 The  experiment  procedure  can  be  summarized  as  below:    

1.  Open  the  dynamometer  water  intake  vane  slightly  so  that  a  trickle  of  water  stream  is  fed   into   the   dynamometer   to   prevent   harming   the   bearing   sealing’s   of   the  dynamometer.  2.  Turn  the  engine  on-­‐off  button  to  position  1.  3.  Keep  the  throttle  slightly  open  and  pull  the  recoil  starter  lever  to  start  the  engine.    4.   Slowly   increase   the   throttle   to   the  maximum   speed   (100%  open   throttle   position)  while  opening  the  dynamometer  water   intake  vane  and  close   the  vane  which  controls  the  exit  water  flow  from  the  dynamometer.  5.  By  adjusting  the  water  vane  on  the  dynamometer,  set  the  engine  speed  to  5000  RPM  (you  have  to  give  this  process  enough  time  to  let  the  engine  speed  reach  to  a  constant  value).  6.  Read  and  take  note  of  the  torque  and  pressure  value  in  the  slant  manometer.  7.   Close   the   vane   controlling   the   fuel   intake   to   the   pipette   from   the   tank   and   let   the  engine  burn   the   fuel   inside   the  pipette.  Make   sure   to   record   the   time   that   the   engine  takes  to  consume  a  specific  amount  of  fuel  in  the  pipette  (i.e.  8ml).  

15

8.    Repeat  the  steps  5-­‐7  by  decreasing  the  engine  speed  500  RPM  at  a  time.    9.  Open  the  exit  and  intake  water  vanes  and  decrease  the  throttle  of  the  engine  to  idle  condition.    

6.  Formulations    6.1.  Brake  Power  

     The   mechanical   brake   power   of   the   engine   is   the   product   of   the   torque   on   the  

crankshaft  and  the  rotational  speed  of  the  crankshaft.      

2b

b

N TN Tn

ω

π

=

=    

Where  Nb  =  Brake  power  (Watt)    T  =  Torque  (N-­‐m)    ω  =  Engine  Speed  (rad/sec)        n  =  engine  speed  (rev/sec)  1kW  =  1.36  HP    6.2.  Corrected  Brake  Power  

 Test  results  must  always  be  referred  to  a  known  datum  so  that  comparisons  between  

different   engines   may   readily   be   made   or   the   effect   of   modifications   easily   seen.   All  measurements   taken   should   ideally   be   corrected   to   standard   atmospheric   conditions.  To  find  the  corrected  brake  horsepower,  multiply  the  measured  value  by  the  following  correction  factor.    

  2 .bc aN Tnπ α=  

The   correction   factor,   αa,   for   spark-­‐ignition   engines   shall   be   as   calculated   from   the  formula  

1.2 0.699298aT

Pdα

⎛ ⎞ ⎛ ⎞⎜ ⎟= ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠  

Where  

Ta  :  Absolute  temperature  of  the  intake  air  expressed  in  Kelvin  Pd  :  Dry  atmospheric  pressure  expressed  in  kilopascals  calculated  as    

.d atm vP P P= −Φ  

16

Patm  :  Atmospheric  pressure  expressed  in  kilopascals  Pv:  Saturated  water  vapor  pressure  expressed  in  kilopascals  Φ :  Relative  humidity  expressed  in  percent    Recommended   range   (especially   for   type  approval   testing  and   commercial  purposes):  0.97  ≤  αa  ≤  1.03    6.3.  Corrected  Engine  Torque    Engine   torque   is   the   twisting   or   turning   effort   that   the   engine   applies   through   the  

crankshaft.  Engine  torque  can  be  found  from  the  following  relation:    

2bc

cNTnπ

=  

Where    Tc  =  Engine  corrected  torque  (N-­‐m)    Nbc  =  Corrected  engine  brake  power  (watt)    n  =  engine  speed  (rev/sec)      6.2.  Brake  Thermal  Efficiency    The  thermal  efficiency  of  an  IC  engine  is  the  relationship  between  the  power  output  

delivered  at   the  crankshaft  and  the  energy  available   in   the   fuel   to  produce  this  power  output:      

bcb

f L

NG Q

η =⋅

 

where  Nbc  =  Corrected  brake  power  (kW)    Gf  =  Rate  of  fuel  consumption  (kg/sec)    QL  =  lower  heating  value  of  the  fuel  (kJ/kg)    QL  =  44000  kJ/kg  for  gasoline  fuel  

Also  density  of  fuel  which  will  be  used  for  fuel  flow  rate  can  be  assumed  as:    

fρ  =  740  kg/m3  for  gasoline  fuel    

 6.3.  Brake  Specific  Fuel  Consumption  (bSFC)  

 The  brake   specific   fuel   consumption   is   a  measure   of   efficiency  which   indicates   the  

amount  of  fuel  that  an  engine  consumes  for  the  work  it  produces  and  is  calculated  using  the  below  relation.  

17

fb

bc

Gg

N=  

Where  gb  =  Brake  specific  fuel  consumption  (g/HP-­‐hr)    Gf  =  Rate  of  fuel  consumption  (g/hr)  Nb  =  Corrected  engine  brake  horsepower  (HP)        

6.4.  Brake  Mean  Effective  Pressure  (bMEP)    Although  it  is  a  measure  of  an  engine's  ability  to  do  work,  torque  cannot  be  used  to  

compare   different   engines,   since   it   depends   on   engine   size.   A   more   useful   relative  engine  performance  measure  is  obtained  by  dividing  the  work  per  cycle  by  the  cylinder  volume  displaced  per  cycle.  The  parameter  obtained  thus  is  called  brake  mean  effective  pressure  and  is  defined  shortly  as  the  average  pressure  that  the  gas  exerts  on  the  piston  through  one  complete  operation  cycle.  The  brake  mean  effective  pressure  can  be  found  from  the  following  formula.    

2( )bc

s

Nbmep n iVj=  

Where  bmep  =  Brake  Mean  Effective  Pressure  (kPa)    Nbc  =  Corrected  brake  power  (kW)    n  =  Engine  speed  (rev/sec)    j  =  Number  of  strokes    i  =  number  of  cylinders    Vs  =  Swept  volume  of  a  single  cylinder  (m3)    

 6.5.  Actual  Air-­‐Fuel  Ratio  

 The  actual  air-­‐fuel  ratio  is  calculated  from  values  of  air  and  fuel  mass  flows  obtained  

from  the  airflow  manometer  reading  and  the  time  to  consume,  say,  8  ml.  of  fuel.            

 6.6.  Volumetric  Efficiency  

 Volumetric  efficiency  is  the  ratio  between  the  amount  of  air-­‐fuel  mixture  that  actually  

enters  the  cylinder  and  the  amount  that  could  enter  under  ideal  standard  atmospheric  conditions.    

AF!

"#

$

%&actual

=!mairGf

18

ηv =!mair−actual

!mair−theoretical  

Where   vη  =  Volumetric  Efficiency  (%)  

!mair−actual  =  Actual  Air  Flow  Rate  (  kg/s)    

  !mair−theoretical =  Theoretical  Air  Flow  Rate  (kg/s)      The  amount  of  theoretical  air  that  could  enter  into  a  cylinder  can  be  found  from;  

!mair−theoretical =2nj

"

#$

%

&'⋅ i ⋅Vs ⋅ρSTD

 

STDSTD

air STD

PR T

ρ =⋅

 

 Where   !mair−theoretical   =  Amount   of   theoretical   air   that   could   enter   a   cylinder   under   ideal  standard  atmospheric  conditions.  (kg/s)  

STDρ =  Standard  air  density  (kg/m3)    Vs  =  Swept  volume  of  a  single  cylinder  (m3)  Pstd  =  Standard  atmospheric  pressure  =  101.325  kPa  Tstd=  Standard  atmospheric  temperature  =  293  K.  D  =  Cylinder  bore  (m)  S  =  Piston  Stroke  (m)    6.7.  Excess  Air  Coefficient  

 

ltheoretica

actual

FAFA)/()/(

=α  

 Where  α =  Excess  air  coefficient    (A/F)actual  =  Actual  air  -­‐  fuel  ratio  (kgair  /kgfuel)    (A/F)theoretical  =  Theoretical  air  -­‐  fuel  ratio  (kgair/kgfuel)    Theoretical  air-­‐fuel  ratio  can  be  taken  as  14.6  for  gasoline  fuel.    7.  Report  Presentation    Reports  for  the  lab  experiment  are  due  final  exam.  In  preparing  your  reports  please  

note  below:  • Title  page  should  include:  

Course  code  and  name  Experiment  name  Student  surname,  name  and  ID  number  

19

Laboratory  group  number  and  experiment  date      

• Object   of   the   test   should   be   briefly   explained   (in   your   own   words)   and   data  collected  during  the  test  should  be  tabulated  

• A  sample  calculation  will  be  performed  for  a  selected  load  condition    • All  results  will  be  presented  in  a  tabulated  form.    • Graphs:  Selected  graphs  from  the  following  will  be  drawn.    

 a) Corrected  brake  horse  power  vs.  RPM  b)  Corrected  brake  torque  vs.  RPM  c)  bMEP  vs.  RPM  d)  bSFC  vs.  RPM    e)  Brake  thermal  efficiency  vs.  RPM  f)    Volumetric  efficiency  vs.  RPM                            

• In  your  discussion  &  conclusions,  you  should  analyze  the  plots  and  comment  on  why   they  show  specific   trends.  Also  discuss   the  possible   sources  of  errors   that  may  be  encountered  in  the  experiment.    

 7.  Important  Notes    

• You   are   supposed   to   read   this   write-­‐up   sheet   carefully   before   coming   to   the  laboratory.    

• There  might  be  some  questions  asked  at  the  beginning  or  during  the  lab  session  about   the   working   principles   of   the   dynamometers,   the   engine,   the   air   flow  meter  and  etc.  so  be  ready.  

• Bring   a   chronometer   or   a   watch   for   measuring   fuel   consumption   time   (This  could  also  be  provided  in  the  lab).  

• The  humidity,  atmospheric  pressure  and  temperature  should  be  recorded  at  the  beginning  of  the  experiment.  

• The   laboratory   ambient   temperature   value   can   be   read   from   the   thermometer  located  beside  the  engine  test  bed.  

• Water  vapor  pressure  should  be  taken  from  the  thermodynamic  tables  using  the  ambient  pressure  and  temperature.  

                     

20

 Date:

Engine Analysis Data Sheet

Barometric pressure: Relative Humidity:.....%

Temperature:

 

Engine Speed RPM

Torque [N.m]

Fuel Amount ml                

Consumption Duration [s]

Manometer Pressure [mmH2O]