LECTURE 5

MEC  2621-­‐  INTRODUCTION  TO  AUTOMOTIVE  ENGINEERING  

By:  Dr.  SANISAH  SAHARIN    

Engine  Performance  

OTTO  Cycle  

ENGINE  POWER  TRAIN  OPERATION  

Ø  CrankshaI-­‐transferring   power   to   the   transmission  shaI  

Ø  The top dead centre (TDC) of an engine refers to the crankshaft being in a position such that θ = 0º.

Ø  The volume in this position is called clearance volume (VC)

Ø  Bottom dead centre (BDC) refers to the crankshaft being θ = 180º.

Ø  The total volume VL is maximum at bottom dead centre (BDC).

Ø What  is  significant  of  the  compression  raNo  for  the  engine  on  vehicle  performance  ?  

Ø Heavy  vehicle  needs  high  torque-­‐  what  about  its  engine  compression  raNo?  

Ø Lighter  passenger  car  needs  low  torque-­‐  what  about  compression  raNo  for  its  engine?  

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o  A 3-litre SI V6 engine that operates on a four-stroke cycle at 3600 rpm. The compression ratio is 9.5, the length of the connecting rods is 16.6 cm, and the engine is square (b=s). At this speed, combustion ends at 20º aTDC.

o  Cylinder bore and stroke length o  Average piston speed o  Clearance volume of one cylinder o  Distance the piston has travelled from TDC at the end of

combustion o  Volume in the combustion chamber at the end of

combustion.

Example  

example  

•  The  peak  pressure  of  a  SI  engine  rotaNng  at  1500  rpm  occurs  0.003  s  aIer  the  spark,  what  will  be  the  spark  Nming  when  peak  pressure  is  at  TDC.  If  the  inlet  valve  opens  at  10  degrees  bTDC  and  closes  at  45  degrees  aBDC,  how  long  the  inlet  valve  opening  period  is  in  seconds    

Ø  Energy  input  to  an  engine  Qin  comes  from  the  combusNng  fuel.  

Ø  Fuel  is  a  HC.  

Ø  Air  is  used  to  supply  the  oxygen  needed  for  this  chemical  reacNon.  

Ø  For  combusNon  Ø  the  proper  relaNve  amounts  of  air  (oxygen)  and  fuel  

must  be  presented.  

Ø  Air-­‐fuel  raNo  (AF)  are  parameters  used  to  describe  the  mixture  raNo.  

Air-­‐Fuel  Ra,o  

Ø The   mean   effecNve   pressure   (mep)   is   the  work  done  per  unit  displacement  volume.    

Ø  Two  useful  mean  effecNve  pressure  parameters:      

Ø imep   :   The   indicated  mean   effecNve   pressure   is   the  net  work  done  per  unit  of  displacement  volume  by  the  gas  during  compression  and  expansion.  

Ø bmep:   is   the   external   sha6   work   done   per   unit  volume  by  the  engine.  

Ø  Volumetric efficiency:

Ø  it is defined as the mass of fuel and air induced into the cylinder divided by the mass that would occupy the displaced volume at the density (ρ) in the intake manifold.

Ø  Engine   power   is   the   power   that   is   available   at   the  crankshaI  to  drive  the  car.  Ø The  unit  of  engine  power  is  in  kW  (or  hp)  Ø Power  is  defined  as  the  rate  of  work  of  engine  

Ø P  =(Force  x  Stroke  length/Nme=F  x  S/t).  

Ø  Three   types   of   works   are   normally   defined   in   case   of  engine  power,  

Ø  Indicated  horse  power  (ihp)  :  is  the  power  developed  inside   the   combusNon   chambers   due   to   the  combusNon.  

o  Engine  power  

Ø  Torque is the static measurement of how much work an engine does.

Torque  

Ø Power is a measure of how fast the work is being done.

Power  

Ø A   two-­‐stroke   gas   engine   has   piston  diameter   of   150   mm,   length   of   stroke  400   mm   and   indicated   mean   effecNve  pressure  5.5  bar.  The  engine  makes  120  explosions   per   minute.   Determine   the  mechanical  efficiency  of  the  engine,  if  its  bhp  is  5  kW.    

Example  

Ø In   a   four-­‐stroke   single   cylinder   gas  engine   the   indicated   mean   effecNve  pressure   is   0.46   MN/m2,   the   brake  power   9   kW,   speed   250   rpm,  mechanical  efficiency,  η  =  0.8,  and  bore  to  stroke  raNo  =  0.66.  Calculate  cylinder  diameter  and  mean  piston  speed.  

Example  

example  

•  A  four  stroke  petrol  engine  delivers  35.75kW  with  a  mechanical  efficiency  of  80%,  the  fuel  consumpNon  of  the  engine  is  0.4  kg  per  brake  power  hour,  and  the  A/F  raNo  is  14:1.The  heaNng  value  of  the  fuel  is  41870  kJ/kg.  Find:  (a)  i.p,  (b)  f.p.,  (c)ηBT  ,  (d)  ηIT  ,  (e)  fuel  consumpNon  per  hour,  (f)  air  consumpNon  per  hour.    

Effect  of  Engine  speed  

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