text: Denotation offers the possibility of immedi- 
  ate destruction of the powerplant. The nor- 
  mal combustion process is initiated by the 
  spark and beginning of flame front propaga- 
  tion 
  is propagated, the 
  combustion chamber pressure and temperature 
  begin to rise. Under certain conditions of 
  high combustion pressure and temperature, 
  the mixture ahead of the advancing flame front 
  may suddenly explode with considerable vi- 
  olence and send strong detonation waves 
  through the combustion chamber. The result 
  is depicted by the graph of figure 2.16, whete:a 
  sharp, explosive increase in pressure takes place 
  with a subsequent reduction of the mean pres; 
  sure during 
  the power stroke. Detonation 
  produces sharp explosive pressure peaks many 
  times greater than normal combustion1 Also, 
  the exploding gases radiate considerable heat 
  and cause excessive temperatures for many local 
  the engine. The effects of heavy 
  parts of 
  detonation are so severe that structural damage 
  is the immediate result. Rapid rise of cylinder 
  head temperature, rapid drop in BMEP, and 
  loud, expensive noises are evidence of detona- 
  tion. 
  
  As the flame front 
  
  Two aberrations of the combustion process 
  
  are preignition and detonation. Preignition 
  is simply a premature ignition and flame f&t 
  propagation due to hot spots in the combustion 
  chamber. Various lead and carbon deposits 
  and feathered edges on metal surfaces can sup- 
  ply a glow 
  ignition spot and begin a flame 
  propagation prior 
  to normal spark ignition. 
  As shown on the graph of figure 2.16, pre- 
  rise of 
  causes a premature 
  ignition 
  pressure during the piston travel. As a result, 
  preignition combustion pressures and tempera- 
  tures will exceed normal combustion values and 
  are very likely 
  to cause engine damage. Be- 
  cause of the premature rise of pressure toward 
  the end of the compression stroke, the net work 
  of the operating cycle is reduced. Preignition 
  is evidenced by a rise in cylinder head tempera- 
  ture and drop in BMEP or torque pressure. 