NAVWEPS 00-8OT-RO  AIRPLANE PERFORMANCE  Obviously, spark ignition timing is an impor-  tant factor controlling the initial rise of pres-  sure in the combustion chamber.  The ignition  of the fuel mixture must begin at the proper  time to allow flame front propagation and the  release  of heat to build up peak pressure  for the  power stroke .  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  As the flame front 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  parts of the engine.  The effects of heavy  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.  The speed of flame front propagation is a  major factor affecting the power output of the  reciprocating engine since this factor controls  the rate of heat release and rate of pressure  rise  in the combustion chamber.  For this reason,  dual ignition is necessary  for powerplants of  high specific power output.  Obviously, nor-  mal combustion can be accomplished more  rapidly with the propagation of two flame  fronts rather than one. The two sources of  ignition are able to accomplish the combus-  tion heat release and pressure  rise in a shorter  period of time.  Fuel-air ratio is another factor  affecting the flame propagation speed in the  combustion chamber. The maximum flame  propagation speed occurs near a fuel-air ratio  of 0.08 and, thus, maximum power output for  a given airflow will tend to occur at this value  rather than the stoichiometric value.  Detonation is not necessarily confined to. a  period after the beginning of normal flame front  propagation. With extremely low grades of  fuel, detonation can occur before normal igni-  tion.  In addition, the high temperatures  and  pressure  caused by preignition will mean that  detonation is usually a corollary of preigniticn.  Detonation results from a sudden, unstable de-  composition of fuel at some critical combina-  tion of high temperature and pressure. Thus,  detonation is most likely to occur at any op  erating condition which produces high com-  bustion pressures and temperatures. Gener-  ally, high engine airflow and fuel-air ratios for  maximum heat release  will produce the critical  conditions. High engine airflow is common  to high MAP and RPM and the engine is most  sensitive to CAT and fuel-air ratio in this  region.  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-  ignition  causes a premature  rise  of  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.  140 