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 . 
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. 
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. 
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. 
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