Detonation There is a limit, however, to the amount of compression and  the degree of temperature rise that can be tolerated within an  engine cylinder and still permit normal combustion. All fuels  have critical limits of temperature and compression. Beyond  this limit, they ignite spontaneously and burn with explosive  violence. This instantaneous and explosive burning of the  fuel/air mixture or, more accurately, of the latter portion of  the charge is called detonation. Detonation is the spontaneous combustion of the unburned  charge ahead of the flame fronts after ignition of the charge.  [Figure 10-43]  During normal combustion, the flame fronts  progress from the point of ignition across the cylinder. These  flame fronts compress the gases ahead of them. At the same  time, the gases are being compressed by the upward movement  of the piston. If the total compression on the remaining  unburned gases exceeds the critical point, detonation occurs. The explosive burning during detonation results in an  extremely rapid pressure rise. This rapid pressure rise and  the high instantaneous temperature, combined with the  high turbulence generated, cause a scrubbing action on the  cylinder and the piston. This can burn a hole completely  through the piston. The critical point of detonation varies with the ratio of fuel to  air in the mixture. Therefore, the detonation characteristic of  the mixture can be controlled by varying the fuel/air ratio. At  high power output, combustion pressures and temperatures  are higher than they are at low or medium power. Therefore,  at high power, the fuel/air ratio is made richer than is needed  for good combustion at medium or low power output. This  is done because, in general, a rich mixture does not detonate  as readily as a lean mixture. Unless detonation is heavy, there is no flight deck evidence  of its presence. Light to medium detonation does not cause  noticeable roughness, temperature increase, or loss of power.  As a result, it can be present during takeoff and high-power  climb without being known to the flight crew. In fact, the effects of detonation are often not discovered  until after teardown of the engine. When the engine is  overhauled, however, the presence of severe detonation  during its operation is indicated by dished piston heads,  collapsed valve heads, broken ring lands, or eroded portions  of valves, pistons, or cylinder heads. The basic protection from detonation is provided in the design  of the engine carburetor setting, which automatically supplies  the rich mixtures required for detonation suppression at high  power; the rating limitations, which include the maximum  operating temperatures; and selection of the correct grade of  fuel. The design factors, cylinder cooling, magneto timing,  mixture distribution, degree of supercharging, and carburetor  setting are taken care of in the design and development of the  engine and its method of installation in the aircraft. The remaining responsibility for prevention of detonation  rests squarely in the hands of the ground and flight crews.  They are responsible for observance of rpm and manifold  pressure limits. Proper use of supercharger and fuel mixture,  and maintenance of suitable cylinder head and carburetor- air-temperature (CAT) must be adhered to. Pre-ignition, as the name implies, means that combustion  takes place within the cylinder before the timed spark jumps  across the spark plug terminals. This condition can often be  traced to excessive carbon or other deposits that cause local  hot spots. Detonation often leads to pre-ignition. However,  pre-ignition may also be caused by high-power operation on  excessively lean mixtures. Pre-ignition is usually indicated  in the flight deck by engine roughness, backfiring, and by a  sudden increase in cylinder head temperature. Any area within the combustion chamber that becomes  incandescent serves as an igniter in advance of normal timed  ignition and causes combustion earlier than desired. Pre- ignition may be caused by an area roughened and heated by  detonation erosion. A cracked valve or piston, or a broken  spark plug insulator, may furnish a hot point, that serves as  a glow plug. Pre-Ignition Intake valve Exhaust valve Piston Crankcase Connecting rod Figure 10-43.  Detonation within a cylinder.  10-30