content_image/585355.jpg
corrected, can quickly lead to failure of the piston, cylinder, or valves. In less severe cases, detonation causes engine overheating, roughness, or loss of power. 

Detonation is characterized by high cylinder head temperatures and is most likely to occur when operating at high power settings. Some common operational causes of detonation include: 

• Using a lower fuel grade than that specified by the aircraft manufacturer or operating the engine after it has been sitting for an extended period; after 3 weeks or as indicated by the POH, drain old fuel and replenish with fresh fuel. 

• Operating the engine at high power settings with an excessively lean mixture. 

• Extended ground operations. 

Detonation may be avoided by following these basic guidelines during the various phases of ground and fl ight operations: 

• Make sure the proper grade of fuel is being used. Drain and refuel if the fuel is old. 

• Develop a habit of monitoring the engine instruments to verify proper operation according to procedures established by the manufacturer. 

Preignition occurs when the fuel/air mixture ignites prior to the engine’s normal ignition event. Premature burning is usually caused by a residual hot spot in the combustion chamber, often created by a small carbon deposit on a spark plug, a cracked spark plug insulator, or other damage in the cylinder that causes a part to heat sufficiently to ignite the fuel/air charge. Preignition causes the engine to lose power and produces high operating temperature. As with detonation, preignition may also cause severe engine damage because the expanding gases exert excessive pressure on the piston while still on its compression stroke. 

Detonation and preignition often occur simultaneously and one may cause the other. Since either condition causes high engine temperature accompanied by a decrease in engine performance, it is often diffi cult to distinguish between the two. Using the recommended grade of fuel and operating the engine within its proper temperature and RPM ranges reduce the chance of detonation or preignition. 

## Fuel Systems 

The fuel system is designed to provide an uninterrupted fl ow of clean fuel from the fuel tank to the engine. See Chapter 3, Components and Systems, for more information on fuel tanks. See earlier section in this chapter for specifics on fuel 

injection systems. The fuel must be available to the engine under all conditions of engine power, altitude, attitude, and during all approved fl ight maneuvers. [Figure 4-17] 

<img src='content_image/585355.jpg'>

## Fuel Pumps 

WSC aircraft with carburetors have engine-driven fuel pump systems. A diaphragm pump is the primary pump in the fuel system for two-stroke engines. Air pulses in the crankcase actuate a diaphragm and provide fuel under pressure to the carburetor. Four-stroke engines have a mechanical pump driven directly off the engine. 

Sometimes an electric auxiliary pump is provided for use in engine starting and in the event the engine pump fails. The auxiliary pump, also known as a boost pump, provides added reliability to the fuel system. The electric auxiliary pump is controlled by a switch in the fl ight deck. 

