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cases, detonation causes engine overheating, rough- ness, or loss of power. 

Detonation is characterized by high cylinder head temperatures, and is most likely to occur when operat- ing 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 your POH you should drain old fuel out and replenish with fresh fuel. 

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

• Detonation also can be caused by extended ground operations. 

Detonation may be avoided by following these basic guidelines during the various phases of ground and flight 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 insula- tor, or other damage in the cylinder that causes a part to heat sufficiently to ignite the fuel-air charge. Preig - nition causes the engine to lose power, and produces high operating temperature. As with detonation, pre- ignition may also cause severe engine damage, be- cause the expanding gases exert excessive pressure on the piston while still on its compression stroke. 

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

## Fuel Systems 

The fuel system is designed to provide an uninterrupt- ed flow of clean fuel from the fuel tank to the engine. See Chapter 3 for more information on fuel tanks. The 

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fuel must be available to the engine under all condi- tions of engine power, altitude, attitude, and during all approved flight maneuvers. [Figure 4-12] 

## Fuel Pumps 

Powered parachutes have fuel pump systems. The main pump system is engine-driven and sometimes an electrically-driven 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 electrically-driven auxiliary pump is controlled by a switch in the cockpit. 

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. 

## Fuel Plunger Primer 

The fuel plunger primer is used to draw fuel from the tanks to supply it directly into the cylinders prior to starting the engine. This is particularly helpful during cold weather when engines are hard to start because there is not enough heat available to vaporize the fuel in the carburetor. For some powered parachutes, it is the only way to deliver fuel to the engine when first starting. After the engine starts and is running, the fuel pump pushes fuel to the carburetors and begins 

