The cooling system fan pulls air across the fins in the radiator to transfer the heat from the coolant. Its main function is to prevent overheating when the vehicle is not moving or not moving very fast and the air transfer across the radiator is decreased. There are two basic types of fans, engine-powered and electricpowered. The engine-powered fan is run off a drive belt from the crankshaft pulley. There are also three types of engine-powered fans. A flex fan has thin flexible blades. As the engine is at idle, requiring more air, the blades are curved and draw a lot of air; however, as the engine speeds up, the blades flex until they are almost straight, drawing little air but at the same time reducing used engine power. The fluid coupling fan is designed to slip at higher engine speed. As the engine is at idle, the fluid engages the blade to turn it; when the engine speeds up, the fluid

NAVEDTRA 14264A 2-15 is not able to keep up and allows the blade to slip. This allows for a reduction of engine power consumed. The thermostatic fan clutch has a temperature sensitive metal spring that controls the fan speed. The spring controls oil flow in the fan clutch. When the spring is cold, it allows the clutch to slip. As the spring heats up, the clutch locks and forces air circulation. The thermostat senses the temperature of the engine and opens or closes to control water flow as required. The thermostat has a wax-filled pellet contained in a cylinder. A spring holds the piston and valve in a normally closed position. As the temperature increases, the wax heats up and expands, allowing the valve to open. As the temperature decreases, the wax cools, retracts, and closes the valve.

2.2.6 Fuel Type An engine burns fuel as a source of energy. Various types of fuel will burn in an engine: gasoline, diesel fuel, gasohol, alcohol, liquefied petroleum gas, and other alternative fuels. Gasoline is the most common type of automotive fuel. It is abundant and highly flammable. Extra chemicals like detergents and antioxidants are mixed into it to improve its operating characteristics. Antiknock additives are introduced to slow down the burning of gasoline. This helps prevent engine ping, or the knocking sound produced by abnormal, rapid combustion. Gasoline has different octane ratings. This is a measurement of the fuels ability to resist knock or ping. A high octane rating indicates that fuel will not knock or ping easily. Highoctane gasoline should be used in high-compression engines. Low-octane gasoline is more suitable for low-compression engines.

Diesel fuel is the second most popular type of automotive fuel. A single gallon of diesel fuel contains more heat energy than a gallon of gasoline. It is a thicker fraction or part of crude oil. Diesel fuel can produce more cylinder pressure and vehicle movement than an equal part of gasoline. Since diesel fuel is thicker and has different burning characteristics than gasoline, a high-pressure injection system must be utilized. Diesel fuel will not vaporize as easily as gasoline. Diesel engines require the fuel to be delivered directly into the combustion chamber. Diesel fuel has different grades as well: No. 1, No. 2, and No. 4 diesel. No. 2 is normally recommended for use in automotive engines. It has a medium viscosity (thickness or weight) grade that provides proper operating traits for the widest range of conditions. It is also the only grade of diesel fuel at many service stations.