supplied to the main power bus through the battery solenoid.  Both the starter and the starter switch draw current from the  main bus, but the starter will not operate until the starting  solenoid is energized by the starter switch being turned to the  “start” position. When the starter switch is released from the  “start” position, the solenoid removes power from the starter  motor. The starter motor is protected from being driven by  the engine through a clutch in the starter drive that allows  the engine to run faster than the starter motor. In a four-stroke engine, the engine oil system performs  several important functions, including: Four-stroke engines use either a wet sump or dry sump  oil system. Refer to the Pilot’s Handbook of Aeronautical  Knowledge for more information on four-stroke oil  systems. The burning fuel within the cylinders produces intense heat,  most of which is expelled through the exhaust system. Much  of the remaining heat, however, must be removed, or at least  dissipated, to prevent the engine from overheating. While the oil system in a four-stroke engine and the fuel-oil  mix in a two-stroke engine is vital to the internal cooling of  the engine, an additional method of cooling is necessary for  the engine’s external surface. WSC engines operate with  either air-cooled or liquid-cooled systems. Many WSC aircraft are equipped with a cylinder head  temperature (CHT) gauge. This instrument indicates a direct  and immediate cylinder temperature change. This instrument  is calibrated in degrees Celsius or Fahrenheit. Proper CHT  ranges can be found in the POH/AFM/AOI for that machine.  [Figure 4-20] Air cooling is accomplished by air being pulled into the  engine shroud by a cooling fan. Baffles route this air over  fins attached to the engine cylinders where the air absorbs the  engine heat. Expulsion of the hot air takes place through one  or more openings in the shroud. If cylinder head temperatures  rise too much in an air-cooled engine, it is because of  lubrication problems, cooling fan drive belt damage or wear,  Liquid cooling systems pump coolant through jackets in  the cylinders and head. The heated liquid is then routed to  a radiator where the heat is radiated to the atmosphere. The  cooled liquid is then returned to the engine. If the radiator is  mounted close to the propeller, the propeller can constantly  move air across the radiator and keep the engine cool even  when the WSC is not moving.  [Figure 4-21]  Radiators  mounted away from the propeller make it more diffi cult for  the radiator to cool the engine unless the WSC is moving.  [Figure 4-22] Breaking in an engine through ground runs on a hot day  is when radiator placement is most critical. Liquid-cooled  engines can overheat for a number of reasons, such as coolant  • Lubricating the engine’s moving parts. • Cooling the engine by reducing friction. • Removing heat from the cylinders. • Providing a seal between the cylinder walls and  pistons. • Carrying away contaminants. Oil Systems Engine Cooling Systems or air blockage in the cooling fins by a bird or insect nest.  [Figure 4-1] Water Cooler Oil Cooler Figure 4-20. Cylinder head temperature probe (yellow wire) is  under spark plug. Figure 4-21. Cooling radiators—oil cooler is on top and water  cooler is on bottom. 4-17