Cooling Systems.docx

*Cooling Systems* AMT 177 Reciprocating Engine Theory *Cooling Systems* Excessive heat is undesirable in any internal combustion engine for three principal reasons: If the temperature inside the engine cylinder is too great, the fuel-air mixture is preheated, and combustion occurs before the de...

*Cooling Systems* AMT 177 Reciprocating Engine Theory *Cooling Systems* Excessive heat is undesirable in any internal combustion engine for three principal reasons: If the temperature inside the engine cylinder is too great, the fuel-air mixture is preheated, and combustion occurs before the desired time Since premature combustion causes detonation, knocking, and other undesirable conditions, there must be a way to eliminate heat before it causes damage *Cooling Systems* About one-fourth of the heat released by aircraft fuel is changed into useful power The remainder of the heat must be dissipated so that it is not destructive to the engine In a typical aircraft powerplant, half of the heat goes out with the exhaust and the other is absorbed by the engine Circulating oil picks up part of this soaked-in heat and transfers it to the airstream through the oil cooler The engine cooling system takes care of the rest *Cooling Systems* *Cooling Systems* Cooling is a matter of transferring the excess heat from the cylinders to the air, but there is more to such a job than just placing the cylinders in the airstream A cylinder on a large engine is roughly the size of a gallon jug ***Its outer surface is increased by the use of cooling fins on the head and barrel so that it presents a barrel-sized exterior to the cooling air*** ***If too much of the cooling fin area is broken off, the cylinder cannot cool properly, and a hotspot develops*** *Cooling Systems* Cowling and baffles are designed to force air over the cylinder cooling fins The baffles direct the air close around the cylinders and prevent it from forming hot pools of stagnant air while the main streams rush by unused ***In some applications, blast tubes are built into the baffles to direct jets of cooling air onto accessories to prevent overheating*** *Cooling Systems* *Cooling Systems* Reciprocating engines use some type of inter-cylinder and cylinder head baffles to force the cooling air into close contact with all parts of the cylinders The air baffle blocks the flow of air and forces it to circulate between the cylinder and the deflectors The air baffle prevents the air from passing away from the cylinder head and forces it to go between the head and deflector Although the resistance offered by baffles to the passage of the cooling air demands that an appreciable pressure differential be maintained across the engine to obtain the necessary airflow, the volume of cooling air required is greatly reduced by employing properly designed and located cylinder deflectors *Cooling Systems* The most common means of controlling cooling is the use of cowl flaps These flaps are opened and closed by electric motor-driven jackscrews, by hydraulic actuators, or manually in some light aircraft When extended for increased cooling, the cowl flaps produce drag and sacrifice streamlining for the added cooling On takeoff, the cowl flaps are opened only enough to keep the engine below the red-line temperature ***During ground operations, the cowl flaps should be opened wide since drag does not matter and cooling needs to be set at maximum*** *Cooling Systems* ***If an aircraft is equipped with cowl flaps, there must be a cylinder head temperature gauge installed*** As cowl flaps vary the amount of cooling air available to the engine, it is thus possible for both over-temperature and under-temperature conditions to exist The system usually consists of an indicator, electrical wiring, and a thermocouple The thermocouple end that connects to the cylinder is either the bayonet or gasket type To install the bayonet type, the knurled nut is pushed down and turned clockwise until it is snug The gasket type fits under the spark plug and replaces the normal spark plug gasket *Cooling Systems* ***Some aircraft use augmentors to provide additional cooling airflow*** Augmentors use exhaust gas velocity to cause airflow over the engine so that cooling is not entirely dependent on the prop wash The engine is pressure cooled by air taken in through two openings in the nose cowling, one on each side of the propeller spinner A pressure chamber is sealed off on the top side of the engine with baffles properly directing the flow of cooling air to all parts of the engine compartment Warm air is drawn from the lower part of the engine compartment by the pumping action of the exhaust gases through the exhaust ejectors This type of cooling system eliminates the use of controllable cowl flaps and assures adequate engine cooling at all operating speeds *Cooling Systems* **FAR 43 - Appendix A** **Major Alterations, Major Repairs, and Preventive Maintenance** **(a) Major alterations** \(xii) **Changes to the basic design of the** fuel, oil, **cooling**, heating, cabin pressurization, electrical, hydraulic, de-icing, or exhaust **systems**. *Cooling Systems* In order for engine baffles to efficiently and effectively direct cooling air flow around the cylinder fins, steps must be taken to separate the high pressure zone inside the cowling from the low pressure zone This is accomplished through the use of gap seal material which is attached to the engine baffles *Cooling Systems* The baffles and deflectors normally are inspected during the regular engine inspection, but they should be checked whenever the cowling is removed for any purpose Checks should be made for cracks, dents, loose fasteners, and torn baffle seals The most common cause of baffle damage is engine vibration, which causes severe fatigue; damage resulting from this condition is aggravated and accelerated by poor maintenance practices *Cooling Systems* During baffle inspections, equal attention should be paid to the cowling Damage to cowlings can be caused by *Cooling Systems* Helicopters powered by reciprocating engines present a unique challenge in cooling, due to: ***To solve these conditions, a fan is usually mounted to the engine along with any necessary ducting and baffling***