Answers - Aircraft Engine Cooling Systems | Aircraft Systems

Answers - Aircraft Engine Cooling Systems

1. b
Many reciprocating engines utilize cylinder baffles to help direct cooling air into close contact with all cylinder parts.

2. b
In an augmenter system, engine exhaust gases are discharged into a stainless steel augmentor tube. The flow of high velocity exhaust gases within the tube creates an area of low pressure at the augmentor inlet that draws additional air from within the cowl into the augmentor tube where is it discharged overboard with the exhaust. This process increases the airflow over the engine and aids in cooling.

3. c
Cylinder deflectors and baffles are designed to force air over the cylinder cooling fins to ensure proper cooling. Therefore, if a cylinder baffle or deflector is damaged, it should be repaired as soon as possible to prevent a loss of cooling efficiency. Even a small amount of damage could cause a localized hot spot and an eventual engine malfunction.

4. b
Cracks in the cooling fins of a cylinder are allowed, provided they are within the manufacturer's allowable limits. To repair a cracked cooling fin, you should remove the damaged fin then contour file the affected area.

5. b
When performing repairs to a cylinder's cooling fins, the engine manufacturer's service or overhaul manual should be consulted to ensure the repair is within limits.

6. b
If a cooling fin is inadvertently bent on an aluminum cylinder head and no crack forms, the fin should be left alone. Aluminum cooling fins are very brittle, and any attempt to straighten them could cause them to crack or break.

7. c
On an air-cooled reciprocating engine, cooling fins are located on the cylinder head, cylinder barrel, and on the underside of the piston head.

8. c
Cowl flaps are typically located on the bottom of an engine cowl and provide a means of controlling the amount of air that exits the cowl which, in turn, controls the amount of air flowing around the cylinders. For example, opening the cowl flaps increases the air exit area which effectively increases the amount of air that can circulate over the cylinder fins. Furthermore, the outside airstream flowing over an opened cowl flap creates a low pressure area which further assists in removing heat from the engine compartment.

9. a
Cowl flaps are small doors at the rear of an engine cowling that are opened to vary the amount of cooling air that flows through the engine compartment. In normal cruise flight, the forward motion of the aircraft typically produces enough airflow over the engine that the cowl flaps can remain closed.

10. c
Cylinder baffles are sheet metal shields located in an engine compartment that channel air around the cylinders for cooling. If a small crack develops in a cylinder baffle it is acceptable to stopdrill the crack.

11. b
An engine's cowling and baffles are designed to channel air over the engine cylinders to aid in removing heat from the engine. The cowling is responsible for receiving impact air and making it flow around the engine while the baffles direct the air close to the cylinder fins to prevent hot spots from forming.

12. b
Cowl flaps are small doors at the rear of an engine cowling that are used to vary the amount of cooling air that flows through the engine compartment. When an engine is operated on the ground, the airflow through the cowl is limited due to the lack of forward motion. Therefore, to keep the engine from overheating, the cowl flaps should be placed in the full open position to allow the maximum amount of cooling air to flow through the engine compartment.

13. a
In a turbine engine, the fuel/air mixture is burned in the combustors, then flows into the first stage turbine nozzle guide vanes. Therefore, of the choices given, the guide vanes operate at the highest temperatures in a turbine engine.

14. a
Any time an electrical component fails to operate, the first action should be to check the component's fuse or circuit breaker.

15. a

16. a
The cooling fins on reciprocating aircraft engines are designed with a precise surface area to dissipate a certain amount of heat. Therefore, if a large piece of cooling fin breaks off from a cylinder, a hot spot can develop.

17. c
The hottest area on a cylinder head is around the exhaust valve and, therefore, requires the greatest fin area per square inch.

18. b
When operating a reciprocating engine-powered helicopter, ram air pressure from the rotor system is usually not sufficient to cool the engine, particularly when the helicopter is hovering. Therefore, many helicopters utilize large engine-driven fans to maintain a strong flow of air around the engine.

19. b
In a typical aircraft reciprocating engine, about 40 percent of the heat generated in the engine is carried out with the exhaust gas while approximately 30 percent is removed by the oil and the engine's cooling system. The remaining 30 percent is converted into useful power. Therefore, the majority of heat generated by combustion is carried out with the exhaust gases.

20. b
Although a broken cooling fin reduces cooling efficiency, it is not necessarily cause for rejection. For example, if the manufacturer's service limits have not been exceeded, the cooling fin may be filed to produce smooth contours and remain in service.

21. b
After a flight and a few minutes of taxiing, an engine typically will not become excessively warm and, therefore, can be shut down almost immediately. However, if an engine becomes excessively hot as indicated by the cylinder head temperature gauge and the oil temperature gauge, you should allow the engine to cool at idle speed for a short time before shutdown.

22. c
Cylinder head temperature is usually measured with a thermocouple sensing device. A thermocouple consists of a circuit with two dissimilar metal wires that are joined at both ends to form two junctions. When one junction is heated, the thermocouple generates an electric current that can be measured by a galvanometer. The hotter the high temperature junction, the greater the current produced. By calibrating the galvanometer in degrees, it becomes a thermometer.

23. a
At higher power settings, a very lean mixture does not allow any excess fuel into the engine to aid in cooling and, therefore, high cylinder temperatures typically result.

24. c
When air is compressed, its temperature rises. Therefore, some turbocharger systems utilize an intercooler that cools the compressed air before it enters the carburetor.

25. c
When an engine is operated for a long period at idle rpm, a rich fuel/air mixture must be used to keep cylinder head temperatures within acceptable limits. However, after prolonged operation, the excess fuel has a tendency to build up and foul out the spark plugs.

26. a
Almost all reciprocating engine cylinder barrels have cooling fins machined directly onto their outside surfaces to help dissipate heat. These fins allow heat to be conducted away from the inside of the cylinder, allowing the use of stronger, lighter alloys.

27. c
Many reciprocating engines use blast tubes to direct cooling air to inaccessible areas of an engine compartment. A blast tube is simply a small pipe or duct that channels air from the main cooling air stream onto heat-sensitive components such as spark plugs and alternators.

28. a
Approximately 25 percent of the air passing through a turbine engine's combustion chamber is used to support combustion while the other 75 percent is used to propel the turbine and cool the engine.

29. b
Volumetric efficiency is the ratio of the volume of the fuel/air charge drawn into a cylinder to the actual volume of the cylinder. Therefore, anything that limits airflow through an engine's induction system will cause a decrease in volumetric efficiency. At part throttle operation, the partially closed throttle valve restricts airflow to the cylinders and, therefore, causes a decrease in volumetric efficiency.

30. b
The majority of aircraft engine pistons are machined from aluminum alloy forgings. On some pistons, cooling fins are provided on the underside of the piston to facilitate the transfer of heat to the engine oil.

31. a
Cowl flaps are used to control the amount of air that flows over an engine. During ground operations, many aircraft engines have a tendency to overheat due to the decreased airflow into the cowling. Therefore, during most ground operations the cowl flaps are typically left fully open to provide maximum cooling.

32. b
Volumetric efficiency is the ratio of the volume of the fuel/air charge drawn into a cylinder to the actual volume of the cylinder. With high engine temperatures the air entering an engine heats up and becomes less dense before it enters the cylinders. Because of this, less oxygen reaches the engine's cylinders and volumetric efficiency decreases.