Answers - Powerplant Fuel Metering Systems | Aircraft Systems

Answers - Powerplant Fuel Metering Systems

1. b
Automatic fuel control units sense power lever position, engine rpm, compressor inlet air temperature and density, and burner pressure or discharge pressure. Since turbine engine aircraft do not utilize a mixture control, it is not a factor in the operation of the fuel control unit.

2. a
In order to stabilize internal components and ensure consistent results, most engine manufacturers specify that all trim adjustments be made in the increase direction. In other words, the engine will idle at a value just above the minimum idle speed and maximum thrust will be obtained slightly before the power levers reach the full forward position.

3. b

4. a
A supervisory electronic engine control (EEC) includes a computer that monitors several engine operating criteria and uses this information to adjust a standard hydromechanical fuel control unit (FCU) to obtain a constant thrust for a given power lever position.

5. b
A full-authority electronic engine control (EEC) performs all of the functions required to operate an engine. In other words, it receives data from the aircraft and engine systems and then issues commands to various actuators that control engine operating parameters.

6. c
Any fault in a supervisory electronic engine control (EEC) automatically causes the EEC to relinquish engine control to the hydromechanical control unit. At the same time, the EEC sends a signal to the cockpit that illuminates an annunciator light to inform the flight crew of the change in operating mode.

7. b

8. a
Turbine engines are designed to operate using a specific type of fuel with a given BTU value and specific gravity. Therefore, if a performance check is made on an engine using an alternate fuel, the specific gravity setting should be checked and changed as necessary on the fuel control unit to ensure proper performance. Although an alternate fuel is likely to cause an engine to perform differently.

9. c
Part of the procedure used for trimming an engine includes measuring engine inlet barometric pressure and ambient temperature. To ensure a temperature value that is accurate, it is common practice to hang a thermometer in the shade of the nose wheel-well.

10. c

11. a
When high-pressure air is bled from the compressor for various aircraft functions, it has the same effect as decreasing the compressor's efficiency. Therefore, if a trim adjustment is made with the bleeds on, an inaccurate or overtrimmed condition will result. Because of this, most engine manufacturers require that an engine be trimmed with the engine bleeds off.

12. c
An automatic mixture control utilizes a sealed, helium-filled bellows that fluctuates with changes in air density. As air density decreases due to an increase in altitude or temperature (answer C), the helium pressure inside the bellows causes the bellows to expand and move a poppet valve that adjusts the fuel/air mixture.

13. c
For an engine to develop maximum power at full throttle, the fuel mixture must be richer than that used at cruise power settings. The additional fuel is used to cool the engine and prevent detonation. One way to make sure the engine gets this additional fuel is with an economizer valve which automatically enriches the fuel/air mixture at throttle settings above 60 to 70 percent power.

14. a
The force that is responsible for discharging fuel into the throat of a float-type carburetor is a result of the differential pressure between the fuel discharge nozzle within the venturi and the pressure exerted on the fuel within the float chamber. As engine speed increases the amount of air flowing past the venturi increases causing a greater pressure differential and corresponding increase in fuel flow.

15. b
The main air bleed in a float-type carburetor allows air to be drawn into the carburetor venturi along with the fuel. The additional air helps decrease the fuel density and destroy surface tension, resulting in better vaporization and control of fuel discharge at lower engine speeds. If the main air bleed becomes clogged, it stands to reason that less air will be drawn into the engine and the fuel/air mixture will become excessively rich at high power settings.

16. b
Most carburetor floats are adjusted by adding or removing shims between the needle seat and the throttle body. This method of adjustment is much more convenient and precise than adjusting the length of the float shaft or changing the angle of the float arm pivot.

17. b

18. c

19. b
During engine operation, the carburetor float is responsible for maintaining the appropriate amount of fuel within the float bowl. When fuel is drawn from the bowl, the float lowers and fuel is allowed into the bowl. By the same token, if a float should become punctured, it would fill with fuel and sink. With the float resting on the bottom of the bowl, fuel would be allowed to continually enter the bowl and eventually enrich the fuel/air mixture.

20. b
The back-suction type mixture control system used on some float-type carburetors utilizes low pressure from the venturi to control the amount of air pressure within the float chamber. By varying the pressure acting on the fuel in the float chamber, the pressure differential between the carburetor throat and float bowl can be controlled which, in turn, provides control of the mixture being supplied to the engine.

21. c
As both altitude and temperature increase, the air becomes less dense. Therefore, as an airplane climbs or as the air temperature increases, the amount of oxygen drawn into an engine decreases. In either of these situations if an engine is not equipped with a carburetor that can be adjusted for increases in altitude and temperature, the fuel/air mixture will become excessively rich.

22. c

23. a

24. a

25. b
In a back-suction type mixture control system a certain amount of low pressure air from the venturi acts on the fuel in the float chamber. By controlling the amount of low pressure air that is vented to the float chamber you can control the pressure differential between the carburetor throat and float chamber which, in turn, dictates the amount of fuel that flows into the engine. With this type of system, when the mixture control is placed in idle cutoff position, all atmospheric pressure is removed from the float chamber and the fuel is placed under negative pressure. This stops fuel flow which, in turn, stops the engine.

26. a

27. c
With the throttle valve closed at idling speed, air velocity through the venturi is so low that it cannot draw enough fuel from the main discharge nozzle to keep an engine running. Therefore, in order to allow the engine to idle, a fuel passageway called an idling jet is incorporated in the low pressure area between the throttle valve and throttle body that discharges fuel into the throttle body.

28. a
In a pressure-type carburetor, fuel follows in the same path at idling as it does when the main metering system is in operation. However, because of the low air velocity through the venturi, insufficient differential pressure exists to displace the diaphragm that holds the poppet valve open and allows fuel to flow. Therefore, a spring is used to physically hold the poppet valve off its seat so fuel can flow while the engine is idling.

29. b

30. c
The automatic mixture control on a pressure carburetor contains a tapered needle valve that is connected to a pressurized metallic bellows that expands and contracts with changes in air pressure. With this type of system, when the aircraft climbs and the atmospheric pressure decreases, the bellows expands and pushes the tapered needle valve into the atmospheric passage which restricts the flow of air to the regulator unit. With the flow of air to the regulator decreased, the amount of fuel allowed to flow to the engine is also decreased. Based on this, if the bellows in an automatic mixture control were to rupture, fuel flow would continue normally and the engine would run rich.

31. b
In a float-type carburetor, a float chamber is provided between the fuel supply and the metering system to provide a nearly constant level of fuel to the main discharge nozzle. The fuel level in the float chamber is set slightly lower than the discharge nozzle outlet to allow differential pressure to draw the fuel into the carburetor throat. If the float chamber fuel level was higher or at the same level as the nozzle outlet, fuel would run out of the carburetor when the engine was shut down.

32. a

33. c
The fuel level in the float chamber of a carburetor should be one-eighth inch below the main discharge outlet. When measuring this level, your measurements should be taken away from the edge of the float chamber since the fuel tends to cling to the walls of the chamber. If a measurement were taken at the edge, an inaccurate measurement may result.

34. c

35. a
If a float type carburetor leaks when an engine is shut down, either the needle valve is not firmly seated or the float level is adjusted too high.

36. a
The main discharge nozzle cannot be used at idle speeds because there is insufficient airflow through the venturi to create enough of a pressure differential to force fuel from the nozzle. Because of this, float-type carburetors employ an idle jet or idle discharge nozzle that takes advantage of the low pressure area between the throttle valve and throttle body. This idle jet provides sufficient fuel flow to allow an engine to run at low rpm.

37. c

38. b
In a float-type carburetor, the throttle valve controls the mass airflow through the venturi and, therefore, must be located downstream of both the venturi and the main discharge nozzle. However, in pressure injection carburetors the throttle valve is located after the venturi and just before the main discharge nozzle.

39. b
Carburetors are calibrated at sea level, and the correct fuel/air mixture is established at that altitude with the mixture control set in the FULL RICH position. However, as altitude increases, the density of air entering the carburetor decreases while the density of the fuel remains the same. This means that at higher altitudes, the mixture becomes progressively richer. Therefore, the purpose of the mixture control is to allow the pilot to control the amount of fuel that is mixed with the incoming air.

40. b

41. a
In float-type carburetors with a back-suction type mixture control system, a certain amount of low pressure air from the venturi is vented to the float chamber to control the pressure differential between the float chamber and venturi. By varying the pressure acting on the fuel in the float chamber, the system varies the amount of fuel being supplied to the engine. With this type of system, when the mixture is placed in the idle cutoff position, the float chamber is filled with low pressure air from the venturi eliminating the pressure differential. With no difference in air pressure, there is no force to pump fuel into the engine and, therefore, the engine quits running.

42. c

43. a

44. c
In a float-type carburetor, the throttle valve controls the mass airflow through the venturi and, therefore, must be located downstream of the venturi and upstream of the engine.

45. b

46. a
When the throttle valve is opened quickly, a large volume of air rushes through the carburetor. To ensure that enough fuel is mixed with the onrush of air, carburetors are equipped with a small fuel pump called an accelerator pump that provides a short burst of fuel when the throttle is advanced rapidly.

47. b
A mixture control on a carburetor controls the ratio of the fuel/air mixture by allowing a pilot to regulate the amount of fuel introduced into the mixture. Depending on the type of carburetor, the mixture control can be a manual or an automatic device.

48. c

49. b
Some continuous-flow fuel injection systems used on aircraft engines have a fuel discharge nozzle located in each cylinder head. The nozzle outlet is directed into the intake port where fuel and air are mixed just prior to entering the cylinder. Although some systems inject fuel directly into each cylinder, this is not the case with continuous-flow systems.

50. c

51. b
The accelerator pump in a pressure injection carburetor responds to changes in manifold pressure. For example, when the throttle valve is opened rapidly manifold pressure increases and causes the accelerator pump to inject additional fuel into the carburetor throat to maintain the proper fuel/air mixture.

52. c
Fuel metering in a float-type carburetor is accomplished with either an idle jet or a main metering jet. At idle speeds, airflow through the venturi is not great enough to draw fuel from the main discharge nozzle, so an idle jet located between the throttle valve and throttle body supplies fuel at low speeds when the throttle valve is closed. However, at all speeds above idle, the main metering jet supplies the necessary fuel to keep the engine running.

53. c
This question is asking when the fuel injection system discharges fuel into the intake port, not when the fuel enters the cylinders. In a continuous cylinder fuel injection system, the injector pump is not timed to inject fuel into the intake port at a specific time. Instead, fuel is always available at all intake ports.

54. b

55. b

56. a

57. c

58. c
Backfiring is a condition that occurs when the fuel/air mixture within the induction system ignites and explodes when the intake valve opens. In a direct cylinder fuel injection system, the intake valve only allows air to enter the cylinder, while the fuel is injected through a separate nozzle. This eliminates any mixing of fuel and air in the induction system which, in turn, prevents backfiring.

59. b
The throttle valve limits the airflow through the carburetor at all throttle settings except full throttle. At full throttle, the throttle valve is opened all the way leaving only the venturi to limit the airflow.

60. a

61. c
When carburetor heat is applied, warm air is directed into the carburetor intake. Warm air is less dense than cool air and, therefore, the application of carburetor heat results in a richer fuel/air mixture.

62. a
When adjusting a carburetor's idle speed or mixture, the engine should be warmed up and operating in its normal temperature range, since fuel vaporization qualities are different in an engine when it is cold.

63. a
On radial engines, fluids tend to seep into the lower cylinders and cause liquid lock. To prevent adding to this problem, the priming system on a radial engine only primes the cylinders that are horizontal or pointing upward. Therefore, on a nine-cylinder radial engine, only cylinders one, two, three, eight, and nine are primed.

64. c
The idle jet in a float-type carburetor utilizes an idle air bleed that allows air to be mixed with the fuel before it enters the carburetor throat. This aids in vaporizing fuel before it is drawn into the cylinder.

65. c

66. c
At idle speeds, an engine requires a rich fuel/air mixture so there is additional fuel available for cooling. However, as engine speed is increased to a cruise setting, the additional ram airflow created by the propeller and forward section of the aircraft cool the engine sufficiently and fuel requirements decrease. Then, as power output approaches maximum, additional fuel is again required to aid in cooling and help prevent detonation. Based on this, curve number two best represents an aircraft engine's fuel/air ratio throughout its operating range.

67. a
Pressure carburetors employ vapor venting systems that take the excess fuel vapor created in the fuel system and direct it back to the fuel tank. With this type of system, air enters the vented chamber and displaces the fuel thereby lowering the fuel level. When the fuel level reaches a predetermined level, the float pulls down on the vapor vent valve and allows fuel vapor to flow back to the fuel tank. Based on this, if the vapor vent valve should stick open or the float become filled with fuel and sink, fuel and fuel vapor continuously flow back to the fuel tank.

68. a
Idle speed on engines using a float-type carburetor is adjusted by limiting how far the throttle valve will close. This is usually accomplished with an adjustable throttle stop or linkage. Although an orifice and adjustable tapered needle are often used on automobile engines, this arrangement is not used on aircraft engines.

69. a

70. c

71. c
The ideal conditions for trimming a turbine engine are no wind, low humidity, and standard temperature and pressure. Because standard day conditions seldom exist, engine manufacturers produce trim charts to compensate for nonstandard conditions.

72. c

73. b
To ensure efficient combustion, fuel must be properly mixed with air, or atomized, before it enters the cylinders. The more fully a mixture is vaporized, the greater the efficiency of the combustion process. On the other hand, if the fuel is not fully vaporized, less fuel mixes with the intake air and the mixture becomes lean even though there is an abundance of fuel present.

74. c
When adjusting the mixture on an idling engine, you should observe the engine rpm gauge on aircraft with a fixed-pitch propeller and the manifold pressure gauge on aircraft equipped with constant-speed propellers. In either case, the power output will increase to a maximum when the mixture is set properly.

75. c

76. a

77. c
On engines that do not use a manifold pressure gauge, you must observe the tachometer for an indication of correct idle mixture. In most cases, the idle mixture should be adjusted so that when the mixture control is pulled toward the idle cutoff position, a 10 to 50 rpm rise occurs prior to a rapid decrease as the engine ceases to fire.

78. c

79. b

80. b
Water vapor is a non-combustible gas. Therefore, when humid air is drawn into an engine, the engine's volumetric efficiency decreases, causing a decrease in engine power.

81. b
At low engine speeds there is insufficient airflow through the carburetor to allow the main discharge nozzle to operate properly; therefore, a separate idle jet is installed in the low pressure area between the throttle valve and throttle body to supply fuel for idling. If an idle jet should become clogged, the engine will not idle.

82. a

83. b
A water injection system adds an alcohol-water compound to the fuel/air mixture to allow an engine to achieve higher manifold pressures and corresponding increase in power without promoting detonation.

84. a

85. b

86. c

87. c
In order for an engine to effectively operate at high power settings, the fuel/air mixture must be rich so there is additional fuel available to aid in cylinder cooling. On engines equipped with an economizer system, when the throttle is advanced beyond approximately 70 percent power the economizer valve opens and automatically enriches the fuel/air mixture.

88. c

89. b

90. c
Of the air consumed by a turbine engine, only about 25 percent is used to support combustion. Because of this, high atmospheric humidity has very little effect on the thrust produced by a jet engine. Furthermore, neither engine pressure ratio nor compressor and turbine rpm decreases noticeably.

91. a
On an aircraft that utilizes a pressurization and dump valve, the pressurizing part of the valve separates the primary and secondary fuel flows. At all engine speeds below approximately 30 to 50 percent N1, the pressurization valve is closed and fuel flows through the primary only. Therefore, when the engine is shut down, the pressurization valve is closed, and stays closed until the engine is running at a medium power setting. The dump part of the valve drains, or dumps the fuel manifold when the engine is shut down. Therefore, when an engine with a pressurization and dump valve is shut down, both valves are open.

92. b
To compensate for decreases in air density, an automatic mixture control decreases fuel flow from the carburetor as an aircraft climbs to higher altitudes. However, if the automatic mixture control were to malfunction and stick in the extended position, the fuel/air mixture would become progressively leaner as the aircraft descends to lower altitudes. With a lean mixture, there is little excess fuel available to aid in engine cooling, so engines that run on a lean mixture typically have high cylinder head temperatures.

93. c

94. a
Detonation is the explosive, or rapid combustion of unburned fuel in a cylinder that results in an extremely rapid pressure rise. Detonation can happen any time an engine overheats or if the improper fuel grade is used.

95. a
If a carburetor leaks fuel from the discharge nozzle, it is an indication that the fuel level is too high in the float chamber. A high fuel level can be caused by a float that is adjusted too high, a leaking or saturated float, dirt trapped between the needle and seat, or a worn needle and seat. Based on this and the choices given, the only logical choice would be to replace the needle valve and seat.

96. c
The RSA fuel injection system relies on both air and fuel forces to provide the correct pressure differential across the primary metering jet. The Teledyne-Continental injection system, on the other hand, uses a special fuel pump to produce the fuel metering pressure.

97. a
Some Teledyne Continental fuel injection systems employ an altitude compensating aneroid valve to prevent an overly rich mixture during sudden acceleration. To accomplish this, an evacuated bellows responds to upper deck pressure to control the size of a variable orifice. When the throttle is suddenly opened, the aneroid holds the orifice open until the volume of air flowing into the engine increases.

98. c