Answers - Aircraft Reciprocating Engine (Part 2) | Aircraft Systems

Answers - Aircraft Reciprocating Engine (Part 2)

56. a

Excessive valve clearance describes a condition where there is too much clearance between a rocker arm and the end of a valve stem. The excessive clearance results in the valve opening late due to the time required for the rocker arm to contact the valve stem. Furthermore, the valve will close early due to the decreased dwell time.

57. a
Plain bearings used in aircraft engines are usually made of nonferrous metals, such as silver, bronze, aluminum, and various alloys of copper, tin, or lead. If this type of material is found in the oil sump of an engine and on the surface of the oil filter, it is an indication that the bearings may be experiencing abnormal wear.

58. c
One characteristic of dyna-focal engine mounts is that the shock mounts point toward the engine's center of gravity. This design feature helps prevent vibration from being transmitted to the airframe. The shock mounts consist of a piece of rubber inside a round metal mount. The rubber within the mount helps absorb vibration and allows some torsional flexing.

59. b
Metal particles on engine oil screens or magnetic sump plugs are generally an indication of partial internal engine failure. However, due to the construction of aircraft oil systems, it is possible that metal particles could have collected in the oil system sludge at the time of a previous engine failure. At any rate, the cause or source of the particles should be determined before the engine is returned to service.


60. a
The most likely cause of oil pressure fluctuating between zero and normal oil pressure is a low oil supply. If you have an engine with a low oil supply, the oil pressure will be normal as long as the oil is being picked up by the pump. However, momentary losses of oil pick-up will cause the oil pressure to drop to zero.

61. c
When adjusting the valves on a radial engine with a floating cam ring, the clearance between the cam ring and cam bearing must be eliminated so the cam is in a definite position prior to adjusting the valve clearance. To do this, specific valves must be depressed and released simultaneously to remove the spring tension from the side positions on the cam. This permits the cam to slide away from the valves you are adjusting.

62. a
When the valve clearance on a reciprocating engine is inadequate, the push rods open the valves earlier and close them late. In addition, as the engine warms up, the valve clearances tend to increase as the cylinders expand. Answer A is correct because the valves will not remain seated as long during engine starting and warmup.

63. b
When there is excessive clearance between the valve stem and rocker arm (valve clearance), the valves will not open as wide or remain open as long during engine operation. This reduces the overlap period and the cylinder's volumetric efficiency.

64. c
Valve overlap represents the number of degrees that both the exhaust and intake valves are open. Two benefits of valve overlap are improved scavenging and cooling characteristics and increased volumetric efficiency.

65. c
In a four-stroke engine, each cylinder fires once every two crankshaft revolutions. Therefore, in order for a cylinder to fire 200 times a minute, the crankshaft must rotate at a speed of 400 rpm (200 x 2 = 400).

66. b
Engine crankshaft runout is typically checked when the crankshaft is separated from the engine. Therefore, crankshaft runout is usually checked during an engine overhaul. Furthermore, manufacturers generally require runout checks after sudden stoppage or a sudden reduction in speed, such as a prop strike.

67. c
Whenever a radial engine remains shut down for more than 30 minutes, oil and fuel may drain into the combustion chambers of the lower cylinders or accumulate in the lower intake pipes. These fluids can cause a liquid lock, or hydraulic lock, which can damage the engine if a start is attempted. To check for a liquid lock, the propeller should be turned by hand in the normal direction of rotation a minimum of two complete revolutions.

68. a
The cold cylinder check determines the operating characteristics of each cylinder of an air-cooled engine. The tendency for any cylinder or cylinders to be cold or to be only slightly warm after the engine was running indicates either a lack of combustion or incomplete combustion. If an engine misses in both the right and left positions of the magneto switch, combustion is not taking place in one or more cylinders. Any time there is a lack of combustion or incomplete combustion, the cylinder(s) affected will feel cooler than the cylinder(s) where complete combustion is occurring.

69. b
Exhaust valve blow-by occurs when the exhaust valve does not seat properly, allowing a portion of the fuel/air charge to escape before combustion takes place. Exhaust valve blow-by is identified by a hissing or whistling sound coming from the exhaust stacks.

70. b
On engines that are equipped with a compensated oil pressure relief valve, a higher oil pressure is maintained when the oil is cold. This helps ensure adequate lubrication when the oil is partially congealed. However, as the oil heats up, the relief valve automatically lowers the system pressure to the normal operating range.


71. a
Some lubrication systems provide a means of diluting oil with fuel. When oil is diluted, less power is needed for starting and starting is accomplished more rapidly. However, if an oil dilution valve leaks, the oil will become excessively thin and cause a reduction in oil pressure. Furthermore, because fuel thinned-oil cannot transfer heat as readily as normal oil, an engine operated with diluted oil will have higher oil temperatures.

72. c
The fuel/air mixture which will result in the highest engine temperature varies according to the engine's power setting. For example, at high power settings, lean mixtures produce the highest temperature. Therefore, a mixture leaner than a manual lean mixture of .060 will produce very high temperatures at high power settings.

73. b
Before removing a cylinder from an engine, the piston should be at top dead center on the compression stroke. Having the piston in this position helps prevent damage to the cylinder, piston, and valves and helps relieve pressure on both the intake and exhaust rocker arms.

74. b

75. c

76. b
Generally, standard aircraft cylinder oversizes are 0.010 inch, 0.015 inch, 0.020 inch, or 0.030 inch. The reason aircraft cylinders cannot be oversized as much as automobile cylinders is because aircraft cylinders have relatively thin walls and may have a nitrided surface.

77. c
A magneto check is conducted with the propeller in the high rpm position at a speed between approximately 1,000 and 1,700 rpm. During this check, the ignition switch is moved from the BOTH to the RIGHT position, and then the BOTH to the LEFT position. While switching from BOTH to a single magneto position, a slight but noticeable drop in rpm should occur.

78. c

79. a
If the valve guides of an engine are worn, there will be excessive clearance between the valve guide and the valve stem. The excessive clearance allows oil to seep by the valve stems and enter the intake and exhaust ports, causing high oil consumption.

80. a
When the piston in cylinder number one is rotated 260 degrees past top dead center on the compression stroke, cylinder number three will be 180 degrees past top dead center between the end of the compression stroke and the beginning of the exhaust stroke. When the number three piston is in this position, the exhaust valve is open and the cylinder would not be able to hold pressure.

81. a
Valve overlap identifies the period when both the intake and exhaust valves are open in a cylinder. The only time valve overlap occurs is at the end of the exhaust stroke and the beginning of the intake stroke.

82. b
Metallic-sodium is used in some valves because it is an excellent heat conductor. In a metallic-sodium filled valve, the sodium melts at approximately 208°F. When this happens, the reciprocating motion of the valve circulates the liquid sodium enabling it to carry away excess heat, thereby reducing valve operating temperatures.

83. c
The only way to adjust the valve clearance on an engine using hydraulic lifters is to insert a different sized push rod.

84. a
The viscosity of oil is affected by temperature and it is not uncommon for some grades of oil to become extremely stiff in cold weather. As you know, stiff oil does not circulate well within an engine and, therefore, if a reciprocating engine is operated at high power settings before it is properly warmed up, oil starvation to some parts is likely to occur.

85. c
Manifold pressure represents the absolute pressure of the fuel/air mixture prior to entering the cylinders. Therefore, an increase in manifold pressure for a given rpm represents a higher pressure fuel/air mixture entering the cylinders. This higher pressure produces a corresponding increase in brake mean effective pressure and power output. Any time the brake mean effective pressure or power output is increased, additional force is transmitted through the pistons to the crankshaft and bearing load increases.

86. a
When rigging any carburetor control linkage, the component being moved must contact its stop prior to the stop in the cockpit is reached. This ensures full control travel.

87. a
A combustion engine relies on a specific air/fuel mixture to produce a given amount of power. Any deviation from this mixture affects power output. As an aircraft climbs, air density decreases thereby decreasing the amount of air in the fuel/air mixture. This results in an excessively rich air/fuel mixture which causes a reduction in engine power.

88. a
When an engine takes in air with a high water vapor content, there is less oxygen available for combustion. Any time there is less oxygen available for combustion there is a corresponding decrease in engine power for any given rpm and manifold pressure.


89. c

90. b
After an engine is installed it should be pre-oiled prior to starting. Pre-oiling helps prevent excessive wear or failure of the engine bearings.

91. c
After a push/pull control rod has been adjusted you should check the number of threads engaging the rod end. To check for the proper amount of engagement, an inspection hole is typically provided in which a piece of safety wire is inserted. If the safety wire can pass through the hole, there is insufficient thread engagement.

92. c
Detonation is the uncontrolled burning of the fuel/air mixture. Typical causes of detonation include use of fuel with too low an octane rating, high manifold pressure, high intake air pressure, and engine overheating.

93. c
As a rule, an unsupercharged reciprocating engine operated at altitude produces less power than it does at sea level. This is because, at higher altitudes, less dense air is drawn into the cylinders resulting in a less potent fuel/air charge.

94. c
An extremely lean mixture will either not burn at all or burn so slowly that combustion continues until the intake valve opens near the end of the exhaust stroke. When this happens, the flame in the cylinder ignites the contents in the intake manifold causing an explosion, or backfire within the induction manifold.

95. b
When an engine is being pre-oiled, a line from the inlet side of the engine-driven oil pump must be disconnected to permit the pre-oiler tank to be connected. Then, a line near the nose of the engine is disconnected to allow oil to flow out. Once oil flows out of the engine pre-oiling is complete.

96. c
When reducing power on an engine equipped with a constant speed propeller, care should be taken to never let the manifold pressure get too high for a given rpm. To do this, the throttle should be pulled back to reduce the manifold pressure first followed by a slow reduction in rpm.

97. a
For a given rpm, the fuel/air mixture that results in best power is always richer than the mixture used for best economy. One way to remember this is to associate best power with using more fuel and best economy with using less fuel.

98. a
An extremely lean mixture will either not burn at all or burn so slowly that combustion continues until the intake valve opens near the end of the exhaust stroke. When this happens, the flame in the cylinder ignites the contents in the intake manifold causing an explosion known as a backfire.

99. b
Detonation is the uncontrolled burning of the fuel/air mixture. Typical causes of detonation include use of fuel with too low an octane rating, high manifold pressure, high intake air pressure, and engine overheating.

100. c
A small induction system air leak will have the most noticeable effect on engine operation at idle. The reason for this is that at low engine speeds the volume of air entering the induction system is small. Therefore, the additional air coming in through a crack will lean the fuel/air mixture appreciably.

101. a
When reducing power on an engine equipped with a constant speed propeller, care should be taken to never let the manifold pressure get too high for a given rpm. To do this, the throttle should be pulled back to reduce the manifold pressure first followed by a slow reduction in rpm.

102. c
One way of determining combustion chamber problems is by examining the condition of the spark plugs. For example, normal operation is indicated by a spark plug having a relatively small amount of light brown or tan deposit on the nose of the center electrode insulator. However, if heavy oily deposits are found on the spark plugs, it is a good indication that the rings or valve seals are worn and allowing oil to seep into the cylinder.

103. a
All piston rings let some combustion chamber pressure into the engine crankcase. This pressure is vented to the atmosphere through a crankcase breather. Therefore, if a crankcase breather becomes plugged, pressure will build up inside the crankcase.

104. c
Valve overlap represents the time when both the intake and exhaust valves are open simultaneously. When there is too much valve clearance, the valves do not open as wide or remain open as long as they should. If the valves are not open as long, the amount of overlap decreases.

105. a
A reciprocating engine's critical altitude is that altitude at which the engine can maintain sea level power. Any increase above an engine's critical altitude results in a decrease in available horsepower. The critical altitude of a typical turbocharged engine is generally between 8,000 and 16,000 feet MSL.

106. c
Excessive leakage past the piston rings can be detected by the sound of escaping air at the engine breather tube or oil filler cap. Leakage at the exhaust valve (answer A) is detected by listening for air at the exhaust outlet, and intake valve leakage (answer B) is detected at the air intake.

107. c
Afterfiring refers to a condition when unburned fuel from an excessively rich fuel/air mixture combines with air in the exhaust stacks and ignites, or fires, in the exhaust system. Sticking intake valves (answer A) cause timing conflicts that can lead to flash back in the intake pipe and high engine temperatures, whereas an excessively lean mixture (answer B) causes backfiring.



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