Answers - Aircraft Reciprocating Engine (Part 1) | Aircraft Systems

Answers - Aircraft Reciprocating Engine (Part 1)

1. b
Both ball bearings and roller bearings are used in aircraft engines. However, since the steel balls in a ball bearing offer less surface contact than the rollers of a roller bearing, ball bearings produce less rolling friction. Therefore, ball bearings are generally used in high-powered reciprocating engines to keep friction to a minimum.

2. c
Before starting a radial engine that has been shut-down for more than 30 minutes, the propeller should be pulled through by hand in the direction of normal rotation to detect hydraulic lock. Hydraulic lock is more likely to occur in radial engines because the inverted lower cylinders may allow oil to collect in the combustion chambers.

3. c
If a bearing fails or is in the process of failing, metal to metal contact is occurring. The friction which accompanies this metal to metal contact generates a great deal of heat and can cause high oil temperatures (answer B). The higher the oil temperature, the more oil is consumed (answer A). Since the question asks for the condition that is least likely to be caused by a failed bearing, answer (C) is the best choice.

4. b
The amount of horsepower an engine produces is directly related to the engine's rpm. Therefore, most aircraft engines must run at a speed in excess of 2,000 rprn to develop sufficient power. However, propeller efficiency at these speeds decreases rapidly. Therefore, in order to maintain an acceptable level of propeller efficiency a reduction gear is used. A reduction gear allows an engine to run at the high rprn needed to produce more horsepower while at the same time, allow the propeller to rotate at a lower, more efficient rpm.

5. c
Volumetric efficiency is a comparison of the volume of a fuel/air charge inducted into all cylinders to the total piston displacement. Factors that reduce volumetric efficiency include part-throttle operation, long, small diameter intake pipes, sharp bends in the induction system, excessive carburetor air temperatures, excessive cylinder head temperatures, incomplete scavenging, and improper valve timing. Of the five choices given in the question only #3, #4, and #5 (answer c) affect volumetric efficiency.

6. c
Special deep-groove ball bearings are used as thrust bearings in most radial engines. This type of bearing generates the least amount of friction of all the types of bearings listed while still being able to withstand both the thrust and radial loads.

7. b
Master rod bearings on radial engines are generally subjected to radial loads only and, therefore, plain bearings are used.

8. a
As a radial engine warms up, the aluminum alloy cylinder heads expand causing the rocker arm in the head to move away from the crankcase. At the same time, the pushrod also expands but at a lesser rate than the cylinder head. The difference in expansion amounts between the cylinder head and pushrod effectively increases the space between the valve stem and rocker arm (valve clearance).

9. c
To determine the displacement of the entire engine, multiply the displacement of each cylinder by the total number of cylinders. The total engine displacement is 1,282.95 cubic inches (1 42.55 x 9 = 1,282.95).

10. c

11. a
The firing order within an engine is designed to provide for balance and to eliminate vibration to the greatest extent possible.

12. c
The time of ignition varies from 20 to 35 degrees before top dead center to ensure complete combustion by the time the piston is slightly past the top dead center position.

13. c
A thin edge on a poppet valve is called a feather edge. Valves with a feather edge are likely to overheat and burn away in a short period of time. Both of these conditions can lead to preignition.

14. a
Each time a cylinder on an engine fires, a pulse of energy is transferred to the crankshaft. Since the power stroke occurs out of line with the throw of the crank, torsional vibrations occur. To dampen these vibrations, moveable counterweights, or dynamic dampers, align to dampen out the vibrations.

15. c

16. a
The master rod in a radial engine is subjected to radial loads only and, therefore, plain bearings are typically used as master rod bearings.

17. b
Brake horsepower is the horsepower that is delivered to the propeller shaft. One way to determine an engine's brake horsepower is to subtract an engine's friction horsepower from its indicated horsepower.

18. a
A cam-ground piston is constructed with a slightly oval cross-section. In other words, the piston's diameter perpendicular to the piston pin is slightly larger than the diameter parallel to the piston pin. This oval shape holds the piston square in the cylinder when an engine is cold and allows the greater mass of the piston pin bosses to expand more freely at operating temperatures. Once expanded, cam-ground pistons provide a better fit within the cylinder.

19. b
One complete revolution of a crankshaft and piston takes 360 degrees, with top dead center (TDC) being 0 degrees and bottom dead center (BDC) 180 degrees. The only time both valves are closed in a four-stroke engine is during a portion of the compression and combustion strokes. This question indicates that the intake valve closes 45 degrees after bottom dead center, or 135 degrees before top dead center (I 80 - 45 = 135). The exhaust valve, on the other hand, opens 70 degrees before bottom dead center, or 11 0 degrees after top dead center (1 80 - 70 = 11 0). Therefore the number of degrees both valves are seated is 245 degrees (1 35 + 11 0 = 245).

20. a
Most cylinders have a certain degree of choke, or taper. This means that the cylinder barrel is slightly narrower at the cylinder head than at the cylinder skirt. A choked cylinder allows for cylinder expansion resulting from the higher operating temperatures near the head. Once a choked cylinder reaches operating temperature, the choked area expands to match the bore at the skirt, and the entire bore becomes straight.

21. a
In a zero-lash or zero-clearance hydraulic valve lifter, oil pressure forces the lifter outward until all clearance between the rocker arm and the valve stem is removed. This condition in an engine is normal.

22. b
The timing disk is a more accurate crankshaft positioning device than timing reference marks. When setting up the ignition timing on an engine, a timing disk should be attached to the engine to measure the crankshaft rotation in degrees.

23. c
As a piston leaves top dead center (TDC) and bottom dead center (BDC), it accelerates and attains its maximum speed at 90 degrees after TDC and 90 degrees after BDC.

24. c
The distance an intake valve may be opened before TDC is limited by several factors. For example, if the intake valve opens too early hot gases remaining in the cylinder may flash back into the intake pipe and induction system causing a backfire.

25. a
The cylinder barrel of a reciprocating engine is made of a steel alloy forging with its inner surface hardened to resist wear. One method used to harden cylinders is nitriding. In the nitriding process, the cylinder is heated and exposed to ammonia or cyanide gas. Nitrogen from the gas is absorbed by the steel causing iron nitrides to form on the steel's surface.

26. a

27. b
At the top of a stroke, a piston is subjected to extreme heat, pressure, and a more erosive environment than at the bottom of a stroke. These factors tend to cause greater piston movement at the top of a cylinder. Therefore, cylinder walls tend to wear more at the top than at the bottom.

28. b
Intake and exhaust valves can be checked for stretch by one of two methods. One involves checking the diameter of the valve stem near the neck of the valve with a micrometer. If the diameter is smaller than normal, the valve has been stretched. The second method involves checking the valve with a radius or contour gauge. The contour gauge is designed to fit along the underside of the valve head. If the contour of the gauge and that of the valve do not match, it indicates that the valve has been stretched.

29. b
In a reciprocating engine, the fuel/air charge is fired by means of an electric spark shortly before the piston reaches top dead center on the compression stroke. The time of ignition varies from 20 degrees to 35 degrees before top dead center, depending upon the engine requirements.

30. b
Ignition occurs at 28 degrees before top dead center. Therefore, the piston will travel 28 degrees to complete the stroke. As the piston moves from top to bottom dead center on the power stroke, the crankshaft turns another 180 degrees. According to the question, the intake valve opens 15 degrees before top dead center of the exhaust stroke, or 165 degrees after bottom dead center. Therefore, total crankshaft travel is 375 degrees (28 + 180 + 165 = 373).

31. c
The stems of some valves have a narrow groove cut in them just below the lock ring groove that allows for the installation of safety circlets or spring rings. The circlets are designed to prevent the valves from falling into the combustion chamber should the valve tip break during engine operation.

32. a
Valve overlap represents the degree of crankshaft travel in which both the intake valve and exhaust valve are open (off their seat). On most reciprocating engines, the intake valve opens before top dead center on the exhaust stroke, while the exhaust valve closes after the piston has passed TDC and started the intake stroke. This results in a valve overlap of anywhere from 40 to 75 degrees.

33. b
When operating properly, an engine equipped with hydraulic lifters will have a valve clearance of zero. However, when the lifters are flat or empty, the lifter must have a specified clearance greater than zero in order to provide the proper operating range.

34. c

35. c

36. c
A full-floating piston pin gets its name from the fact that the pin is free to rotate in both the piston and in the small end of the connecting rod.

37. c
Reciprocating engines are timed so that both the intake and exhaust valves are open near the end of the exhaust stroke and into the beginning of the intake stroke. This valve overlap allows a larger quantity of the fuel/air charge to be drawn into the cylinder which, in return, increases volumetric efficiency. Furthermore, overlap helps to expel the exhaust gases from the previous power stroke and lower operating temperatures.

38. b
When an engine is hot, the clearance between the rocker arm and the valve stem is greater than when the engine is cold. Therefore, if the valves are set to hot clearances when the engine is cold, the overall valve clearance will be excessive and the valves will open late and close early.

39. b
Each valve on a reciprocating engine is closed by two or three helical-coiled springs. If only a single spring were used to close a valve, the spring would vibrate or surge at certain speeds. However, with multiple springs, each spring vibrates at a different engine speed resulting in rapid dampening of all spring surge vibrations.

40. b
Water-mixed degreasing compounds usually contain some form of alkali. If allowed to remain in the pores of metal engine parts when the engine is returned to service, the alkali will contaminate the oil and cause oil foaming.

41. a
The more cylinders an engine has, the closer together the power impulses occur and the smoother the engine operates.

42. c
The compression ratio of an engine is a comparison of the volume of a cylinder when the piston is at the bottom of a stroke to the volume of the same cylinder when the piston is at the top of a stroke.

43. a
The total indicator reading for runout on a crankshaft is the sum of the plus (+) and minus (-) readings. In this case, the runout is .005 in. (.002 + .003 = .005).

44. c
As a general rule, chrome rings should never be used in a chrome cylinder. However, it is extremely important that only approved piston ring and cylinder combinations be used. If approved combinations are not used, excessive cylinder and/or piston ring wear could result.

45. c
The end gap clearance of piston rings is checked by placing a piston ring in the cylinder and inserting a thickness, or feeler gauge, between the two ring ends to determine the amount of clearance.

46. c
An engine's compression ratio is a comparison of the volume of a cylinder when the piston is at the bottom of a stroke to the volume of the same cylinder when the piston is at the top of the stroke. In this question, the ratio is 70: 10 which reduces to 7: 1.

47. a
Water-mixed degreasing compounds usually contain alkali or soap which, if allowed to remain in the pores of the metal, will react with hot oil and cause foaming.

48. b
The basic principle behind doing a power check is to measure the performance of an engine against an established standard and determine if the engine is performing satisfactorily.

49. c
If all the ring gaps happen to be aligned when performing a differential pressure compression check they will allow air to escape from the cylinder and give the same indication as if the rings were defective or worn. To remedy this problem, run the engine for a period of time so the ring gaps have a chance to shift.

50. a
When valve clearance is excessive, the valves will not open as wide or remain open as long as they should. Therefore, both the intake and exhaust valves will open late and close early resulting in a reduced valve overlap period.

51. c
One of the most widely used automatic oil temperature control devices is the floating control thermostat. This unit provides both manual and automatic control of the amount of air that passes through the oil cooler by controlling the oil cooler air-exit door.

52. a
When performing a power check on a reciprocating engine, it should be noted that with a constant air density, a given propeller and blade angle will always turn at the same rpm for a given horsepower. Therefore, if an engine is producing a lower than normal rpm at a full throttle setting, the engine may be weak.

53. b

54. c
When a spark occurs between the electrodes of a spark plug, metal is taken from one electrode and deposited onto another. Therefore, when a spark plug fires positively, the ground electrode wears more than the center electrode and when a spark plug fires negatively, the center electrode wears more than the ground electrode. Furthermore, lead and other impurities produced during the combustion process tend to precipitate to the lower spark plugs, causing them to wear. To help equalize spark plug wear, each time spark plugs are removed they should be replaced in the cylinder next in the firing order to the one from which they were removed and switched from top to bottom.

55. c.
When a differential compression test is being performed on an aircraft engine, the piston should be at top dead center when the air pressure is introduced into the cylinder. If the piston is past top dead center, the air pressure will force the piston to the bottom of the cylinder causing the propeller to rotate in the normal direction of rotation.