Answers - Aircraft Reciprocating Engine


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
The four-stroke cycle begins when the piston starts moving down in the cylinder on the intake stroke. When the piston reaches bottom center it reverses direction and starts moving up on the compression stroke. Near the top of the compression stroke, the spark plug fires causing ignition of the fuel/air mixture. As soon as the fuel/air mixture begins to burn, the piston is forced down in the power stroke. As the piston approaches bottom center, the exhaust valve opens and the piston reverses direction to begin the exhaust stroke. Therefore, the five events of a four stroke engine are intake, compression, ignition, power, and exhaust.

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