Reciprocating Engines

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1. What is the main advantage of a horizontally opposed engine over a radial engine for powering modern aircraft?
The horizontally opposed engine has a much smaller frontal area and is easier to streamline than a radial engine.
2. How many throws are there in the crankshaft of a six-cylinder horizontally opposed engine?
Six.
3. What kind of connecting rod arrangement is used in a radial engine?
A master rod connects the single throw of the crankshaft with a piston. All of the other pistons are connected to the master rod with link rods.
4. Of what material are most piston rings made?
Gray cast iron.
5. What is the reason for using hydraulic valve lifters in an aircraft engine?
Hydraulic valve lifters keep all of the clearance out of the valve operating mechanism. This decreases the wear of the valve train components.
6. At what speed does the camshaft turn
relative to the crankshaft speed in a horizontally opposed engine? ,The camshaft turns at one half of the crankshaft speed.
7. What kind of main bearings are used in a horizontally opposed engine?
Steel-backed lead-alloy bearing inserts.
8. On what stroke is the piston of a reciprocating engine when the intake valve begins to open?
On the exhaust stroke.
9. On what stroke is the piston of a reciprocating engine when the exhaust valve begins to open?
On the power stroke.
10. Why are both the hot and cold valve clearances given for most radial engines?
The hot clearance is given for valve timing purposes. The timing is adjusted with the valves in cylinder number one set with the hot clearance. When the timing is set all of the valves are adjusted to their cold clearance.
11. What is meant by a cam-ground piston?
A piston that is not perfectly round. Its dimension parallel with the wrist pin is several thousandths of an inch less than its dimension perpendicular to the wrist pin. When the piston reaches operating temperature the metal in the piston pin boss expands enough that the piston becomes perfectly round.
12. Where is the piston in a reciprocating engine when the ignition spark occurs?
About 30 degrees of crankshaft rotation before the piston reaches top center on the compression stroke.
13. What is meant by a full-floating wrist pin?
A wrist pin that is not clamped in either the piston or the connecting rod. Full-floating wrist pins are kept from scoring the cylinder walls by soft metal plugs in their ends.
14. Why do most aircraft reciprocating engines use more than one spring on each valve?
By using more than one spring and having the wire diameter and pitch of the springs different valve float is minimized. The springs have different resonant frequencies so at least one spring will always be exerting a force on the valve.
15. Would excessive valve clearance cause the valves to open early or late?
Excessive clearance will cause the valve to open late and close early.
16. What is the purpose of valve overlap in a reciprocating engine?
Valve overlap allows the inertia of the exhaust gases leaving the cylinder to help the fresh induction charge start flowing into the cylinder.
17. What type of piston rings are installed on the pistons of an aircraft reciprocating engine?
Compression rings oil control rings and oil wiper rings.
18. Why are some exhaust valves partially filled with metallic sodium?
The metallic sodium melts at engine operating temperature and sloshes back and forth inside the hollow valve. It picks up heat from the valve head and transfers it into the valve stem so it can be transferred to the cylinder head through the valve guide.
19. What causes detonation in an aircraft engine?
Excessive heat and pressure in the engine cylinder causes the fuel-air mixture to reach its critical pressure and temperature. Under these conditions the mixture explodes rather than burns. This explosion is called detonation.
20. Why is a compression check important for determining the condition of an aircraft reciprocating engine?
A compression check can determine the condition of the seal between the piston rings and the cylinder walls and the seal between the intake and exhaust valves and their seats.
21. What is meant by the compression ratio of a reciprocating engine?
The ratio of the volume of the cylinder with the piston at the bottom of its stroke to the volume with the piston at the top of its stroke