Explanations: Reciprocating Engines

Question 1

What type of reciprocating engine uses ball bearings?

Answer 1

They are used in high powered reciprocating engines, where keeping friction to a minimum is important.

Question 2

What is an axial load?

Answer 2

The force acting along the axis or centerpiece of a structure, producing a uniform effort.

Also known as a thrust load.

Question 3

What is a radial load?

Answer 3

The maximum force that can be applied to the shaft in the radial direction (any direction perpendicular to the motor shaft axis).

Question 4

Why does a ball bearing produce less rolling friction than roller bearings?

Answer 4

It has a smaller contact area.

Question 5

Why does hydraulic lock sometimes occur in radial and inverted engines?

Answer 5

When the engine is idle, some cylinders sit below the crankcase. Oil will leak from the crankcase, past the piston rings, and fill the combustion chamber. This condition is called hydraulic lock.

Question 6

What will happen if an engine is started before oil is removed from a cylinder containing hydraulic lock?

Answer 6

The piston will move against the noncompressible oil and cause serious damage.

Question 7

Why do some reciprocating engines have reduction gears?

Answer 7

The higher the RPM, the greater the power, however the efficiency of a propeller decreases as the blade tips approach the speed of sound.

Reduction gears allow the engine to reach maximum power but keep the propeller tips moving below the speed of sound.

Question 8

What is the volumetric efficiency of a reciprocating engine?

Answer 8

The ratio of…

the weight of the fuel/air charge taken into the cylinder,

to…

the weight of a charge that would completely fill the entire volume of the cylinder at the same pressure.

Question 9

What is the “groove” of a ball bearing?

Answer 9

The depth of the raceway into which the ball fits.

Question 10

What is the master rod of a reciprocating engine?

Answer 10

The rods that connect the pistons to the crankshaft.

Question 11

What is a rocker arm?

Answer 11

a rocking lever in an engine, especially one in an internal combustion engine which serves to work a valve and is operated by a pushrod from the camshaft.

Question 12

Why is the operating temperature valve clearance of a radial engine greater than that of the cold valve clearance?

Answer 12

When the radial engine is operating, the cast aluminum alloy cylinder head expands far more than the steel push rod.

As the cylinder expands, the rocker arm moves away from the cam ring and the hot, or running, valve clearance becomes much greater than the cold clearance.

Question 13

What is piston displacement?

Answer 13

The total volume swept by the pistons in one revolution of the crankshaft.

Piston Displacement = Piston Area (in. squared) x stroke (in.)

Total Piston Displacement = Volume of one cylinder (in. cubed) x number of cylinders.

Question 14

What is an opposed engine?

Answer 14

An opposed-piston engine is a piston engine in which each cylinder has a piston at both ends, and no cylinder head.

Question 15

When does the ignition of the fuel/air mixture in the cylinder of a reciprocating engine begin?

Answer 15

It begins when the piston is about 20 to 30 degrees before reaching top center on the compression stroke. If timed correctly, it will all be burned up shortly after the piston passes over top center.

Question 16

Why will feather edged valves cause preignition?

Answer 16

Because the heat in the cylinder will cause the thin area to glow red hot and this will ignite the fuel/air mixture before the correct time for ignition.

Question 17

What are torsional vibrations and what are they caused by?

Answer 17

The angular vibration of an object - commonly a shaft - along its axis of rotation.

It is caused by firing impulses of the engine.

Question 18

How do movable counterweights reduce torsional vibration?

Answer 18

Movable counterweights, called dynamic dampers, suspended from certain crank cheeks, rock back and forth and act as pendulums, changing the resonant frequency of the rotating elements, thus reducing the torsional vibration.

Yikes. Word vomit much?

Question 19

What is a cam-ground piston?

Answer 19

A piston whose diameter is a few thousandths of an inch greater in a plane perpendicular to the wrist pin boss than it is parallel to the boss.

When the piston reaches its operating temperature, the large mass of metal in the piston pin boss expands enough that the piston becomes round.

Since the piston is round at its operating temperature, it provides a better seal than it would if it were round while cold and expanded to an out-of-round condition when hot.

Question 20

What is a taper-ground cylinder?

Answer 20

The large mass of the cylinder head expands more when heated than the smaller mass of the cylinder barrel, so the diameter of a choke-ground cylinder becomes uniform when the engine is at its operating temperature.

Question 21

Why are hydraulic valve lifters used?

Answer 21

Because they remove all of the clearance between the rocker arm and the tip of the valve stem.

Question 22

Why does the top of a cylinder wear more than the bottom?

Answer 22

The heat of combustion decreases the efficiency of the lubrication at the top of the cylinder.

Question 23

What are dynamic dampeners?

Answer 23

movable counterweights that rock back and forth and act as pendulums, changing the resonant frequency of the rotating elements, thus reducing the torsional vibration.