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(ASA) Powerplant Oral Study Guide
Questions and Correct Answers
What is the main advantage of a horizontally opposed engine over
a radial engine for powering modern aircraft? Ans: The
horizontally opposed engine has a much smaller frontal area and
is easier to streamline than a radial engine
How many throws are there in the crankshaft of a six-cylinder
horizontally opposed engine? Ans: Six
What kind of connecting rod arrangement is used in a radial
engine? Ans: 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
Of what material are most piston rings made? Ans: Gray cast iron
What is the reason for using hydraulic valve lifters in an aircraft
engine? Ans: Hydraulic valve lifters keep all of the clearance out
of the valve operating mechanism. This decreases the wear of the
valve train components
At what speed does the camshaft turn, relative to the crankshaft
speed in a horizontally opposed engine? Ans: The camshaft turns
at one half of the crankshaft speed
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, 2 | Page
What kind of main bearings are used in a horizontally opposed
engine? Ans: Steel-backed, lead-alloy bearing inserts
On what stroke is the piston of a reciprocating engine when the
intake valve begins to open? Ans: On the exhaust stroke
On what stroke is the piston of a reciprocating engine when the
exhaust valve begins to open? Ans: On the power stroke
Why are both the hot and cold valve clearances give for most
radial engine? Ans: 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.
What is meant by a cam-ground piston? Ans: 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
Where is the piston in a reciprocating engine when the ignition
spark occurs? Ans: About 30 degrees of crankshaft rotation
before the piston reaches top center on the compression stroke
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, 3 | Page
What is meant by a full-floating wrist pin? Ans: 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
Why do most aircraft reciprocating engines use more than one
spring on each valve? Ans: 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
Would excessive valve clearance cause the valves to open early or
late? Ans: Excessive clearance will cause the valve to open late
and close early
What is the purpose of valve overlap in a reciprocating engine?
Ans: Valve overlap allows the inertia of the exhaust gases leaving
the cylinder to help the fresh induction charge start flowing into
the cylinder
What type of piston rings are installed on the pistons of an aircraft
reciprocating engine? Ans: Compression rings, oil control rings,
and oil wiper rings
Why are some exhaust valves partially filled with metallic sodium?
Ans: The metallic sodium melts at engine operating temperature
© 2025 All rights reserved
, 4 | Page
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
What causes detonation in an aircraft engine? Ans: 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
Why is a compression check important for determining the
condition of an aircraft reciprocating engine? Ans: 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
What is meant by the compression ratio of a reciprocating engine?
Ans: 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
What are the two basic sections of a turbine engine? Ans: The hot
section and the cold section
What are the two basic types of compressors that are used in
aircraft turbine engines? Ans: Axial-flow and centrifugal
© 2025 All rights reserved
(ASA) Powerplant Oral Study Guide
Questions and Correct Answers
What is the main advantage of a horizontally opposed engine over
a radial engine for powering modern aircraft? Ans: The
horizontally opposed engine has a much smaller frontal area and
is easier to streamline than a radial engine
How many throws are there in the crankshaft of a six-cylinder
horizontally opposed engine? Ans: Six
What kind of connecting rod arrangement is used in a radial
engine? Ans: 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
Of what material are most piston rings made? Ans: Gray cast iron
What is the reason for using hydraulic valve lifters in an aircraft
engine? Ans: Hydraulic valve lifters keep all of the clearance out
of the valve operating mechanism. This decreases the wear of the
valve train components
At what speed does the camshaft turn, relative to the crankshaft
speed in a horizontally opposed engine? Ans: The camshaft turns
at one half of the crankshaft speed
© 2025 All rights reserved
, 2 | Page
What kind of main bearings are used in a horizontally opposed
engine? Ans: Steel-backed, lead-alloy bearing inserts
On what stroke is the piston of a reciprocating engine when the
intake valve begins to open? Ans: On the exhaust stroke
On what stroke is the piston of a reciprocating engine when the
exhaust valve begins to open? Ans: On the power stroke
Why are both the hot and cold valve clearances give for most
radial engine? Ans: 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.
What is meant by a cam-ground piston? Ans: 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
Where is the piston in a reciprocating engine when the ignition
spark occurs? Ans: About 30 degrees of crankshaft rotation
before the piston reaches top center on the compression stroke
© 2025 All rights reserved
, 3 | Page
What is meant by a full-floating wrist pin? Ans: 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
Why do most aircraft reciprocating engines use more than one
spring on each valve? Ans: 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
Would excessive valve clearance cause the valves to open early or
late? Ans: Excessive clearance will cause the valve to open late
and close early
What is the purpose of valve overlap in a reciprocating engine?
Ans: Valve overlap allows the inertia of the exhaust gases leaving
the cylinder to help the fresh induction charge start flowing into
the cylinder
What type of piston rings are installed on the pistons of an aircraft
reciprocating engine? Ans: Compression rings, oil control rings,
and oil wiper rings
Why are some exhaust valves partially filled with metallic sodium?
Ans: The metallic sodium melts at engine operating temperature
© 2025 All rights reserved
, 4 | Page
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
What causes detonation in an aircraft engine? Ans: 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
Why is a compression check important for determining the
condition of an aircraft reciprocating engine? Ans: 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
What is meant by the compression ratio of a reciprocating engine?
Ans: 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
What are the two basic sections of a turbine engine? Ans: The hot
section and the cold section
What are the two basic types of compressors that are used in
aircraft turbine engines? Ans: Axial-flow and centrifugal
© 2025 All rights reserved
