Reciprocating Engines Flashcards
What is a constant-volume cycle?
In a reciprocating engine heat energy is added to the air
What is a four-stroke also refereed to?
Otto-cycle
How many cycles are in a four stroke?
Five-event cycle
What are the five cycles of a stroke?
Intake, compression, ignition, power, and exhaust
What is a stroke?
The distance the piston moves inside the cylinder
How is the duration of a stroke measured?
In degrees of crankshaft rotation
How many degrees is each stroke?
180°
Why are more cylinders better?
More cylinders the more power and the smoother the operation
The firing impulses are closer together
What is the bore of a cylinder?
The inside diameter of a cylinder
Explain the five event cycles
-Intake-draws the fuel-air mixture into the cylinder
-Compression-compresses the mixture
-Ignition-ignites the mixture
-Power-the mechanical energy is produced
-Exhaust-spent gases are forced out of the cylinder
When does ignition occur?
Before the piston reaches TDC
What happens during the power stroke?
Both valves are closed. The piston is forced down by the pressure. The only time useful work is done
What happens if the ignition timing is correct?
The fuel-air mixture will completely burn shortly after the piston passes TDC on the power stroke
What is valve overlap and why is it important?
-The portion of the operating cycle of a four-stroke reciprocating engine during which both the intake and exhaust valves are open at the same time
-Its important for volumetric efficiency and proper cooling
When does the intake valve open?
During the exhaust stroke just before the piston reaches TDC
When does the exhaust valve closes?
During the intake stroke just after the piston passes TDC
When does the intake valve close?
During the compression stroke just after the piston passes BDC
When does ignition occur?
During the compression stroke just before the piston reaches TDC
When does the exhaust valve open?
During the power stroke just before the piston reaches BDC
1hp equals?
550ft-lbs per second
What are the three types of horsepower involved in recip engines?
Indicated horsepower
Friction horsepower
Brake horsepower
What is brake horsepower?
The actual horsepower delivered to the propeller shaft
What is torque?
The force that produces a twisting or rotational motion of the crankshaft
What are the three major efficiency groups in a recip engine?
Thermal efficiency
Mechanical efficiency
Volumetric efficiency
What is brake thermal efficiency?
The ratio of useful work done by an engine to the heat of the fuel it uses
What is volumetric efficiency?
It’s the ratio of the volume of fuel-air charge inside the cylinder to the single cylinder displacement
What factors affect engine power?
Piston displacement
Compression ratio
Valve timing
Ignition timing
Air density
What is compression ratio?
The ratio of the volume of the cylinder with the piston at the BDC to the volume of the cylinder with the piston at TDC
What is detonation?
An uncontrolled explosion inside the cylinder caused by fuel exceeding it’s critical temperature and pressure
What is pre-ignition?
Ignition of the fuel-air mixture inside the cylinder before the normal spark occurs
What major parts make up the top end of an engine?
Cylinders
Valve assemblies
Pistons
What is choke-ground cylinder?
The barrels are ground so its diameter at the top is slightly smaller than diameter at the bottom
-The top of the barrel expands more than the rest
What is cylinder honing?
-The barrel is honed with a 45° cris-cross finish
-holds oil for lubrication
What are the two ways cylinder walls are hardened?
Chrome plating-orange
Nitriding-blue
What is an interference fit?
The barrel is chilled and the head is heated and then screwed together
How can you tell the difference between the exhaust and intake side of a cylinder head
The exhaust side has more fins
Why does the exhaust valve hollow in the middle?
Its filled with metallic sodium that aids in cooling the valve
To what degree are intake valves ground?
30° to provide for better airflow into the cylinder
To what degree are exhaust valves ground?
45° to make sure of proper seating
What is interference angle?
The difference between the angles of the valve seat and the valve face
How are valves open and closed?
Opened by pressure from the camshaft and closed by the force of two or more coil springs
Why are two springs used for valves?
So if a spring has a resonant frequency it will lose spring pressure
What are cam-ground pistons?
The boss area absorbs more heat because of extra metal and expands more
What are the function of piston rings?
-Sealing action to prevent gas leakage
-transfer heat from the piston to the cylinder wall
-oil control on the cylinder wall preventing oil entering the combustion chamber
Why should you use a different material on piston rings from the piston
They will not wear out the rings initially to provide a good seal
What are the three critical measurements for piston rings?
Eng-gap clearance
Tension
Side clearance
Why should compression rings be staggered?
To minimize leakage
