Aviation Powerplants: Internal-Combustion Engine Theory and Performance
Terms in this set (25)
Why are recip engines for aircraft called heat engines?
Because they utilize heat energy to produce the power for propulsion.
Define the terms bore and stroke
Stroke- The distance through which the piston travels
Bore- The internal diameter of a piston head
What are the TDC and BDC positions of the piston?
Top dead center and bottom dead center
List the four stroke cycle of a piston engine
Intake stroke, compression stroke, power stroke, and exhaust stroke.
What are the positions of the intake and exhaust valves during the power stroke?
Intake valve is closed while exhaust valve opens.
At what point in the operating cycle of an engine does ignition take place?
On the compression stroke
Why is ignition timed to take place at this point?
To allow time for complete combustion of the fuel.
What is valve overlap?
The angular distance through which both valves are open.
Define the terms valve lead and valve lag
Valve lag- the opening or closing of the intake or exhaust valves after TC or BC
Valve lead- the opening or closing of the intake or exhaust valves before BC or TC
Define the term power
It is the rate of doing work.
What is indicated horsepower?
It is the horsepower developed by the engine, that is, the total horsepower converted from heat energy to mechanical energy.
Define the term brake horsepower
The actual horsepower delivered by an engine to a propeller or other driven device.
Define the term friction horsepower
It is the part of the total horsepower necessary to overcome the friction of the moving parts in the engine and its accessories.
How are ihp, fhp, and bhp related?
Bhp= ihp - fhp;
What is the purpose of a dynamometer or prony brake?
It is used to measure the torque, or turning moment, produced by an engine.
Define mechanical and thermal efficiencies
Mechanical Efficiency- It is the measure of an engines ratio of the brake horsepower, or shaft output, to the indicated horsepower, or power developed in the cylinders.
Thermal Efficiency- It is a measure of the heat losses suffered in converting the heat energy of the fuel to mechanical work.
Define volumetric efficiency
It is the ratio of the volume of the fuel-air charge burned by the engine at atmospheric pressure and temperature to the piston displacement.
List some of the causes that could reduce volumetric efficiency
-Improper valve timing
-High engine rpm
-High carburetor air temperature
-Improper design of the induction system
-High combustion chamber temperature
List some of the likely causes of detonation
-High inlet-air temperature
-Insufficient fuel octane rating
-Excessive engine load
-Overadvanced ignition timing
-Excessively lean fuel-air mixture
-Excessive compression ratio
Describe the cause of preignition
It is caused when there is a hot spot in the engine that ignites the fuel air mixture before the spark plug fires.
Define compression ratio
It is a factor that controls the maximum horsepower which can be developed by the engine, other factors remaining the same.
What factors limit the compression ratio of an engine?
If the compression ratio of the internal combustion engine is much greater than 10:1, preignition or detonation may occur and cause overheating, loss of power, and damage to the engine. As the compression ratio increases, the pressure in the cylinder will also increase.
Define brake specific fuel consumption
It is the number of pounds of fuel burned per hour for each bhp produced.
What effect does carburetor air temperature have on engine operation?
It affects the density, and therefore the quantity, of air taken into the engine. If the CAT is too high, detonation results.
What effect does exhaust back pressure have on engine performance?
It has a decided effect on engine performance because any pressure above atmospheric at the exhaust port of a cylinder will reduce volumetric efficiency.
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