Engine Flashcards
Two main classes of aircraft engine
Piston and Gas
Four types of piston engine
Radial, Inline, Inverted inline, Horizontally opposed
Radial engine cylinder arrangement
Cylinder are arranged in a circle around the crankshaft
Inline engine cylinder arrangement
Cylinder arrangement in one or more lines, either upright or inverted
Standard piston engine today
Horizontally opposed as it is
- Low bulk and streamlined to reduce drag,
- Ease of cooling
- High power/weight ratio
- Positioning of the thrust line to give enough propeller ground clearance
- Good visibility from the cockpit
Horizontally opposed cylinder arrangement
Cylinders arranged horizontally in two lines on opposite sides of the crankshaft
What do piston (reciprocating) engines use as fuel
Aviation gas (AVGAS)
What does the piston do
Moves back and forth within the cylinder and provides a gas-tight seal
What does the connecting rod do
Connects the piston to the crankshaft
What does the valves do
Open and close alternatively to admit the fuel air mixture to the cylinder and to enable the exhaust gases to be expelled
What does the cylinder do
Provides the enclosed space for combustion to take place
What does the spark plugs do
Ignite the compressed fuel air mixture in the cylinder
What does the crankshaft do
Converts the reciprocating movement of the piston to a turning motion and transmits the power developed in the cylinders to the propeller
How does a piston engine work
The piston moves up and down inside the cylinder, in petrol engines a mixture of vaporised fuel and air is burned, the heat generated causes the gases in the cylinder to expand rapidly and push the piston down the cylinder. The reciprocating movement is converted into a rotation of the crankshaft and the attached propeller.
Energy changes in the piston engine
Chemical energy in the fuel is first converted to heat energy by burning it in the cylinder and then to mechanical energy to turn the propeller and develop the thrust required for powered flight
What is one stroke
The full movement of the piston either up or down in the cylinder
How many revolutions are in a four stroke cycle
Two
What are the four strokes
Intake, compression, power, exhaust
What happens during the intake stroke
The fuel air mixture is sucked into the cylinder past the open inlet valve at the top, as the piston descends toward the crankshaft, pressure in the cylinder is reduced and the fuel air mixture flows in as a result of the higher pressure in the intake manifold.
What is the intake manifold
The system of pipes which leads the mixture from the carburettor to each cylinder
What is the charge
The volume of mixture drawn in on each induction stroke
What happens in the compression stroke
Early in the compression stroke the inlet valve is now closed and the piston moves back toward the cylinder head. With both valves now closed, the charge is fully contained within the cylinder; therefore it becomes compressed and as a result its temperature rises considerably. As the piston is completing the compression stroke, the fuel-air charge is ignited by the spark plugs and controlled combustion begins. This causes the gases in the cylinder to expand rapidly and exert a strong pressure on the piston which has now passed the top of its stroke.
What happens in the power stroke
The piston is pushed with considerable force back down the cylinder, just prior to the completion of the power stroke the exhaust valve opens.
What happens in the exhaust stroke
The piston moves back up the cylinder and the burnt gases are forced out of the cylinder to the atmosphere via the exhaust manifold, as the piston is approaching the cylinder again the intake valve opens in preparation for the next induction stroke
How does a single cylinder engine get enough momentum to keep the crankshaft turning during the 3 strokes that don’t provide power to turn the crankshaft
A heavy flywheel is attached to the crankshaft
What is essential for smooth operation of the four stroke engine
Multiple cylinders as the the engine will receive multiple power impulses during the 2 revolutions of the crankshaft
In level flight or climbing in an aircraft with a fixed pitch propeller, what indicates power output
rpm gauge (tachometer)
The higher the rpm the ____ the power
Greater
Why does the rpm gauge no longer give a good indication of engine power output during descent
The oncoming airstream increasingly helps to drive the propeller
How is power output measured in engines fitted with a variable-pitch propeller and constant-speed unit
A combination of manifold pressure and engine rpm
How is power varied in engines fitted with a variable-pitch propeller and constant-speed unit
By adjusting both the manifold pressure (with the throttle) and rpm (with the pitch control)
What does the intake valve do
Admits the fuel air mixture to the cylinder
What does the exhaust valve do
Allows the burnt gases to be expelled
Camshaft function
Ensures that valves open and close at right times
What speed does the camshaft operate at
Crankshaft speed
What does valve lead, lag and overlap do
Help engine breath better
Valve lead
Early opening of the exhaust valve
Valve lag
Late closing of the intake valve
Valve overlap
Both valves open at the same time
Ignition
High voltage spark occurs across the points of spark plugs fitted to each cylinder, which enables a controlled flame to start moving through the compressed fuel-air charge so that max cylinder pressure develops at the most advantageous stage of the power stroke
