Power plant Flashcards
Aeroplanes have 2 types of engines, what are they?
Combustion (reciprocating - constant volume) and gas turbine (constant px)
How can piston engines (cylinds) be arranged?
Inline, inverted inline, radial, V or H and horizontally.
How does the piston work?
The fuel/air mix is pushed into each cylinder head and this gas is burned to create a combustion.
The engine here is converting chemical (fuel) NRG to heat NRG (combustion) to mechanical NRG (combustion turns the crankshaft which turn the prop).
What is a piston engine?
It’s an engine where a piston inside a cylind is powered by the combustion of an air/gas mix. It’s called a 4 stroke engine as it requires 4 strokes of the piston travelling up and down the cylind to create a power stroke. AKA Otto cycle after Nikolaus Otto who developed the 4 stroke eng.
What are the 4 strokes in an Otto cycle?
- Intake (suck)
- Compression (squeeze)
- Combustion (bang)
- Exhaust (blow)
What is the intake stroke?
Where the piston travels from TDC to BDC while air/gas mixture is intro into the combustion chamber in the cylind head via the inlet valve. The inlet valve closes just as the piston reaches BDC.
What is the compression stroke?
As the piston travels from BDC to TDC it is now compressing the mixture in the combustion chamber, increasing the pressure. Just before the piston reaches TDC the spark plugs charge (aka lead charge). As the piston reaches TDC and begins to travel BDC again, the magnetos deliver the voltage to the spark plugs which then spark.
What is the power stroke?
As the piston is travelling from TDC to BDC, the spark plugs have already ignited and the mixture gas is burning (combustion). This combustion creates a powerful force pushing the piston down (to BDC) rapidly creating what is termed the power stroke. This power is then used to turn a crankshaft connected to the bottom of the cylind. As the crankshaft is directly connected to the prop, this turns the prop.
What is the exhaust stroke?
Just before the piston arrives BDC from the power stroke, the exhaust valve opens. As the piston reaches BDC and travels TDC again, the burned gases in the cylind is then pushed out via the exhaust valve in the cylind head as the piston travels TDC.
As the last gases exhaust, and the piston is about to travel BDC again, the exhaust valve closes and the inlet valve opens to start the process all over again.
What is pre ignition?
Progressing burning of the air/fuel mix before the spark from the spark plugs. This is caused by hot spots in the cylind becoming red hot and igniting the mixture prematurely.
Result - rough running, possible back firing and increased CHT.
What is detonation?
When poor quality fuel or/and wrong fuel is used the anti-knock qualities are not present to withstand the compression (and thus heat) within the cylind hence causing an uncontrolled burning of the gases (instantaneous explosion) compared to a controlled, progressive and smooth burning of the mix in a combustion.
Consequences of excessive high CHT?
Pre ignition. Piston suffering high friction - cylinder walls not lubricated enough.
Consequences: weaken engine components, rough running.
How is the engine cooled?
On each cylinder head, extra surfaces are placed around it to spread out the heat across a larger sfc. These sfcs are called cooling fins.
The engine itself is cooled by taking the air from behind the prop (high pressure) and directing it around the engine via baffles as this creates low pressure. After working it’s way around the engine, the air is then exhausted through the back and bottom of the engine.
What happens to the engine cooling at different spds?
High RPM, low IAS e.g. TO/GA - as engines are cooled via intake air, at low IAS (min air intake) and high RPM (where engine working hard), the engine is not efficiently cooling.
Low RPM, high IAS e.g. descending - cooling maybe too effective thus shock cooling may occur. This is where the engine is cooled to much and too quickly. This causes cracks (breaks) in engine parts due to the stress.
Therefore ideally would want some pwr on while descending. Also why we do engine warm(s) in PFLs. Some complex AC have cowl flaps to aid cooling and warming.
What are the main functions of oil?
Carries away contaminants.
Lubricates.
Creates a seal around the cylind wall and piston.
Anti corrosion qualities.
Acts as hydraulic oil to move prop in constant pitch prop.
