Piston Engines Flashcards
Two Types of Engines
Piston engines
Gas turbines
Engine Layouts
Radial In-line upright In-line inverted Vee Horizontally Opposed
The Piston
Moves within the cylinder and forms one of the combustion chamber walls
Piston Rings
Help to seal the cylinder, ensuring better power production and stops oil from entering the combustion chamber
Top 3 are compression rings
Bottom 3 are oil rings
The Crankshaft and Connecting Rod
Turns the linear motion of the piston into a rotating motion of the crankshaft
Absorbs the power produced by each cylinder and transfers it to the propellor
The Inlet Valve
Allows the fuel/air mixture into the cylinder
Is the larger valve
The Valves
Use rocker arms to open the valves and springs to close them
The Exhaust Valve
Allows the exhaust gases from the combustion process out of the cylinder to the exhaust manifold and then through the exhaust pipe
Is the smaller valve
Can be hollowed out and contain metallic sodium to cool and dissipate the exhaust gases
The Camshaft
Responsible for the timing of when the valves open and close
Rotates at half crankshaft speed
Operates rocker arms and push pull rods that open and close the valves at the correct times
Contains holes called oil gulleys to cool
Lycoming Piston Engine
Grey
Camshaft sits above the crankshaft
Continental Piston Engine
Yellow
The Cylinder
The chamber in which the piston moves
Where the fuel/air mixture is compressed and combusted under pressure
Externally air-cooled via baffles and cooling fins
The Spark Plugs
Used to ignite the fuel/air mixture and allow combustion to occur
Firing controlled by the magnetos
Two per cylinder, firing at the same time, for redundancy and an increased efficiency in the burn of the mixture
The Combustion Chamber
Top part of the cylinder where the mixture is compressed and ignited resulting in combustion
Size may change accordingly to the size of the cylinder
The Otto Cycle
4 stroke cycle
Converts heat energy to mechanical energy
Intake/Induction Stroke
From TDC to BDC
Inlet valve is open, exhaust valve closes
Pressure reduces, drawing mixture from the carburettor into the cylinder
Compression Stroke
Inlet valve closes in the early stages
Piston moves from BDC to TDC
Increased pressure and temp of mixture as volume decreases
Fuel/air mixture is ignited by the spark plugs just prior to TDC
Power/Combustion Stroke
Both valves remain closed
From TDC to BDC
Mixture is being burnt and exerting a strong force pushing the piston down
Piston drives the crankshaft and other pistons
Exhaust valve opens just prior to TDC
Exhaust Stroke
Exhaust valve is open
From BDC to TDC
Exhaust gases are pushed out
Inlet valve opens just prior to TDC
Crankshaft Rotation
All cylinders fire within two revolutions (720 degrees)
Modified Otto Cycle
Valve timing and ignition timing are adjusted for each cylinder to ensure that the power strokes of each cylinder are at different times, making for a more efficient engine
Ineffective Crank Angle
For 40 degrees of rotation, there is very little linear motion, therefore the valves stay open during this time to allow more fuel/air mixture in
Valve Lead
Inlet vale opens just before TDC on the exhaust stroke
Exhaust valve opens just before BDC on the power stroke
Valve Lag
Inlet valve closes just after BDC on the compression stroke
Exhaust valve closes just after TDC on the induction stroke
Valve Overlap
Both valves are open at the start of the intake stroke
Ignition Timing
Spark ignites the mixture just prior to the piston reaching TDC on the compression stroke
The spark is retarded during startup
Compression Ratio
Volume of the cylinder with the piston at BDC/Volume of the cylinder with the piston at TDC
(Swept volume + Combustion volume)/Combustion volume
Generally 8:1
Power
The rate of doing work
Power produced is reduced considerably towards the end of the stroke
Indicated Horsepower (IHP)
Horsepower available at all cylinders
An indication of how much heat energy from the fuel is converted into mechanical energy
IHP = BHP + FHP
Brake Horsepower (BHP)
A measure of the useful work done by the piston engine
Equals IHP - FHP
Friction Horsepower (FHP)
Part of the IHP absorbed by friction and driving the valves, ignition, lubrication systems, etc
Shaft Horsepower (SHP)
The brake horsepower of a rotating shaft
A measure of the thrust produced by a turboprop engine
Thrust Horsepower (THP)
A measure of the thrust produced by a turbojet or turbofan engine
Torque
A turning force or moment about a point
Engine Torque
The turning force about the centre-line of the crankshaft
A function of:
- Cylinder pressure, which is controlled by the throttle
- The distance or arm between the crankshaft centre-line and the big end of the connecting rod
Cylinder Head Temperature Gauge
Between 300 - 400 degrees Fahrenheit
Often on back cylinders as they tend to run hotter
Can be controlled by the cowl flap
Causes of Excessive Engine Temperatures
High power settings
Low airspeed
Over-lean mixture
Low oil level
Fins and Baffles
Aid cooling by increasing the surface area of the metal that can absorb heat from the cylinders and allows air to flow in the small gaps and carry the heat away
Help to circulate the air around the cylinders
Oil Temp Gauge
Monitors the viscosity of the oil
Oil Pressure Gauge
Fluctuating means there is insufficient oil
0 is generally a pump failure
Exhaust Gas Temperature (EGT) Gauge
Will show temperature changes more quickly than a CHT gauge
High EGT possible risk of detonation
