Piston Engines Flashcards
Other names for piston engines
- Internal combustion engine
- Reciprocating engine
What does a piston engine do?
Convert heat energy into useful work - turns the propeller and provides thrust.
How many cycles does a piston engine operate on?
Four stroke cycle:
- Induction
- Compression
- Power
- Exhaust
What is engine output measured in?
Horsepower
What do pistons do?
- Move up and down
- Burn fuel/air mixture (charge)
- Expands the gases
What is chemical energy converted into?
Heat energy and then mechanical energy - this is converted to rotary motion of the crankshaft, turning the propeller.
What does a gas turbine (jet) engine do?
Generates thrust by expelling high-velocity air in the opposite direction.
What is different about a piston engine compared to a jet engine?
- Operates on a continuous cycle.
Jet engine:
- Air is accelerated at a high speed and pushed rearward, creating forward propulsion.
- Processes happens continuously and simultaneously
Piston engine:
- Can only do one stage at a time
What is an in-line engine?
Cylinders arranged in a straight line along the crankcase
What is a radial engine?
Cylinders arranged in a circular pattern around central crankshaft.
What is an inverted in-line engine?
Crankshaft located above the cylinders - provides better forward visibility for the pilot in a single-engine aircraft.
What is a horizontally opposed engine?
- AKA ‘flat engine’
- Cylinders arranged horizontally on opposite sides of a central crankshaft.
- Most common type of engine in GA
What is a cylinder?
- Chamber where piston executes its up and down movement
- Within is the combustion chamber - where the burning of fuel-air mixture happens
- Aircraft engines usually have four or more cylinders in different configurations
What are cooling fins used for?
- To save weight
- Enhance engine cooling
What is a cylinder head?
- Located top of each cylinder
- Seals cylinder and houses components such as valves, camshafts and spark plugs
- Have cooling fins
What is the valve operating mechanism?
- Where the inlet and exhaust valves are actuated by the camshaft which is gear driven by the crankshaft.
- Camshaft moves the pushrods and the rocker arms.
- Rocker arms pushes on a valve, the valve opens.
- A valve spring closes the valve when the rocker arms stop pushing.
What do spark plugs do?
- Two plugs per cylinder
- Ignites the fuel-air mixture
- Operated electrically
- Two spark plugs for safety - also provides better flame front (better amount of combustion).
What is an injector?
- Located on top of cylinders
- Used for fuel injected engines (rather than carburetted engines).
- Sprays fuel into combustion chamber
What is the ideal gas equation?
PV = RT
- P = pressure
- V = volume
- R = gas constant
- T = temperature
Piston engine operating principle
Moving pistons up and down, burning fuel-air mixture, expanding gases, and pushing the piston down the cylinder.
What will gas do when heated?
Expand in volume.
What does the term cycle refer to?
The sequences of operations the engine goes through to convert duel into motion.
What does stroke refer to?
The distance the piston moves up or down within the cylinder.
Explain ‘induction’.
- Piston moves down
- Fresh charge of fuel-air mixtures is drawn into the cylinder
Explain ‘compression’.
- Piston move up
- Compression and ignition of the fuel-air mixtures
Explain ‘power’ (combustion).
- Piston moves down
- Expansion of the ignited fuel-air mixture and absorption of the energy.
Explain ‘exhaust’.
- Piston moves up
- Expulsion of waste combustion products.
Define: Top Dead Centre’ (TDC)
When the piston is at maximum distance from the crankshaft.
- Piston is at its highest point within the cylinder.
Define ‘Clearance Volume’
The space/volume above the piston within the cylinder when it is at TDC
Define: ‘Bottom dead centre’ (BDC).
Minimum distance from the crankshaft.
- Piston is positioned at its lowest point within the cylinder
Define: ‘Swept volume’
Volume that the piston displaces in the cylinder between the TDC and the BDC.
What is the compression ratio?
Compression ratio = (swept volume + clearance volume)/clearance volume.
- The ratio of the proportion between the volume above the piston when it’s at BDC to the volume above the piston when it’s at TDC.
Define: ‘bore’
Internal diameter of the cyclinder.
What is the firing interval?
Firing interval = number of degrees per cycle/ number of cylinders
- The number of degrees of crankshaft rotation that occurs between the firing of each cylinder in the engine.
What during interval does a four cylinder engine have?
180 degrees
Define: ‘firing order’
- The sequence in which the ignition system (magneto) supplies the electrical current to the park plug in a piston engine.
Define: ‘Manifolds’
System of pipes and ducts.
- Intake manifold - directions fuel-air mixtures form the carbuerettor to individual cylinders
- Exhaust manifold which takes the gases away from the cylinders
Define: ‘Manifold pressure’.
- Pressure of the fuel-air mixture existing at any given time in the intake manifold
- Manifold air pressure (MAP) gauge can be used to measure this.
Define: ‘Crank angle’
Position of the crankshaft in relation to the piston and is measure in degrees.
Often used to describe the timing of events within the four-stroke engine cycle.
Define: ‘Ground power unit’ (GPU).
- A vehicle or portable device that provides electrical power to Parker aircraft when their engines are not running.
- Used to start larger engines where the capacity of an aircraft battery is not sufficient.
What is detonation?
- Spontaneous combustion (exploding) of fuel-air mixture after normal ignition.
- Caused by induction of fuel-air mixture at too high a temperature/pressure (carb heat on and lean mixture)
- Typically occurs when engine is running at high power settings, with high manifold pressure and low RPM, especially if the fuel’s octane rating is too low.
What can detonation lead to?
- Multiple flame fronts within the cylinder
- Sharp increase in pressure and temperature, causing overheating of the engine.
- Broken pistons, damaged cylinder heads, and cracked spark plugs
What do you do if detonation is experienced?
- Increase airspeed to allow to cool the cylinders
- Reduce throttle setting to reduce the amount of oxygen entering the cylinders. Less oxygen will lead to less flame in the cylinders
- Enrich the mixture to cool cylinders
What is pre-ignition?
Ignition of fuel-air mixture by other than spark plugs - occurs before the spark plug provides a spark.
What is pre-ignition caused by?
- Prolonged detonation
- Very hot glowing carbon deposits inside the cylinder AKA ‘hot spots’.
What can pre-ignition lead to?
- High cylinder pressure and temperatures which can causes severe engine damage as the expanding combustion gases exert force on the piston while it is still on its compression stroke.
- Deterioration of bearing surfaces
- Engine knock and loss of power
What is a diesel engine?
- Diesel = compression fuel
- DOES NOT REQUIRE SPARK FROM SPARK PLUG
- Does not used a carburettor - direct fuel injection
- Ignites when the diesel-air mixture is compressed to the required amount.
What is diesel knock?
Diesel Knock = detonation equivalent
- Caused when the diesel-air mixture is compressed to the required amount
What does diesel knock cause?
- Rapid and violent fluctuations of pressure in the cylinders
Define: ‘valve lag’.
Late closing of the inlet valve on the induction stroke.
- 60 degrees crank angle
Define: ‘Valve lead’
Early opening of the exhaust valve on the power stroke.
- 55 degrees crank angle
Define: ‘Valve Overlap’
Both inlet and exhaust valve opened at the same time.
- Occurs during a short period around TDC
- 30 degrees crank angle
- Happens on the fourth stroke
What are the advantages of valve overlap?
- Increased amount of fuel-air mixture is induced into the cylinders
- Assists with scavenging the burnt gases
- Enhances cooling of the upper parts of the cylinder
- Improves volumetric efficiency
- Improves overall engine performance
Define: ‘Ignition timing’
The point in the engine’s cycle at which the park plugs fires.
- Measured in degrees before the piston reaches TDC in the compression stroke.
- Occurs before TDC to 15 -20 degrees after TDC.
- Incorrect ignition timing should be varied with engine speed
Define: ‘Spark Advance’
- When the spark plug ignites the fuel-air mixture before TDC on the compression stroke.
- Ensures the most power is obtained from the cylinder.
- Typically 15 to 40 degrees prior to TDC
- Not instantaneous
