Aircraft - Powerplant Flashcards
Boyles Law
Pressure and volume inversely proportional.
P x V = constant
Assuming constant temperature
Charles Law
Volume and temperature are proportional.
V / T = constant
Assuming constant pressure
Combined Gas Law
P x V / T = constant
Bournoulli’s equation
Static pressure + dynamic pressure = total pressure (constant)
Newton’s laws
1) A body will continue in rest or in uniform motion in straight line, unless acted upon by a force
2) Rate of change of momentum is proportional to (and in direction) of applied force
3) Action and reaction are opposite and equal
Work
Force x distance
Power formula
Force x distance / time
[the rate of doing work]
Force equation
Mass x acceleration
Thrust
A force based on acceleration and mass of air pushed backwards:
F = M x (V(jet) - V(flight))
Where V(jet) = speed of air existing turbine or propeller
V(flight) = speed of air encountered
Horsepower measurements
- Indicated
- Friction
- Brake
- Thrust
Indicated HP - Theoretical reciprocating engine power, based on pressure developed in cylinders, ignoring work done in the engine.
Friction HP - Work done within the engine
Brake HP - Power delivered to the shaft
Thrust HP - Power converted to thrust (brake HP with propeller efficiency)
Relationship between BHP, IHP and FHP
BHP = IHP - FHP
Piston engine efficiency
- overall
- thermal
- volumetric
Overall: 80%
Thermal: 33% (heat lost through exhaust)
Volumetric: 80%
Piston energy type chain
Chemical energy ->
Heat energy ->
Pressure energy ->
Mechanical energy
Piston strokes
Induction
Compression
Power
Exhaust
Connecting rod
- description
- small and big ends
Joins piston to crankshaft.
“Small end” at piston
“Big end” at crankshaft
Top dead centre & bottom dead centre
Based on piston and combustion chamber at the top and crankshaft & connecting rod at the bottom.
So TDC is small cylinder volume, BDC is max cylinder volume.
Piston engine
- Mean Effective Pressure (MEP)
The average pressure exerted on the piston during the power stroke.
This determines the theoretical power of the engine.
3 issues with theoretical 4 stroke engine
Air has momentum.
At BDC and TDC linear movement of piston is very small despite rotational movement of crankshaft being constant (“area of ineffective crank angle”).
Combustion of fuel and increase in pressure is not instantaneous.
Changes in practical 4 stroke engine based on the issues with theoretical
Valve timing
Ignition timing
4 stroke valve timing
Both valves lead and lag.
This maximises the amount of mixture that can enter and reduces required valve speeds.
Some loss of power due to exhaust opening before power stroke is complete, but this is minimal due to limited piston movement at BDC.
Valve overlap
Lead and lag of the inlet and exhaust valves leads to a period when both are open.
This allows the momentum of exiting exhaust gas to “pull in” new mixture, called scavenging.
Valve timing - fixed or variable?
Valve timing is optimised based on the shape of the cam, which is fixed, so valve timing is fixed.
It doesn’t vary with RPM.
[Note: determined in terms of angles, not speed, obviously higher RPM leads to faster physical valve speeds, but same point in the cycle]
4 stroke ignition timing
- General
- Variable?
Ignition is “advanced”, so happens before TDC, to ensure that power peaks as piston leaves period of ineffective crank angle.
Timing DOES change based on RPM (unlike valve timing) as combustion delay is fixed, so as the engine gets faster the spark needs to happen earlier to achieve the right timing.
Radial engines
- description
- # cylinders
Pistons mounted radially around the crank shaft. Need to have an odd number of cylinders to allow timing (e.g. 1, 3, 5, 2, 4).
Creates a big area with a lot of drag, and upside down cylinders have issues with oil pooling.
Crank throw
This is half of the stroke distance.
It is the range of movement of the connecting rod connection at the crankshaft, between central point (centre of crankshaft) and its TDC or BDC point.
Alternatively could be described as the amplitude of the vertical movement of the crankshaft/connecting rod connection.
Crank throw - considerations around weight
Shorter crank throw reduces weight as components can all be smaller.
Thus aircraft engines tend to be “short stroked”.
Camshaft
Controls valve timing. Driven via gears by the crankshaft and rotates at half the speed of the crankshaft, so valves each open and close once per cycle.
Piston engine
- Crankcase
Main structural element of engine.
Mounting points to mount to airframe, for accessory gearbox and for cylinders.
Internal passages allow oil to flow and a breather allows air to vent.
Piston Engine
- Crank Assembly
Crank shaft
Connecting Rods
Pistons
NOT Crank Case
Piston engine
- Accessory gearbox
Drives oil, ignition, hydraulic, pneumatic systems and sometimes engine starter motor.
Exhaust valve cooling
Exhaust valve is hollowed and partly filled with sodium. This melts at high temperature and transfers heat from the valve head to the stem.
Valve bounce
Caused when spring closing valve vibrates at its resonant frequency. This prevents the valve from staying gas tight.
Resolved by having two counter wound helical springs (one within the other) holding the valve closed, which have different resonant frequencies.
Valve gear - tappet
Fitted between the camshaft and push rod, creates the right clearance to ensure valve movement range is correct.
Has some clearance for thermal expansion so it can fully close when hot, but too much clearance means the valve won’t fully open.
Limitation on normally aspirated engines
Engine power limited by the amount of air that enters the cylinders. The greater the mass of air in the cylinder, the more combustive power can be created.
Naturally this is less at higher altitude where outside air pressure is lower.
Manifold Absolute Pressure (MAP)
Mixture pressure at inlet manifold just before going into cylinder. Can be measured by a MAP gauge and is a good indicator of MEP and thus engine power.
Controlled by the throttle.
What does MAP gauge show when:
- Engine off
- Engine idle
- Engine full throttle
Engine off - shows outside pressure, so 29.92 in Hg at MSL in ISA.
Engine idle - zero
Engine full throttle - Just less than 29.92
Calculating power of piston engine
- MEP method
Power = Force x distance / time
Use PLANE
Pressure (MEP) & Area (of piston crown)
Length (Stroke length)
Number of cylinders
Engine (RPM / 2 - only one power stroke per 2 rotations)
Calculating power of piston engine
- Torque method
Power = Force x distance / time
Distance x force x rpm
= torque x rpm
Torque measured at the gearbox between engine and propeller
Fixed vs variable pitch propeller engine power monitoring
Fixed pitch generally just look at rpm.
Variable pitch power is a function of rpm and manifold pressure, so have a MAP gauge as well as rpm.
Piston engine
- Clearance volume
- Total volume
- Swept volume
- Compression ratio
Clearance volume = Volume @ TDC
Total volume = Volume @ BDC
Swept volume = the difference
Compression ratio =
Total volume : Clearance volume
High piston compression ratio effects
High compression ratio improves thermal efficiency, but increases likelihood of detonation.
Squared or square cage engine
Piston engine where piston diameter = stroke.
Volumetric efficiency
- definition
Ratio of mixture induced in cylinder to mass of air (at standard temp/pressure) that would fill the swept volume.
In normally aspirated engine this is affected by the ease of inflow and ease of exhaust flow, along with density of the air.
Effect on volumetric efficiency of:
- Altitude
- RPM
Assuming constant MAP (big assumption), volumetric efficiency increases with altitude due to easier exhaust flow.
Also increases with RPM due to increased momentum of gases.
Power at altitude for piston engines
Consider variable pitch propeller where MAP and rpm can be controlled
At low altitude throttle is less than full for given MAP, so as you climb volumetric efficiency increases and throttle can be opened to maintain MAP.
When throttle is fully open you are at peak power and above that altitude power will decrease as MAP falls.
Note: MAX power is always decreasing as throttle always full and MAP decreasing with altitude.
Engine oil designations
Number indicates viscosity (higher number - higher viscosity).
Letters in front indicate additives.
[New engines run in with zero additive oil to reduce lead in time]
Dry sump oil system
Used oil falls into a sump at the bottom of the engine. A scavenge pump (stronger than the main oil pressure pump to prevent pooling in the sump) pumps it through the cooler into the main oil tank (onto a de-aerator plate).
Hot well
This is a perforated metal cylinder over the oil tank outlet. When engine started it lets the warmer oil through more easily than the more viscous colder oil. This allows an initial good flow of warm oil before the entire tank is warm.
Oil system filters
2 coarse filters fitted before pumps (suction filter before pressure pump, scavenge filter before scavenger pump) to protect the pumps from damage.
A fine pressure filter fitted after pressure pump to protect components, has a bypass valve.
Magnetic plugs
AKA chip detectors.
Magnetic parts on oil return lines (from sump pump via scavenger pump to coolers and main oil tank) to provide WARNING of bits of metal.
Pressure regulation in oil system (piston)
Pumps are engine driven and higher pressure than required to ensure they work. Have pressure relief valves fitted in parallel that return oil from outlet to inlet side of pump if pressure is too high.
Controlling oil cooling
Might have shutters to allow level of oil cooling to be controlled.
An anti-surge valve allows coolers to be bypassed in case of blockage or excessive cooling causing “oil coring” (congealing in the cooling matrix).
Location of oil cooler in dry and wet sump systems
In dry sump cooler is between scavenger pump and the oil tank (in the return line).
In wet pump it is fitted in the supply to the engine.
Impact of oil temperature on oil pressure
Cold oil is more viscous and gives higher oil pressure
Impact of low oil quantity on oil pressure
Low amount of oil has to do more work so temperature increases. This temperature increase results in reduced oil pressure.
Wet sump system
Similar to dry sump but no oil tank, all oil is stored in the sump at bottom of the engine.
Uses “splash lubrication” where crankshaft moving through oil flings it around the engine.
Wet sump disadvantages
Sump affected by inverted flight so wet sump can have oil starvation (dry sump has a tank with ready supply).
Splash lubrication at high rpm can cause excessive oiling, plus the splash lubricating oil isn’t filtered or cooled.
Usually need more oil for wet sump.
Checking oil level dry vs wet sump
Oil level in dry sump needs to be checked immediately after flight as some will drain back to sump.
Wet sump wait until cooled.
Hydraulicing
When oil pools in inverted cylinders (inverted or radial engines) and causes damage when engine starts as it is not compressible.
Need to remove spark plugs to drain and start with a manual cranking to check.
Cylinder Head Temperature (CHT)
Hottest part of the engine so useful to measure. Usually measure the hottest cylinder (last one, with worst air cooling).
Functions of engine oil
Lubrication
Cooling
Cleaning
Protection
& HYDRAULIC operations
Magneto system description
Engine drives a rotating magnet within a U-shaped stator, with primary (few, thick) and secondary (many, thin) windings around it. This generates an AC current.
The primary winding low tension (LT) circuit is regularly broken, which induces a pulse in the secondary winding high tension (HT) circuit.
Large number of secondary windings means high voltage, which is distributed to a spark plug by the distributor.
NOTE: Tension = Voltage
Prevention of arcing in the magneto system
Arcing can occur at the breaker points in the low tension circuit when the circuit is broken due to high induced current.
A capacitor is installed parallel with the contact breaker to prevent this (called a condenser) which also assists in breaking of the primary circuit.
Capacitor in magneto circuit PREVENTS ARCING.
Ignition system & magnetos
Works opposite to normal system. In normal operation the ignition switches are open, so the LT breaker controls the primary windings. When a magneto ignition switch is grounded, the primary coil is connected to earth so the breaker does nothing and the magneto fails.
As a result of this a break in the ignition wiring will result in permanent ignition firing, not a failure of ignition.
So NOT fail safe on the ground.
Reference to ignition switching system. What does ignition switch activated mean?
This is what is activated when you turn the key to magneto 1 or magneto 2.
So its action is to connect the primary system to earth.
In this sense in the “dual” setting the ignition switch system is considered disactivated, not activated.
Pressurised magnetos
Magnetos use insulating property of air to isolate components. So at high altitudes may need to be pressurised to prevent arcing.
Spark plug augmentation (3)
Need sufficient voltage even at low rpm. Gearbox can do this at low rpm (idle) but need extra help at startup:
i) Impulse magneto has a spring-loaded clutch that stores energy and releases it (twice per engine revolution)
ii) HT Booster coil supplies a high voltage directly to the distributor, by powering primary coil with battery and collapsing it with a trembler interrupter.
iii) LT Booster coil uses aircraft battery directly to power the primary coil.
Spark plug retardation
The 3 augmentation methods (impulse magneto, HT booster, LT booster) all have methods for retarding ignition during starting to prevent sparking too far ahead of TDC which would damage engine.
Detonation
AKA knocking
Rapid spontaneous combustion of mixture after burning has begun (i.e. after ignition). Flame rate about 1000ft per second instead of 60 to 80ft.
Energy produced is given up as heat and shock rather than piston movement.
Occurs AFTER normal ignition.
Pre-ignition
- description
- effect of rpm
When mixture ignites before the spark.
Can cause engine to “run on”, which is continuing to run after ignition is switched off.
Unlike detonation, this gets WORSE with higher rpm.
Octane rating
Characteristic of fuel indicated anti-knock value. Above 100 it is a “performance rating”.
Based on the anti-knock qualities of a mix of X% iso-octane and (100-X)% heptane.
It is ok to temporarily run with fuel of a higher rating, but NEVER with a lower rating.
Factors influencing detonation
Increased temperature or pressure of mixture increases risk of detonation.
- Carb heat on with higher power;
- High power & low rpm
- Overheated cylinders
Impact of carb heat on mixture
Air less dense so rich mixture.
Stoichiometric mixture
Chemically correct mixture which achieves complete combustion, 15:1 by mass.
Typical range of mixture
8:1 - rich
20:1 - lean
Appropriate mixture settings
At 15:1 (stoichiometric) high risk of detonation due to excess latent heat.
10% rich will absorb this heat, but use 15% in practice to ensure all cylinders are ok.
At high power settings use 20% rich.
Take-off use 30% rich.
Power chart against mixture settings
Effect of lean mixture on temperatures
Less fuel means the gas burns cooler, however it also burns slower so more heat is transferred to the engine.
- Low gas burn temp
- High cylinder head temp
- Low exhaust gas temp
Other issues with mixture settings
- Popping back
- Idling
Lean mixture can lead to “popping back” when mixture is burning when the inlet valve opens.
At low (idling) rpm exhaust gas momentum is reduced so retention of some exhaust gas dilutes the mixture. So need rich mixture at idle.
However, prolonged idling with rich mixture (e.g. on the ground) causes spark plug fouling.
Fuel volatility
This is readiness to vaporise.
We need fuel to vaporise in the mixture to enable combustion, but not in the fuel tanks or lines.
Gravity feed ceiling
More likely in jet craft, this is a ceiling for use of gravity feed of fuel.
Climbing without the positive pressure from a fuel pump can lead to “foaming”, the vaporising of fuel in lines due to the decrease in air pressure.
Vane type fuel pump
Two double ended vanes split circular housing into 4 segments. They rotate and also slide across each other so segments change in size. Segments large at inlet and small at outlet so pressure changes create the flow.
Basic float carburettor
Float in the carburettor operates a needle valve that cuts of fuel supply at a certain level (fuel delivered under pressure from fuel tanks).
Exits via a u-bend through a “jet” narrowing that controls flow, then further through an exit at the choke/throat venturi part of carburettor, before the throttle.
Venturi effect as air flows past fuel outlet causes fuel to be sucked out.
Carb
- Throttle effect
Throttle controls the amount of air (air meaning air & fuel mixture) allowed through by the butterfly valve.
The higher the flow of air, the greater the venturi effect and so the more fuel is extracted to maintain the right ratio.
Carb
- Pressure balance duct
At high air speed pressure increases in carb throat due to ram air, thus reducing amount of fuel drawn out. Conversely at high altitude lower density of air would en-richen the mixture.
Therefore a pressure balance duct joins the fuel float chamber to the carb intake to ensure correct mix ratio across AIRSPEEDS and FLIGHT CONDITIONS.
Carb
- Diffuser
This is a perforated piece sitting after the jet in the tube from the carb fuel chamber to the nozzle. It has a pressure connection after it to the fuel chamber which adjusts rate of fuel flow to ensure correct mixture ratio across ENGINE SPEEDS.
Required as fuel flow from nozzle isn’t linear with engine power otherwise.
ALSO helps atomise fuel
Mixture control - functionality and purpose
Can function in a variety of ways.
Purpose is to adjust mixture to take account of air density and also to reduce fuel consumption.
Carb
- Slow running jet
This is an outlet positioned by the throttle butterfly valve when almost closed to deliver a rich supply at idle. It has an idle-cut out that closes it to allow the engine to shut down.
Power enrichment
Provides additional fuel for rich mixture required at high power settings (to provide cooling at high power settings).
Can be provided via a variable size main jet, or a separate power jet controlled via a cam connected to the throttle controls.
Carb
- Accelerator pump
Connected to the throttle, provides an injection of fuel into the venturi when throttle is opened, to prevent weak or lean cut.
Power vs altitude for ground boosted supercharger
Power vs altitude for altitude boosted supercharger
Altitude boosted engine
- Full throttle height chart
Components of a centrifugal impeller/compressor
Axial compressor safety margin chart
Variable pitch prop positions
