AGK - Engines Flashcards
What is Boyle’s law?
Assuming constant temperature:
PV = Constant
What is Charle’s law?
Assuming constant pressure:
v ~ T
What is the combined gas law?
Combining Boyle’s (PV = Constant)
and Charle’s (V~T)
PV/T = Constant
What is it called when all the ends of an engine line up?
Top dead centre (TDC)
Bottom dead centre (BDC)
What are the four strokes?
Induction
Compression
Power
Exhaust
How is the practical 4 stroke cycle different from the theoretical one?
Practical cycle takes into account the gases having momentum.
During TDC/BDC there is little linear movement of the piston fo the same angular movement of the crank.
This is ineffective crank angle, there is little change in cyclinder pressure so gasses struggle to enter/exit.
Valve and ignition timing are adjusted in the form of valve lead/lag to account for this, this is the practical 4 stroke cycle.
What is valve lead? (Inlet)
As exhaust stroke comes to the end, but before the induction stroke, the inlet valve opens allowing the momentum of the gases leaving the exhaust to use pressure to pull more fresh mixture in.
What is valve lag? (Inlet)
The inlet valve remaining open beyond BDC after the induction stroke, allowing the intertia of the gas to pull more mixture in at this ineffective crank angle.
What is the equation for thrust?
F = M * (Vjet - Vflight)
What is 1 horsepower in watts?
1 horsepower = 740 watts
What is thermal efficiency?
Ratio between heat energy converted into power output and total heat energy within the fuel.
30% Typical value
What is mechanical efficiency?
The power output of the engine at propshaft compared to the theoretical power output.
Typically 80%
What is volumetric efficiency?
The volume of the charge compared to the avaliable volume.
What is an adiabatic process?
Change in pressure caused by change in volume with no heat energy added or removed.
What is an isochoric process?
Change in pressure with no change in volume.
What is stroke?
Distance the piston travels between the lowest and highest points (TDC & BDC)
When does ignition occur?
Between compression and power strokes
How far does the crank rotate for a stroke?
180 Degrees
What is ineffective crank angle?
Small linear movement of the piston at TDC/BDC for the same angular rotation of the crank.
What does the theoretical cycle assume about combustion time?
It’s instant
What two things do we change in the practical Otto cycle?
Valve timing
Ignition timing
When does valve lead occur on exhaust?
At the end of the power stroke the exhaust valve opens early as most of the energy has been imparted in the inertia of the gases remaining.
When does valve lag occur on exhaust?
Valve remains open during the start of the induction stroke to account for inertia of gases allowing maximum mixture to enter.
During this point we get valve overlap from the inlet valve being open at the same time as the exhaust.
What is valve overlap?
When the exhaust valve lag from the exhaust stroke and the inlet valve lead for the induction stroke, both valves are open at the same time.
What are valve lead/lag measured in?
Degrees of crank rotation
In practical cycle when does compression occur?
When inlet and exhaust are closed
Why do we use valve lead/lag
Increase volumetric efficiency.
Why do we change ignition timing?
Rate of combustion is constant but RPM isn’t!
At high RPM maximum pressure from combustion might be past TDC so we need to advance the spark times so it occurs more early!
Likewise at low RPM peak pressure from combustion could occur before TDC so we must retard spark timing so it occurs later.
What do we do to spark timing at high RPM?
Advance spark timing
What do we do to spark timing at low RPM?
Retard spark timing
Speed of camshaft vs crankshaft?
1/2 Speed of the crankshaft
What determines engine power?
Mass of the mixture ignited in cyclinder
What is the firing interval of a multi-cylinder engine in degrees?
720/Number of cylinders
Radial vs rotary engine?
Rotary has a fixed crankshaft (cylinder rotate around it)
Radial has a moving crankshaft and fixed cylinders
What is a disadvantage and can happen in radial engines?
Oil collects in the inverted cylinders, during compression can cause excessive pressure, blowing cylinder head off
How many cylinders in a radial cylinder bank?
Must be an odd number
How can we add more power in a radian engine?
Add more odd numbered cylinder banks
What 3 parts comprise a cylinder?
Barrel (Steel)
Gas tight gasket
Head (Alloy)
How many spark plugs per cylinder and why?
2 for redundency Ensures quicker (ignited both sides of cylinder), more even burn
Why is the exhaust valve hollow and partially filled with sodium?
Sodium will melt at high temperatures and transfer heat
Why are there 2 valve springs?
To prevent valve bounce
They are wound in different directions
Helps stop valves rotating
What is crank throw?
Half the stroke (distance from TDC to BDC)
What is crank assembly?
Crankshaft, connecting rods and pistons
What is tappet clearance?
Allows for expansion of engine while cold
What happens if too much/little tappet clearance?
Reduction in performance
What is the primary function of lubrication?
Moving parts lubricated to reduce friction and wear
What are 4 secondary functions of lubrication?
Cooling
Cleaning
Protection from corrosion
Hydraulic operations
How does viscosity of oil change with temp?
Reduces with increased temp
What must aero engine oil do? (5 items)
Maintain suitable viscosity over a wide range of temperatures and pressures
Have a low evaporation rate
Inhibit corrosion
Not react with materials it comes into contact with
Discourage the formation of sludge
What is mineral oil?
No additives
What is compound oil?
Mineral oil with additives
What identifies compound oil on the packaging?
Letter preceding viscosity number,
eg
W80 or AD100
What can we use straight/mineral oil for?
New engines to reduce run in time
How is the Saybolt rating for Oil related to S.A.E
Saybolt is twice the S.A.E
What is a dry sump?
Oil is held in a tank mounted remotely from the engine.
What is a wet sump?
Oil is held at the bottom of the crank case
Where is the temp gauge in a dry sump system?
After hot well in oil tank
Where is the pressure gauge in a dry sump oil system?
After the pressure pump
What is a magnetic chip detector for in a dry sump system?
Warns of impending engine failure
Name an advantage of dry sump system?
Separate oil tank ensures supply of filtered and cool oil in all flight conditions
When do we check the oil level of a dry sump system?
After engine shutdown
As overnight oil from the reservoir can trickle to the sump giving an incorrectly low reading.
Dry sump vs wet sump (4 differences)
Same components but no
scavenge pump or oil tank
Pressure oil feeds bearings and splash lubrication
Oil cooler can’t be placed in return line so is placed in pressure line
When do we check the oil level of a wet sump system?
20 minutes after shutdown
Where is the oil cooler in a dry sump system?
In the return line
Where is the oil cooler in a wet sump system?
In the pressure line (supply)
What is oil coring?
Excessive cooling causes oil to conjeal in cooler and block it, oil temperature will rise with oil cooler flap fully open
How do we identify oil coring and prevent it?
Rise in oil temperature with oil flap fully open
Close oil cooler flap
Temp should start to drop
Can now manage system properly
Where do we find the oil temperature gauge?
After cooler before the pressure pump
Where do we find the oil pressure gauge?
At the outlet of the pressure pump
When should oil pressure indicate during startup?
Within 30 seconds or shutdown engine
How do we prevent hydraulicing?
Turn the engine manually to check for resistance
What is compression ratio?
Total volume/Clearance volume
Where do we measure CHT?
At the hottest cylinder
What is engine coolant made of?
Water and glycol
What is CHT a good indicator of?
Engine health
What do we measure CHT with and how is it powered?
Thermocouple and is self powered (produces voltage which varies with temp) from heat so can drive a gauge
What is CHT affected by? (4 things)
Power setting
Aircraft speed
Temperature of cooling air
Ratio of the mixture
When is aircraft most likely to overheat?
Low speed and high power
A climb
How can we regulate an air cooled engine?
Cowl flaps
How long can the starter gear be active for?
30 seconds otherwise it has failed to disengage and we have to shut the engine down.
How are magnetos powered?
Engine rotation
What do magnetos generate?
High tension electricity to spark plugs
How do we ensure redundency of critical ignition parts?
Have 2 or more systems
How does a spark plug work?
A preset gap is present
A high temperature spark arcs
Ignites the charge (mixture)
Is a magneto part of an aircraft’s electrical system?
No, a separate DC alternator provides power to the aircraft.
Magnetos and the ingition system work independantly of this incase it fails.
What is the job of a magneto?
To provide sparks at precisely the right moment to ignite the mixture
How many spark plugs does each magneto power?
One spark plug
How do we turn a magneto off?
Ground the primary winding to earth
How does a mag work?
Collapse of field in a primary winding induces a larger emf in the secondary winding with more turns.
A capacitor stops arcing and loss of field strength across a breaker which times this collapse to when the spark is required. A distributor then applies it to the appropriate spark plug.
What is the purpose of a capacitor in a magneto?
Capacitor stops spark arcing on contact breakers meaning rapid collapse of primary winding causing large voltage in secondary winding.
After how many engine revolutions should the mag generate a spark?
1 spark per 2 revolutions.
What is the p lead in a magneto?
The grounding lead
Why are magnetos pressurised?
Stop internal arcing at altitude
What is an additional advantage of 2 spark plugs?
Accelerates combustion process as flames spread out from both sides of the cylinder simulataneously.
Quicker rise to peak cylinder pressure and hence a rise in power
Why do we see RPM drop with mag switched off?
The combustion time is increased as only one flame front so peak pressure is now not at the desired point and is after TDC due to finite combustion time.
Why do we need auxilary spark augmentation?
Magneto output voltage is directly proportional to engine RPM, so at startup (low RPM) the magneto struggles to provide enough power to produce a spark.
What is a magneto impulse coupling?
Spring loaded clutch between drive spingle and magneto shaft, winds spring up at low RPM and clutch releases spinning the magneto rapidly.
This retards the spark too
What is a magneto high tension booster coil?
Supplies high voltage impulses through a transformer to a trailing (retarded) brush on the distributor rotor.
This system bypasses normal magneto circuit and is driven by the battery.
What is a magneto low tension booster coil?
It supplies low voltage to the magneto primary during startup this additional voltage to the low voltage at low RPM is enough to give a good spark to start the engine.
How do we control low tension and high tension booster coils in an ingition system?
They have to be manually turned on before startup and off after startup.
During mag checks what does no drop in RPM suggest?
That half of the ignition system isn’t earthed
During mag checks what happens if we get a dead cut of the engine?
Failure of that half of the ignition system.
What colour is AVGAS 80?
Red
What colour is AVGAS 100
Green
What colour is AVGAS 100LL
Blue
What colour is Jet A (Avtur)?
Colourless or straw
What is MOGAS?
Motor gasoline
Why don’t we want to use MOGAS?
Carb icing and vapour locks more likely
What is octane reading?
Percentage of Isooctane to heptane in fuel
80 Octane is 80% Isooctane
Ability of fuel to resist knocking/detenation.
What do we read a fuel in if octane is over 100?
Performance numbers
What type of lead is used in fuel and what is it’s purpose?
Tetra-ethyl-lead and it is used to increase anti-knock properties.
What is detenation?
Abnormally, the last bit of mixture (the end gas) may detonate due to excessive heat or pressure.
It causes a rapid increase in heat and pressure after normal ignition causing mechanical shock.
How can we avoid detonation? (4)
Carb heat at high power
Overheating cylinders with lean mixture
High power setting at low RPM
On variable pitch props - Reducing RPM before reducing power
What is pre-ignition?
Ignition of charge before the spark.
Usually caused by hot spot in the engine.
What is air to fuel ratio?
It is the ratio of air to fuel in by MASS
Perfect air fuel ratio?
15:1
What happens to EGT when we get richer?
Richer:
EGT increases up to 15:1 but before it’s cooler due to fuel cooling
What happens to EGT when we get leaner?
EGT will decrease from 15:1 as we get leaner due to air cooling
What mixture releases the most heat?
15:1
What do richer settings to do CHT?
Keep it cooler, fuel acts as coolant.
What do leaner settings do to CHT?
Make it rise dangerously to the point it could cause detonation.
This is because the mixture burns more slowly so will heat up the engine instead of pushing the cylinder down
What does a lean mixture do to EGT and CHT
Lower EGT (No longer at perfect 15:1, air cooling) Raise CHT (Combustion slower so releases more heat to cylinder instead of pushing piston down)
Best fuel consumption mixture?
20% lean
Purpose of Carb pressure balance duct?
To ensure same pressure in float chamber as ram air in inlet
Purpose of Carb diffuser?
Stop enrichment at high speed (fuel higher density) ensures same mixture ratio at high/low RPM by placing air in the fuel from pressure duct
Purpose of Carb idling jet?
At low throttle, pressure drop in venturi very low so not enough fuel to keep engine running, idling jet ensures rich mixture at idle by using pressure drop in constriction caused by butterfly valve.
Purpose of Carb power enrichment?
At high power gives extra rich mixture as heat and pressure are rising. This prevents detonation.
Purpose of Carb accelerator pump?
When throttle opened rapidly airflow increases almost instantly but there is a delay before fuel is sucked in, this would cause a lean cut of the engine.
Accelerator pump forces fuel into the engine with a piston when the throttle is opened to prevent this.
Why is carb icing an issue inside?
Carb produces a drop in temp due to reduction in pressure through the venturi and the evaportation of fuel.
Can drop temperature by as much as 25 C.
What causes carb icing?
Humidity as temp drop across carb 25C so moisture in air will freeze.
Symptoms of carb icing (6)
Loss RPM Reduced performance Reduced inlet manifold pressure Decreased EGT Rough running Jammed throttle
What is direct fuel injection?
Injects fuel directly into cylinder (diesel usually)
What is indirect fuel injection?
Delivers fuel to inlets before inlet valve continously
How is metering pump connected in indirect injection?
Throttle and inlet butterfly.
Why is it bad to excess prime the engine?
Cause fire in cylinder and wash oil of side of cylinders.
How can we prime an engine (3)?
Manually operating plunger to force float down in carb
Throttle excersised to activate accelerator pump
Manually or electrically operated priming pump
Why do fuel injection systems have alternate air sources?
Impact icing can block the intake air filter, so there is an alternate unfiltered air supply for an injected engine.
What is the EGT gauge used for?
Tuning mixture settings.
Lean/Richen from peak temperature at 15:1
Temp drops off either side of this peak
