AGK - Pistons

Combustion occurs at a constant…

volume

(jet is constant pressure)

also the process is intermittent, where as jet is constant.

Piston - where is torque measured

Measured at the gear box located between the engine and the propeller, it is the turning moment.

Diesel Engines:

• compression ignition, uses heat generated from compression to ignite the mixture.

• no spark plugs, but does have glow plugs to assist with starting.

• uses direct fuel injection

  • so doesn’t need intake icing, as there is no venturi where ice would build.

• higher compression ratio 14:1 to 28:1, compared with 9:1 for petrol.

• fuel flow determines power

• less powerful than petrol engines

• diesel is less inflammable, harder to burn, so can be compressed more

• no mixture control

• thermal efficiency is higher than petrol.

Swept Volume:

length of stroke x piston crown area

volume difference between TDC and BDC.

Multiply it by the total number of cylinders, it gives the total displacement

Compression Ratio:

The ratio of the largest volume to the smallest volume

Total displacement

Piston stroke x piston area x number of cylinders

Manifold Pressure

measured after turbochargers just before air enters the engine, downstream of the throttle valve.

When engine is off on ground, the pressure will be atmospheric.

When the engine is running, it will be lower pressure than atmospheric, as the engine is sucking air in.

If throttle is opened, the pressure increase (more air)

If turbocharger is used, then incoming air is increase, so pressure can be above atmospheric.

Specific Fuel Consumption

The mass of fuel required to produce a unit of power per a time.

Piston Lbs per hour per horsepower

Turbine: fuel flow / thrust.

Detonation:

Uncontrolled explosion of the air-fuel mixture in the cylinder, leading to a knocking noise and potential engine damage.

Unburnt parts of the of the mixture reach a high temp and ignite, earlier than usual.

Low octane fuel means it ignites easier, so can cause detonation.


The problem: TOO HIGH TEMP:
To correct: reduce manifold pressure = less throttle and fully enrich the mixture. (cools it down)

Pre-ignition:

A condition where the air-fuel mixture ignites earlier than intended before the piston reaches the top of its compression stroke.

Something in the cylinder is too hot, and can ignite too early, to avoid use the correct mixture setting.

Octane and Compression Relationship:

The more compression the higher octane requires, as with higher compression the temp is higher, so need more resistance to detonation.

Carburettor Icing

Caused by expansion of air after the venturi, the air expands and cools, also the fuel evaporating further reduces the temp.

Ice accretion can occur in both carb and fuel injection systems, but less so in the fuel injection, as it is only really inktake icing, as there is no venturi.

Parts of Carburettor

Mixture: Controlled by the needle valve, allows more fuel in when its opened.

Accelerator pump: when throttle is increased quickly, the carb is slow so not enough fuel would be mixed with the air, so a accelerator pump, linked to the thorttle is used.

Diffuser: Keeps fuel/air mixture steady at both low and high power settings.

Effects of Carburettor icing:

Constant speed propellor:
RPM remains the same (as its constant).
So the indicator would be loss of manifold pressure, as less air is getting through.

Fixed pitch: RPM reduces (less combustion).

Manifold Pressure increases after carb heat turned on? why?

It means Ice was forming, (when ice was there, less air was getting through so pressure was low), with the ice removed, more air gets through and pressure rises.

If you put carb heat on, and pressure drops, there was no ice (before colder denser air, now warmer less dense air = less pressure).

Carb heat and mixture

With carb heat on, air is less dense, so if fuel remains the same, then it becomes richer, as less air for the same amount of fuel.

Wet Sump:

• oil is collected at bottom of engine casing

• no external reservoir - limits oil quantity.

• uses just a pressure pump to take fuel from bottom, to top / to spray on components.

• difficult to manage temp of oil, as always in the engine.

Dry Sump:

• uses scavenger pump, to collect oil at bottom of case and return to reservoir.

• can hold more oil due to reservoir

• easier to control temp as it is taken outside of the engine.

• scav pump is stronger than pressure, to ensure casing stays dry.

High Viscosity:

• highly viscous = thick, honey

• oil more viscous at higher temperature.

Oil

Yellow = Single grade - used in new engines / refurbs
Red = multi-grade - normal ops

The number is the viscosity, the higher the number the more viscous.
W= winter, better for low temps.

yellow = sun = single.

Fuel Oil Heat Exchanger

Usually the pressure of oil is higher than fuel, so if there is a leak it will leak into the fuel as this is less of an issue as it just combusts.

If fuel gets into the engine, it could cause a fire.

How does a Magneto Work:

Magnets rotated by engine drive
this creates magnetic field in the primary winding
a contact breaker opens, this collapses the magnetic field
a very high voltage is produced in the secondary winding
this is is sent to the distributor, which sends out the voltage to a spark plug at the right time.


When the magneto is OFF, the circuit is grounded, so the build up cannot happen.

once the engine is running the system is independent of aircraft electrical system.

Capacitor: Prevents arcing across contact breaker points

Grounded wire = stopped.

Magneto: Impulse Coupling:

Basically a spring that allows magneto to build up enough speed, when engine is slow. So it can build enough voltage.

the spring building it up also delays the spark, to ensure valves are closed when it ignites.

Ideal Mixture Ratio:

15:1 but this is not used in normal ops at the combustion temp would risk detonation.

We set mixture around 10% richer, the excess fuel absorbs some latent heat, and cools the charge below detonation.

At high power, (high pressure and temp), risk of detonation increases, so we further increase mixture to absorb this heat.

When mixture is too high, much of the fuel isn’t burnt and it is exhausted.

a lean mixture means a slower burn, which heats the engine up more, as it has longer to burn.

Mixture Graph

1. Take off at sea level, full rich mixture, isn’t peak power, but reduces risk of detonation.

2. Best power is with a little leaner mixture.

3. 3 to 5, the learner the mixture the less power it produces.

4. 15:1 is most effcient, chemically correct ratio.

Propellors: ATM and CTM

ATM = aerodynamic twisting moment - the propeller is being driven by the engine, the resultant ATM rends to drive the blade to course.

CTM = Centrifugal twisting moment, tends to drive the blade to fine pitch.

As these forces do not balanxce it is always more difficult to move the blade from fine to course.

ATM → Course (you need a course to make money from the ATM)
opposite = A = C
C = F.
CTM → Fine

Constant Speed what happens to the propellor angle.

• as TAS increases, the air coming from the front increases, so the blade angle will increase to meet it (at less of an angle), to reduce the drag.
  
  TAS increase = Blade angle increase

Thinking about the pitch of propellers:

Think of the blades as wings, and the airflow is vertical to the propeller circle (the propeller plane)
As the propeller moves around it hits the air

In this case, a low angle is facing flat down to earth, a high angle 85° would be pointing forward.

a low angle would be a fine pitch.

Feather → Course → Fine →Reverse
FCFR


a fine pitch is used when the chord of the propeller is more inline with the plane of rotation.

when going faster, more airflow from the front, means we need to rotate the blade to be more parallel to the airflow, so this is course, used in cruise. To take bites into the air to pull is forward.

Synchroniser vs Synchrophasor:

Synchroniser: Equalise RPM only (N-rpM)

Synchrophasor: Equalise RPM , AND blade phase = less vibration = less noise.

Propeller Pitch Lever

Pull back to feather (think about shutting down, back for feather)

also, think forward, go around all forward (fine pitch for slow speed).

Angle range of propellors:

Alpha range =. inflight

beta = reverse.

Critical Engine:

The engine in case of a failure, would result in the worst aircraft handling.

• when a prop spins, the downward blade produces more thurst, so for a clockwise spin, the right side of the propeller will produce more thrust.

• so for the right engine, the thrust vector is further out from the aircraft centre line. creating more yaw motion which needs to be counteracted.

• so for clockwise spinning, left engine is critical.
  
  

• worse crosswind direction: coming from the same side as engine failure as it would push more on the rudder, and add to the yaw from the working engine.

Stress on rudder questions

• AFT CoG is worse - if the aft is closer to the rudder, the arm is shorter, so more force is needed on the rudder to stabilise us.

• to keep wings level in flight, we need to turn into the ok engine to keep our heading, but it means we sideslip with the dead side into the airflow, the ball would be central, but there would be a lot of pressure on the rudder.

• instead, we use a small bank to the working engine, this reduces the side slip, meaning the rudder workload is reduced. the ball will show a side deflection.

So to cause the most strain on rudder, have a AFT CoG,, and a large rudder input using just yaw to keep the wings level. this would show 0 on the ball indicator.

Another answer for high stresses.

‘Ah Can’t Get No Sleep’ - Aft CG, No Slip Indication

Least strain: forward CoG, turn into the working engine.

Mechanism to change the blade angles:

variable pitch: a hydraulic piston and cylinder in the hub, usually engine oil.

springs provide force in opposite direction, so when pressure is removed it foes to a safe position, typically feather.
(should disable this when shutting down so that on engine start they aren’t in feather position)

Auto Feather:

Used at low speed / high power, take off and landing. To auto feather so that control of aircraft is not lost.

Effect of high humidity on Piston

Decreases piston power as the air density is lower, because water takes up more room.

Rated power, vs take off power

Rated power = max continuos power

Take off power = the max performance, but has time limitation.

Super vs Turbo Charger

• super uses engine to drive compressor, the turbocharger uses engine exhaust.

• 
  

• boost is monitored by MAP.

• compressor and turbine mounted on common shaft.

• driven by exhaust system

• air enters eye of impeller and leaves tangental

• power available is less effected by altitude

Ground boosted: increased MAP above normal sea level pressure, allowing engine to produce more power

Altitude boosted: compensates for loss of atmospheric pressure as altitude increases

Waste Gate:

Controlled by hydraulic waste gate actuator

When gate is closed, all gas is directed to the turbine (waste closed means none is wasted).

FADEC = Buzz Word

FADEC = Ignition timing / mixture control.

Intercooler:

after supercharger, before MAP, cools compressed air, reduces risk of detonation, increases its density