aerospace propulsion study for shenyang aerospace university by lale420 (mid_term_3)

Piston Engine

Ignition System

The Ignition System

Purpose

To provide a spark discharge at the spark plug at the correct time

There is no electrical connection between the aircraft electrical system and the ignition system

Failure of the electrical system will not affect the ignition system

The Ignition System

The source of electricity to create a spark in an aircraft electrical system is the magneto, a self container generator totally independent of the electrical system

The source of electricity to create a spark in a car is the battery, therefore if a car’s electrical system fails so does the ignition system

The primary aim of using a magneto instead of battery ignition is the safety provided by its independence

Battery Ignition System

Magneto Ignition System

The Ignition System The ignition system provides the spark that ignites the fuel/air mixture in the cylinders and is made up of: • Magnetos • Spark plugs • High-tension leads • Ignition switch • Breaker points for spark timing • Distributor: to direct the magneto electrical

energy to the correct spark plug

The Ignition System

The Ignition System

Magneto • Uses a permanent magnet to generate an electrical

current completely independent of the aircraft’s electrical system.

• The magneto generates sufficiently high voltage to jump a spark across the spark plug gap in each cylinder.

• The system begins to fire when you engage the starter and the crankshaft begins to turn.

• It continues to operate whenever crankshaft rotates. • The operation of the magneto is controlled in the

cockpit by the ignition switch.

The Ignition System

Magneto

• Airplanes incorporate a dual ignition system with two individual magnetos

• Each magneto operates independently to fire one of the two spark plugs in each cylinder.

• If one of the magnetos fails, the other is unaffected.

• The engine will continue to operate normally, although you can expect a slight decrease in engine power.

The Ignition System

A magneto is simply an electrical generator consisting of a coil and a magnet

All piston aircraft engines have dual magnetos for safety should one fail

The magneto is driven by the crankshaft, so once the engine is running the magneto is self sustaining

The Ignition System

The Starter Relay Relays and solenoids are used when it is necessary to open or close a circuit carrying a large amount of current from a remote location and with a small switch

This component of the ignition system wouldn’t be necessary if you were happy hand swinging the prop

R

R Magneto R Spark Plug

L Magneto L Spark Plug

Key on Left Magneto

Turning the key to the left magneto position earths the right magneto

R



Key on Right Magneto

Turning the key to the right magneto position earths the left magneto

R



Key on Both Magnetos

With the key in the BOTH position, neither magneto is earthed

Key on Both Magnetos

R



Key on OFF position

With the key in the OFF position, both magnetos are earthed

Key on OFF position

The Impulse Magneto

Being a generator (as compared to an alternator), at the RPM experienced during start-up the magneto electrical output is very low and this will result in the spark not being “hot” enough to ensure ignition of the fuel/air mixture takes place

To provide the necessary voltage to produce a “hot” spark for starting an impulse coupling magneto is used

To produce a “hot” spark we need to increase the voltage of the low revolving magneto (generator)

The Impulse Coupling

The impulse coupling magneto uses a heavy duty coiled spring which is tensioned during the starting process. Once sufficient tension is achieved, one end of the spring, attached to the magnet is released, spinning the magnet very quickly

The tension spring is coupled to two flyweights such that once the engine starts the spring mechanism disconnects from the magneto mechanism


Another benefit of the impulse magneto is its ability to retard the spark for the slow RPM which may otherwise result in engine “kick-back”

Only one of the two magnetos is an impulse magneto and is used on its own during the starting process

After start-up both magnetos engage for normal operations

Booster Coil

The booster coil is separate from the magneto and it can generate a series of sparks on its own. During the start cycle, these sparks are routed to the distributor rotor and then to the appropriate cylinder ignition lead.

Booster Coil

The Ignition System

For a spark to occur, very high voltage is required. This is normally in excess of 20,000 volts

At these high voltages leakage from the high tension leads can occur, especially at high altitudes. As the air becomes less dense, its insulating capability decreases

The high voltages that pulsate along the leads emit radio frequency which will interfere with VHF radio operation

The Ignition System

To prevent this from occurring the ignition harnesses are covered with a protective coating

Occasionally the protective coating cracks. We can tell by the RF noise generated on the VHF radio

Checking Magneto Operation

Magnetos need to be checked twice during a flight operation, each of these for very specific and different reasons

During the pre-takeoff checks we check the magnetos at 1700 RPM to ensure they are both working

Prior to shut down after a flight we check the magnetos to ensure that the ignition key mechanism works to ground both of them

Detonation

It is an uncontrolled,

explosive ignition of

the fuel/air mixture

within the

combustion

chamber.

Detonation

• It causes excessive temperatures and pressures which, if not corrected, can quickly lead to failure of the piston, cylinder, or valves. In less severe cases, detonation causes engine overheating, roughness, or loss of power.

• Detonation is characterized by high cylinder head temperatures, and is most likely to occur when operating at high power settings.

Detonation

What causes detonation?

• Using a lower fuel grade

• Operating with very high manifold pressures in conjunction with low r.p.m.

• Operating the engine at high power settings with excessively lean mixture.

• Detonation also can be caused by extended ground operations, or steep climbs where cylinder cooling is reduced.

Pre-ignition

It occurs when the fuel/air mixture ignites prior to the engine’s normal ignition event. • Premature burning is usually caused by a

residual hot spot in the combustion chamber, often created by a small carbon deposit on a spark plug, a cracked spark plug insulator, or other damage in the cylinder that causes a part to heat sufficiently to ignite the fuel/air charge.

Pre-ignition

• Pre-ignition causes the engine to lose power, and produces high operating temperature.

• As with detonation, pre-ignition may also cause severe engine damage, because the expanding gases exert excessive pressure on the piston while still on its compression stroke.

• Detonation and pre-ignition often occur simultaneously and one may cause the other

An aircraft electrical system operates at approximately constant

A. voltage

B. amperage

C. resistance

And….. the answer is………..

If the switch wire leading from the impulse magneto to the cockpit key was severed, the effect on the engine would be

A. the engine would not start as it is affecting the impulse magneto

B. the engine will continue to run if both magnetos are turned off

C. the impulse mechanism will be in-operative

D. the engine will stop when the other magneto is switched off


A centre-zero ammeter placed between the battery and the bus bar will indicate

A. the electrical potential of the battery

B. the total current of the electrical system

C. the total alternator output

D. whether the battery is being charged or discharged


If the alternator failed in flight the outcome would be

A. all electrical instruments would immediately fail

B. the engine would continue to run

C. the engine would stop when the battery went flat

D. the engine would immediately stop


It is advisable to check both magnetos to make sure they are capable of being switched off. This check should be made

A. during the 1700 RPM engine run-up prior to flight

B. before shutting the engine off after flight

C. before starting the engine


On conducting a magneto check just prior to take-off, no discernible drop is observed when the key is placed on the left magneto position even though the other magneto key position tests correctly. This could be due to

A. the magnetos being in first class condition

B. a sticky tachometer

C. the earth wire to the left magneto is severed

D. the earth wire to the right magneto is severed


The earth wire coming from the magneto is connected to

A. the nearest metal part of the aircraft

B. the case of the magneto

C. the magneto key switch

D. any metal part of the aircraft


With the ignition key in the OFF position, if the magneto earth wires are disconnected, turning over the propeller may cause

A. engine flooding because the fuel-air mixture isn’t being burnt

B. a short circuit in the magneto as the electrical charge has nowhere to go

C. no spark at the spark plug

D. the engine to start


If your aircraft battery is completely flat and you attempt to start the engine by swinging the propeller, which of the following could occur when the engine starts

A. the alternator may get damaged

B. the alternator may overcharge the battery resulting in the battery fluid boiling

C. the alternator may not function once the engine is operating normally

D. the alternator must not be turned on until the centre zero ammeter is showing a positive charge


The capacity of a battery is measured in

A. amps

B. amp hours

C. volt hours

D. volts


aerospace propulsion study for shenyang aerospace university by lale420 (mid_term_3)

Engineering