Systems Review 1ATC Chapter 4

Aim

To review principals of aircraft propulsion

Objectives1. Name the basic components of the

reciprocating engine and describe the Otto cycle2. State the operation of the ignition system3. State the causes of and ways to prevent

abnormal combustion

1. Describe the Otto cycleEngine Components

Cylinders• Barrel in which combustion takes place and the piston moves• The exterior is normally manufactured with cooling fins• Typical arrangement on light aircraft is horizontally opposed 4 or 6 cylinder

1. Describe the Otto cycleEngine Components

Piston• Moves within the cylinder• Three or more grooves are machined around the piston in which are set

steel piston rings• The first ring prevents combustion gases from escaping• The lower rings prevent oil from entering the combustion space

1. Describe the Otto cycleEngine Components

Inlet and Exhaust valves• Fitted to cover ports in the cylinder heads• Inlet valve allows the fuel/air mixture into the cylinder• Exhaust valve allows spent combustion gases to escape• When not open they are held in place by springs

1. Describe the Otto cycleEngine Components

Spark Plugs• Ignites the fuel air mixture at the correct time• Typically arranged with one at the top and one at the bottom of the

cylinder

1. Describe the Otto cycleEngine Components

Connecting Rod• Connects the piston to the crankshaft

1. Describe the Otto cycleEngine Components

Crankshaft• A shaft designed with a crank for each cylinder• Turns the up and down motion of the piston into the rotary motion

required for the propeller

1. Describe the Otto cycleOtto Cycle

Engines typically found on light aircraft are of the four stroke type, as the name suggest it takes four strokes of the cylinder to complete one cycle

IntakeThe piston is moving in a downwards direction, reducing the pressure in the cylinder, sucking in the fuel/air mixture (also known as the charge)

1. Describe the Otto cycleOtto Cycle

CompressionThe inlet valve closes and piston moves back towards the cylinder head compressing the charge.As the charge is compressed the temperature rises considerably, diesel engines will use this increase in pressure and temperature to ignite the charge

1. Describe the Otto cycleOtto Cycle

PowerThe charge is ignited by the spark plugs resulting in rapid expansion of the gases in the cylinder, increasing pressure n the top of the piston forcing it to travel down

1. Describe the Otto cycleOtto Cycle

ExhaustJust prior to completion of the power stroke the exhaust valve opens and as the piston moves back up towards the cylinder head the exhaust gases are forced out of the cylinder via the exhaust manifold

2. Operation of the ignition systemIgnition system

The purpose of the ignition system is to ignite the charge at the precise time in order to create a controlled flame front Most light aircraft have dual independent ignition systems for redundancy and to create more efficient combustion

The ignition system is made up of:• An electrical source, in this

case a magneto• A distributor• High tension ignition leads• Spark plug

2. Operation of the ignition systemMagneto

The magneto works on the principal of electro magnetic inductionA magnet rotates near a conductor which has a coil of wire wrapped around it inducing a currentMost magnetos will have a primary and secondary coilMost aircraft engines will have two magnetos named left and right respectively

2. Operation of the ignition systemSpark plug

Operate by providing a gap between the high tension wire coming off the distributor and earthAll aircraft spark plugs are shielded to prevent radio interferenceMost aircraft engines will have two spark plugs for each cylinder, one at the top, one at the bottom

3. Abnormal CombustionAbnormal Combustion

Normal combustion can be defined as the process in which the compressed fuel/air charge in the cylinder is ignited at the intended time by the spark plugs with the resulting flame front spreading across the cylinder head in a smooth, uniform manner until the charge has been consumed.Generally speaking there are two types of abnormal combustion:• Detonation• Pre-ignition

3. Abnormal CombustionDetonation

Typically occurs when the pressure or temperature of the charge is too high resulting in spontaneous explosive combustionIndicated by rough running, high CHT or possible engine failureContributory factors include:• Expired fuel• Operation of carb heat at high power, low airspeed• Using fuel with a lower than recommended octane• Over lean mixture• Excessive manifold pressure• Over heated engineIf detonation is suspected:• Enrichen the mixture• Reduce the throttle• Increase airspeed• Open cowls

3. Abnormal CombustionPre-ignition

Occurs when the charge prematurely ignitesTypically caused by a hot spot on the cylinder igniting the chargeIndicated by:• Rough running• Back fire• Sudden rise in CHT• Engine failureCan be caused by:• Hot spots on the cylinder from carbon deposits• High power settings with a lean mixture• Overheated spark plugs

Questions?