1. Systems Review 1
ATC Chapter 4

2. Aim
To review principals of aircraft propulsion

3. Objectives
Name the basic components of the reciprocating engine and describe the Otto cycle
State the operation of the ignition system
State the causes of and ways to prevent abnormal combustion

4. 1. Describe the Otto cycle
Engine 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

5. 1. Describe the Otto cycle
Engine 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

6. 1. Describe the Otto cycle
Engine 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

7. 1. Describe the Otto cycle
Engine 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

8. 1. Describe the Otto cycle
Engine Components
Connecting Rod
Connects the piston to the crankshaft

9. 1. Describe the Otto cycle
Engine 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

10. 1. Describe the Otto 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
Intake
The piston is moving in a downwards direction, reducing the pressure in the cylinder, sucking in the fuel/air mixture (also known as the charge)

11. 1. Describe the Otto cycle
Compression
The 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

12. 1. Describe the Otto cycle
Power
The 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

13. 1. Describe the Otto cycle
Exhaust
Just 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

14. 2. Operation of the ignition 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

15. 2. Operation of the ignition system
Magneto
The magneto works on the principal of electro magnetic induction
A magnet rotates near a conductor which has a coil of wire wrapped around it inducing a current
Most magnetos will have a primary and secondary coil
Most aircraft engines will have two magnetos named left and right respectively

16. 2. Operation of the ignition system
Spark plug
Operate by providing a gap between the high tension wire coming off the distributor and earth
All aircraft spark plugs are shielded to prevent radio interference
Most aircraft engines will have two spark plugs for each cylinder, one at the top, one at the bottom

17. 3. Abnormal Combustion Abnormal 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

18. 3. Abnormal Combustion Detonation
Typically occurs when the pressure or temperature of the charge is too high resulting in spontaneous explosive combustion
Indicated by rough running, high CHT or possible engine failure
Contributory 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 engine
If detonation is suspected:
Enrichen the mixture
Reduce the throttle
Increase airspeed
Open cowls

19. 3. Abnormal Combustion Pre-ignition
Occurs when the charge prematurely ignites
Typically caused by a hot spot on the cylinder igniting the charge
Indicated by:
Rough running
Back fire
Sudden rise in CHT
Engine failure
Can be caused by:
Hot spots on the cylinder from carbon deposits
High power settings with a lean mixture
Overheated spark plugs

20. Questions?