1. CGS Ground School Technical The piston engine
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2. Principle of operation
The piston engine is also known as the internal combustion engine because the air/fuel mixture is burnt within the engine.
The engine consists of a number of moving parts that turn fuel into heat energy and then into rotational movement.
On an aircraft, this rotational movement is what drives the propeller.

3. Parts of the engine Cylinder Piston Connecting rod Crankshaft
Magneto drive
Magneto
Spark plug

4. Parts of the engine Exhaust cam Inlet cam Pushrods Rockers Inlet valve
Exhaust valve

5. The four stroke cycle
The engine converts fuel into heat energy and then into rotational movement over a series of four events.
Induction – the intake of the fuel/air mix.
Compression – of the fuel air mix.
Power – the spark plug ignites the fuel air mix.
Exhaust – the waste gas is expelled from the engine.

6. Induction During induction the inlet valve is open.
The piston travels down.
Causing a suction that draws in the fuel/air mixture.

7. Compression During compression both valves are closed.
The piston starts to travel up, compressing the fuel/air mixture therefore increasing its pressure.

8. Power
The compressed mixture is now ignited by a spark from the spark plug.
The mixture combusts and rapidly heats up, this causes it to expand.
The expanding mixture forces the piston down, therefore providing the driving force.

9. Exhaust The exhaust valve now opens.
The piston once again travels up forcing the waste gases out.
The piston is now back in position to start the induction part of the cycle again.

10. Axis of crankshaft rotation
The Vigilant Engine
The Vigilant has a four cylinder horizontally opposed engine.
At any given time, each cylinder is at a different point in the four stroke cycle.
All four pistons are joined together on a crankshaft.
Which provides a smooth, constant power output.
Direction of flight
Induction
Power
Exhaust
Axis of crankshaft rotation
Compression

11. The Vigilant Engine
The Vigilant has a four cylinder horizontally opposed engine.
At any given time, each cylinder is at a different point in the four stroke cycle.
All four pistons are joined together on a crankshaft.
Which provides a smooth, constant power output.
The sequence in which each cylinder ignites is known as the firing order. In the Vigilant the firing order is This firing order (as opposed to ) reduces the stresses on the crankshaft.
Direction of flight 1 2 3 4

12. The Vigilant Engine
The firing order remains in sequence due to the distributor.
The distributor sends a current from the magneto (which will be covered later) to the spark plugs.
It does this using a rotating electrode inside the distributor called a rotor arm.
As the rotor arm rotates it passes over four contact points that then send a spark to the individual spark plugs.
Direction of flight 1 2 3 4

13. Valve timing
When the piston is at the lowest point of travel it is said to be at Bottom Dead Centre (BDC).
BDC

14. Valve timing
When the piston is at the lowest point of travel it is said to be at Bottom Dead Centre (BDC).
When the piston is at the highest point of travel it is said to be at Top Dead Centre (TDC).
The previous slides suggested that the inlet valve opens at TDC at the start of the induction stroke, and closes at BDC.
Likewise they suggested that the exhaust valve opens at BDC and closes at TDC at the each end of the exhaust stroke.
TDC

15. Valve timing
As the piston approaches and passes TDC and BDC it has very little vertical movement compared to the amount of rotational movement of the crankshaft.
The part of the crankshaft’s cycle where these small vertical movements occur are called the ineffective crank angles.
Because the piston is almost stationary, this gives engine designers a chance to improve the performance and efficiency of the engine without having to increase its size.

16. Inlet valve timing
In practice the timing has to be adjusted to account for:
The time taken for the valve to open.
The time for air/fuel mix to start flowing.
By opening the inlet valve shortly before the piston reaches TDC and closing after the piston has reached BDC, a larger amount of fuel air mixture can enter the cylinder.

17. Exhaust valve timing
In practice the timing has to be adjusted to account for:
The time taken for the exhaust gas to flow out.
By opening the exhaust valve shortly before the piston reaches BDC and closing after the piston has reached TDC, the rising piston forces out the burnt fuel/air mixture and creates a low pressure area in the cylinder to assist the intake of a new fuel/air mixture.

18. Valve overlap
It therefore follows that there must be an occasion during the cycle when both valves are open at the same time.
This is known as valve overlap and occurs either side of TDC.
This improves engine efficiency because the incoming fuel/air mixture is at a slightly higher pressure than the exhaust gases, therefore force the exhaust out quicker.

19. Ignition timing
As with the valve timing, the ignition also has to be timed so that it ignites the fuel/air mixture at the optimum moment.
As such the spark occurs shortly before the piston is at TDC.

20. Engine cooling
Due to the repeated combustion of fuel/air mix and the friction of parts moving in the engine, a substantial amount of heat can build up.
This heat can be reduced in a number of ways but principally by lubricating the moving parts with oil.
The Vigilant has a wet sump system, which means the oil sits underneath the moving parts of the engine whilst it is switched off.

21. Engine cooling
However, when the engine is running, oil is pumped (via an engine driven oil pump) from the sump onto the crankshaft and rockers.
This oil lubricates the moving parts and is also sucked into the bottom of the cylinders below the pistons.
The suction is caused every time the piston travels up the cylinder, this is achieved by the use of piston rings.

22. Piston rings
Three chrome vanadium rings are inserted into slots around the outside of the piston.
The top two provide a seal to ensure no leakage of the fuel/air mixture during the compression stroke and no loss of power during the power stroke. These are known as compression rings.
The lower ring is called the oil control ring and it’s function is to apply a fine film of oil to the cylinder walls. As the piston moves up it creates suction underneath it, this pulls oil into the cylinder which is distributed by the oil control ring.

23. Engine cooling
As well as providing lubrication and sealing the combustion chamber, the oil also absorbs a lot of heat from the engine, therefore acting as a coolant.
During the combustion process, small particles of sludge and carbon residue are produced. The oil carries these away from the cylinders to the oil filter where the oil is cleaned.
The oil also protects the metal parts of the engine from corrosion.
The other form of cooling on the Vigilant engine is air cooling. On the outside of the cylinders are cooling fins, these provide more surface area for the air cooling to act up on.

24. Volts Amps
The ignition system
The Vigilant is fitted with an engine driven generator that is used solely to power the spark plugs. It is called a magneto.
The magneto consists of:
A magnet rotated by the engine
Inside a heavy wire coil
When the magnet rotates it produces a low voltage/high amperage current in the coil.

25. Volts Amps
The ignition system
The low voltage/high amperage is transformed into a high voltage/low amperage by fitting another coil that is made of thinner wire.
The higher voltage produces a much hotter spark when supplied to the spark plug.

26. The ignition system
When the engine is started, it is done so by the starter motor.
This initially turns the engine (and therefore the magneto) too slowly to produce a spark sufficiently hot to ignite the fuel.

27. The ignition system
To overcome this, the magnet in the magneto is attached to the engine through a coil spring.
This allows the spring to turn and build up tension.
When sufficient tension is built up, the spring rotates the magnet at the optimum speed, thus producing a hot spark.
This is known as an impulse magneto.

28. Magneto earthing
The magneto is earthed through the LT lead (the lead that runs from the magneto to the distributor).
When the ignition is “OFF”, the magneto is earthed. When the ignition is on it is live. The magneto is earthed in this way to ensure if it fails, it does so safely.

29. THE END
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30. Master slide