lecture 2

Propulsion: 

Propulsion Flt Lt Jackson

Lecture 2: 

Lecture 2 Carburetion and Ignition systems

Introduction: 

Introduction The carburettor and Ignition system are both vital components for the piston engine Carburettor:- Provides the fuel air mixture to the piston and cylinder Ignition system:- Provides the source of ignition for the fuel and air mixture at the correct moment

Fuel systems: 

Fuel systems 2 different ways of provide fuel and air to the cylinder Carburettor Direct Fuel injection

Carburettor: 

Carburettor Consists of a tube with a jet inside of the tube Air passes through the tube creating a vacuum through which fuel is pulled from the jet. The amount of air flowing through tube dictates the amount of fuel leaving the jet Thus the correct fuel air mixture is obtained

Carburettor: 

Carburettor The amount of air flowing through the tube is controlled by a butterfly valve called the throttle plate Venturi (Choke Tube):- This is the narrowing of the tube through which the air flows creating the vacuum

Consolidation: 

Consolidation E A B C D F G

Carburettor: 

Carburettor

Carburettor: 

Carburettor When the engine is idling the throttle is said to be closed However the throttle cannot be fully closed but a small gap is left at the edge This small gap doesn’t permit enough fuel to pass into the cylinder Thus a small jet is used to spray fuel directly above the throttle by the same mechanism

Fuel:air Ratio: 

Fuel:air Ratio Correct air:fuel ratio is about 15:1 by weight although this can vary depending on type of fuel The simple carburettor described above doesn’t provide for efficient running as the mixture tends to richen as the throttle is opened

Fuel:air Mixture: 

Fuel:air Mixture This can be overcome by Partly blocking the main jet using a neddle attached to the throttle Diffuser action

Other Refinements: 

Other Refinements Manually altering the mixture Using a tapered needle as discussed Using 2 or more main jets which can operate at different times Having secondary choke tubes which operate only when needed

Accelerator pump: 

Accelerator pump This pumps fuel into the air intake to account for the lag caused during accelerating This is caused because the fuel is heavier than air so takes longer to reach the intake than the air when the throttle is opened. This creates an acceleration lag when the throttle is opened

Special Adaptations: 

Special Adaptations As altitude increases the air gets thinner thus causing the mixture to richen This is solved by use of an aneroid which expands due to decreased air pressure and pushes a tapered needle into the main jet This however causes a decrease in powered available at altitude

Supercharging: 

Supercharging A pump is used to increases the pressure of the incoming fuel:air mixture into the cylinders. This compensates for the above problems and can significantly increases power output

Fuel Injection: 

Fuel Injection

Ignition Systems: 

Ignition Systems Aero engines use a system of magnetos to produce the high tension spark required to ignite the fuel air mixture This method is more reliable than the battery and high tension coil used in most car engines

Magneto: 

Magneto It works like a small dynamo in creating a constant small current through the movement of a magnet This is converted into high tension impulses. These impulses are used to create the spark in each cylinder in the correct order.

Componants: 

Componants The permanent rotating magnet The armature containing the coils The contact Breaker:- Mechanically operated switch The capacitor The distributor

Rotation of the Magnet: 

Rotation of the Magnet

Primary Circuit: 

Primary Circuit The magneto contains 2 circuits(Primary and Secondary) The primary circuit contains the constant low current created by the movement of the magnet. The contact breaker points are opened when a spark is required

Mechanism: 

Mechanism This causes the magnetic field created by the primary circuit to collapse and produces a high voltage impulse in the secondary winding. This passes via the distributor to the spark plugs and creates a spark

Distributor: 

Distributor This distributes the impulse to the correct spark plug in the firing order for the pistons.

Spark Plugs: 

Spark Plugs 3 Sections Screened gland nut Centre assembly Steel body

Spark plugs: 

Spark plugs Materials Central electrode:- Nickel Rod with silver inserts Earth electrodes:- Platinum Gap size is important and must be maintained at the manufacturers recommendations

ANY QUESTIONS?: 

ANY QUESTIONS?

QUESTIONS 1: 

QUESTIONS 1 1. What is the best fuel/air ratio for correct mixture? 2. What are the two main functions of a carburettor? 3. Give three essential components of a float chamber type carburettor? 4. How does the float chamber mechanism maintain the correct fuel pressure at the jet? 5. What governs the amount of fuel passing through the main jet? 6. What happens if the pressure differential between the float chamber and the jet is increased? 7. Give two common methods of varying the mixture strength? 8. How does a supercharger work? 9. What is a boost control? 10. What is a turbo-charger?

QUESTIONS 2: 

QUESTIONS 2 1. Why do all aero-engines have dual ignition systems? 2. What is the effect of an incorrect contact breaker gap? 3. What is the purpose of the capacitor? 4. What governs the strength of the electromotive force? 5. How is the high voltage current required to provide a spark at the plugs produced? 6. Do the cylinders fire in numerical sequence - and if not - why not?