Piston Engine Operations PISTON ENGINES Part 5 Piston Engine Operations (Fuel & Ignition)

The Carburettor Remember that the fuel is pumped to the carburettor. The carburettor is a fuel reservoir and air delivery tube. It controls the mixing of air and atomized fuel, and the flow of the mixture to the engine, therefore control of the fuel flow, via the carburettor, is control of the engine.

The Carburettor Fuel is mostly supplied through the main jet nozzle, AIR/FUEL MIXTURE FLOWS TO ENGINE Fuel is mostly supplied through the main jet nozzle, on the same level as fuel in the float chamber. Air is sucked through the Venturi by air pressure, as the piston goes DOWN on an Induction Stroke. This pressure reduction pulls fuel from the float chamber into the main jet. VENTURI FUEL FLOW AIR VENT FUEL NOZZLE FUEL LEVEL FLOAT FLOAT CHAMBER RESERVOIR LOWER AIR PRESSURE FUEL FEED

The Carburettor As fuel leaves the float chamber, AIR/FUEL MIXTURE FLOWS TO ENGINE As fuel leaves the float chamber, air enters via the vent, the float drops, opening a needle valve, which allows fuel to be replaced by the fuel pump. Fuel entering the chamber forces air out of the vent, the float rises, closing the needle valve again. VENTURI FUEL FLOW NEEDLE VALVE AIR VENT FUEL NOZZLE THIS ONLY HAPPENS WHEN ONE PISTON IS ON THE INDUCTION STROKE FUEL LEVEL FLOAT FLOAT CHAMBER RESERVOIR LOWER AIR PRESSURE FUEL FEED

LINK TO THROTTLE LEVER/PEDAL The Throttle Valve AIR/FUEL MIXTURE FLOWS TO ENGINE The throttle valve consists of a plate, mounted in a central spindle, linked to the throttle lever (or foot pedal). The plate can be any shape that closely fits the tube in the carburettor. VALVE PLATE VALVE SPINDLE FLOAT LINK TO THROTTLE LEVER/PEDAL FLOAT CHAMBER RESERVOIR

The Throttle Valve An Open Throttle (Full Power) AIR/FUEL MIXTURE FLOWS TO ENGINE An Open Throttle (Full Power) allows the Venturi to work at maximum efficiency. A Low Throttle setting (Idle or Ticking Over) means the Venturi do not work very well, with only sufficient Fuel/Air mixture to the engine. LOW THROTTLE OPEN THROTTLE FLOAT So an alternative, more effective, Venturi is needed. FLOAT CHAMBER RESERVOIR

The Throttle Valve The Edge Gaps now become the Venturi AIR/FUEL MIXTURE FLOWS TO ENGINE The Edge Gaps now become the Venturi for the Low Air Flow. The Fuel Nozzle is re-designed, with an extension to the throttle valve. At Idle – Slow Running, the extension jet controls the fuel flow. LOW THROTTLE OPEN THROTTLE FLOAT At Full Throttle the main Venturi comes into operation. FLOAT CHAMBER RESERVOIR

Balanced Air Pressure AIR/FUEL MIXTURE FLOWS TO ENGINE Air Filters can cause restrictions to the Air Flow, with a slight vacuum before the carburettor. This creates Pressure Differences between: So the Float Chamber vent is closed off, and the Float Chamber is connected to the Carburettor Air Intake. Equalising pressure at Low Speed, and preventing too much Fuel mixing with the Air. Here AND Here FLOAT FLOAT CHAMBER RESERVOIR VACUUM

Mixing Air and Fuel Diffuser Tube AIR/FUEL MIXTURE FLOWS TO ENGINE Efficient air/fuel mixing is essential. So fuel needs to be ‘atomized’ (broken down into the smallest particles possible). Fuel in liquid form doesn’t burn, it’s fuel vapour which burns, so atomizing fuel creates a greater vapour surface. A more efficient mix of fuel & air can be obtained by fitting a Diffuser Tube onto the main jet. FLOAT FLOAT CHAMBER RESERVOIR So let’s take a look here.

Mixing Air and Fuel The Diffuser Tube HIGH-POWER SETTING IDLE SETTING Increased flow of mixed Air/Fuel AIR INLET HIGH-POWER SETTING The throttle valve is fully open and maximum airflow is passing through the main venturi. Fuel level in the Float Chamber IDLE SETTING the diffuser is full of fuel but very little air, fuel supplied via idle jet CRUISE SETTING air bubbles through the diffuser holes, improving atomization OPEN THROTTLE idle jet stops working, main Venturi comes into operation Air mixes with Fuel DIFFUSER TUBE MAIN JET Fuel feed from the Float Chamber THROTTLE VALVE

The Pressure Capsule 15:1 Air at higher altitudes is ‘thinner’, causing lower pressure. But the piston travels the same distance whatever the altitude it is working in. Therefore at higher altitudes less air is taken into the engine. Less airflow means a richer mixture, (more fuel than air), which would exceed the ideal ratio of 15:1

The Pressure Capsule A method of adjusting the fuel and air flow is required to maintain the idea 15:1 ratio. Since air pressure is easily sensed, the device used is an Aneroid Capsule. The aneroid capsule is a sealed chamber made from concertinaed thin metal. The inside is a partial vacuum, the outside senses the air pressure, (termed as ‘ambient’ pressure). FLOAT FLOAT CHAMBER RESERVOIR ANEROID CAPSULE

The Pressure Capsule At LOW Altitude – Higher Atmospheric Pressure, the capsule is compressed by the outside air pressure, allowing the correct quantity of fuel through, for the weight of air passing into the engine. FLOAT FLOAT CHAMBER RESERVOIR

The Pressure Capsule At HIGH Altitude – Lower Atmospheric Pressure, the capsule expands, pushing the needle into the main jet, allowing a lower quantity of fuel for the lower weight of air passing into the engine. The result is the air/fuel ratio is maintained at 15:1 at all atmospheric conditions. FLOAT FLOAT CHAMBER RESERVOIR

Fuel Injection Systems The injection system differs from the carburettor system in the way the fuel is delivered. In carburettors, fuel is mixed with air before it enters the cylinders. In injection systems, fuel is injected directly into the air, either in the inlet manifold, or even directly into the cylinders. Injection systems squirt high pressure fuel through a small nozzle, or jet, to atomise the fuel.

Fuel Injection Systems Hydro-Mechanical Injection System The airflow is sensed by an ‘Air Sensor Plate’, which regulates the fuel flow to the engine in the Mixture Control Unit/Fuel Distribution Valve. In this system sufficient fuel is continuously injected into the inlet manifold. Warm Up Regulator Air Inlet from air filter Throttle Valve Cold Start Valve Auxiliary Air Device Idle Speed Adjuster Fuel injector Mixture Control Unit (Fuel Distribution Valve ) Air Sensor Plate

Fuel Injection Systems Electronic Injection System The electronic system is very much simpler, at least with the number of manufactured parts. Pressure is limited by the Pressure Regulator, and the injectors fed from a common ‘Rail’ by the fuel pump. Injection Control Unit Pump Air Filter Throttle Valve Common ‘Rail’ The Injection Control Unit (ICU) controls the timing of the injector valve opening, in conjunction with the Engine Control Unit (ECU). Engine ECU Fuel Injectors Pressure Regulator

Check of Understanding In the carburettor when is air sucked into the Ventri? On the Compression up stroke On the Induction up stroke On the Compression down stroke On the Induction down stroke

Check of Understanding What shape are Throttle Valves? Any Shape Always Circular Always Square or Oblong Never Triangular

Check of Understanding When does the Venturi work at maximum efficiency? When the engine is idle When the engine is at full power When the engine is cruising When the engine is at low power

Check of Understanding What happens in a float chamber carburettor if an air filter causes restrictions to the air flow? The pressure drops rapidly A vacuum is formed The pressure rises rapidly A vortex is formed

Check of Understanding In the diagram below, which arrow points to the main jet? W X Y Z

Check of Understanding At what throttle setting is a diffuser tube full of fuel but with little air? Never Full Power Cruising Ticking Over

Check of Understanding In a float chamber carburettor, what is the function of the device marked with a ‘X’? To atomize the fuel To prevent the fuel/air ratio from being too rich when the aircraft climbs To supply extra fuel for acceleration To prevent the fuel/air ratio from being too rich when the throttle valve is opened

Check of Understanding In a fuel injection system, which of the following does not apply? Fuel can be injected into the inlet manifold The fuel is not atomized Fuel can be injected into the cylinder A throttle valve is not needed

Check of Understanding What is the normal fuel to air ratio in a piston engine? 1 : 5 1 : 15 5 : 1 15 : 1

PISTON ENGINES End of Presentation