AVIA-1065 Week 2 Day 3 2022 Reciprocating Engines PDF

RECIPROCATING ENGINE AVIA 1065 Carburetor Systems 1 Carburetor Systems To provide for engine operation under various loads and at different engine speeds, each carburetor has six syste...

RECIPROCATING ENGINE AVIA 1065 Carburetor Systems 1 Carburetor Systems To provide for engine operation under various loads and at different engine speeds, each carburetor has six systems: 1. Main metering 2. Idling 3. Accelerating 4. Mixture control 5. Idle cutoff 6. Power enrichment or economizer Each of these systems has a definite function. It may act alone or with one or more of the others (m*i*a*m*i*p) 2 Carburetor Systems main metering system supplies fuel to the engine at all speeds above idling fuel discharged by this system is determined by the drop in pressure in the venturi throat 3  4 Carburetor Systems Idling necessary for idling because the main metering system can be erratic at very low engine speeds low speeds the throttle is nearly closed The velocity of the air through the venturi is low and there is little drop in pressure differential pressure is not sufficient to operate the main metering system, so no fuel is discharged from this system most carburetors have an idling system that supplies fuel to the engine at low engine speeds Carburetor Systems Idle Jet 5 Float-Type Carburetors Idling System 6 Carburetor Systems accelerating system supplies extra fuel during sudden increases in power, when the throttle is opened airflow through the carburetor increases to obtain more power from the engine the main metering system then increases the fuel discharge With the sudden acceleration, the increase in airflow is so rapid that there is a slight time lag before the increase in fuel in supplying extra fuel during this period, it prevents a temporary leaning out of the mixture and gives smooth acceleration 7 Float-Type Carburetors 8 Accelerating System When the throttle valve is opened quickly, a large volume of air rushes through the carburetor the amount of fuel that is mixed is less than normal due to the slow response rate of the main metering system the fuel/air mixture leans out momentarily and can cause the engine to accelerate slowly or stumble as it tries to accelerate Float-Type Carburetors Accelerating System to overcome this tendency, the carburetor is equipped with a small fuel pump called an accelerating pump consists of a simple piston pump that is operated through linkage by the throttle control a passageway opening into the main metering 9 system or the carburetor barrel near the venturi Float-Type Carburetors Accelerating System when the throttle is close, the piston moves back and fuel fills the cylinder If the piston is pushed forward slowly, fuel seeps past it back into the float chamber if pushed rapidly, it sprays fuel in the venturi and enriches the mixture 10 Float-Type Carburetors Accelerating System A common type of accelerating system used in float carburetors is illustrated in Figure 2- 18. An example of a cutaway accelerator pump is shown in Figure 2-19. 11 Carburetor Systems mixture control system determines the ratio of fuel to air in the mixture 12 cockpit controlled manual mixture control can select the mixture ratio to suit operating conditions  mixture control system Carburetor Systems necessary to have a means of adjusting because as the airplane climbs and the atmospheric pressure decreases there is a corresponding decrease in the weight of air passing through the induction system volume remains constant volume of airflow that determines the pressure drop at the throat of the venturi 13 Carburetor Systems mixture control system the carburetor tends to meter the same amount of fuel to this thin air as to the dense air at sea level the natural tendency is for the mixture to become richer as the airplane gains altitude automatic mixture control prevents this by decreasing the rate of fuel discharge to compensate for the decrease in air density many carburetors have automatic mixture controls so that the fuel/air ratio, once selected, it does not change with variations in air density https://www.aircraftsystemstech.com/p/float-type- carburetors-float-type.html 14 Float-Type Carburetors Mixture Control System As altitude increases – air becomes less dense – at an altitude of 18,000 feet the air is only half as dense as it is at sea level – means that a cubic foot of space contains only half as much air at 18,000 feet as at sea level an engine cylinder full of air at 18,000 feet contains only half as much oxygen as a cylinder full of air at sea level 15 Float-Type Carburetors Mixture Control System low pressure area created by the venturi is dependent upon air velocity rather than air density the venturi draws the same volume of fuel through the discharge nozzle at a high altitude as it does at a low altitude the fuel mixture becomes richer as altitude increases can be overcome either by a manual or an automatic mixture control 16 Float-Type Carburetors Mixture Control System float-type carburetors two types of purely manual or cockpit controllable devices are in general use for controlling fuel/air mixtures, the needle type and the back-suction type. [Figures 2-16 and 2-17] 17 Float-Type Carburetors Mixture Control System needle-type system manual control is provided by a needle valve in the base of the float chamber can be raised or lowered by adjusting a control in the cockpit moving the control to “rich,” opens the needle valve wide permits the fuel to flow unrestricted to the nozzle moving the control to “lean,” partially closes the valve restricts the flow of fuel to the nozzle 18 Float-Type Carburetors Economizer System mixture control 19 Reciprocating Engines BREAK! RETURN AT 2 20 Float-Type Carburetors Mixture Control System quadrant in the cockpit is usually marked “lean” near the back end and “rich” at the forward end extreme back position is marked “idle cutoff” and is used when stopping the engine 21 22 Float-Type Mixture Control System Carburetors Float-Type Carburetors Mixture Control System needle-type mixture control placing the mixture control in idle cutoff seats the needle valve, shutting off fuel flow completely back-suction mixture controls a separate idle cutoff line leads to the extreme low pressure on the engine side of the throttle valve 23 Carburetor Systems idle cutoff system part of the manual mixture control stops the fuel discharge from the carburetor completely when the mixture control lever is set to the “idle cutoff” position aircraft engines are stopped by shutting off the fuel rather than by turning off the ignition 24 Carburetor Systems idle cutoff system If the ignition is turned off with the carburetor still supplying fuel, fresh fuel/air mixture continues to pass through the induction system to the cylinders As the engine is coasting to a stop and if it is excessively hot, this combustible mixture may be ignited by local hot spots within the combustion chambers can cause the engine to continue running or kick backward 25 Carburetor Systems idle cutoff system Unburned gases may pass through the cylinder and be ignited in the hot exhaust manifold a combustible mixture remains in the induction passages, the cylinders, and the exhaust system – an unsafe condition since the engine may kick over after it has been stopped and seriously injure anyone near the propeller the engine is shut down by means of the idle cutoff system the spark plugs continue to ignite the fuel/air mixture until the fuel discharge from the carburetor ceases 26 Carburetor Systems idle cutoff system this alone should prevent a combustible mixture in the cylinders some engine manufacturers suggest that just before the propeller stops turning, the throttle be opened wide so that the pistons can pump fresh air through the induction system, the cylinders, and the exhaust system as an added precaution against accidental kick-over after the engine has come to a complete stop, the ignition switch is turned to the “off” position 27 Carburetor Systems power enrichment system automatically increases the richness of the mixture during high power operation varies fuel/air ratio necessary to fit different operating conditions at cruising speeds, a lean mixture is desirable for economy reasons at high power output, the mixture must be rich to obtain maximum power and to aid in cooling the engine cylinders it is a valve that is closed at cruising speeds and opened to supply extra fuel to the mixture during high power operation 28 Carburetor Systems power enrichment system it increases the fuel flow at high power the power enrichment system is actually a fuel saving device without this system, it would be necessary to operate the engine on a rich mixture over the complete power range The mixture would be richer than necessary at cruising speed to ensure safe operation at maximum power sometimes called an economizer power compensator 29 Carburetor Systems 30 carburetor functions as a unit with one system in operation does not necessarily prevent another from functioning main metering system is discharging fuel in proportion to the airflow while the mixture control system determines whether the resultant mixture is rich or lean If the throttle is suddenly opened wide the accelerating and power enrichment systems act to add fuel to that already being discharged by the main metering system Carburetor Types AVIA-1065 31 Carburetor Types float-type carburetor the most common of all carburetor types several distinct disadvantages effect that abrupt maneuvers have on the float action fuel must be discharged at low pressure leads to incomplete vaporization difficulty in discharging fuel into some types of supercharged systems 32 Carburetor Types Carburetor Types chief disadvantage of the float carburetor is its icing tendency float carburetor must discharge fuel at 33 a point of low pressure the discharge nozzle must be located at the venturi throat the throttle valve must be on the engine side of the discharge nozzle means that the drop in temperature due to fuel vaporization takes place within the venturi as a result, ice readily forms in the venturi and on the throttle valve  pressure-type carburetor discharges fuel into the airstream at a pressure well above atmospheric results in better vaporization permits the discharge of fuel into the airstream on the engine side of the throttle valve drop in temperature due to fuel vaporization takes place after the air has passed the throttle valve and at a point where engine heat tends to offset it Carburetor Types 34  pressure-type carburetor fuel vaporization icing is practically eliminated effects of rapid maneuvers and rough air on the pressure-type carburetors are negligible since its fuel chambers remain filled under all operating conditions have been replaced mostly by fuel injection systems and have limited use on modern aircraft engines Carburetor Types 35 Float-Type Carburetors https://www.youtube.com/watch?v=NhdIh8c aqUc&list=PLhtIpSeAnaZ- m0NC72Nx195W_B2jBCsDx https://www.youtube.com/watch?v=caPd96 m8J7w&t=11s 36  consists essentially of six subsystems The essential subsystems of a float-type Float-Type carburetor are illustrated in Figure 2-10. These systems are: Carburetors 1. Float chamber mechanism system 2. Main metering system 3. Idling system 4. Mixture control system 5. Accelerating system 6. Economizer system 37  Float Chamber Mechanism System provided between the fuel supply and the main metering system of the carburetor float chamber, or bowl serves as a reservoir for fuel in the carburetor [Figure 2-11] Float-Type Carburetors 38  Float Chamber Mechanism System provides a nearly constant level of fuel to the main discharge nozzle which is usually about 1⁄8" below the holes in the main discharge nozzle fuel level must be maintained slightly below the discharge nozzle outlet holes to provide the correct amount of fuel flow to prevent fuel leakage from the nozzle when the engine is not operating Float-Type Carburetors 39  Float Chamber Mechanism System fuel level is kept nearly constant by a float-operated needle valve and a seat needle seat is usually made of bronze needle valve is made of hardened steel may have a synthetic rubber section which fits the seat Float-Type Carburetors 40 Reciprocating Engines BREAK! RETURN AT 3 41  Float Chamber Mechanism System with no fuel in the float chamber, the float drops toward the bottom of the chamber allows the needle valve to open wide as fuel enters from the supply line the float rises (floats in the fuel) closes the needle valve when the fuel reaches a predetermined level Float-Type Carburetors 42  Float Chamber Mechanism System When the engine is running fuel is being drawn out of the float chamber the valve assumes an intermediate position valve opening is just sufficient to supply the required amount of fuel to keep the level constant Float-Type Carburetors 43  Float Chamber Mechanism System https://youtu.be/9BYm0HnLGRU With the fuel at the correct level discharge rate is controlled accurately by the air velocity through the carburetor venturi a pressure drop at the discharge nozzle causes fuel to flow into the intake airstream atmospheric pressure on top of the fuel in the float chamber forces the fuel out the discharge nozzle a vent or small opening in the top of the float chamber allows air to enter or leave the chamber as the level of fuel rises or falls Float-Type Carburetors 44 Float-Type Carburetors Main Metering System The main metering system supplies fuel to the engine at all speeds 45 above idling and consists of: – 1. Venturi – 2. Main metering jet – 3. Main discharge nozzle – 4. Passage leading to the idling system – 5. Throttle valve Float-Type Carburetors Main Metering System the throttle valve controls the mass airflow through the carburetor venturi considered a major unit in the main metering system 46 typical main metering system Figure 2-12 Float-Type Carburetors Main Metering System venturi performs three functions: 1. Proportions the fuel/air mixture 2. Decreases the pressure at the discharge nozzle 3. Limits the airflow at full throttle fuel discharge nozzle located in the carburetor barrel 47 end is in the throat or narrowest part of the venturi Float-Type Carburetors Main Metering System main metering orifice, or jet, placed in the fuel passage between – the float chamber – the discharge nozzle – to limit the fuel flow when the throttle valve is wide open when the engine is cranked 48 – with the carburetor throttle open – https://youtu.be/9BYm0HnLGRU Float-Type Carburetors 49 Float-Type Carburetors Main Metering System venturi performs three functions: – 1. Proportions the fuel/air mixture – 2. Lowers the pressure at the discharge nozzle – 3. Limits the airflow at full throttle fuel discharge nozzle – located in the carburetor barrel 50 – end is in the throat or narrowest part of the venturi where the lowest pressure takes place Float-Type Carburetors Main Metering System The main metering orifice, or jet is placed in the fuel passage between the float chamber and the discharge nozzle Meant to limit the fuel flow when the throttle valve is wide open when the engine is cranked, with the carburetor throttle open air flows from the air intake through the carburetor barrel into the intake manifold volume of airflow depends upon the degree of throttle opening 51 as the air flows through the venturi, its velocity increases Float-Type Carburetors Main Metering System this velocity increase creates a low pressure area in the venturi throat the fuel discharge nozzle is in low pressure Since the float chamber is vented to atmospheric pressure, a pressure drop across the discharge nozzle is created this pressure difference, or metering force, causes fuel to flow from the discharge nozzle 52 The fuel comes out of the nozzle in a fine spray, the tiny particles of fuel in the spray quickly vaporize in the air Float-Type Carburetors Main Metering System The metering force (pressure differential) in most carburetors increases as the 53 throttle opening is increased fuel must be raised in the discharge nozzle to a level at which it discharges into the airstream pressure differential of 0.5 "Hg is required Float-Type Carburetors Main Metering System The metering force is considerably reduced at low engine speeds fuel delivery from the discharge nozzle decreases, if an air bleed (air metering jet) is not incorporated 54 The decrease in fuel flow in relation to airflow is due to two factors: fuel tends to stick to the walls of the discharge nozzle and break off intermittently in large drops instead of forming a fine spray part of the metering force is required to raise the fuel level from the float chamber level to the discharge nozzle outlet Float-Type Carburetors Main Metering System The basic principle of the air bleed can be explained by simple diagrams, as shown in 55 Figure 2-13 A the same degree of suction is applied to a vertical tube placed in the container of liquid the suction is sufficient to lift the liquid about 1 inch above the surface Float-Type Carburetors Main Metering System B a small hole is made in the side of the tube above the surface of the liquid bubbles of air enter the tube and the liquid is drawn up in a continuous series of small slugs or drops 56 air “bleeds” into the tube tending to retard the flow of liquid through the tube the large opening at the bottom of the tube prevents any great amount of suction from being exerted on the air bleed hole or vent Also an air bleed hole that is too large in proportion to the size of the tube would reduce the suction available to lift the liquid Float-Type Carburetors Main Metering System C by placing a metering orifice in the bottom of the tube 57 air is taken in below the fuel level by means of an air bleed tube a finely divided mixture of air and liquid is formed in the tube Float-Type Carburetors Main Metering System In a carburetor, a small air bleed is bled into the fuel nozzle slightly below the fuel level 58 open end of the air bleed is in the space behind the venturi wall where the air is relatively motionless and at approximately atmospheric pressure low pressure at the tip of the nozzle draws fuel from the float chamber and draws air from behind the venturi Float-Type Carburetors Main Metering System Air bled into the main metering fuel system decreases the fuel density 59 destroys surface tension results in better vaporization control of fuel discharge especially at lower engine speeds Float-Type Carburetors Economizer System to develop maximum power at full throttle the fuel mixture must be richer than for cruise additional fuel is used for cooling the combustion chambers to prevent detonation An economizer is essentially a valve that is closed at throttle settings below approximately 60–70 percent of rated power This system, like the accelerating system, is operated by the throttle control 60 Float-Type Carburetors Economizer System consists of a needle valve which begins to open when the throttle valve reaches a predetermined point near the wide-open position as the throttle continues to open, the needle valve is opened further additional fuel flows through it this additional fuel supplements the flow from the main metering jet direct to the main discharge nozzle 61 The End 62