Air Scoop Usually a converging duct that will make use of the ram air effect of the aircraft s motion.

Transcription

1 1 Induction & Fuel Metering Aviation Maintenance Technology Induction = those locations of the engine nacelles where air entering cannot avoid entering the cylinders. There are two basic types 1. non-supercharged 2. supercharged 3 Non-supercharged components 1. Air scoop, 2. Air filter, 3. Ducting, 4. Hot air selector box, heat muff, and ducting 5. Fuel metering device, 4 6. Sensors 7. De-icing devices 8. Ducting to intake manifold, air reservoir is made by long convergent duct. Supercharged systems usually contain one of more of the above items, and they will also contain some form of high volume air pump. 5 Air Scoop Usually a converging duct that will make use of the ram air effect of the aircraft s motion. 6 7 At scoop inlet, ram effect can increase the air pressure at the fuel metering inlet in a super charging effect because air demand from throttle is less then ram air going in. This requires a diverging duct and an air reservoir.

2 8 The velocity then decreases and pressure increases as the divergent duct forms into a reservoir. They may make scoops square and install guide vanes to reduce coriolis swirling Air filters. Wetted type mesh, 25 hrs; Pleated paper element, up to 500 hrs; Impregnated foam, 11 polyurethane foam, fuel resistant tacky stuff, Made by Brackett w/stc, good for 100 hrs, do not clean foam element, replace only, has no problem w/water 12 Decrease all replacement intervals if dusty and dirty. Most common contaminant is dirt, Is organic materials mixed with silica. The organics create sludge, and the silica grinds up metal parts. 13 Ducting Preformed rigid aluminum or stainless.020" Semi-rigid flexible, scat tubing = red -80 to 450 f Cat tubing = black -65 to 300 f, commonly between air filter box and carb. 14 These are metal wire reinforced silicon paper,

3 Can thread one piece into another. Most common size is -8 = 2", Size increments are 1/4 15 Hot air valve, muff and ducting Alternate air = Heat source provides alternate air source which melts carb ice. Carb heat control is in for cool, out for heat. 16 Lindbergh s mechanic invented the first hot air device on the Spirit of St. Louis. Heat selector box has cold filtered inlet, hot unfiltered inlet, outlet, and flap valve Flap is on a shaft w/ pivot bushings, chafe occurs on shaft. 17 If steel type find old seat assemblies from carb to replace bushings. Heat muff is a shroud around muffler to be used as a hot air source. Also may use inter-cylinder baffles, turbine side of turbo, oil sump, as alternate hot air sources. 18 Fuel metering devices 1. pressure carburetors - TBI 2. float carburetors 3. continuous fuel injection 4. pulsed fuel injection 19 Sensors MAP gauges are plumbed into the supercharger blower chamber, reading the amount of boost. Normally aspirated engine is plumbed somewhere between carb and engine

4 Latent heat of vaporization is the energy required to convert a liquid to a gas, without changing the temperature. Laws of evaporation. 1. The rate increases as temperature increases The rate increases as surface area exposed increases. 3. The rate increases when atmospheric pressure decreases. 4. The rate varies with the nature of the liquid The rate increases for water when the humidity decreases. 6. The rate varies with the rate of air contact across the surface. 7. Evaporation causes a cooling effect or loss of heat. 24 Deicing systems Three types of ice. 1. Vaporization ice 2. Throttle ice 3. Impact ice 25 Vaporization ice is fuel related. Fuel gives up heat as it evaporates, cooling the air mass causing freezing moisture. Occurs from fuel discharge area onwards in the airstream. More of a problem with float carbs 26 Throttle ice occurs on the back side in the low pressure stall area of throttle plate, This ice commonly occurs from long slow descent. In a pressure carb fuel nozzles are after Venturi thereby reducing vaporization and throttle icing. 27 Impact ice occurs when visible moisture at edge of freezing or colder, that impacts and freezes or impacts and collects Air frame subfreezing, temp 35 f or lower, droplets impact, explode and

5 freeze Can install carb temp probe into intake manifold with a green, yellow, red gauge scale. CAT = Carburetor Air Temperature Constant speed = drop in MAP, fixed pitch = drop in RPM. 30 Heating is done by routing warm air from a shroud around the exhaust system into the intake to melt the ice. The less dense hot air will initially give a loss in power, but if power then climbs, you are melting the ice. Some systems used alcohol in a reservoir to melt the ice. This will also add fuel to the engine inlet. 31 Intake system ducting 1. intake manifold, oil bath & non oil bath 2. cylinder distribution assembly 3. intake runner tubes 4. connection hoses Cylinder attachment flanges and hardware. 6. Support bracketing It is common to see intake interiors slightly rough, from casting process. Although this causes slight boundary layer turbulence it reduces fuel condensation traveling into engine in droplet form. 35 Polished intake interiors work well for high end racing performance but provide terrible, stumbling low end performance.

6 36 Porting and blue printing is the process of enlarging intake interiors, and hand grinding all joints to remove any interior ridges at the joint. Porting and polishing should not be done on aircraft engines unless approved by the manufacturer. 37 MAP Manifold Absolute Pressure This is the pressure found between the air throttle valve, and the engine intake valves. Absolute means zero on the scale = complete vacuum, ambient = approx Hg 38 Any engine is primarily an air pump. It draws air through the intake systems, and expels it, mixed with exhaust gasses, out the exhaust systems. 39 The air throttle valve is the major RPM or Power controlling device for any spark ignited reciprocating engine. (Turbines and some Diesels are controlled by fuel metering only). It acts as a variable restriction to the air flow being drawn into the engine. 40 Therefore when it is fully open the engine will attempt to maximize power and RPM. RPM may be limited by load or by maximum, open throttle air/fuel input. MAP will be very close to ambient pressure in a nonsupercharged engine, and may exceed ambient in a supercharged version. 41 When the throttle is closed there will still be a slight amount of air passing by it. RPM and Power will be minimized to the lowest level possible while continuing to run reasonably smooth. (If the flyweight momentum gets too low the power pulses tend to lope and stumble causing low idle misfires). 42

7 MAP will be very low, anywhere from 14 Hg - 22 Hg depending on engine type, health and current ambient conditions. As the engine warms up it becomes more efficient, so look for this during warm-up cycles. 43 MAP and engine power have a direct correlation to BMEP in the cylinders. The maximum BMEP allowed in any given cylinder is set by the antidetonation factor of the fuel being used. Lower octane fuels will be more likely to detonate in high BMEP engines. 44 ALWAYS USE THE APPROPRIATELY RATED FUEL. Note: Ignition timing can also effect BMEP, advanced timing will raise BMEP to a point. 45 Common problems, failures and diagnostic techniques Leaks - small Typically will go unnoticed. May not get a slight rise in RPM during fuel cut-off. 46 May get a slightly rough idle; this can be very hard to notice due to prop thrusting. Can cause mixture control to be shifted out of normal range. Detection by visual inspection, or 1 psi air source, and soapy solution. Smoke generators are now becoming available. 47 Leaks - large Engine won t start, or runs very poorly. Runs only in full rich position. Severe back firing or after firing. 48 Can be caused by closed throttle starting back fire. MAP will indicate leaks as higher than normal, or lower if in boosted

8 range. 49 High power flat - deep hum sometimes - OK at idle and low power Restricted air filter, or induction blockage (pressure and floatcarbs will react differently depending on the restriction) 50 Restricted exhaust (very high EGTs) Restricted fuel flow - (no hum with this, just power drop, high EGTs) MAP will indicate the last two as higher than normal, and the first as lower than normal. 51 Supercharging Its purpose is to increase the mass of the air/fuel charge going into the engine for each revolution. 52 Most supercharged engines also have constant speed propellers They are designed to keep full power, manifold pressure at or above sea level pressure. Early systems could not exceed ambient pressures. 53 Critical altitude = that altitude where the boosted manifold pressure can no longer exceed sea level pressure. As system technology developed we eventually designed supercharged engines that exceeded ambient pressure in the intake system. 54 This can also be done at lower altitudes for maximum power during critical flight operations, Is sometimes used for limited duration s. Maximum boost is determined by the strength of the intake system, or the detonation characteristics of the engine 55 General gas law = volume of a gas is inversely proportional to the absolute pressure and is directly proportional to the temperature.

9 (V1 * P1) / T1 = (V2 * P2) / T2 56 Many things create high cyl pressure. 1. Advanced ignition timing 2. High density air 3. Low octane, hot burning fuels 4. Excessive compression (wrong engine parts) 5. High intake temperatures 57 High pressure creates high temperature. At a certain temperature and pressure threshold, the pressure shock wave, which travels faster then the heat/burn wave, starts to spontaneously ignite the air/fuel mixture in an uncontrolled fashion. 58 This is called detonation, - FAA declares this to be a state of uncontrolled burning. Pre-ignition is when any source ignites the air/fuel charge prior to the spark plug ignition. 59 Air Density Standard day conditions = at sea level, at 15 c or 59 f, at 45 N lat. Factors effecting density Water and hi temps displace air molecules creating less density. 60 Avogadro s law states that any gas molecule will take up the same space at the same temperature. N2 has about twice the mass per molecule as H2O therefore the wet air has less mass per unit of volume. (Lower density) 61 Density altitude = common pilot nomenclature, higher numbers means lower density. 62 Supercharger is a non-positive displacement centrifugal air pump (compressor) that increases the air density in the intake systems and the

10 cylinders. Can be single or multi speed up to 13 to 1 ratio to crankshaft, in low blower up to 42 inhg. hi blower = up to 80 inhg 63 When switching to high speed the manifold pressure will go up and oil pressure will drop. Supercharging is the overall term for pumping air into the engine regardless of how this is accomplished. 64 But is also the term for mechanically pumping air with a gear driven compressor. (Some auto racing applications use a 4 wide belt drive) Turbo supercharging uses exhaust to drive a turbine/compressor pump. 65 In most super chargers fuel is mixed with air before supercharging (internal), but not all (external). This helps to vaporize the fuel. Turbo superchargers have the air compressed prior to fuel metering 66 If engine cannot boost above 30.00"Hg then it is a normalized boosted engine. It can be said the normalized engine regains normal ambient pressure, up to the point where the turbo is at maximum output. 67 Those that go above 30.00"Hg are known as supercharged. Intercooler can be used on superchargers to cool the air charge after the compressor 68 With some engines there is a distribution impeller, but this nota charging impeller. (more later) On the R-985 it has an actual supercharger impeller at a 7.5 to 1 ratio to the crankshaft. 69

11 It is possible to have superchargers that have several stages of charging much like a turbine engine, and, or they can have several speeds of compressor RPM. Stages and speeds. Can have Stages: Speeds: More then one compressor impeller creates the multiple stages Only the gear driven Supercharger can have different speeds (Automatic Gear box) Dual speeds are controlled by solenoid switch. Engine must have low compression ratios to prevent blow ups with high pressures. Supercharger clutches are engaged with oil pressure. 73 Supercharging casing can be airfoil, vaned, venturies Supercharger efficiency is by speed of rotation, and size of impeller 74 Advantages 1. More hp, 2. shorter takeoff, 3. bigger payload, 4. Greater fuel efficiency, 5. higher altitude 75 Disadvantages, 1. premature engine wear, 2. more pre ignition and detonation, 3. closer monitoring and maintenance, 76

12 4. must use higher fuel grades, 5. Harder to work on higher costs, 6. low altitude restrictions due too much boost. 77 Need to beef up the engine to handle the turbocharger or supercharger. Intercooler is heat exchanger that cools air before going into engine to increase density. 78 Two common current producers of turbo-superchargers are Ray-Jay and Garrett AirResearch. 79 RayJay were the original type of turbo, with electric on/off solenoid, then came the automatic controls. VAPC, APC, Density controller, Distribution impeller = a mixing impeller, connected directly to the crankshaft that does not provide any boosting capacity. 80 Never install a supercharger onto an engine that was not designed from the ground up to be supercharged. Conversions always loose reliability and longevity, and increase the possibility of sudden catastrophic engine departation. 81 Supercharger and *Turbosupercharger components. 1. Compressor housing 2. *Turbine housing 3. Compressor impeller 4. *Turbine impeller 5. Drive shaft Transmission assembly (Geared superchargers only) 7. Diffuser 8. Bearing sections 9. Lubrication plumbing

13 10. Air ducting 11. *Exhaust ducting 12. Control systems 83 Compressor housing is made of cast aluminum, cast, then machined. Compressor impeller, centrifugal, is the upper deck pressure side, directs air from the center to the outside. Turbine housing, cast iron or steel, 84 Turbines are impulse or reaction types, can be lost wax investment casted then machined or ground to final shape. Investment casting will lessen the porosity of the casting. 85 Turbine temperatures are normally 1,700 f Turbo superchargers can operate at speeds up to 100,000 RPM. 86 Drive shaft connects the compressor impeller to the transmission assembly or to the drive turbine impeller. Transmissions are either simple geared drivelines or they can be multispeed transmissions that are hydraulically, or electro-hydraulically controlled. 87 These latter units will include a clutch systems coupled to a sun/planetary gear system, similar to an automatic automotive transmission. Diffusers are two main types: 1. Airfoil converging/diverging duct 2. Straight vane 88 The venturi-type diffuser is equipped with plain surfaces, This type has been most widely used on medium powered, supercharged engines On large volume engines ranging from 450 hp upwards, either a vane or airfoil type diffuser is widely used.

14 89 Bearing housing, is usually either a floating type plain bearing with pressurized oil (turbo superchargers) or they may use a double set of ball bearings with an oil bath spray. Shaft RPM can be as high as 40,000 to 50,000 RPM on some turbo superchargers. 90 Lubrication plumbing includes standard AN hardware, hoses, and tubing, designed to handle petroleum products at or below 100 psi. The impeller shaft and gear are usually forged integrally of very high grade steel. 91 Air ducting can be anything from high pressure scat tubing to cast aluminum runners and distribution manifolds. This all needs to be considerably stronger than normally aspirated induction assemblies. Do not interchange these parts. 92 Exhaust ducting is usually made from the same materials (Stainless Steel) as the rest of the exhaust system. Since the system has so much motion from vibration and heat expansion there will probably be flex, or expansion joints installed. 93 This may be encased in heat shielding of some form, particularly when installed on aircraft with streamlined cowlings. DO NOT leave this shielding off the aircraft for any flight operations. 94 Control systems Some early systems were manually controlled via a push/pull vernier near the throttle control. 95 During specific altitude and power conditions the pilot would engage the supercharger clutches via oil pressure controls, or start closing the turbosupercharger wastegate.

15 96 Wastegate is a device used on turbo superchargers that allows exhaust gasses to bypass the drive turbine. Typically a closed wastegate means full turbine output, (the bypass is closed). Deck pressure is that pressure just after the compressor. 97 Throttle valve is between compressor and intake system. Fuel is injected into intake close to intake valves. 98 Several systems are described in the following slides. There are numerous configurations. They all function by use of pressure control or density control, or both. 99 A density regulator will measure for pressure and temperature. A pressure regulator will only regulate for pressure. The most simple system is a outside adjustable bypass screw. 100 Other smaller systems may rely on only density regulation. As temperature or pressure changes the oil pressure is allowed to build or release on the wastegate controller. These usually default to open with no pressure. 101 These controllers are on the return side of the actuator. Pressure controllers can measure pressure, density or be differential, comparing two differing pressures. 102 One Lycoming system has four main components 1. Turbo supercharger 2. Density controller 3. Differential controller 4. Bypass assembly (wastegate) 103

16 Turbo supercharger is a sea level boosted engine with a critical altitude of 19,000 ft. Density controller controls oil pressure of wastegate actuator only during full power operation. It is density sensing, temp. and press. 104 Bellows opens oil valve when deck pressure is close to ambient, otherwise it stays closed. Differential controller controls oil pressure of wastegate actuator by means of a differential diaphragm with deck pressure on one side and post throttle MAP on the other side. 105 The differential controller will try to maintain a pressure drop across the throttle valve of 2-4 Hg. When it opens the waste gate opens. This valve will remain closed during wide open throttle operation since there is not enough differential to open it. 106 Another system uses a variable-pressure controller instead of a density and differential controllers. This device works similar to the differential controller but it also has a cam and plunger that is actuated by throttle linkage. 107 Over high deck pressure pushes the poppet down and open on the top end, and the cam pushes the poppet follower up and open from the bottom end during lower power settings. A vacuum filled bellows and several balancing springs work to keep the poppet valve closed. 108 Continential had earlier units which used three controllers and later units combined these into one unit with cam/throttle linkages. These are: absolute pressure controller rate of change controller

17 pressure-ratio controller 109 In similar fashion as they bleed off waste gate control oil opening the waste gate. They are hooked up in parallel so that each may independently dump exhaust gas. 110 Absolute pressure controller is a maximum upper deck pressure limiter. Rate of change controller bypasses wastegate oil if the deck pressure changes faster then 6.5 in Hg/S 111 The engine can over boost momentarily during rapid throttle application. This may only oscillate the power, or it may cause detonation for a short period. 112 Pressure ratio controller will maintain a pressure ratio between the deck pressure and inlet ram air of 2.2:1 at any altitude above 16,000 ft. This prevents excessive heat buildup in the deck airstream which can cause detonation. 113 The variable absolute pressure controller combines these three units and it functions in a manner identical to the Lycoming variable controller. This system also incorporates a overpressure MAP relief valve. 114 It uses varible fuel pressure as the operating force. Another turbo supercharger control system uses an adjustable orifice that also includes an overpressure relief valve 115 Adjustment criteria, at component replacement, or engine overhaul Test run on ground, Calibrate MAP gauge, Slowly advance throttle watching MAP

18 116 Install thermocouple to know temperature at specified port in compressor, then use chart to check temp against pressure read. Slow smooth throttle movement. 117 Frequent oil changes for any turbo supercharged engines. Plan on higher service requirements inspection wise as well. One quote is $1.50 per hour flight time in additional costs. 118 Bootstrapping is when as you increase power, turbo increases more, then you are over powered in a cyclic manner. It is an undesirable cycle of turbocharging events causing the manifold pressure to drift in an attempt to reach a state of equilibrium. 119 Bootstrapping occurs when the waste gate is closed or mostly closed. Rapid movement of the throttle can cause a certain amount of manifold pressure drift in a turbocharged engine. Bootstrapping is when the drift persists. 120 Intercoolers may exist on any of these devices. They provide pressurized intake air cooling. Sonic venturis are used to increase cabin air volume input, sacrificing limited amounts of air pressure. 121 Over boost is when the MAP upper pressure limit has been exceeded. This will break parts, cause detonation and break more parts. 122 Turbo compounding The turbocompound engine consists of a conventional, reciprocating engine in which exhaust driven turbines are coupled to the engine crankshaft. 123

19 This system of obtaining additional power is sometimes called a PRT (power recovery turbine) system. Power recovery turbine systems, because of weight and cost considerations, are used exclusively on very large reciprocating engines. 124 The net yield from them is not sufficient to justify their use on smaller engines. 125 Turbo supercharger problems 1. Aircraft fails to reach critical altitude. 2. Exhaust system leaks. 3. Faulty turbocharger bearings. 4. Waste gate will not close fully. 5. Waste gate will not open Engine surges. 7. Bootstrapping. 8. Waste gate malfunction. 9. Controller malfunction. 10. Waste gate bypass valve bearings tight. 11. Oil inlet orifice blocked. 12. Broken waste gate linkage. 127 Differential controller malfunctions. 1. Seals leaking. 2. Diaphragm broken. 3. Replace controller. 4. Controller valve stuck.

20 1 Aircraft Fuel Metering 2 Purpose of metering To adequately mix the proper amount of fuel with the combustion air entering into an engine, during all phases of engine operation. 3 Section Outline Terms Characteristics of fuels Characteristics of air Engine operational needs Basic system outline 4 Section Outline Carburetion Principles Air metering Fuel metering Enrichment / derichment Acceleration systems 5 Section Outline Pressure Carburetors Fuel Injection Systems Diagnostic and troubleshooting Basic strategy development Common failure areas 6 Terms Take-off power = maximum power allowed for take-off operations. usually limited to 1-5 minutes Rated Power = highest power Mfg.. will guaranty the engine to produce continuously at a given altitude. 7 Terms Absolute altitude = highest altitude which the engine will run.

21 Friction horsepower = the horsepower required to overcome friction and accessories. 8 Terms Mechanical efficiency = BHP/IHP Brake horse power = the power available to perform work. Indicated horse power = P.L.A.N.K / 33,000 ft.lbs/min 9 Terms P = mean effective pressure L = length of stroke in Ft. A = area of bore in sq/in N = # of power strokes per min (rpm/2) K = # of cylinders 10 Terms MEP = mean effective pressure 11 Terms Otto cycle = the four stroke cycle Intake Compression ignition (event only) Power Exhaust 12 Terms Otto pressure cycle 13 Terms Density altitude = the effective altitude of prevailing conditions presented in terms of standard conditions. Carburetor = a device that meters air and fuel into the intake system. Metering = to provide in measured quantities (Websters) 14 Terms

22 Fuel injection = similar to carburetion but fuel distribution occurs at a different location from air metering, usually just behind the intake valve. 15 Terms Octane rating determined by the percentage of iso-octane that must be mixed with normal heptanes to reduce detonation. more iso-octanes reduce detonation to a point. 16 Terms Air metering force Those forces created by air being drawn into the engine because of pressure differential. Fuel metering force Air metering forces that are used to determine the air mass entering the engine for fuel metering. 17 Terms Fuel distribution forces Those forces used to distribute and atomize the fuel into the intake air stream. Fuel metering and distribution forces may or may not be the same force. 18 Terms Full rich: mixture control setting that provides the maximum fuel flow for any given throttle setting. Engine will run cooler but will consume fuel at the greatest rates. 19 Terms Rich best power: the richest mixture control setting that does not reduce RPM or power. May not be used during takeoff, or may be used for a limited duration above certain power settings. 20 Terms Lean best power: The leanest mixture control setting that does not cause a lower RPM.

23 This is higher than best economy. Engine temp goes up as it gets leaner. Not used for high power settings. 21 Terms Best economy mixture: The leanest mixture control setting that will not damage the engine. This setting is close to a Stoichiometric ratio. One where all fuel and oxygen will be consumed. Produces more heat than the others. 22 Terms Best power mixture: The mixture control setting that produces the best overall power without damaging the engine. Will be different for different throttle settings. Between best rich and best lean. 23 Terms Flame propagation is slower when rich or lean from lean best power. Temperature is higher when lean, to a point - misfiring EGT gauge is very useful for setting mixture. 24 Terms 25 fuel characteristics Aviation gasoline = hydrocarbon fuel C 5 H 12 - C 8 H 18 = common fuel range 2 C 5 H O 2 => 10 CO H 2 O 2 C 5 H O 2 => 6 CO H 2 O + 2 NO x + 4 CO = incomplete burn NO x = the brown stuff of smog CO= the most toxic part of smog 26 fuel characteristics Aviation gasoline has a high rate of vaporization.

24 Volatile = readily vaporizeable at a relatively low temperature. Has come in a number of different grades. Most common grade available = 100LL 27 fuel characteristics Avgas is made up of many differing molecules of similar structure. All gasoline's are composed of many grades of fuel oils. This causes them to have boiling points that vary within a range. Typical boiling point will range from 100 f f 28 fuel characteristics Octane rating is a rating of resistance to detonation. Higher is better. Octane calculated by three methods RON = Research Octane Number MON = Motor Octane Number (RON+MON)/2 = average of both Latter is the most common rating. 29 fuel characteristics Octane rating may be achieved by mixing high octane iso-octanes with lower octane heptanes. The iso-octanes reduce detonation The heptanes provide more Btu's Today many different blends and additives are used in gasoline's. 30 fuel characteristics Dual octane ratings indicate the lean octane value and the rich octane value. 100 octane would have the anti-knock qualities of 100% iso-octane. TEL is used to achieve higher than 100% iso-octane ratings. 31 fuel characteristics

25 T.E.L. Tetraethyl Lead Anti-detonation compound stabilizes the rate of burning under extreme conditions. reduces metal adhesion on exhaust valve to seat contact. Ethylene dibromide scavenges the lead deposits by forming lead bromide. 32 fuel characteristics Ethylene dibromide boils around 250 f T.E.L. boils around 360 f Lead bromide boils between these. This causes the TEL to distribute unevenly from the others. Some cylinders will get over leaded, others will get over scavenged. as much as 96ml/gl from 5.5ml/glgas. 33 fuel characteristics Scouring additives such as Tricresyl phosphate, or triphenol phosphate in the oil or fuel may help scavenging lead deposits as well. They produce lead phosphate instead of lead bromide, which has an even lower boiling point and mixes better. 34 fuel characteristics Color Octane BTU s lead ml/gl Red 80/87 18,000.3 Blue 100LL 18, Green 100/130 19, Purple 115/145 19, fuel characteristics Avgas has a relatively high heat value in BTU s. BTU = British Thermal Unit = energy required to raise one lbs of water 1 f. Heat energy does not directly relate to burn rate or temperature

26 generated. 36 fuel characteristics Ignition temperatures of fuels. Natural gas 1,100 f Gasoline's f Diesels f Jet fuels f Spark plug arc f Best arc temp f 37 fuel characteristics Lower atmospheric pressures will increase rate of vaporization will also lower boiling temperatures Fuel must be vaporized to oxidize or burn. Fuel must be liquid to be pumped. 38 fuel characteristics The primary purpose of a fuel distribution device is to administer fuel in an atomized state that is close to vapor. Heat energy must then complete the vaporizing process. Excess fuel causes excess vaporizing which cools the combustion process. 39 fuel characteristics Fuel can vaporize in warm engine compartment fuel lines. Worse on hot soaked engine being restarted. Any turbulence generators in fuel delivery system can accelerate vapor lock conditions 40 fuel characteristics Vapor lock areas

27 sharp bends high rise areas routing close to hot components pump inlets carburetor inlets 41 fuel characteristics Booster pumps will reduce possibility of vapor lock. Gravity feed systems not likely to vapor lock. Higher volatility fuels more likely to vapor lock. 42 fuel characteristics Typical method to repair vapor lock Mixture full rich Throttle cracked - slightly open Magnetos on Start engine If it quickly dies then it was probably vapor locked 43 fuel characteristics Bleed the systems with aux pump. Mixture full lean or at idle cutoff - prevents flooding Throttle full open boost pump on Electric pump on Wait twenty seconds to purge vapor. Start normally. 44 fuel characteristics Automotive fuels Several STCs exist for the use of auto gas. May only require an identifier hose clamp on a push rod tube. Generally only good for low compression engines.

28 45 fuel characteristics won t have a loss of power but may have reduced high power duration. Auto fuel has many additives that are unknown or proven incompatible with aviation systems. Some auto gas blends have higher a volatility then avgas and are more likely to vapor lock. 46 fuel characteristics Methyl tertiary butyl ether can attack rubber causing swelling can make humans sick Methyl Alcohol can attack rubber causing swelling can make humans sick can rust components 47 fuel characteristics 48 air characteristics Air is a compound of several gasses 76% nitrogen 21% oxygen 2.1% carbon dioxide and rising the rest is the remaining heavier gasses. lighter gasses tend to rise away. 49 air characteristics All of these gasses are displaced by moisture. Since moisture content varies from none to complete saturation it is not considered to be a part of air. 50

29 air characteristics Fuel requires the oxygen for combustion which releases large amounts of heat. Elements other than the moisture and oxygen, resist involvement in the combustion process. 51 air characteristics Nitrogen = N molecular weight of 14 forms in molecules of two or more very resistant to molecular interaction with fuel or oxygen will bond with hydrogen to form ammonia's 52 air characteristics Oxygen = O molecular weight of 8 forms molecules of two commonly easily interacts with many substances. interaction will consume or release energy 53 air characteristics Water vapor = H 2 O molecular weight of 10 will weakly bond with itself releases high amounts of energy when forming. displaces heavier N 2 and O 2 molecules. 54 air characteristics Avogadro s law molecules of any gas demand the same space for a given temperature. A water molecule will demand the same space as a nitrogen or oxygen molecule.

30 This reduces the overall density. 55 air characteristics Low pressure areas do not attract humidity. They are created by humidity. Low pressure will reduce the lifting capacity of the wing. It will reduce the power output of the engine by decreasing air mass. 56 air characteristics This will reduce the fuel metering force that is created by the air mass entering the engine. It will reduce the rate of fuel vaporization in the engine due to saturation levels of the air. It can reduce detonation. 57 engine operational needs The primary operational need for an engine is parts of air to 1 part of vaporized fuel, by weight. This is known as the stochiometric ratio. This is the best power to fuel ratio. C 5 H O 2 => 5 CO H 2 O 58 engine operational needs Engines can run at 26:1 (super lean) 5:1 (super rich) Common ranges for aviation engines are: 16:1 to 12:1 for normal ops 8:1 to 10:1 for enrichment cooling 59 engine operational needs There are several differing phases of operation during the typical running

31 cycle of an engine. Each phase, or mode of operation requires distinctly different fuel metering needs. Some different operating modes are: 60 engine operational needs Starting Cold and warm operations Idle operations Transition operations Cruise operations High power operations Shut down operations Differing altitudes and pressures 61 engine operational needs Starting operations: calls for fuel to be delivered to the cylinders with a minimum of cranking. fuel metering and atomization forces will be at their weakest. compression and ignition will be at their weakest. 62 engine operational needs Starting and cold operations: engine will be at its coldest operating condition because vaporization is poor when cold, extra fuel is needed to make up for the lean vapor condition. 10% fuel distribution will be very uneven. 63 engine operational needs Idle operation, normal or cold: the intake is at its least efficient phase. compression efficiency is lowest. fuel metering forces are the least stable.

32 fuel vaporization and distribution can be very poor. 64 engine operational needs Transitional operations: engine will be trying to accelerate may be operating partially on several fuel metering systems air fuel ratios may need to cycle between lean and rich as throttle and RPM are transitioned. 65 engine operational needs Cruise operations: power demand is low to medium. best fuel economy is desired. highest portion of flight time. vaporization is good. cooling demands are low. most likely to develop icing 66 engine operational needs High power settings: for take off and climb. may have duration limits. most cooling needed. best vaporization condition. most likely detonation phase. compression and ignition at their best. 67 engine operational needs Shut down operations: post metering fuel system needs to be scavenged of burnable mixture. combustion chamber needs to be vented of varnish developing gasses. engine needs to be safely shut down and defused, pending magneto

33 switch failures and or hot spots. 68 engine operational needs Altitude - pressure compensation: appropriate air:fuel ratios must be maintained at any altitude or pressure. can be manual, automatic or both. air:fuel ratios may need manual compensation for unusual operations. such as starting, stopping, hot, cold, emergencies, vapor lock, etc. 69 engine operational needs 70 basic system outline Fuel storage and delivery systems must store fuel until needed, then deliver fuel to the fuel metering system, as needed. Fuel storage is either in wings, fuselage, or both. 71 basic system outline Fuel storage systems regulated by FAR 23, covered in AMT 100. Common source of fuel contaminants Materials compatibility is critical Rubber, sealants, seals, composites 72 basic system outline Must have Debris / moisture sumps w/ drains Sealed Fuel caps w/labeling Fuel quantity indicator Fuel outlet screen Pre-firewall off/on/selector valve w/ detents for each engine. 73

34 basic system outline valve can t pass through OFF Fuel outlet screen must be 8-15 mesh. must have surface area 5 times the outlet cross-sectional area. Gascolater mesh 25-1 Carb strainer mesh basic system outline Venting systems must be present and functional. It must prevent negative pressure differential. Cannot allow fuel to be siphoned. Must vent overboard safely. 75 basic system outline Delivery systems can be gravity feed or pump systems. Gravity feed systems must provide 150% of maximum fuel demand. Pump systems must provide 125% maximum fuel demand. 76 basic system outline No fuel delivery system device can cause stoppage of fuel flow for any reason or during any failure mode.(all must contain bypass systems) 77 basic system outline Mechanical engine powered pump is the most common delivery system. Pump systems can be single or dual pump systems. Non-positive displacement centrifugal pumps are used for fuel sump pick-up when line head pressure is too low. 78 basic system outline Positive displacement pumps are usually an engine driven primary pump. They can be geared, pulsed, stroked, vaned, wobble or gerotor.

35 They may use a loop system with fuel return to pump inlet. 79 basic system outline Tubing, hoses, fire sleeve, and filtration as per FAR 23, 33, and 34. Fire sleeve forward of the firewall. Main filtration at the lowest point. Firewall sealed and fuel flow shut off required. Alternate or hot air source required. 80 basic system outline Air metering system must be able to vary air volume entering the engine is primary power/rpm control Fuel metering system must have a means to determine approximate air mass entering into combustion chamber. 81 basic system outline Fuel distribution system must have a means to atomize fuel evenly into the combustion air stream. (complete fuel vaporization is the ideal situation) Air / air-fuel compressor system. Pre or post fuel metering/distribution 82 basic system outline Induction systems carries meter air and possibly metered fuel into cylinder heads. Combustion chamber intake valve regulates air/fuel charge into combustion chamber is the end of fuel metering/induction 83 Carburetion Principles Air metering principles

36 Fuel metering principles Fuel distribution principles Enrichment / derichment principles 84 Carburetion Principles Air Metering Air metering is critical to spark ignited engines. because gasoline will combust over a wide range of mixture ratios regulating RPM/power with fuel control only would not be responsive. deceleration won t occur until the engine suddenly stops firing. 85 Carburetion Principles Air Metering True rotary engine. no air metering throttle. fuel metering off / on only. take off and cruise was at full power descending and landing was at engine off, no power. 86 Carburetion Principles Air Metering Air metering the most common device used is a throttle butterfly valve. it is a coin shaped disc that is bisected by a shaft. the shaft rotates opening or closing the throttle bore. 87 Carburetion Principles Air Metering Most reciprocating engines will consume six pounds of air per hour per brake horsepower 88 Carburetion Principles Air Metering Closed, engine off or at idle 89 Carburetion Principles

37 Air Metering Mid-throttle, engine cruising 90 Carburetion Principles Air Metering Full-throttle, engine full power 91 Carburetion Principles Air Metering Another less common air metering device is the slide throttle. uses a cylindrical tube that slides in or out of the throttle bore, at right angles to the bore. 92 Carburetion Principles Air Metering 93 Carburetion Principles Air Metering 94 Carburetion Principles Air Metering 95 Carburetion Principles Air Metering 96 Carburetion Principles Air Metering 97 Carburetion Principles Air Metering One final function of air metering is to determine which carburetor fuel metering mode or system should be used. Most carburetors will have more than one fuel metering pathway. Commonly this is a direct function of throttle plate position.

38 98 Carburetion Principles Fuel Metering Fuel metering principles measure the rate of air flow provide a pre-metered staging area for fuel to control metering pressure. using predetermined values, meter the appropriate fuel to the distribution devices. 99 Carburetion Principles Fuel Metering Bernoulli s Principle a fluid moving through a converging / diverging duct will increase velocity and decrease pressure. by comparing reduced pressure with ambient pressure one has an accurate estimation of fluid volume passing the restriction. 100 Carburetion Principles Fuel Metering 101 Carburetion Principles Fuel Metering This is known as the Venturi effect. the force created by this pressure differential can be used to directly meter and distribute fuel. or it can be used for metering purposes only, with fuel distribution accomplished elsewhere. 102 Carburetion Principles Fuel Metering Fuel staging area is usually regulated by a poppet valve. provides a collection area where pre-metered fuel pressure is stabilized. can be controlled by floats or pressure diaphragms. 103 Carburetion Principles Fuel Metering Fuel metering is usually accomplished with removable metering ports (jets). is similar to measuring the air flow. controlling a pressure differential across a known restrictor allows

39 accurate metering of fuel flow. venturi differential can be the only source for fuel metering forces 104 Carburetion Principles Fuel Metering Fuel metering ports are removable so they may be replaced with different sizes. Drilling to adjust usually destroys them. The metered fuel circuit may be vented with an air bleed jet to increase metering force and aid in atomization. 105 Carburetion Principles Fuel Discharge Centrally located in the airstream of the carburetor. Primary purpose is to carry metered fuel to the center of the airstream and discharge it. Secondary purpose is to atomize fuel as much as possible. 106 Carburetion Principles Fuel Discharge Fuel discharge nozzle can be a simple pipe jutting into the airstream. Can include a secondary venturi to provide additional pressure differential and stable airflow. May have a concentric nozzle head with equally spaced fuel ports. 107 Carburetion Principles Fuel Discharge Nozzle head may be shaped to increase venturi effect. May include an air bleed mixing chamber in discharge head. May be located on either side of throttle plate. (pressure vs. float) 108 Carburetion Principles enrichment / derichment Fuel enrichment/derichment is caused by several common techniques. physically alter venturi pressure differential. turn off or on a separate fuel path. turn off or on a separate air path. or any combination of the above. 109 Carburetion Principles enrichment / derichment Two methods for altering venturi pressures. 1. choke = another butterfly valve upstream of the throttle valve and the fuel metering venturi.

40 closing causes greater fuel metering force, thereby enriching the mixture. rarely used in aviation. 110 Carburetion Principles enrichment / derichment 2. Alter fuel staging area pressure at the point where it is metered. this can be done several ways depending on carburetor system. pre-metered fuel may be at ambient pressure, above ambient. or below ambient. 111 Carburetion Principles enrichment / derichment Turning on or off enrichment air bleed or fuel circuits or pathways is done by: linkages to the throttle control. placement or position of the throttle plate 112 Carburetion Principles enrichment / derichment enrichment / derichment may be used for: hot or cold starting. high altitude / low pressures. full power cooling. fuel conservation. 113 Carburetion Principles Automatic Mixture Controls These devices are used on all Stromberg Pressure carbs and many of their float carbs. Consists of an aneroid bellows filled with nitrogen and or light oil. They are responsive to both temperature and pressure changes 114 Carburetion Principles Automatic Mixture Controls This means they are density controllers. The internal oil in the webs of the bellows act as dampers for vibration The plunger valve is cut with two different tapers. 115 Carburetion Principles

41 Automatic Mixture Controls This helps to correct for the non-linear changes in temperature and pressure in the atmosphere. 116 Carburetion Principles acceleration systems Acceleration systems may be needed to add fuel for rapid crankshaft acceleration. normal fuel metering forces cannot respond fast enough to meet the extra demand. 117 Carburetion Principles acceleration systems Most common system is a one shot per throttle application piston pump system. may or may not have a separate distribution path. used more commonly on higher powered systems. 118 Carburetion Principles acceleration systems Pressure Carburetors commonly use a diaphragm type pump. Can be mechanically driven by throttle or: Can use MAP to drive pump. low MAP draws diaphragm back filling chamber. High MAP releases this into air. 119 Carburetion Principles 120 Carburetion Principles 121 Carburetion Principles 122 Carburetion Principles 123 Carburetion Principles 124 Carburetion Principles This can be a one check valve or two check valve pump. the one valve type draws fluid into the pump chamber faster than low

42 pressure venturi air can be back drawn through the distribution nozzle 125 Carburetion Principles The two check valve systems draw fuel in through one valve and push fuel out through the other one. These check valves are usually a ball type valve, that may or may not incorporate a spring. They fall out and disappear easily during repair or overhaul. 126 Carburetion Principles 127 Manufacturers Marvel-Schebler Bendix/Stromberg Bing 128 Model identification systems Marvel/Schebler MA = updraft HA = sidedraft Bigger secondary numbers mean larger engine application. Use specific Marvel/Schebler P/N 129 Model identification systems Stromberg-Bendix 5 to 6 groups First Group: NA - Natural Atomization P - Pressure Q - Small Pressure 130 Model identification systems Second Group: Barrel types

43 Third Group Barrel Size from 1 3/16, 1/4 increments 131 Model identification systems Fourth Group: Model letter = major modification signifying model change. Fifth Group: Model number = minor modification not enough for new model change. 132 Model identification systems Sixth Group: Sub-mod letter = very minor change. 133 Model identification systems Bing Uses a number commonly Describes the throttle bore in millimeters. Usually cable slide throttle type. Can be vacuum slide with butterfly 134 Float carburetors are so named because they maintain a fuel staging area at approximately ambient pressure with a float valve. Fuel level is maintained to tight tolerances because fuel metering is a function of float level. Higher levels make it richer. 135 As fuel is drawn from the bowl area the float drops opening the float valve. Fuel pump pressure causes the bowl to refill, floating the valve closed. Under normal operations the float valve remains slightly open to very

44 open, keeping the level constant Floats can be concentric or eccentric. Concentrics are levers, first or second class. Eccentrics are a slide float. Floats can be adjusted by shims under the valve, or by adjusting a valve contact tab. 139 Floats need to be carefully inspected for leaks and possible deterioration. Be very exact in using the manufacturer s guidelines, and in float level measurements. Always leave the float chamber spotless. 140 The main source of fuel metering force comes from the pressure differential between the low pressure area within the venturi and the ambient pressure in the float chamber, or bowl. This is called fuel metering force This fuel metering force draws fuel into a discharge nozzle, or venturi nozzle that introduces fuel into the center of the airstream. Air bubbles can be bled into the fuel gallery just before the discharge device to assist in atomization and distribution Idle circuits will exist that feed fuel through separate ports. These are located just down stream of the throttle plate. There may also be transition ports to assist throttle transitionfrom idle to midrange.

45 145 Idle circuits are very similar on all models. Most will have a idle fuel ratio adjusting screw that regulates fuel flow. Some rare applications may use an air screw to adjust idle fuel ratio. 146 Both series of ports are active when throttle plate is closed or transitioning. They may drip slightly during other phases. Are usually a completely separate fuel circuit from the main fuel metering. May also have air bleed systems. 147 Idle and transition circuits function because the throttle plate is causing high velocity air to flow close to the port. Once throttle plate opens enough the port stops delivering fuel flow Most of these carburetors are up draft or side draft. The systems are identical with the position of the fuel discharge venturis and idle circuits upstream of the throttle valve regardless of the airflow direction

46 Float lever arrangements are the same for all carbs. Some use a first class lever, others use a second class lever. Acceleration systems, when used are similar. Some auto systems use a diaphragm pump rather than a piston. 156 Fuel enrichment circuits for Marvel-Schebler units. Mixture control is accomplished by a rotary valve that regulates fuel flow to the power jet. This valve acts as the main fuel metering jet 157 Enrichment/economizer is caused by full throttle linkage and a needle valve restricting bleed air. This causes more fuel and less air to be drawn to the discharge nozzle. The additional high velocity air makes up for the loss of bleed atomization