Induction & Fuel Metering
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1 Induction & Fuel Metering Aviation Maintenance Technology 253 - 254 2 INDUCTION • 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 INDUCTION • 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 INDUCTION • 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 INDUCTION • Air Scoop • Usually a converging duct that will make use of the ram air effect of the aircraft’s motion. 6 INDUCTION 7 INDUCTION • 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. 8 INDUCTION • 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. 9 INDUCTION 10 INDUCTION • Air filters. • Wetted type mesh, 25 hrs; • Pleated paper element, up to 500 hrs; • Impregnated foam, 11 INDUCTION • 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 INDUCTION • 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 INDUCTION • 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 INDUCTION • These are metal wire reinforced silicon paper, • Can thread one piece into another. • Most common size is -8 = 2", • Size increments are 1/4” 15 INDUCTION • 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 INDUCTION • 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 INDUCTION • 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 INDUCTION • Fuel metering devices • 1. pressure carburetors - TBI • 2. float carburetors • 3. continuous fuel injection • 4. pulsed fuel injection 19 INDUCTION • 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. 20 INDUCTION 21 INDUCTION • 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. 22 INDUCTION • 2. 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. 23 INDUCTION • 5. 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 INDUCTION • Deicing systems • Three types of ice. • 1. Vaporization ice • 2. Throttle ice • 3. Impact ice 25 INDUCTION • 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 INDUCTION • 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 INDUCTION • 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 freeze. 28 INDUCTION 29 INDUCTION • 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 INDUCTION • 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 INDUCTION • Intake system ducting • 1. intake manifold, oil bath & non oil bath • 2. cylinder distribution assembly • 3. intake runner tubes • 4. connection hoses 32 INDUCTION • 5. Cylinder attachment flanges and hardware. • 6. Support bracketing 33 INDUCTION 34 INDUCTION • 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 INDUCTION • Polished intake interiors work well for high end racing performance but provide terrible, stumbling low end performance. 36 INDUCTION • 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 INDUCTION • 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. 29.92 ”Hg 38 INDUCTION • 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 INDUCTION • 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 INDUCTION • 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 INDUCTION • 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 INDUCTION • 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 INDUCTION • 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 anti- detonation factor of the fuel being used. • Lower octane fuels will be more likely to detonate in high BMEP engines. 44 INDUCTION • ALWAYS USE THE APPROPRIATELY RATED FUEL. • Note: Ignition timing can also effect BMEP, advanced timing will raise BMEP to a point. 45 INDUCTION • 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 INDUCTION • 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 INDUCTION • Leaks - large • Engine won’t start, or runs very poorly. • Runs only in full rich position. • Severe back firing or after firing. 48 INDUCTION • Can be caused by closed throttle starting back fire. • MAP will indicate leaks as higher than normal, or lower if in boosted range. 49 INDUCTION • 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 INDUCTION • 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 INDUCTION • Supercharging • Its purpose is to increase the mass of the air/fuel charge going into the engine for each revolution. 52 INDUCTION • 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 INDUCTION • 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 INDUCTION • 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 INDUCTION • General gas law = volume of a gas is inversely proportional to the absolute pressure and is directly proportional to the temperature.