Induction, Cooling, & Exhaust

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Induction, Cooling, & Exhaust

Aviation Maintenance Technology

111 B - 112 B

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h 1998INDUCTION

• Induction = those locations of the enginenacelles where air entering cannot avoidentering the cylinders.

• There are two basic types

• 1. non-supercharged

• 2. supercharged

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h 1998INDUCTION

• Non-supercharged components

• 1. Air scoop,

• 2. Air filter,

• 3. Ducting,

• 4. Hot air selector box, heat muff, andducting

• 5. Fuel metering device,

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INDUCTION• 6. Sensors

• 7. De-icing devices

• 8. Ducting to intake manifold, air reservoir ismade by long convergent duct.

• Supercharged systems usually contain one ofmore of the above items, and they will alsocontain some form of high volume air pump.

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• Air Scoop

• Usually a converging duct that will makeuse of the ram air effect of the aircraft’smotion.

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h 1998INDUCTION

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• At scoop inlet, ram effect does increase theair pressure at the mouth in a super chargingeffect because air demand from throttle isless then ram air going in.

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• The velocity then increases and pressuredrops as the convergent duct develops in theventuri area.

• They make scoops square and install guidevanes to reduce coriolis swirling.

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h 1998INDUCTION

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h 1998INDUCTION

• Air filters.

• Wetted type mesh, 25 hrs;

• Pleated paper element, up to 500 hrs;

• Impregnated foam,

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h 1998INDUCTION

• 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

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h 1998INDUCTION

• Decrease all replacement intervals if dustyand dirty.

• Most common contaminant is dirt,

• Is organic materials mixed with silica.

• The organics create sludge, and the silicagrinds up metal parts.

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h 1998INDUCTION

• Ducting

• Performed rigid aluminum or stainless .020"

• Semi-rigid flexible, scat tubing = red -80° to450°f

• Cat tubing = black -65° to 300°f, commonlybetween air filter box and carb.

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h 1998INDUCTION

• These are metal wire reinforced siliconpaper,

• Can thread one piece into another.

• Most common size is -8 = 2",

• Size increments are 1/4”

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h 1998INDUCTION

• Hot air valve, muff and ducting

• Alternate air =

• Heat source provides alternate air sourcewhich melts carb ice.

• Carb heat control is in for cool, out for heat.

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h 1998INDUCTION

• Lindbergh’s mechanic invented the first hotair device on the Spirit of St. Louis.

• Heat selector box has cold filtered inlet, hotunfiltered inlet, outlet, and flap valve

• Flap is on a shaft w/ pivot bushings, chafeoccurs on shaft.

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h 1998INDUCTION

• If steel type find old seat assemblies fromcarb to replace bushings.

• Heat muff is a shroud around muffler to beused as a hot air source.

• Also may use inter-cylinder baffles, turbineside of turbo, oil sump, as alternate hot airsources.

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h 1998INDUCTION

• Fuel metering devices

• 1. pressure carburetors - TBI

• 2. float carburetors

• 3. continuous fuel injection

• 4. pulsed fuel injection

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h 1998INDUCTION

• Sensors

• MAP gauges are plumbed into thesupercharger blower chamber, reading theamount of boost.

• Normally aspirated engine is plumbedsomewhere between carb and engine.

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Unliscen

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h 1998INDUCTION

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• Latent heat of vaporization is the energyrequired to convert a liquid to a gas, withoutchanging the temperature.

• Laws of evaporation.

• 1. The rate increases as temperatureincreases.

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• 2. The rate increases as surface areaexposed increases.

• 3. The rate increases when atmosphericpressure decreases.

• 4. The rate varies with the nature of theliquid.

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• 5. The rate increases for water when thehumidity decreases.

• 6. The rate varies with the rate of air contactacross the surface.

• 7. Evaporation causes a cooling effect orloss of heat.

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h 1998INDUCTION

• Deicing systems

• Three types of ice.

• 1. Vaporization ice

• 2. Throttle ice

• 3. Impact ice

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Unliscen

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• Vaporization ice is fuel related.

• Fuel gives up heat as it evaporates, coolingthe air mass causing freezing moisture.

• Occurs from fuel discharge area onwards inthe airstream.

• More of a problem with float carbs

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h 1998INDUCTION

• Throttle ice occurs on the back side in thelow pressure stall area of throttle plate,

• This ice commonly occurs from long slowdescent.

• In a pressure carb fuel nozzles are afterVenturi thereby reducing vaporization andthrottle icing.

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h 1998INDUCTION

• Impact ice occurs when visible moisture atedge of freezing or colder, that impacts andfreezes or impacts and collects

• Air frame subfreezing, temp 35°f or lower,droplets impact, explode and freeze.

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Unliscen

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h 1998INDUCTION

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h 1998INDUCTION

• Can install carb temp probe into intakemanifold with a green, yellow, red gaugescale.

• CAT = Carburetor Air Temperature

• Constant speed = drop in MAP, fixed pitch= drop in RPM.

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INDUCTION• Heating is done by routing warm air from a

shroud around the exhaust system into theintake to melt the ice.

• The less dense hot air will initially give a lossin power, but if power then climbs, you aremelting the ice.

• Some systems used alcohol in a reservoir tomelt the ice. This will also add fuel to theengine inlet.

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h 1998INDUCTION

• Intake system ducting

• 1. intake manifold, oil bath & non oil bath

• 2. cylinder distribution assembly

• 3. intake runner tubes

• 4. connection hoses

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h 1998INDUCTION

• 5. Cylinder attachment flanges andhardware.

• 6. Support bracketing

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h 1998INDUCTION

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• It is common to see intake interiors slightlyrough, from casting process.

• Although this causes slight boundary layerturbulence it reduces fuel condensationtraveling into engine in droplet form.

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h 1998INDUCTION

• Polished intake interiors work well for highend racing performance but provide terrible,stumbling low end performance.

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h 1998INDUCTION

• Porting and blue printing is the process ofenlarging intake interiors, and hand grindingall joints to remove any interior ridges at thejoint.

• Porting and polishing should not be done onaircraft engines unless approved by themanufacturer.

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• MAP Manifold Absolute Pressure

• This is the pressure found between the airthrottle valve, and the engine intake valves.

• Absolute means zero on the scale =complete vacuum,

• ambient = approx. 29.92 ”Hg

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Unliscen

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• Any engine is primarily an air pump.

• It draws air through the intake systems, andexpels it, mixed with exhaust gasses, out theexhaust systems.

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• The air throttle valve is the major RPM orPower controlling device for any sparkignited reciprocating engine. (Turbines andsome Diesels are controlled by fuelmetering only).

• It acts as a variable restriction to the airflow being drawn into the engine.

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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 ina nonsupercharged engine, and may exceedambient in a supercharged version.

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• When the throttle is closed there will still be aslight amount of air passing by it.

• RPM and Power will be minimized to thelowest level possible while continuing to runreasonably smooth. (If the “flyweight”momentum gets too low the power pulses tendto lope and stumble causing low idle misfires).

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h 1998INDUCTION

• 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 moreefficient, so look for this during warm-upcycles.

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• MAP and engine power have a directcorrelation to BMEP in the cylinders.

• The maximum BMEP allowed in any givencylinder is set by the anti-detonation factorof the fuel being used.

• Lower octane fuels will be more likely todetonate in high BMEP engines.

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h 1998INDUCTION

• ALWAYS USE THE APPROPRIATELYRATED FUEL.

• Note: Ignition timing can also effect BMEP,advanced timing will raise BMEP to apoint.

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• Common problems, failures anddiagnostic techniques

• Leaks - small

• Typically will go unnoticed.

• May not get a slight rise in RPM during fuelcut-off.

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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 ofnormal range.

• Detection by visual inspection, or 1 psi airsource, and soapy solution.

• Smoke generators are now becoming available.

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• Leaks - large

• Engine won’t start, or runs very poorly.

• Runs only in full rich position.

• Severe back firing or after firing.

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• Can be caused by closed throttle startingback fire.

• MAP will indicate leaks as higher thannormal, or lower if in boosted range.

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h 1998INDUCTION

• High power flat - deep hum sometimes -OK at idle and low power

• Restricted air filter, or induction blockage(pressure and float carbs will reactdifferently depending on the restriction)

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• Restricted exhaust (very high EGTs)

• Restricted fuel flow - (no hum with this, justpower drop, high EGTs)

• MAP will indicate the last two as higherthan normal, and the first as lower thannormal.