Overheating of a piston engine is likely to result from an excessively ? Review > pilot
Jaredleto
egt relative to temperature of charge as it burns within cylinder cht measures temperature of engine cylinder
the highest egt achieved at a ratio of 15 parts of air to one part of fuel
if ratio reduced ie rich charge temperature reduces due to excess fuel being left in cylinder if ratio greater than 15 1 ie lean or weak charge cooled excess air to appreciate why cht increases with lean mixtures we must consider speed at which fuel burns ie its flame rate lean mixtures burn slower than rich mixtures this means more heat transferred to engine components with lean mixtures
example if you move a lighted match quickly under your hand no problem but move it slowly you will burn your hand! slow burning lean mixtures will increase cht.
In an engine equipped with a float type carburettor the low temperature that causes carburettor ice is normally the result of ?
In an engine equipped with a float type carburettor low temperature that causes carburettor ice normally result of Vaporization of fuel expansion of air in carburettor. One disadvantage of float type carburetor its icing tendency the vaporization of fuel combined with expansion of air as it flows through carburetor causes a sudden cooling of mixture the temperature of air passing through carburetor may drop significantly within a fraction of a second water vapor in air 'squeezed out' this cooling and if temperature in carburetor reaches 0°c or below moisture will be deposited as frost or ice inside carburetor passages even a slight accumulation of this deposit will reduce power may lead to complete engine failure particularly when throttle partly or fully closed
the formation of carburetor ice may reduce or block fuel/air flow to engine.
In addition to the fire hazard introduced excessive priming should be avoided because ?
In addition to fire hazard introduced excessive priming should be avoided because It fouls spark plugs. One disadvantage of float type carburetor its icing tendency the vaporization of fuel combined with expansion of air as it flows through carburetor causes a sudden cooling of mixture the temperature of air passing through carburetor may drop significantly within a fraction of a second water vapor in air 'squeezed out' this cooling and if temperature in carburetor reaches 0°c or below moisture will be deposited as frost or ice inside carburetor passages even a slight accumulation of this deposit will reduce power may lead to complete engine failure particularly when throttle partly or fully closed
the formation of carburetor ice may reduce or block fuel/air flow to engine. With respect to a piston engine aircraft ice in carburettor May form at oat's higher than + °c. One disadvantage of float type carburetor its icing tendency the vaporization of fuel combined with expansion of air as it flows through carburetor causes a sudden cooling of mixture the temperature of air passing through carburetor may drop significantly within a fraction of a second water vapor in air 'squeezed out' this cooling and if temperature in carburetor reaches 0°c or below moisture will be deposited as frost or ice inside carburetor passages even a slight accumulation of this deposit will reduce power may lead to complete engine failure particularly when throttle partly or fully closed
the formation of carburetor ice may reduce or block fuel/air flow to engine. As flight altitude increases if no leaning made with mixture control The density of air entering carburettor decreases the amount of fuel remains constant.
density decreases as we gain altitude but volume of air entering in carburettor does not change as the volume of fuel (fuel density does not change with altitude). Cooling air a reciprocating engine can be obtained means of The density of air entering carburettor decreases the amount of fuel remains constant.
density decreases as we gain altitude but volume of air entering in carburettor does not change as the volume of fuel (fuel density does not change with altitude). The main advantage of a constant speed propeller as compared to a fixed pitch propeller a High efficiency in a much wider speed range of aeroplane.
advantages of a constant speed propeller compared to a fixed pitch propeller
the power setting of a piston engine defined a combination of manifold pressure (boost) rpm where separate power lever rpm lever control provided it possible to vary one while leaving other constant so optimising operation of engine/propeller combination to give best efficiency/fuel economy least engine wear tear in order to achieve this a 'variable pitch' propeller must be used enabling pilot to select a propeller pitch thus to vary rpm independently of manifold pressure provided that propeller operating between its internal fine coarse pitch stops once an rpm has been selected a control unit (csu constant speed unit or pcu propeller control unit) will automatically vary propeller pitch angle therefore its angle of attack to prevailing relative airflow in order to maintain selected rpm despite airspeed manifold pressure variations a constant speed propeller has a high efficiency in a much wider speed range than a fixed pitch propeller. 8 From cruise with all parameters correctly set if altitude reduced to maintain same mixture fuel flow should High efficiency in a much wider speed range of aeroplane.
when altitude increases air density decreases
to maintain a correct mixture ratio of 1 to 15 (fuel mass / air mass) if altitude reduced we need to increase fuel flow to maintain a constant ratio (air volume entering carburettor will not change but air mass will increase). 9 A turbocharger in a reciprocating engine consists of a Compressor turbine mounted on a common shaft.
when altitude increases air density decreases
to maintain a correct mixture ratio of 1 to 15 (fuel mass / air mass) if altitude reduced we need to increase fuel flow to maintain a constant ratio (air volume entering carburettor will not change but air mass will increase). 10 An impulse coupling inhibited at higher than normal rotational starting speeds Compressor turbine mounted on a common shaft.
impulse coupling magneto
the impulse coupling a spring loaded mechanical coupling which delays rotation of magneto during an engine start after a short period of rotation device allows magneto to catch up in effect spinning magneto shaft faster than engine turning it the faster it turns hotter spark the bigger voltage impulse making easier starting after engine running centrifugal forces move spring loaded arms which make impulse coupling work at low speeds out of way the impulse coupling only works at very low engine speeds. 11 The very rapid magnetic field changes flux changes in primary coil of a magneto are accomplished the Contact breaker points opening.
inside primary circuit a set of contact points that make flux reversals more abrupt when they open close at critical moments
when points are closed current flows in primary coil a flux field builds up around it when a spark needed they open which causes field in primary to collapse induce a current in secondary winding as field moves across it which fed to spark plugs through distributor
the more suddent breaking of primary circuit is more current will be induced in secondary coil. 12 In a compressor stage of a jet engine sequence Contact breaker points opening. Compr es sor highlighted rs means correct order so rotor stator
turbine stage opposite. 13 The accessory units driven the accessory gearbox of a turbo jet engine are 1 thrust reverser pneumatic motors2 ac generator through its constant speed drive csd 3 oil pumps4 hydraulic pumps5 high pressure fuel pumpsthe combination regrouping all correct statements Contact breaker points opening.
the accessory gearbox driven a radial shaft that meshes with a bevel gear driven the n2 (or gas producer rotor) is used to drive essential engine components such as fuel pump oil pump fuel control unit generators hydraulic pumps the accessory gear box also used to drive engine rpm feedback tacho generators is means which starter drives engine these are systems which must operate as soon as engine starts
structure containing gearbox can be utilised as a support as well
each component that attaches to accesssory gearbox has its own mounting pad with its unique gear reduction speed rotational direction. 14 Consider a jet engine whose control based on engine pressure ratio epr 1 with a constant epr thrust decreases when altitude increases2 with a constant epr thrust independent of mach number3 at same environmental conditions a given epr setting maintains thrust irrespective of engine wear due to ageing 4 epr determined the impact pressure difference between turbine outlet and compressor inlet5 on take off in event of icing not detected the crew indicated epr lower than real eprthe combination regrouping all correct statements Contact breaker points opening. Engine pressure ratio (epr) a measure of thrust of a jet engine obtained dividing pressure at back of engine the pressure at front
epr settings vary with altitude temperature when altitude increases thrust decreases
answer 4 the epr determined the impact pressure difference between turbine outlet the compressor inlet wrong this a ratio not a difference. 15 In axial flow compressor of a turbo jet engine flow duct tapered its shape calculated so as to Maintain a constant axial speed in cruising flight. The duct in question the convergent duct formed between compressor engine compressor casing
the airflow through a jet engine varies dependant on thrust selection since engine runs most of time at cruise engine designed to give correct flow rates at this thrust setting. 16 In stator of a turbine gas velocity v and static pressure ps vary as follows V increases ps decreases. If you have multiple stage turbines there a set of guide vanes between each turbine disc to redirect flow again before next turbine blade they are stationary so are sometimes called stators
with turbine you want a high velocity so turbine blades are designed to extract power from hot expanding gasses to turn compressor using high velocity hitting turbine blade the guide vanes straighten flow before next set of turbine blades but in effect it decreases pressure increases velocity this also prevents excessive backpressure which could work its way forward affect combustion process
the compressor on other hand designed to increase pressure through each of stages so you can deliver a good mass flow to combustion chambers however increasing pressure will try to reverse flow so convergent duct designed to keep velocity constant through whole compressor so that momentum of airflow prevents reverse flow in effect velocity therefore increased in each rotor section
velocity increases decreases through each set of rotors stators by using a convergent duct velocity therefore stays constant but pressure steadily increasing through each stage. 17 Epr can be defined as ratio of The total turbine outlet pressure to total compressor inlet pressure. If you have multiple stage turbines there a set of guide vanes between each turbine disc to redirect flow again before next turbine blade they are stationary so are sometimes called stators
with turbine you want a high velocity so turbine blades are designed to extract power from hot expanding gasses to turn compressor using high velocity hitting turbine blade the guide vanes straighten flow before next set of turbine blades but in effect it decreases pressure increases velocity this also prevents excessive backpressure which could work its way forward affect combustion process
the compressor on other hand designed to increase pressure through each of stages so you can deliver a good mass flow to combustion chambers however increasing pressure will try to reverse flow so convergent duct designed to keep velocity constant through whole compressor so that momentum of airflow prevents reverse flow in effect velocity therefore increased in each rotor section
velocity increases decreases through each set of rotors stators by using a convergent duct velocity therefore stays constant but pressure steadily increasing through each stage. 18 A gas turbine engine turbine section may employ active clearance control To provide enhanced clearances between blade tips the casing.
active clearance control (acc) a method used in gas turbines to improve fuel efficiency this achieved controlling tip clearance it particularly effective on turbofans since they are required to respond quickly to changes of thrust setting
the active clearance control consists of acc valve which receives air from bypass duct routes air to flow over pipes surrounding turbine casing which in turn reduces thermal expansion of turbine case maintains accurate clearance between turbine case the blade tip this clearance should be maintained accurately which essential the engine efficiency its performance the acc valve opening adjusted automatically the fadec system depending on throttle position. 19 In a gas turbine engine maximum gas temperature attained Within combustion chamber.
active clearance control (acc) a method used in gas turbines to improve fuel efficiency this achieved controlling tip clearance it particularly effective on turbofans since they are required to respond quickly to changes of thrust setting
the active clearance control consists of acc valve which receives air from bypass duct routes air to flow over pipes surrounding turbine casing which in turn reduces thermal expansion of turbine case maintains accurate clearance between turbine case the blade tip this clearance should be maintained accurately which essential the engine efficiency its performance the acc valve opening adjusted automatically the fadec system depending on throttle position. 20 A pressure relief valve that does not fit on its seat properly would result in Within combustion chamber. To prevent too high a pressure a pressure release valve fitted it comprises a stainless steel ball bearing a spring
the ball forced the spring against an oil port when oil pressure sufficiently high ball if forced off its seat oil allowed to bypass this restriction thus making sure that pressure never gets too high
if pressure relief valve not hermetic there will be a leak the pressure decreases. Exclusive rights: Content under private and protected license, reproduction prohibited and prosecuted.
