Engine Instruments Flashcards
Describe the use of the tachometer
Used to indicate the rotational speed of the engine. They measure crankshaft rpm.
Describe the function and principle of operation of the following instruments:
(a) tachometers (centrifugal, drag cup, electrical and electronic types);
Centrifugal - use rotating flyweights to convert rotation of the drive cable(proportional to engine rpm) into movement of the pointer.
Drag cup - magnet attached to the end of the drive cable which produces a rotating magnetic field. Surrounding the magnet is a drag cup. As magnet rotates, it sets up eddy currents in the drag cup which interacts with the fields creating a force on the cup which is connected to the pointer to read rpm.
Electrical - use a small engine-driven generator to transmit electrical energy to the cockpit to give an indication of rpm.
Electronic - special points are put on the engine magneto and are proportional to the engine rpm. Electronic circuits are used to measure the frequency and move the pointer to indicate rpm.
Describe the function and principle of operation of the following instruments:
Manifold pressure and boost gauges
- required on aircraft with CSU or are supercharged so that engine power can be accurately set and is the pressure which exists in the inlet manifold downstream of the carburettor, FCU, supercharger.
- utilises an aneroid capsule. The pressure measuring port in the manifold is connected by tubing and hose to the capsule in the instrument any increase or decrease in manifold pressure causes the capsule to expand or collapse moving the pointer.
- boost gauge is a gauge used if a turbo is fitted.
Describe the function and principle of operation of the following instruments:
Oil pressure gauges (direct, remote and ratiometer)
Direct reading - uses a bourdon tube(curved hollow tube) One end of the tube is anchored while the other is free and is connected to the instrument pointer. As fluid inside it hardens and causes the tube to straighten it which correlates to the pointer moving
Remote indicating - remote indication of of oil pressure by transmitting the data electrically
Ratiometer- oil pressure is applied to a diaphragm or bourdon tube mounted to the engine and the resulting movement of the sensor is used to vary the resistance in an electrical circuit.
Describe the function and principle of operation of the following instruments:
Fuel pressure gauges
Usually of the diaphragm type and the data is transmitted electrically to a ratiometer type instrument. Measures the pressure of fuel
Describe the function and principle of operation of the following instruments: vacuum gauge
Gives a direct indication of the suction of a vacuum pump using a bourdon tube. When system pressure is applied, since its less then the surrounding air, the bourdon tube increases its curvature which is converted to pointer movement.
Describe the function and principle operation of cylinder head temperature and exhaust gas temperature.
To measure the temperature of cylinder heads.
- a thermocouple is a circuit made up from two dissimilar metals. When one of the junctions (hot junction) between the metals is heated and a temperature difference exists between it and the ‘cold junction’ a small voltage is produced and this is sent to the instrument pointer
- EGT used primarily as an aid to leaning the mixture to give best fuel economy and for turbo fitted aircraft prevent overheating.
- uses a thermocouple like CHT except that the probe is mounted in the exhaust manifold and gives a measure of the temperature of the exhaust gases which pass by it.
Describe the function and principle of operation of the following instruments: oil temperature gauge (mechanical and ratiometer)
Mechanical bulb - consists of a probe filled with a volatile chemical, which at normal temp is part liquid and gas and the bulb being connected to a bourdon tube. When heated, liquid vaporises and gas is contained and its pressure rises which is applied to the bourdon tube which moves the pointer.
Ratiometer/electric - a probe containing a coil of fine nick wire which when heated increases in resistance and this change is resistance results in movement of the pointer.
Describe the function and principle of operation of the following instruments: outside air temperature gauge (direct reading and mechanical bulb)
Direct reading air temp gauge - bi-metallic strip, two thin strips of metal with different expansions are joined together in a helix. they coil/uncoil depending to changes in temperature which is then seen on the gauge.
Mechanical bulb air temp gauge - like the direct reading, except operating independently of electrical power.
Describe the function and principle of operation of the following instruments: fuel quantity gauge
variable resistance gauge - utilises a float in the tank which varies the resistance of an electrical circuit. As fuel is used and float is lowered, the resulting change in resistance alters the current flow causing movement of the pointer.
Capacitance indicators - use of two concentric tubes which form the plates of a capacitor. Dielectric (non conducting medium) between plates consist of fuel at bottom and air at top and they both have different amounts of capacitance built up between them. As Fuel is used the change in capacitance sends a signal which moves the pointer.
Describe the function and principle of operation of the following instruments: fuel flow gauge
Usually used with supercharged piston engines.
A vane type sensor located in the fuel feed line to the engine. As fuel flows past the vane it rotates through an arc by an amount proportional to the volumetric rate of fuel flow. This deflection of the vane is relayed to the instruments.
