ENGINE INSTRUMENTATION

A basic understanding of how engine instruments work is an important part of the knowledge you must have as an engine operator and troubleshooter.

Part of this knowledge includes knowing how to interpret the instrument markings so you can com-prehend how an engine is performing. Instrument markings establish operational ranges as well as minimum and maximum limits. In addition, the markings allow you to distinguish between normal operation, time limited operation, and unauthorized operating conditions. Engine instrument range markings are based on limits found in the engine's Type Certificate Data Sheet. Traditionally, instru-ment markings consist of green, blue, yellow, and red lines or arcs and intermediate blank spaces.

Green arcs are the most widely used of all the instrument markings and usually indicate a safe, or normal range of operation. Usually, the upper end of a green arc indicates the maximum limit for contin-uous operation while the low end indicates the

min-Figure 2-1. For standardization purposes, the primary engine controls are arranged from left to right beginning with the throttle, propeller control, and mixture. In addition, each lever is color coded and uniquely shaped.

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imum limit for continuous operation. Operation within a green arc is typically not time restricted.

Blue arcs are used to indicate an allowable range of operations under a unique set of circumstances. For example, a blue arc may indicate an acceptable fuel flow when flying above a specific altitude. Blue arcs are rarely used and may only be seen on certain engine instruments, such as the tachometer, fuel flow, manifold pressure, cylinder head temperature, or torque meter.

A yellow arc indicates a precautionary range of time limited operation permitted by the manufacturer.

However, in some cases, a yellow arc may be omitted on instruments if it is too small to be clearly visible. When this is the case, the manufacturer's instructions will specify a caution range. Engine operation in the yellow arc is typically an indica-tion of an impending problem or a warning to change an operational setting.

A red line indicates a maximum or minimum safe operating limit. Operation beyond a red line typi-cally results in a dangerous operating condition. In addition, other limits of a transient or momentary nature may be indicated by a red triangle, dot, or diamond mark. A red arc, on the other hand, indi-cates a restricted operating range where excessive vibration or other stresses could endanger the engine or airframe.

The colored arcs and lines on engine instruments are typically painted directly on the instrument face. However, some older aircraft instrument mark-ings may be painted on the instrument glass. If this is the case, a white line is used as an index mark between the instrument glass and case. Any discon-tinuity in the white radial line indicates the instru-ment glass has moved. Since misaligninstru-ment between the glass and case renders the instrument range and limit markings inaccurate and unreliable, the glass must be repositioned and secured.

Now that you understand how to interpret the mark-ings on engine instruments, the following discus-sion looks at some of the typical instruments found on reciprocating engine powered aircraft. As each instrument is discussed, it is important to keep in mind that the performance limits of individual engine models vary considerably. Therefore, the procedures and operational ranges that are used in this section do not necessarily correspond to any specific engine.

CARBURETOR AIR TEMPERATURE

Carburetor air temperature (CAT) is measured at the carburetor entrance by a temperature sensing bulb in the ram air intake duct. The sensing bulb senses the air temperature in the carburetor and then sends a signal to a cockpit instrument that is calibrated in degrees Centigrade. The primary purpose of a CAT gauge is to inform a pilot when the temperature at the carburetor can support the formation of ice.

[Figure 2-2]

In addition to identifying the conditions necessary for the formation of ice, excessively high carburetor air temperatures can indicate the onset of detona-tion. For example, if a CAT gauge has a red line identifying a maximum operating temperature, engine operation above that temperature increases the chance of detonation occurring.

Observation of the CAT before engine startup and just after shutdown provides an indication of fuel temperature in the carburetor body. During startup, this information can be used to determine if the fuel is warm enough to support vaporization. On the other hand, a high CAT after engine shutdown is a warning that fuel trapped in a pressure-type carbu-retor could expand and produce potentially damag-ing fuel pressures. High CAT temperatures after shutdown can also indicate the onset of vapor lock, which is the formation of vaporized fuel bubbles in a fuel line that interfere with the flow of fuel to the engine. Any time high CAT readings are observed after shutdown, the fuel selector valve and throttle

Figure 2-2. The carburetor air temperature gauge depicted above indicates that the danger of induction system icing exists when the temperature is between - 15℃ to + 5℃

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should be left open until the engine cools. This helps relieve fuel pressure by allowing expanding fuel to return to the tank.

Information provided by CAT gauges can also be useful in troubleshooting. For example, if severe enough, backfiring may cause a momentary rise in CAT. On the other hand, an induction fire would be indicated by a steady increase in CAT readings.

FUEL PRESSURE

Some engines have a fuel pressure gauge that dis-plays the pressure of fuel supplied to the carburetor or fuel control unit. Most fuel pressure instruments display fuel pressure in pounds per square inch (psi), and provide indications to the pilot that the engine is receiving the fuel needed for a given power setting. A pilot also uses fuel pressure gauges to verify the oper-ation of an auxiliary fuel pump. [Figure 2-3]

One type of fuel pressure gauge uses a Bourdon tube which is a metal tube that is formed in a circular shape with a flattened cross section. One end of the tube is open, while the other end is sealed. The open end of the Bourdon tube is connected to a capillary tube containing pressurized fuel. As the pressurized fuel enters the Bourdon tube, the tube tends to straighten. Through a series of gears, this movement

Figure 2-3. The top fuel pressure gauge is typical for an engine equipped with a float type carburetor. Notice the minimum fuel pressure of approximately 0.5 psi and the maximum of approximately 8 psi. However, the bottom fuel pressure is mr e typical for engines equipped with a fuel injection system.

Figure 2-4. As pressurized fuel enters a Bourdon tube, the tube straightens and causes an indicator needle to move.

Since the amount a Bourdon tube straightens is propor-tional to the pressure applied, they are often used to indi-cate pressure.

is used to move an indicating needle on the instru-ment face. [Figure 2-4]

Another type of fuel pressure indicator utilizes a pressure capsule, or diaphragm. Like the Bourdon tube, a diaphragm type pressure indicator is attached to a capillary tube, which attaches to the fuel system and carries pressurized fuel to the diaphragm. As the diaphragm becomes pressurized, it expands, causing an indicator needle to rotate.

[Figure 2-5] , - - ,

Figure 2-5. Pressure applied internally to a diaphragm causes it to expand, thereby causing rotation of a sector gear. The sector gear, in turn, rotates a pinion gear which is attached to a pointer on a common shaft.

Reciprocating Engine Operation, Maintenance, Inspection, and Overhaul 2-5

A third type of fuel pressure indicator uses a bel-lows that is attached to a capillary tube. The advan-tage of bellows over a Bourdon tube or diaphragm is its ability to provide a greater range of motion. The bellows inside the instrument case expands and moves an indicator needle as the fuel pressure increases. [Figure 2-6]

Electric fuel pressure indicating systems are used when the distances between the engine and cockpit become prohibitive for the use of capillary tubing.

Another reason for using electric fuel pressure indi-cators is to avoid bringing fuel into the cockpit.

Aircraft with electric fuel pressure indicating sys-tems typically use pressure sensors, or transducers, that transmit electrical signals proportional to the fuel pressure to the cockpit. An electric fuel pres-sure gauge in the cockpit receives the signals and displays fuel pressure information.

On aircraft with direct fuel injection or continuous-flow fuel injection systems, fuel pressure is measured as a pressure drop across the injection nozzles. The pressure drop at the nozzles is proportional .to the amount of fuel being supplied to the engine as well as engine power output. Therefore, the gauges in this type of fuel pressure indication system may also be calibrated in percentages of power as well as psi.

FUEL FLOW INDICATOR

A fuel flow indicator measures the rate of fuel an engine burns in gallons per hour or pounds per hour. This provides the most accurate indication of an engine's fuel consumption. In addition, when combined with other instrument indications, the amount of fuel an engine burns can be used to deter-mine the power settings necessary to obtain maxi-mum power, speed, range, or economy. [Figure 2-7]

On aircraft with a continuous-flow fuel injection system, the fuel flow indicator measures the pres-sure drop across the fuel injection nozzles to deter-mine fuel flow. This can be done because, as you recall, fuel pressure in a direct fuel injection system is proportional to fuel flow. With this type of sys-tem, a higher fuel flow produces a greater pressure differential across the injectors, and a correspond-ing increase in fuel flow is indicated in the cockpit.

However, if an injector nozzle becomes clogged, the pressure differential across that nozzle increases, producing a false, or high fuel flow reading.

Another type of fuel flow measurement system measures the volume of fuel flowing to an engine.

This type of system is commonly referred to as an autosyn system and incorporates a movable vane that is in the fuel line leading to the engine. As fuel flows through the fuel line, the vane is displaced an amount proportional to the fuel flow. A spring force opposes the fuel flow and returns the vane to neu-tral when no fuel is flowing. The vane movement is

Figure 2-7. A typic jlays the number

of gallons or pounds per hour an engine consumes. In the fuel flow gauge above, normal fuel flow is from 4.5 to 11.5 gallons per hour.

Figure 2-6. A bellows offers the advantage of a greater range of motion than a Bourdon tube or diaphragm and, therefore, is sometimes used with aircraft.

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electrically transmitted to the flow indicator in the cockpit where an indicator needle indicates the fuel flow.

The movement of the vane must he linear to get an accurate flow measurement. To do this, the restric-tion created by the vane must get larger as the vane increases its rotation. This increase is calibrated into the design of the flowmeter sender unit.

Because the volume of jet fuel changes dramatically with temperature, the fuel flow on several turbine engines is measured in terms of mass rather than volume. A typical mass flow system works on the magnesyn principle. The system consists of two cylinders, an impeller, and a turbine which are mounted in the main fuel line leading to the engine.

The impeller is driven at a constant speed by a three-phase motor which is powered by the aircraft electrical system. The impeller imparts an angular swirling motion to the fuel as it proceeds to the tur-bine. As the fuel impacts the turbine, the turbine rotates until a calibrated restraining spring force balances the rotational force. The deflection of the turbine positions a permanent magnet in a trans-mitter to a position corresponding to the fuel flow in the line. The position of the permanent magnet is then transmitted electrically to a permanent magnet in a receiver which positions the indicator needle in the cockpit. This system is by far the most accu-rate means of monitoring fuel flow because it accounts for changes in fuel viscosity and volume caused by changes in temperature.

In addition to a fuel flow gauge, some aircraft are equipped with fuel totalizers. A computerized fuel system (CFS) with a fuel totalizer is used in both reciprocating and turbine engine aircraft and pro-vides a pilot with a digital readout on the amount of fuel used, fuel remaining, current rate of fuel con-sumption, and the time remaining for flight at the current power setting. Early totalizers were mechan-ical counters that responded to electric pulses, how-ever, new systems utilize electronic counters with digital readouts. Totalizer indicators are usually mounted on the instrument panel and are electri-cally connected to a flowmeter installed in the fuel line to the engine. When the aircraft is serviced with fuel, the counter is set to the total amount of fuel in all tanks. As the fuel passes through the metering element of the flowmeter, it sends a signal to the microprocessor that automatically calculates the amount of fuel remaining.

MANIFOLD PRESSURE

A manifold absolute pressure (MAP) gauge mea-sures the absolute pressure of the fuel/air mixture within the intake manifold. A MAP gauge is used on all aircraft that have a constant-speed propeller to indicate engine power output. Since MAP directly affects a cylinder's mean effective pressure (mep), a pilot uses MAP gauge indications to set the engine power at a pressure level that will not damage the engine. This is especially true for aircraft with tur-bocharged engines because it helps the pilot to avoid excessive manifold pressure. [Figure 2-8]

Before an engine is started, the manifold pressure gauge displays the local ambient, or atmosphere pressure. However, once the engine is started, the manifold pressure drops significantly, sometimes to half the existing ambient air pressure. At full power, the manifold pressure in normally aspirated engines will not exceed ambient pressure, however, in tur-bocharged engines the manifold pressure can exceed ambient pressure.

A manifold pressure gauge consists of a sealed diaphragm constructed from two discs of concentri-cally corrugated thin metal which are soldered together at the edges to form a chamber. The cham-ber is evacuated, creating a partial vacuum which can be used as a reference point to measure absolute pressure. Depending on the type of gauge, the

Figure 2-8. A manifold absolute pressure gauge displays absolute air pressure in the engine's intake manifold in inches of mercury. The green arc indicates the normal oper-ating range and the red line shows the maximum allowable manifold pressure.

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engine manifold pressure is either applied to the inside of the diaphragm or to the outside of the diaphragm. If the engine manifold pressure is applied to the outside of the diaphragm, the instru-ment case must be completely sealed. In either case, when pressure is applied to the diaphragm, the diaphragm movement is transmitted to an indicator pointer through mechanical linkage.

Another manifold pressure gauge uses a series of stacked diaphragms or bellows which are particu-larly useful for measuring low or negative pressures.

In a MAP gauge, one of the bellows measures ambi-ent atmospheric pressure while the other measures pressure in the intake manifold. Differential pres-sure between the two bellows causes motion, which is transmitted to the gauge pointer through a mechanical linkage. Regardless of which type of sealed chamber exists in the instrument, the pres-sure line from the manifold to the instrument case must contain a restriction to prevent pressure surges from damaging the instrument. In addition, the restriction causes a slight delay in gauge response to changes in manifold pressure, preventing jumpy or erratic instrument pointer motion. [Figure 2-9]

Some aircraft instrument installations provide a purge valve that allows you to purge moisture that collects in the pressure line near the MAP gauge.

With the engine running at idle, the purge valve is opened for 30 seconds or more, then closed. When this is done, the engine's vacuum creates a strong suction through the purge valve which effectively removes the moisture from the pressure line.

Whenever you run an engine with a manifold pres-sure gauge, you should check the gauge for proper operation. For example, before the engine is started, the MAP gauge should indicate the local atmos-pheric pressure. However, once the engine is started, the MAP should drop. If this does not

hap-Figure 2-10. The green arc between 75蚌 and 245蚌 on this oil temperature instrument shows the desired oil tempera-ture range for continuous operation. The red line is located at 245蚌 and indicates the maximum permissible oil tem-perature.

pen, and the gauge continues to indicate atmos-pheric pressure, the sense line between the instru-ment and induction manifold may be disconnected, broken, or collapsed. When engine power is increased, the manifold pressure should increase evenly and in proportion to the engine power out-put. If this does not occur, the restriction in the sense line is probably too large.

OIL TEMPERATURE

The oil temperature gauge allows a pilot to monitor the temperature of the oil entering the engine. This is important because oil circulation cools the engine as it lubricates the moving parts. Most oil temperature gauges are calibrated in degrees Fahrenheit and sense oil temperature at the engine's oil inlet. [Figure 2-10]

Most modern oil temperature systems are electrically operated and use either a Wheatstone bridge circuit or a ratiometer circuit. A Wheatstone bridge circuit consists of three fixed resistors and one variable resistor whose resistance varies with temperature. [Figure 2-11]

Figure 2-11. A typical Wheatstone bridge has three fixed resistors and one variable resistor. The temperature probe contains the variable resistor, whose resistance varies with the temperature of the oil flowing past the probe. The bridge in the circuit consists of a galvanometer that is cali-brated in degrees to indicate temperature.

Figure 2-9. The differential pressure bellows of a manifold pressure gauge measures the difference between intake manifold pressure and a partial vacuum.

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When power is applied to a Wheatstone bridge cir-cuit and all four resistances are equal, no difference in potential exists between the bridge junctions.

However, when the variable resistor is exposed to heat, its resistance increases, causing more current to flow through the fixed resistor R3 than the vari-able resistor. The disproportionate current flow pro-duces a voltage differential between the bridge junctions, causing current to flow through the gal-vanometer indicator. The greater the voltage differ-ential, the greater the current flow through the indicator and the greater the needle deflection.

Since indicator current flow is directly proportional to the oil temperature, an indicator calibrated in

Since indicator current flow is directly proportional to the oil temperature, an indicator calibrated in