Engine Testing of Reciprocating Engines
The procedures and equipment used in determining that an engine is ready for airworthy service and is in excellent mechanical condition, normally requires the use of a test stand, or test cell, although the aircraft can be used. [Figure 1] The method of engine testing or run-in that takes place during overhaul prior to delivery of the engine is critical to the airworthiness of the engine. It must be emphasized that engine run-in is as vital as any other phase of engine overhaul, for it is the means by which the quality of a new or newly overhauled engine is checked, and it is the final step in the preparation of an engine for service. Thus, the reliability and potential service life of an engine is in question until it has satisfactorily passed the cell test.
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| Figure 1. Test stand |
The failure of any part during engine testing or run-in requires that the engine be returned, repaired, and completely retested. After an engine has successfully completed test requirements, it is then specially treated to prevent corrosion, if it is shipped or stored before being installed in an aircraft. During the final run-in period during testing, the engines are operated on the proper grade of fuel prescribed for the particular kind of engine. The oil system is serviced with a mixture of corrosion-preventive compound and engine oil. The temperature of this mixture is maintained at 105 °C to 121 °C. Near the end of final run-in, corrosion-preventive mixture (CPM) is used as the engine lubricant. The engine induction passages and combustion chambers are also treated with CPM by an aspiration method. CPM is drawn or breathed into the engine.
Test Cell Requirements
The test cell requires an area to mount and hold the engine for testing. The cell needs to have the controls, instruments, and any special equipment to evaluate the total performance of the engine. A test club should be used for testing instead of a flight propeller. [Figure 2] A test club provides more cooling air flow and the correct amount of load. The operational tests and test procedures vary with individual engines, but the basic requirements are generally closely related.
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| Figure 2. Test club |
Engine Instruments
The test cell control room contains the controls used to operate the engine and the instruments used to measure various temperatures and pressures, fuel flow, and other factors. These devices are necessary in providing an accurate check and an evaluation of the operating engine. The control room is separate from, but adjacent to, the space (test cell) that houses the engine being tested. The safe, economical, and reliable testing of modern aircraft engines depends largely upon the use of instruments. In engine run-in procedures, the same basic engine instruments are used as when the engine is installed in the aircraft, plus some additional connections to these instruments, and some indicating and measuring devices that cannot be practically installed in the aircraft. Instruments used in the testing procedures are inspected and calibrated periodically, as are instruments installed in the aircraft; thus, accurate information concerning engine operation is ensured.
Engine instruments can operate using different methods, some mechanically, some electrically, and some by sensing the direct pressure of air or liquid. Some of the basic instruments are:
- Carburetor air temperature gauge
- Fuel pressure gauge
- Fuel flowmeter
- Manifold pressure gauge
- Oil temperature gauge
- Oil pressure gauge
- Tachometer
- Exhaust gas temperature gauge
- Cylinder head temperature gauge
- Torquemeter
Instrument markings, ranges of operation, minimum and maximum limits, and the interpretation of these markings are general to all the instruments. Generally, the instrument marking system consists of three colors: red, yellow, and green. A red line, or mark, indicates a point beyond which a dangerous operating condition exists. A red arc indicates a dangerous operating range due generally to an engine propeller vibration range. This arc can be passed through, but the engine cannot be operated in this area. Of the two, the red mark is used more commonly and is located radially on the cover glass or dial face. The yellow arc covers a given range of operation and is an indication of caution. Generally, the yellow arc is located on the outer circumference of the instrument cover glass or dial face. The green arc shows a normal and safe range of operation. When the markings appear on the cover glass, a white line is used as an index mark, often called a slippage mark. The white radial mark indicates any movement between the cover glass and the case, a condition that would cause mislocation of the other range and limit markings.
Carburetor Air Temperature (CAT) Indicator
Measured at the carburetor entrance, carburetor air temperature (CAT) is regarded by many as an indication of induction system ice formation. Although it serves this purpose, it also provides many other important items of information.
The powerplant is a heat machine, and the temperature of its components, or the fluids flowing through it, affects the combustion process either directly or indirectly. The temperature level of the induction air affects not only the charge density, but also the vaporization of the fuel. CAT is also useful for checking induction system condition. Backfiring is indicated as a momentary rise on the gauge, provided it is of sufficient severity for the heat to be sensed at the carburetor air-measuring point. A sustained induction system fire shows a continuous increase of CAT.
The CAT should be noted before starting and just after shutdown. The temperature before starting is the best indication of the temperature of the fuel in the carburetor body, and tells whether vaporization is sufficient for the initial firing, or whether the mixture must be augmented by priming. If an engine has been shut down for only a short time, the residual heat in the carburetor may make it possible to rely on the vaporizing heat in the fuel and powerplant. Priming would then be unnecessary.
After shutdown, a high CAT is a warning that the fuel trapped in the carburetor will expand, producing high internal pressure. When a high temperature is present at this time, the fuel line and manifold valves should be open so that the pressure can be relieved by allowing fuel passage back to the tank. The CAT gauge indicates the temperature of the air before it enters the carburetor. The temperature reading is sensed by a bulb or electric sensor. In the test cell, the sensor is located in the air intake passage to the engine and, in an aircraft it is located in the ram-air intake duct. The CAT gauge is calibrated in the centigrade scale. [Figure 3] This gauge, like many other multi-engine aircraft instruments, is a dual gauge; two gauges, each with a separate pointer and scale, are used in the same case.
Notice the range markings used. The yellow arc indicates a range from –10 °C to +15 °C, since the danger of icing occurs between these temperatures. The green range indicates the normal operating range from +15 °C to +40 °C. The red line indicates the maximum operating temperature of 40 °C; any operation at a temperature over this value places the engine in danger of detonation.
Fuel Pressure Indicator
The fuel pressure gauge is calibrated in pounds per square inch (psi) of pressure. It is used during the test run-in to measure engine fuel pressure at the carburetor inlet, the fuel feed valve discharge nozzle, and the main fuel supply line. Fuel gauges are located in the operator’s control room and are connected by flexible lines to the different points at which pressure readings are desired during the testing procedures.
In some aircraft installations, the fuel pressure is sensed at the carburetor or fuel injection unit inlet of each engine, and the pressure is indicated on individual gauges on the instrument panel. [Figure 4] The dial is calibrated in graduations and is extended and numbered. The numbers range from 0 to 10 in this example. The red line on the dial at the 2 pounds psi graduation shows the minimum fuel pressure allowed during flight. The green arc shows the desired range of operation, which is 2 to 9 psi. The red line at the 9 psi graduation indicates the maximum allowable fuel pressure. Fuel pressures vary with the type of fuel system installation and the size of the engine. When fuel injection systems are used, the fuel pressure range is much higher; the minimum allowable pressure is approximately 10 psi, and the maximum is generally 25 psi.
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| Figure 4. Engine instrument clusters |
Oil Pressure Indicator
The main oil pressure reading is taken at the pressure side of the oil pump. Generally, there is only one oil pressure gauge for each aircraft engine. The oil pressure gauge dial does not show the pressure range or limits for all installations. [Figure 4] The actual markings for specific aircraft may be found in the aircraft specifications or Type Certificate Data Sheets. The lower red line at 25 psi indicates the minimum oil pressure permissible in flight. The green arc between 60 to 85 psi illustrates the desired operating oil pressure range. The red line at 100 psi indicates maximum permissible oil pressure.
The oil pressure gauge indicates the pressure, in psi, that the oil of the lubricating system is being supplied to the moving parts of the engine. The engine should be shut down immediately if the gauge fails to register pressure when the engine is operating. Excessive oscillation of the gauge pointer indicates that there is air in the lines leading to the gauge, or that some unit of the oil system is functioning improperly.
Oil Temperature Indicator
During engine run-in in the test cell, engine oil temperature readings are taken at the oil inlet and outlet. From these readings, it can be determined if the engine heat transferred to the oil is low, normal, or excessive. This information is of extreme importance during the breaking-in process of large reciprocating engines. The oil temperature gauge line in the aircraft is connected at the oil inlet to the engine.
Three range markings are used on the oil temperature gauge. The green arc in Figure 4, on the dial, shows the minimum oil temperature permissible for ground operational checks or during flight. The green mark between 25 °F and below 245 °F shows the desired oil temperature for continuous engine operation. The red mark at 245 °F indicates the maximum permissible oil temperature.
Fuel Flow Meter
The fuel flow meter measures the amount of fuel delivered to the engine. During engine testing procedures, the fuel flow to the engine can be measured by three different methods: a direct flow meter, a pressure-based flow meter, or a turbine senor-based flow meter. The direct reading flow meter uses a series of calibrated tubes located in the control room. The tubes are of various sizes to indicate different volumes of fuel flow. Each tube contains a float that can be seen by the operator, and as the fuel flow through the tube varies, the float is either raised or lowered, indicating the amount of fuel flow. From these indications, the operator can determine whether an engine is operating at the correct fuel/air mixture for a given power setting. Reciprocating engines on light aircraft usually use a fuel pressure gauge that is also used for the flowmeter. This is because the fuel flow is proportional to the fuel pressure in this system. Fuel flow is measured normally in gallons per hour.
In most turbine aircraft installations, the fuel flow indicating system consists of a transmitter and an indicator for each engine. The fuel flow transmitter is conveniently mounted in the engine’s accessory section and measures the fuel flow between the engine-driven fuel pump and the fuel control device. The transmitter is an electrical device that contains a turbine that turns faster as the flow increases, which increases the electrical signal to the indicator. The fuel flow transmitter is connected electrically to the indicator located on the aircraft flight deck, or on the test cell operator’s panel. The reading on the indicator on turbine aircraft is calibrated to record the amount of fuel flow in pounds of fuel per hour.
Manifold Pressure Indicator
The preferred type of instrument for measuring the manifold pressure on reciprocating engines is a gauge that records the pressure as an absolute pressure reading. Absolute pressure takes into account the atmospheric pressure plus the pressure in the intake manifold. To read the manifold pressure of the engines, a specially designed manifold pressure gauge that indicates absolute manifold pressure in inches of mercury ("Hg) is used. The red line indicates the maximum manifold pressure permissible during takeoff.
The manifold pressure gauge range markings and indications vary with different kinds of engines and installations. Figure 5 illustrates the dial of a typical manifold pressure gauge and shows how the range markings are positioned. The green arc starts at 35 "Hg and continues to the 44 "Hg. The red line on the gauge, at 49 "Hg shows the manifold pressure recommended for takeoff. This pressure should not be exceeded.
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| Figure 5. Manifold pressure gauge |
Tachometer Indicator
The tachometer for reciprocating engines shows the engine crankshaft rpm. The system used for testing the engine is the same as the system in the aircraft installation. The tachometer, often referred to as TACH, is calibrated in hundreds with graduations at every 100-rpm interval. The dial shown in Figure 6 starts at 0 rpm and goes to 35 (3,500 rpm). The green arc indicates the rpm range within operation that is permissible. The red line indicates the maximum rpm permissible during takeoff; any rpm beyond this value is an overspeed condition.
Turbine engines use percent rpm indicators due to the high rpm that the engines generally operate. Each rotating assembly in an engine has its own percent rpm indicator. The 100 percent position on the indicator is the highest rpm the engine can operate. Red lines and green arcs operate the same as with reciprocating engines.
Cylinder Head Temperature Indicator
During the engine test procedures, the cylinder head temperatures of various cylinders on the reciprocating engine are normally tested. Thermocouples are connected to several cylinders and, by a selector switch, any cylinder head temperature can be indicated on the indicators. When installed in the aircraft, there is sometimes only one thermocouple lead and indicator for each engine installed in an aircraft.
Cylinder head temperatures are indicated by a gauge connected to a thermocouple attached to the cylinder, that tests show to be the hottest on an engine in a particular installation. The thermocouple may be placed in a special gasket located under a rear spark plug, or in a special well in the top or rear of the cylinder head.
The temperature recorded at either of these points is merely a reference or control temperature; but as long as it is kept within the prescribed limits, the temperatures inside the cylinder dome, exhaust valve, and piston is within a satisfactory range. Since the thermocouple is attached to only one cylinder, it can do no more than give evidence of general engine temperature. While normally it can be assumed that the remaining cylinder temperatures are lower, conditions such as detonation are not indicated unless they occur in the cylinder that has the thermocouple attached.
The cylinder head temperature gauge range marking is similar to that of the manifold pressure and tachometer indicator. The cylinder head temperature gauge is a dual gauge that incorporates two separate temperature scales. [Figure 7] The scales are calibrated in increments of 10°, with numerals at the 0°, 100°, 200°, and 300° graduations. The space between any two graduation marks represents 10 °C.
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| Figure 7. Cylinder head temperature gauge |
Torquemeter
Most torque systems use an oil pressure output from a torque valve to indicate actual engine power output at various power settings. The torquemeter indicates the amount of torque being produced at the propeller shaft. A helical gear moves back and forth as the torque on the propeller shaft varies. This gear, acting on a piston, positions a valve that meters the oil pressure proportionally to the torque being produced. A change in pressure from the valve that is connected to a transducer is then converted to an electrical signal and is transmitted to the flight deck. The torquemeter can read out in pounds-feet of torque, percent of horsepower, or horsepower. The earlier systems read out in psi, and the flight engineer converted this to the correct power setting. [Figure 8] Some systems use strain gauges to attach to the ring gear to provide an electrical signal directly to the readout.
Warning Systems
Many of the miscellaneous gauges and devices indicate only that a system is functioning or has failed to function. On some aircraft, a warning light illuminates when the fuel pressure or oil pressure is low.