EXHAUST AND CYLINDER TEMPERATURES

One of the most useful tools that the engine operator has for monitoring an engine’s performance is the thermocouple pyrometer. The principal use of this device is in the exhaust system (but it can also be used for other purposes) where it is used to measure the exhaust gas temperatures at each cylinder or the common temperature in the exhaust manifold. By comparing the exhaust gas temperatures of each cylinder, the operator can determine if the load is balanced throughout the engine.

The two types of pyrometers in use are the fixed installation and the portable hand-held instrument (figure 3-5). Both types use a thermocouple unit, such as the one shown in figure 3-5, installed in the exhaust manifold.

In its simplest form, a thermocouple consists of two dissimilar metal wires, usually iron and constantan (55% copper and 45% nickel) that are joined at both ends to form a continuous circuit. When the temperatures at the junctions are different an electrical current is produced and flows in the circuit. The greater the temperature difference, the greater the voltage produced.

One junction, known as the hot junction, is contained in a closed-end tube, installed in the exhaust manifold of each cylinder. The other junction called the cold junction, is exposed to room temperature, and is located at the pyrometer wire

Figure 3-4A.—MRC-for measuring compression and firing pressures (front).

Figure 3-4B.—MRC-for measuring compression and firing pressures (back).

Figure 3-5.—Pyrometers used in diesel exhaust systems.

terminals (see figure 3-6). A pyrometer (millivolt meter) measures the voltage produced and shows the results on a scale which has been calibrated to read in degrees of temperature. In fixed installation pyrometers, if the connecting wires are of the same type as those of the thermocouples, the thermocouple element becomes, in effect, extended to the pyrometer terminals and the temperature at the meter (now the cold junction) becomes the reference temperature. Then the selector switch can be rotated to any cylinder and contact can be made between the pyrometer and the hot junction. A reading can then be obtained for that particular point.

The hand-held pyrometer consists of an indicator and a pair of pointed prods attached to a sub-base and supported by a handle. To obtain a reading, the prod points are pressed against the exposed thermocouple terminals. The reading is taken from the scale. A point to remember is that the zero adjuster must be set to indicate room temperature rather than 0° temperature.

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