As more fuel is burned in an engine, the hotter the exhaust gases will become. The device that measures the temperature of these gases is called a PYROMETER. By comparing the

Figure 6-14.-Exhaust manifold of a turbocharged engine.

Figure 6-15.-Pyrometers used in diesel engine exhaust systems.

exhaust gas temperature of each cylinder, the operator can determine if the load is balanced throughout the engine.

There are two types of pyrometers that measure exhaust temperature readings-the fixed installation and the portable hand-recording instrument (fig. 6-15). Both types use a thermo-couple unit, such as the one shown in figure 6-16, installed in the exhaust manifold.

Pyrometers of the fixed installation type (view A of fig. 6-15) have a thermostatically operated control spring which makes the required tem-perature corrections automatically. A selector switch is used to show the readings of one cylinder at a time. The portable hand pyrometer (view B of fig. 6-15) has a zero adjuster which must be set by hand and placed in contact temporarily with the terminals of each thermocouple.

The indicating unit of the pyrometer is calibrated to give a direct reading of temperature. However, the pyrometer actually measures the difference between the electric current produced by heat acting on dissimilar metals in the thermocouple hot junction and cold junction. The metals most commonly used in the thermo-couple are iron and constantin, which are covered by an insulator. The thermocouple is placed so that the hot junction is in contact with the exhaust gases.

When the hot junction is heated by the exhaust gases, a small voltage develops in proportion to the temperature. This voltage is transferred by the wires to the pyrometer indicator. The pyrometer indicator is actually a sensitive voltmeter that has a scale that is graduated to show temperatures between the hot junction and the cold junction.

Figure 6-16.-Sectional view of a thermocouple.

Some variations in exhaust temperatures are normal and are to be expected when engines are operating at other than full power. The amount that exhaust temperatures may differ from cylinder to cylinder will depend on the type of engine. Refer to the appropriate manufacturers technical manual for the allowable limits for your diesel.