Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 18. Fuel Mixture Indicators

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION XVIll - FUEL MIXTURE INDICATORS

81. Purpose and use.-a. The fuel mixture indicator (exhaust gas analyzer) is an instrument that indicates fuel-air ratio of the mixture entering the engine.

b. The instrument is used as a guide to the pilot in setting the mixture control. Heretofore with fixed pitch propellers a method was used for setting the mixture control which was based on the falling off of engine rpm as the mixture was leaned or enriched. With the use of constant. speed propellers, another method of indicating the mixture became necessary since the rpm is not affected even when the mixture is lean enough to damage the engine or rich enough to cause excessive fuel consumption.

82. Description.-a. Fuel mixture indicators are of two general types, those for single-engine airplanes and those for dual-engine airplanes. Although the construction features of the two are different, their principle of operation is exactly the same.

(1) Analysis cell.  (2) Indicator unit.
FIGURE: 36.-Fuel mixture indicator for a single engine (Breeze).

(1) The fuel mixture indicator for single-engine airplanes (fig. 36) consists of an analysis cell and an indicator unit. The indicator unit includes the galvanometer for indicating the fuel-air ratio, a ballast tube for maintaining a substantially constant current through the bridge, a lamp for proper illumination of the scale together with a lamp series resistor, and the necessary coils and resistances, plug and sockets, etc., for making connections to the other units of the assembly. The analysis cell consists of a filter for removing the carbon particles from the exhaust, filled with replaceable stainless steel wool, which is connected by a small opening to the analysis cell proper. In this are four small cells across each of which is stretched a fine platinum resistance wire. Two of these cells are sealed with moistened air (moisture provided by a removable wick) and the other two are connected through the filter to the exhaust gases. The four cells form the legs of a resistance bridge (Wheatstone bridge) across which a galvanometer type indicator is connected. Figure 37 is a wiring diagram of the fuel mixture indicator for the single-engine airplane showing the internal connections of the units. The two legs of the Wheatstone bridge in the analysis cell that are exposed to the exhaust gases are marked "EXP." Also shown are the connections to the various series and shunt resistances, galvanometer, ballast tube, and source of current supply.

FIGURE 37.-Fuel mixture indicator wiring diagram for a single engine (Cambridge).

(2) The component parts of the dual-engine fuel mixture indicator system are shown in figure 38 and consist of two analysis cells, one for each engine, an indicator unit, and a junction box. The analysis cells for this installation are similar to that described for the single-engine airplane and includes the brass block or resistance thermometers for the analysis of the exhaust gas, necessary resistors for completing the bridge circuit, and filter chamber for filtering the carbon particles from the exhaust. The indicator unit contains two galvanometers to each of which is attached a pointer. Both pointers move over a single dial having an upper and a lower scale for indicating the fuel-air ratio of both engines. The junction box unit includes the ballast tube, a switch, a rheostat, two resistance units, and a panel for connections.

b. Multiple conductor shielded cable with conventional type terminals is used to connect the various units. A current supply of from  11.5 to 15 volts from the battery circuit in the airplane is used to energize the electrical system of the unit. This is reduced to 4.1 volts by the ballast tube and variable resistances in the circuit because the indication could be accurate only on a constant voltage.

(1) Analysis cell (left engine).    (2) Analysis cell (right engine).

(3) Indicator unit.

(4) Junction box.
FIGURE 38.-Fuel mixture indicator for dual engines (Cambridge).

83. Operation. a. The engine exhaust is composed of several gases, carbon dioxide (CO₂), carbon monoxide (CO), oxygen (0₂), hydrogen (H₂), and nitrogen (N₂). The proportion of these gases in the exhaust varies and depends upon the fuel-air ratio of the fuel mixture supplied to the combustion chambers by the carburetor. This relationship is definite; consequently, an instrument which will respond to changes in the proportion of certain of the gases can be used readily to indicate the fuel-air ratio of the fuel mixture.

b. A rich mixture (more fuel and less air than normal) results in an increase in H₂., and a decrease in CO₂ and a lean mixture reverses the proportion. H₂ has a thermal conductivity about six times greater than air, while that of CO₂ is approximately one-half that of air. It is on this difference in thermal conductivity of the two gases and their varying proportions in the exhaust that the operation of the instrument is based. The proportion of the other gases varies also but their thermal conductivity is considered as being about equal to that of air as far as it concerns the operation of the instrument.

c. The sampling tubes continuously collect and dispose of gas samples. The gas is collected and brought into the analysis cell block. The cell block contains the Wheatstone bridge which is in electrical balance at 0° C. with no exhaust gases present. The two diametrically opposed branches of the Wheatstone bridge are placed in the path of the sample gas which must flow around the two exposed branches or resistors. The other two diametrically opposed branches of the Wheatstone bridge are sealed in a chamber exposed to moisture saturated air. Exhaust gases are completely sealed off from these two branches or resistors of the Wheatstone bridge.

d. The voltage energizing the Wheatstone bridge is reduced to 4.1 volts by means of a ballast tube. This is merely a resistor sealed in a hydrogen filled glass bulb which reduces to a minimum the influence of outside temperature, pressure, and altitude on the resistor. Any variation in current to the fuel mixture indicator instrument is reduced to such small proportion by the ballast tube as to be negligible. The current stabilizes the temperature of the branches of the Wheat-stone bridge (or resistors) at 260° F. As the exhaust gases flow around the two exposed cells of the resistors, heat will be carried off by the sample of exhaust gas passing the cell. The amount of heat carried off depends upon the proportion of H₂ and CO₂ in the volume of the gas.

e. A greater proportion of the volume of H₂ gas carries the heat away from the cells or resistors more rapidly. This cools and lessens the resistance of the resistor and causes the indicator needle to indicate a richer mixture. The movement of the needle is directly in proportion to the increase of H₂ gas. A greater proportion of CO₂ gas and a decrease of H₂ gas around the cells or resistors will carry off heat less rapidly. The increased temperature of the resistor results in increased resistance causing the needle to deflect to the lean indication.

f. There is one important exception to the normal operation as described. If the mixture is leaned to a point where detonation (explosive combustion of the mixture within the cylinder) occurs, high proportions of H₂ are liberated causing the pointer to swing to the rich side. If this indication is misunderstood and the mixture leaned further, the extent of the detonation will increase and a still richer indication will be shown. If properly understood, this characteristic can be used successfully as a warning of detonation which is quite dangerous and if allowed to continue can result in total engine failure. Slight detonation is indicated by a fluctuation of the pointer.

g. The instrument is automatically in operation when engine is started. When making adjustments of the mixture, approximately 1 minute must be allowed before reading the indicator, as this period of time is required before a change in the carbon dioxide and hydrogen content of the exhaust gases will be indicated. The instrument will accurately indicate values of fuel-air ratios from 0.068 to 0.110, provided there is no detonation present. As the mixture is leaned out if the indicating needle does not show a leaner mixture or backs up the scale toward the rich side, detonation has very likely been encountered and the mixture should be enriched.

h. It is not expected that the fuel-air ratio indicator will supplant the engine cylinder thermometer in the avoidance of engine overheating. As there may be conditions which would cause overheating with normal indications of the fuel-air ratio indicator, both instruments should be watched and, if necessary, the mixture enriched from the recommended settings in order to prevent overheating. If the pointer stays at the center of the scale near the "A" (air-point) position, it may be that the sampling line from the exhaust stack to the analyzing cell is broken.

i. When carburetor air heaters are used, the mixture indication very definitely goes rich and if prolonged running is to be done while using the air heater, the mixture should be reset after applying the heat. Likewise when heat is shut off the pointer will definitely show lean. Therefore when shutting off the carburetor air heater always enrich the mixture first.

84. Installation.-a. General information on installation of instruments is given in section III and specific installation instructions can be obtained from installation drawings for the particular airplane on which this equipment is to be installed. A typical dual-engine installation is shown in figure 39.

FIGURE 39.-Dual-engine fuel mixture indicator installation (Cambridge)

h. The indicator is installed on the panel from the rear. It may be easily removed for replacement when necessary. Each engine requires two identical sampling tubes and one analysis cell assembly. The inlet sampling tube must be installed as far downstream from the last port as will insure an average sample from all the cylinders. The opening faces so as best to catch the exhaust gases. The outlet sample tube must be from 8 to 12 inches downstream from the inlet sampling tube. The analysis cell must be mounted in the nacelle (preferably on the fire wall) upon a surface where the vibrations are at a minimum and the temperature range is between 32° to 125° F. Below 32° F. will freeze the condensate.

c. To avoid accumulation of condensed gases in the analysis cell, the inlet tubing must run upward for a distance of 12 to 18 inches above the analysis cell and then be brought downward to the cell. Likewise the outlet tubing must have a constant downward slope, having no water trap.

85. Maintenance.-a. All general points on maintenance given in section II are applicable to this instrument.

b. Specific items on inspection and maintenance are as follows:

(1) To adjust mechanical zero, set the master switch on the air-plane in the "off" position and note the alinement of the pointer on the indicator; if it is either to the right or left of the "A" point marked on the scale, turn the small screw which is located on the face of the instrument until the pointer is in alinement. Vibrate the instrument and repeat until the alinement becomes stable.

(2) To check and adjust electrical zero requires a balanced humidity condition in the wick and filter chambers. This condition is accomplished by removing the filter material and temporarily replacing it with a water-saturated cloth. The wick must also be removed and saturated with clean water to insure perfectly balanced conditions. The current is then turned on and after 20 minutes, the position of the pointer on the indicator is checked. It should aline with the "A" point on the scale. If the alinement is not correct, it is an indication of unbalanced resistance. This is corrected by loosening the hex nut on top of the cell and with a screw driver turning the core a slight amount until the pointer on the indicator is in perfect alinement with the "A" point on the scale. The indicator should be vibrated lightly during the actual adjusting operation and extreme care must be exercised when tightening the nut after completing the adjustment not to move the core again. This procedure must be strictly followed step by step, otherwise there is a possibility of doing more damage than good.

(3) To clean the sampling lines and nipples, break the connections at the cell, insert a discarded tachometer shaft. into each line, first one then the other, and rotate it several times. It may be necessary to withdraw and repeat the insertion several times in order to dislodge the condensate and carbon residue.

(4) To service the filter which is located in a tube extension at the bottom of each cell assembly, remove the safety wire and the plug. Then take the filter out and wash in gasoline to remove the oil and carbon residue. Then wash it in clean water, shake thoroughly, and replace it in the tube. If the material is corroded and decomposed to any extent, replace it with a new one.