Basic principles of operation are discussed here with no attempt being made to give detailed maintenance instructions. For the specific information needed to inspect or maintain a particular installation or unit, consult the manufacturer’s instructions.
The basic requirement of a reciprocating fuel metering system is the same, regardless of the type of system used or the model engine on which the equipment is installed. It must meter fuel proportionately to air to establish the proper fuel/air mixture ratio for the engine at all speeds and altitudes at which the engine may be operated. In the fuel/air mixture curves shown in Figure 2-1, note that the basic best power and best economy fuel/air mixture requirements for reciprocating engines are approximately the same. The fuel metering system must atomize and distribute the fuel from the carburetor into the mass airflow. This must be accomplished so that the fuel/air charges going to all cylinders holds equal amounts of fuel. Each one of the engine’s cylinders should receive the same quantity of fuel/air mixture and at the same fuel/air ratio.
Due to the drop in atmospheric pressure as altitude is increased, the density of the air also decreases. A normally aspirated engine has a fixed amount or volume of air that it can draw in during the intake stroke, therefore less air is drawn into the engine as altitude increases. Less air tends to make carburetors run richer at altitude than at ground level, because of the decreased density of the airflow through the carburetor throat for a given volume of air. Thus, it is necessary that a mixture control be provided to lean the mixture and compensate for this natural enrichment. Some aircraft use carburetors in which the mixture control is operated manually. Other aircraft employ carburetors which automatically lean the carburetor mixture at altitude to maintain the proper fuel/air mixture.
The rich mixture requirements for an aircraft engine are established by running a power curve to determine the fuel/ air mixture for obtaining maximum usable power. This curve is plotted at 100 rpm intervals from idle speed to takeoff speed. [Figure 2-2] Since it is necessary in the power range to add fuel to the basic fuel/air mixture requirements to keep cylinder-head temperatures in a safe range, the fuel mixture must become gradually richer as powers above cruise are used. [Figure 2-1] In the power range, the engine runs on a much leaner mixture, as indicated in the curves. However, on the leaner mixture, cylinder-head temperature would exceed the maximum permissible temperatures and detonation would occur.
The best economy setting is established by running a series of curves through the cruise range, as shown in the graph in Figure 2-3, the low point (auto-lean) in the curve being the fuel/air mixture where the minimum fuel per horsepower is used. In this range the engine operates normally on slightly leaner mixtures and obviously operates on richer mixtures than the low-point mixture. If a mixture leaner than that specified for the engine is used, the leanest cylinder of the engine is apt to backfire because the slower burning rate of the lean mixture results in a continued burning in the cylinder when the next intake stroke starts.