Internally driven superchargers were used almost exclusively in high horsepower radial reciprocating engines and are engine driven through a mechanical connection. Although their use is very limited, some are still used in cargo carriers and spray planes. Except for the construction and arrangement of the various types of superchargers, all induction systems with internally driven superchargers were very similar. Aircraft engines require the same air temperature control to produce good combustion in the engine cylinders. For example, the charge must be warm enough to ensure complete fuel vaporization and, thus, even distribution. At the same time, it must not be so hot that it reduces volumetric efficiency or causes detonation. All reciprocating engines must guard against intake air that is too hot. As with any type of supercharging (compressing intake air), the air gains heat as it is compressed. Sometimes this air requires cooling before it is routed to the engine’s intake ports. With these requirements, most induction systems that use internally driven superchargers must include pressure and temperature-sensing devices and the necessary units required to warm or cool the air.
The simple internally driven supercharger induction system is used to explain the location of units and the path of the air and fuel/air mixture. [Figure 3-10] Air enters the system through the ram air intake. The intake opening is located so that the air is forced into the induction system, giving a ram effect caused by the aircraft moving through the air. The air passes through ducts to the carburetor. The carburetor meters the fuel in proportion to the air and mixes the air with the correct amount of fuel.
The carburetor can be controlled from the cockpit to regulate the flow of air. In this way, the power output of the engine can be controlled. The manifold pressure gauge measures the pressure of the fuel/air mixture before it enters the cylinders. It is an indication of the performance that can be expected of the engine. The carburetor air temperature indicator measures either the temperature of the inlet air or of the fuel/air mixture. Either the air inlet or the mixture temperature indicator serves as a guide so that the temperature of the incoming charge may be kept within safe limits. If the temperature of the incoming air at the entrance to the carburetor scoop is 100 °F, there is approximately a 50 °F drop in temperature because of the partial vaporization of the fuel at the carburetor discharge nozzle. Partial vaporization takes place and the air temperature falls due to absorption of the heat by vaporization. The final vaporization takes place as the mixture enters the cylinders where higher temperatures exist. The fuel, as atomized into the airstream that flows in the induction system, is in a globular form. The problem, then, becomes one of uniformly breaking up and distributing the fuel, remaining in globular form to the various cylinders. On engines equipped with a large number of cylinders, the uniform distribution of the mixture becomes a greater problem, especially at high engine speeds when full advantage is taken of large air capacity.
One method, used mainly on radial reciprocating engines, of improving fuel distribution is shown in Figure 3-11. This device is known as a distribution impeller. The impeller is attached directly to the end of the rear shank of the crankshaft by bolts or studs. Since the impeller is attached to the end of the crankshaft and operates at the same speed, it does not materially boost or increase the pressure on the mixture flowing into the cylinders. But, the fuel remaining in the globular form is broken up into finer particles as it strikes the impeller, thereby coming in contact with more air. This creates a more homogeneous mixture with a consequent improvement in distribution to the various cylinders, especially on acceleration of the engine or when low temperatures prevail.
To obtain greater pressure of the fuel/air mixture within the cylinders, the diffuser or blower section contains a high speed impeller. Unlike the distribution impeller, which is connected directly to the crankshaft, the supercharger, or blower impeller, is driven through a gear train from the crankshaft.