40,000 feet 24,000 feet 10.92 psi − 2.72 psi = 8.20 psid Ambient pressure—2.72 psi Ambient pressure—5.70 psi 10.92 psi − 5.70 psi = 5.22 psid Cabin altitude—8,000 feet (10.92 psi) Sea level Figure 16-41.  Differential pressure (psid) is calculated by subtracting the ambient air pressure from the cabin air pressure. Reciprocating Engine Aircraft  16-25 Figure 16-42.  A reciprocating engine supercharger can be used as  a source of pressurization if it is upstream of carburetion. enabling high altitude flight with the benefits of low drag and  weather avoidance in relative comfort and without the use of  supplemental oxygen.  [Figures 16-44  and  16-45] Both superchargers and turbochargers are oil lubricated.  The supercharger is part of the fuel intake system and the  turbocharger is part of the exhaust system. As such, there is  a risk of contamination of cabin air from oil, fuel, or exhaust  fumes should a malfunction occur, a shortcoming of these  pressurization sources.  There are three typical sources of air used to pressurize  reciprocating aircraft: supercharger, turbocharger, and engine- driven compressor. Superchargers and turbochargers are  installed on reciprocating engines to permit better performance  at high altitude by increasing the quantity and pressure of the  air in the induction system. Some of the air produced by each  of these can be routed into the cabin to pressurize it. A supercharger is mechanically driven by the engine. Despite  engine performance increases due to higher induction  system pressure, some of the engine output is utilized by  the supercharger. Furthermore, superchargers have limited  capability to increase engine performance. If supplying both  the intake and the cabin with air, the engine performance  ceiling is lower than if the aircraft were not pressurized.  Superchargers must be located upstream of the fuel delivery  to be used for pressurization. They are found on older  reciprocating engine aircraft, including those with radial  engines.  [Figures 16-42  and  16-43]  Turbochargers, sometimes known as turbosuperchargers, are  driven by engine exhaust gases. They are the most common  source of pressurization on modern reciprocating engine  aircraft. The turbocharger impeller shaft extends through  the bearing housing to support a compression impeller  in a separate housing. By using some of the turbocharger  compressed air for cabin pressurization, less is available  for the intake charge, resulting in lower overall engine  performance. Nonetheless, the otherwise wasted exhaust gases are put to work in the turbocharger compressor,  Compressor Air intake Engine gears To engine carburetor and intake To cabin for pressurization