Radial Vs. Axial-Flow Turbochargerby Rob Wagner
Two types of turbochargers power automobile engines and industrial-type power plants. The most common turbo engine is the radial, or centrifugal, turbocharger with air force-fed through a pump to create dynamic pressure to create high speed. An axial-flow turbocharger is equipped with impellers fastened to a shaft that help force the air in an axial direction through a pipe. Radial turbochargers employ a much older technology than the axial flow.
Turbochargers can help power four- and two-stroke engines. Turbochargers are two machines sharing a single shaft. A centrifugal, or radial, turbocharger features a gas turbine at the end of the shaft and receives its power through engine exhaust. At the opposite end of the shaft is a centrifugal compressor to pressurize the air intake of the engine. With an axial flow turbocharger, the shaft at the opposite end has impellers to pressurize the air intake. As the air pressure increases in either turbocharger, the pressure forces higher density air into the cylinder to boost output. For example, the engine’s torque doubles by increasing the air pressure by 15 psi. But the engine also consumes twice as much fuel.
Axial flow turbochargers are similar to gas turbines and are common in aircraft engines. Axial flow compresses air into increasingly smaller spaces using a series of impeller or axial compressor fans. Axial flow versions are better suited for aircraft because of their large size and heavy weight. Yet they are more efficient than centrifugal or radial turbochargers. Newer axial flow turbos need no intercooler. Axial flow turbos are gaining popularity in automobiles as they become smaller.
The radial or centrifugal turbocharger forces air into its intake system from an impeller using centrifugal force to push air out radially through a scroll pump, or centrifugal scroll, that expands in diameter to slow moving air but increases the pressure. Radial turbochargers share the same characteristics as superchargers, except superchargers use step-up gears or a belt to drive centrifugal force instead of the exhaust gases in the turbocharger. Centrifugal turbos produce low pressure at low rpm. Centrifugal turbochargers, like centrifugal superchargers, are compact and can free wheel when the engine requires no boost pressure.
Radial turbochargers install easily in vehicle engine compartments because they are compact. Mounting a radial turbocharger away from the air intake allows installing an intercooler with the turbo. It uses less engine power. Radial turbochargers also use fewer parts than superchargers since there is no drive belt. Yet axial flow turbochargers are more efficient because the exhaust gas is forced directly against the entire turbine wheel while the radial version’s exhaust flows from the side of the wheel and then around the perimeter of the wheel. Axial flow turbochargers have air pressure ratios up to 3-to-1 while radial versions produced since the 1950s have pressure ratios ranging from 1.5-to-1 to 1.7-to-1. Radial turbochargers typically cost less. Axial flow turbos are heavier and more expensive.
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