Development of heat-resistant materials, and high speed bearings for aircraft gas turbines has has resulted in marked improvements in relatively small turbine-compressor sets driven by the exhaust energy of engines. These are known as exhaust turbo-chargers. Two types are available:
Axial flow turbines designed to make full use of the impulsive energy available at the start of the exhaust period, coupled to a centrifugal compressor. These are used on the larger engines which to assist the cooling of both the engine and turbine are usually arranged to have a large valve overlap period 120° - 160°, to avoid interference of the exhaust impulses from the different cylinders the turbine casing is provided with a number of separate entries which connect up with cylinders whose exhaust pulses do not interfere.
Radial flow turbines largely used on the smaller engine sizes, being relatively small and light, this type of turbo-charger can be directly mounted on the exhaust manifolds. As the exhaust gases must flow in radially against centrifugal force mean exhaust pressures are somewhat higher than with the axial flow type. It is not usual to alter the normally aspirated valve timings with this kind of turbo-charging on small high speed truck engines. Overlap can be used and scavenge is obtained on larger engines. A wide variety of sizes and nozzle rings are available to permit the matching of engine and turbo-charger characteristics.