Small Industrial and Marine Engine Turbochargers
Turbochargers designed for small industrial and marine engines, though larger than those of large truck engines, are similar in concept to the automotive turbochargers described above. Radial flow compressor and turbines are used, with an inboard bearing arrangement. Apart from the larger size, they are required to have greater durability and higher efficiency. Thus the designs are usually more complex and expensive.
An industrial engine turbocharger with radial compressor and turbine (Brown Boveri RR series)
Engines designed for these applications operate over a smaller speed range than truck engines, and at greater b.m.e.p., hence higher compressor pressure ratio. It follows that the flow range required from the compressor is smaller, hence vaned diffusers are used. Vaned turbine stator nozzles are also used. This results in higher design point compressor and turbine efficiency. A range of diffuser nozzle angles and turbine stator blade angles are available for matching a basic turbocharger to a particular engine.
The maximum size is governed by precision casting limitations for the radial flow turbine rotor, currently about 300 mm, although most units in this class are smaller. Turbine housings are simple volutes designed to deliver the flow evenly around the circumference of the stator nozzle ring, the latter generating the design gas flow angle at rotor inlet. The turbine housings are supplied in uncooled or water cooled form. Although cooling is undesirable thermodynamically, it is sometimes required for safety reasons due to the potential danger of hot exposed surfaces in small engine rooms.
Bearings are of similar design to those of automotive units, except that clearances, relative to turbocharger size, are smaller. Sometimes cooling air is bled from the compressor to the rear of the turbine hub and bearing area. This also helps prevent exhaust gas leaking down the back of the turbine wheel and reaching the bearings. These techniques help keep the hot end bearing cool, preventing serious oil oxidation deposits. Like the smaller units previously described, the lubricating oil system of the engine is also used for the turbocharger. Since bearing clearances are smaller, rotor movements are small and conventional labyrinth oil seals can be used at the compressor and turbine ends of the rotor shaft.
Turbochargers of this type are made in relatively small numbers, by batch production, hence their cost is high relative to automotive units.