Large Industrial and Marine Diesel Engine Turbochargers
These turbochargers are characterized by having axial flow, single stage, turbines and are fitted to the majority of large industrial and marine engines, both four- and two-stroke. The duty cycles of these engines are more arduous than that of automotive engines and they tend to spend much more of their operating time at high load. Furthermore the consequences of failure are more serious, particularly on a marine engine. As a result, although every attempt is made to keep the designs simple, the primary objectives are a very high level of reliability, high efficiency and versatility to cover a great range of engine types and sizes at reasonable cost. However, design variations from one manufacturer to another are greater than is the case with smaller turbochargers.
Figure 2.10 is a cross-section of a typical large turbocharger, with a radial flow compressor and axial flow turbine. The compressor impeller is made in two separate parts, the inducer and main part of the impeller. The inducer is usually machined from a steel casting or an aluminium forging, and is splined or keyed to the shaft. The impeller is machined from an aluminium forging except for very high pressure ratio requirements when titanium is used due to its superior high temperature properties. The advantage of the two piece compressor is ease of machining, but an additional benefit is some impeller vane damping provided by friction at the inducer-impeller contact surfaces. Compressor diffusers are vaned for high efficiency.
The turbine disc is machined either as an integral part of the shaft or is shrunk on to the shaft. The rotor blades may be cast, forged or machined from a high temperature creep-resistant steel such as Nimonic 8OA or 90. Welded joints or ‘fir-tree’ roots are used to fix them to the disc, the latter design being more common on high pressure units since they provide a degree of vibration damping and allow a wider selection of blade and disc materials to be considered. Additional vibration damping can be provided by wire lacing the blades. The turbocharger manufacturer will offer a range of ‘trims’ or flow capacities with each basic design of turbocharger by varying blade (stator and rotor) height and stator blade angle.
A disadvantage of the axial flow turbine is that it complicates the design of the gas inlet and outlet. The gas inflow section is particularly important hence this is usually located on the end, allowing generous curvature in the inlet ducts to the stator blades for minimum flow distortion and loss. The turbine exit duct acts merely as a collector, hence a compact design can be used, minimizing turbocharger length. However, a recent trend is to utilize some exhaust diffusion to increase turbine expansion ratio and power output.
Most of the larger turbochargers in this class have outboard rolling element bearings (i.e. outside the compressor and turbine, Figure 2.10), with their own oil supply, and resilient mountings to prevent brinelling. The advantages of this are stable shaft mounting and low dynamic loads due to the wide bearing spacing, small bearing diameter, low rolling resistance and good access for bearing maintenance. The use of separate oil supplies for the turbocharger and engine enables a lower viscosity oil to be used, further reducing bearing friction. Low pressure ratio turbochargers use simple rotating steel discs, partially immersed in the oil, to pick up and deliver the oil to the bearings, but with higher bearing loads and speeds, gear pumps are used to spray oil on the bearings. Plain or sleeve bearings are sometimes available as an option and are preferred for durability although their frictional losses are greater.
Turbocharger design is simpler with inboard bearings since this gives greater freedom to design low loss intake ducts. Fewer components are required and the turbocharger is shorter, lighter and cheaper as a result (Figure 2.11). The disadvantage is a less stable bearing system and higher bearing loads. Fully floating sleeve and multi-lobe plain bearings are used, with well damped mountings for stability; the rotors must still be carefully balanced. Relative to rolling element bearings, higher oil pressure and greater oil flow rates are required and the combination of large diameter and width means that frictional losses are greater.
With either bearing system, the turbine outlet casing is the main structure to which the other components are bolted, and incorporates mountings to the engine. The casing is usually water cooled. Bolted to it is the water cooled turbine inlet casing, incorporating the bearing housing (for outboard bearings) and its oil reservoir. Single, two, three and four entry turbine inlets are available, manufactured from high grade cast iron. Between turbine inlet and outlet casings, provision is made for mounting the turbine stator nozzle ring. The compressor inlet and outlet casings are aluminium alloy castings.
The compressor inlet casing incorporates webs to support the bearing housing if outboard bearings are used. These webs must be carefully designed to be far enough away from the impeller to avoid impeller vane excitation. The casing also houses a combined air filter and silencer on most larger turbochargers (Figure 2.10). Sound waves originating at the compressor intake are reflected and reduced in intensity by baffles lined with sound absorbing material.