Diesel Engine Troubleshooting

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Small Industrial and Marine Engine Turbochargers

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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.

industrial engine turbocharger Small Industrial and Marine Engine Turbochargers

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.

Written by Ed

October 18th, 2011 at 3:46 am

Diesel Engines Turbochargers Compressor

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The compressor impeller is an aluminium alloy (LM- 16-WP or C-355T61) investment casting, with a gravity die-cast aluminium housing (LM-27-M). The design of the impeller is a compromise between aerodynamic requirements, mechanical strength and foundry capabilities. To achieve high efficiency, and minimum flow blockage, very thin and sharp impeller vanes are required, thickening at the root (impeller hub) for stress reasons. It is common practice to use splitter blades that start part way through the inducer, in order to maintain good flow guidance near the impeller tip without excessive flow blockage at the eye. Until recently the impeller vanes have been purely radial so that blades were not subjected to bending stress. However most recent designs incorporate backswept blades at the impeller tip since this has been shown to give better flow control and reduces flow distortion transmitted through from impeller to diffuser.

Automotive turbocharger compressor impeller Diesel Engines Turbochargers Compressor

Automotive turbocharger compressor impeller, with splitter blades.

Typical design point pressure ratios fall in the range of 2 to 2.5:1, requiring impeller tip speeds of 300 to 350 m/s, hence small units of typically 0.08 m tip diameter rotate at 72 000 to 83 000 rev/min. In order to match wide differences in air flow requirements from one engine to another, a range of compressor impellers is available to fit the same turbocharger. These will be produced from one or two impeller castings, but with different tip widths and eye diameters generated by machining as shown in Figure 2.6, and matched with appropriate compressor housings. Usually up to ten or more alternative ‘trims’ are available but since the impeller tip diameter is unchanged and the hub diameter at the impeller eye is fixed by the shaft diameter, the flow passage variations alter the efficiency as well as flow characteristics of the impeller.

Diesel Engines Turbochargers Compressor Diesel Engines Turbochargers Compressor

The compressor can be a loose or slight interference fit on the shaft, clamped by the compressor end nut. Impellers of most turbochargers are balanced before assembly onto the shaft, so that components can be interchanged without rebalancing.

Vaneless diffusers are used on all except very high pressure ratio compressors. Relative to the alternative vaned designs, the vaneless diffuser is slightly less efficient due to a longer gas flow path and poorer flow guidance, but has a substantially wider range of high operating efficiency. This is important in truck and passenger car applications where engine speed, and therefore mass flow range, is large. The volute acts not merely as a collector of air leaving the diffuser, but is usually designed to achieve a small amount of additional diffusion in its delivery duct. Generally the volute slightly overhangs the diffuser  in order to reduce the overall diameter of the turbocharger. The volute and impeller casing are invariably formed as a single component.

Written by Ed

October 18th, 2011 at 3:28 am

Automotive Diesel Engines Turbochargers

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Turbochargers in this class are used for passenger car diesel engines rated at 45 kW upwards to larger special heavy truck and construction vehicles rated at up to 600 kW. The most important design factors are cost, reliability and performance. To keep cost low, the design must be simple, hence a single stage radial flow compressor, and a radial flow turbine are mounted on a common shaft with an inboard bearing system. This arrangement simplifies the design of inlet and exhaust casing and reduces the total weight of the turbocharger.

Automotive Diesel Engines Turbochargers Automotive Diesel Engines Turbochargers

An automotive diesel engine turbocharger

Written by Ed

October 18th, 2011 at 3:21 am

Posted in Diesel Engine Theory

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Diesel Engines Turbochargers

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A turbocharger is an exhaust-powered supercharger, that unlike conventional superchargers, has no mechanical connection to the engine (Figs. 9-3b and 9-4). The exhaust stream, impinging against the turbine (or “hot”) wheel, provides the energy to turn the compressor wheel. For reasons that have to do with the strength of materials, turbo boost is usually limited to 10 or 12 psi. This is enough to increase engine output by 30—40%.

6068T turbocharged Diesel Engines Turbochargers

Turbocharging represents the easiest, least expensive way to enhance performance. It is also something of a “green” technology, because the energy for compression would otherwise be wasted as exhaust heat and noise (Fig. 9-5). On the other hand, the interface between sophisticated turbo machinery, turning at speeds as great as 140,000 rpm and at temperatures in excess of 1000°F, and the internal combustion engine is not seamless.

turbocharger Diesel Engines Turbochargers

Unless steps are taken to counteract the tendency, turbochargers develop maximum boost at high engine speeds and loads. The turbine wheel draws energy from exhaust gas velocity and heat, qualities that increase with piston speed and load. The compressor section behaves like other centrifugal pumps, in that pumping efficiency is a function of impeller speed. At low speeds, the clearance between the rim of the impeller and the housing shunts a large fraction of the output. At very high rotational speeds, air takes on the characteristics of a viscous liquid and pumping efficiency approaches 100%. In its primitive form, a turbocharger acts like the apprentice helper, who loafs most of the day and, when things get busy, becomes too enthusiastic.

turbocharger heat balance Diesel Engines Turbochargers

Another innate, but not necessarily uncorrectable, characteristic of turbocharged engines is the lag, or flat spot, felt during snap acceleration. Perceptible time is required to overcome the inertia of the rotating mass. By the same token, the wheels continue to coast for a few seconds after the engine stops.

Written by Ed

February 23rd, 2011 at 1:53 am

Posted in Air Systems

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