Diesel Engine Troubleshooting

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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

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Diesel Engines Air Cleaner

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Older engines are often fitted with oil-bath filters that combine oil-wetted filtration with inertial separation (Fig. 9-1). Air enters at the top of the unit through the precleaner, or cyclone. Internal vanes cause the air stream to rotate, which tends to separate out the larger and heavier particulates. The air then passes through the central tube to the bottom of the canister, where it reverses direction. Some fraction of the remaining particulates fail to make the U-turn and end in the oil reservoir. Most of those that remain are trapped in the oiled mesh.

oil bath air cleaner Diesel Engines Air Cleaner

These devices depend, in great part, upon the velocity of the incoming air to centrifuge out heavier particles. At low engine speeds, the particles remain in the air stream, and filtration depends solely upon the oil-wetted mesh. Under ideal circumstances filtering efficiency is no more than 95% and, in practice, can be much less. If overfilled, subjected to high flow-rates, or tilted much off the horizontal, oil-bath cleaners bleed oil into the intake manifold. The oil can plug the aftercooler and raise exhaust temperatures enough to cause early turbocharger failure.

Figure 9-2 shows a replaceable paper-element filter of the type used on automobiles and light trucks. When used for industrial engines, the paper element is often combined with a prefilter and a centrifugal precleaner. At their most sophisticated, paper-element filters can have an efficiency of 99.99%. Efficiency improves when the filter is lightly impacted with dust. Engines operated in extremely dusty environments also benefit from an exhaust-powered ejector mounted upstream of the filter.

The filter should be mounted horizontally on the manifold to reduce the possibility of dirt entry when the element is changed. Filters that mount vertically, such as the one shown in Fig. 9-2, should include a semi-permanent safety element below the main filter. This element functions to trap dust when the filter is removed for servicing.

single stage air cleaner Diesel Engines Air Cleaner

All engines should have some sort of filter monitoring device that, if not supplied by the manufacturer, can be fabricated by the simple expedient of plumbing a vacuum gauge immediately downstream of the filter. When new, paper-element filters impose a pressure drop of about 6.0 in./H2O and the inlet ducting usually adds about 3.0 in./H2O. Engine performance falls off noticeably when the total system pressure drop exceeds 15 in./H2O.

Service the filter only when the air-restriction indicator trips, since each time the element is removed some dust enters the system. Before removing the element wipe off all dust from the inside of the filter housing. Do not blow out paper elements with compressed air: a tear in the element, so small that it may not be visible to the eye, exposes the engine to massive amounts of dust intrusion.

And before we leave the subject, it should be remarked that “high-performance” filters can provide marginal power increases by reducing pumping losses. But the reduction in pressure drop often comes at the cost of reduced filtration efficiency. Any air filter worthy of its name should have its efficiency certified by a third party under the protocols of the SAE air cleaner test code J726.

Rebuilt engines almost always have shorter lives than new engines. For years it was believed that factory inspection and assembly procedures gave new engines the edge. But evidence is accumulating that abrasives entering from leaks in the flexible tubing couplings downstream of the filter are the culprit (Fig. 9-3a). These couplings, which see boost temperatures of 300°–400°F (149°–204°C), harden with age and are rarely replaced. Nor is it possible to detect leaks by external inspection of the tubing and hose couplings. However, the presence of leaks will be revealed by streaks in the dust film that collects on inlet-tubing inner diameters (IDs). The origin of the streak marks the point of dirt entry.

Flexible couplings should be replaced periodically and as inexpensive insurance for rebuilt engines. Secure the coupling with SAE type F clamps—not worm-gear plumber’s clamps—that provide a 360° seal.

elastomer tubing Diesel Engines Air Cleaner

Written by Ed

February 23rd, 2011 at 1:42 am

Posted in Air Systems

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