Diesel Engine Troubleshooting

Archive for the ‘Air Systems’ Category

Variable Geometry Turbine

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Variable geometry turbine (VGT) turbochargers include a throttling mechanism on the exhaust inlet to generate boost at engine speeds just above idle. With boost comes torque, which results in low-rpm flexibility unmatched since the days of steam. But torque is a side-effect—VGT is an emissions-control expedient, intended to reduce NOx levels in the exhaust.

Until 2004, when EPA Tier 2 regulations went into effect, EGR was required only at high speeds, when exhaust-gas pressure exceeded manifold pressure. All that was needed was a connection between the exhaust pipe and the intake manifold, and exhaust flow would take care of itself. Under the present rules, EGR must be available across the rpm band.

A conventional turbocharger, sized for maximum power output, cannot provide the low-speed boost necessary for exhaust gases to overcome manifold pressure. BMW uses two conventional turbochargers—a small unit that develops boost just off idle and a larger turbocharger for maximum power production. Other manufacturers get around the manifold-pressure problem by throttling the intake with a butterfly valve. At low speeds, the valve closes, creating a vacuum behind it. But this expedient costs power.

The VGR generates boost at all speeds. Figure 9-11 illustrates the construction of adjustable-vane type used on motor vehicles. Pivoted vanes (3), controlled by movement of the adjuster ring (4), surround the turbine housing. At low speeds, the vanes swing closed like a Venetian blind. This flow obstruction increases the velocity of the exhaust gases striking the turbine wheel. The vanes also direct the gas stream toward the outer edge of the wheel for improved leverage. Exhaust-gas volume and energy increase with engine speed. At high speeds, the control vanes swing open to take advantage of the energy now available. In their full-open position, control vanes also function as a boost limiter, to prevent turbocharger overspeeding.

bosch VGT Variable Geometry Turbine

Variable geometry turbine systems are closely monitored for boost, exhaust back pressure, and compressor inlet and outlet temperature, so that failure generates one or more trouble codes. European automobiles control vane pitch and turbo boost with a stepper motor. U.S. heavy truck VGTs are controlled by lube-oil or air pressure acting on a spring-loaded diaphragm. A rod transfers diaphragm movement to the turbo adjustment ring. Replacement actuators include instructions for adjustment, an operation that requires a scanner capable of cycling the unit and a precisely regulated source of pressure. Fig. 9-12 shows a Detroit Diesel air-operated VGT actuator.

air operated VGT Variable Geometry Turbine

Written by Ed

February 23rd, 2011 at 2:39 am

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Diesel Engines Turbochargers Overhaul

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Only the most general instructions can be provided here, because construction details, wear limits, and torque specifications vary between make and model. It should also be remarked that American mechanics do not, as a rule, attempt turbocharger repairs. The defective unit is simply exchanged for another one.

However, there are no mysteries or secret rites associated with turbocharger work. Armed with the necessary documentation and the one indispensable special tool—a turbine-shaft holding fixture—any mechanic can replace the bearings and seals, which is what the usual field overhaul amounts to.

The holding fixture secures the integral turbine wheel and shaft during removal and installation of the compressor nut. Figure 9-9 illustrates plans for one such fixture, used on several Detroit Diesel applications. Dimensions vary, of course, with turbocharger make and model.

turbine holding fixture Diesel Engines Turbochargers Overhaul

Factory-supplied documentation should alert you to any peculiarities of the instrument, such as a left-handed compressor-nut thread or the need to heat the compressor wheel prior for removal. Special precautions include the following:

• Do not use a wire wheel or any other sort of metallic tool on turbo wheels or shaft. Remove carbon deposits with a soft plastic scraper and one of the various solvents sold for this purpose. Light scratches left by wheel contact on the turbo housings can be polished out with an abrasive.

• Do not expose the turbine shaft to bending forces of any magnitude. Pull the shaft straight out of its bearings. Use a double u-joint between the socket and the wrench when removing and torquing the compressor-wheel nut.

• Do send out the rotating assembly for shaft alignment and balancing (Fig. 9-10).

• Do exercise extreme cleanliness during all phases of the operation.

• Do prelubricate the bearings with motor oil and prefill the bearing housing prior to starting the engine.

• Do make certain that all fasteners and small tools are accounted for and not lurking within the turbocharger or its plumbing.

heins balancing Diesel Engines Turbochargers Overhaul

Written by Ed

February 23rd, 2011 at 2:35 am

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Diesel Engines Turbochargers Inspection

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The seven-step inspection procedure outlined here was adapted, with modifications, from material supplied by John Deere.

1. Turbo housing Before disconnecting the oil lines, examine external surfaces of the housing for oil leaks, which would almost certainly mean turbo seal failure.

2. Compressor housing inlet and wheel Inspect the compressor wheel for erosion and impact damage. Erosion comes about because of dust intrusion; impact damage is prima facie evidence of negligence. Carefully examine the housing ID and compressor blade tips for evidence of rubbing contact, which means bearing failure.

3. Compressor housing outlet Check the compressor outlet for dirt, oil, and carbon accumulations. Dirt points to a filtration failure; oil suggests seal failure, although other possibilities exist, such as clogged turbo-oil return line or crankcase breather. Carbon on the compressor wheel might suggest some sort of combustion abnormality, but the phenomenon is also seen on healthy engines. I can only speculate about the cause.

4. Turbine housing inlet Inspect the inlet ports for oil, heavy carbon deposits, and erosion. Any of these symptoms suggest an engine malfunction.

5. Turbine housing outlet and wheel Examine the blades for impact damage. Look for evidence of rubbing contact between the turbine wheel and the housing, which would indicate bearing failure.

6. Oil return port The shaft is visible on most turbochargers from the oil return port. Excessive bluing or coking suggests lubrication failure, quite possibly caused by hot shutdowns.

7. Bearing play measurements Experienced mechanics determine bearing condition by feel, but use of a dial indicator gives more reliable results. Note that measurement of radial, or side-to-side, bearing clearance involves moving the shaft from one travel extreme to another, 180° away (Fig. 9-8A). Hold the shaft level during this operation, because a rocking motion would muddy the results. Axial motion is measured as travel between shaft thrust faces (Fig. 9-8B). In very general terms, subject to correction by factory data for the unit in question, we would be comfortable with 0.002 in. radial and 0.003 in. axial play. Note that Schwitzer and small foreign types tend to be set up tighter.

dial indicator Diesel Engines Turbochargers Inspection

Written by Ed

February 23rd, 2011 at 2:29 am

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Diesel Engines Turbochargers Crankcase Ventilation

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This system removes combustion residues and, in the process, subjects the crankcase to a slight vacuum. In normal operation, fresh air enters through the breather filter and crankcase vapors discharge to the atmosphere (prepollution engines) or to the intake side of the turbo compressor. The latter arrangement, known as positive crankcase ventilation (PCV) is the norm.

Under severe load, blowby gases accumulate faster than they can be vented and escape through the breather filter. These flow reversals, which occur more frequently in turbocharged engines, tend to clog the filter. Restrictions at the filter allow corrosive gases to linger in the crankcase. A partially functional filter can also pressurize the crankcase under the severe blowby conditions that accompany heavy loads. Oil seals and gaskets leak as a consequence.

Under light loads, reduced flow through the breather depressurizes the crankcase. Low crankcase pressures encourage oil to migrate into the turbocharger and collect in the aftercooler on engines with positive crankcase ventilation. Because oil is a fairly good thermal insulator, the efficiency of the aftercooler suffers. Tests of a Detroit Diesel engine, conducted by Diesel Research, Inc., established that oil migration resulting from a 40% efficient breather cost 6% of engine output. The engine in question had 4000 hours on the clock.

Written by Ed

February 23rd, 2011 at 2:24 am

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Diesel Engines Turbochargers Lubrication

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Elevated combustion pressure contaminates the oil with blowby gases and promotes oxidation by raising crankcase oil temperature. That fraction of the oil diverted to the turbocharger can undergo 80°F temperature rise in its passage over the bushings. Change lube oil and filter(s) frequently.

The cost and disposal problems associated with filters make reusable filters attractive for fleet operators. Racor, a division of Parker Hannifin, manufactures a series of liquid filters with washable, stainless-steel elements and a TattleTale light that alerts the operator when the filter needs to be cleaned.

Frequently inspect the turbocharger and its oiling circuitry for evidence of leaks that, if neglected, can draw down the crankcase.

Written by Ed

February 23rd, 2011 at 2:19 am

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Diesel Engines Turbochargers Maintenance

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The first priority is to obtain actual performance data, particularly with reference to turbocharger behavior. The full story would require a dynamometer to extract, but one can gain useful insight by observing the changes in manifold pressure under working loads that, at some point in the test, should be great enough to cause the wastegate to open. The rise in oil temperature and variations in crankcase pressure supply additional parts of the picture.

Do not operate a turbocharged engine unless the air cleaner (or spark arrestor) is in place and the intake-side ducting secure. The compressor acts as a vacuum cleaner, drawing in foreign, objects which will severely damage the unit and might cause it to explode. The troubleshooting chart makes reference to coast-down speed. If you feel it is necessary to observe compressor rotation, cover the turbocharger inlet with a screen to at least keep fingers and other large objects out of the mechanism.

When dismantling a turbocharger and related hardware, make a careful tally of all fasteners, lockwashers, and small parts removed. Be absolutely certain that all are accounted for before starting the engine. Immediately shut down the engine if the turbocharger makes unusual noise or vibrates.

Turbocharging (and supercharging generally) put severe stress on lubrication, air inlet, crankcase ventilation, and exhaust systems.

Written by Ed

February 23rd, 2011 at 2:16 am

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Diesel Engines Turbochargers Aftercoolers

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Compressing air raises its temperature, which reduces change density and tends to defeat the purpose of supercharging. Sophisticated turbocharger installations include a heat exchanger, or aftercooler, between the compressor outlet and intake manifold. The cooling medium can be air, engine coolant, or—for marine applications—water. The plate and tube seawater-fed aftercooler used with the Yanmar 4LH-HTE boosts output to 135 hp, or 30 hp more than an identical engine without aftercooling.

Air-cooled heat exchangers work most efficiently when mounted in front of the radiator. Engine coolant should be taken off at the pump discharge and returned to some point low on the water jacket.

Air-cooled units require little attention, other than an occasional dust off. Liquid cooled heat exchangers should be cleaned as needed to remove scale and fouling, and periodically tested by blowing low-pressure (25-psi maximum) air through the tubes. Water intrusion into the intake tract can be expensive.

Written by Ed

February 23rd, 2011 at 2:13 am

Diesel Engines Turbochargers Wastegate

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All turbocharger installations incorporate some form of boost limitation; otherwise, boost would rise with load until the engine destroyed itself.

Turbocharger geometry, sometimes abetted by inlet restrictions and designed-in exhaust backpressure, limit boost on constant-speed engines. Automotive and light truck engines have surplus turbocharging capability for boost at part throttle. These applications employ a wastegate—a kind of flap valve—that automatically opens at a preset level of boost to shunt exhaust gas around the turbine.

Most wastegates are controlled by a diaphragm, open to the atmosphere on one side and to manifold pressure on the other (Fig. 9-7). The Ford unit shown also incorporates a relief valve. Normally the wastegate opens at 10.7 psi; should it fail to do so, the relief valve opens at 14 psi and, because it is quite noisy, alerts the driver to the overboost condition.

wastegate Diesel Engines Turbochargers Wastegate

Other wastegates are spring-loaded and usually include an adjustment, appropriately known as the “horsepower screw.” As usually configured, tightening the screw increases available boost. Therefore, exercise restraint.

Test wastegate operation by loading the engine while monitoring rpm and manifold pressure. If the installation does not include a boost gauge, connect a 0–20 psi pressure gauge at any point downstream of the compressor. The diaphragm-sensing line (on units so-equipped) serves as a convenient gauge point. Note, however, that the gauge must be connected with a tee fitting to keep the wastegate functional. High gear acceleration from 2500 rpm or so should generate sufficient load to open the gate.

The control philosophy discussed in the preceding paragraphs implies that the turbocharger is used for power enhancement. When mid-range torque is the object, the wastegate opens early, at the engine speed corresponding to peak torque output. Most of these applications employ computer-controlled wastegates, whose response is conditioned by manifold pressure, engine rpm, coolant temperature, and other variables. No generalized test procedure has been developed for these devices.

Written by Ed

February 23rd, 2011 at 2:08 am

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Diesel Engines Turbochargers Construction

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Figure 9-6 illustrates an air-cooled Ishikawajima-Harima turbocharger of the type found on engines in the 100–150-hp range. The inset shows the water-cooled version of the same turbocharger, plumbed into the engine cooling system. All modern small-engine turbochargers follow these general patterns.

Floating bushings, located in the central bearing chamber, support the shaft. These bushings, like the connecting rod bearings specified for the original Ford V-8, float on the ID and OD. Thus, bushing speed is half that of shaft speed, which is to say that the bushings can reach speeds of 60,000 or 70,000 rpm. A floating thrust bushing contains axial motion.

air cooled turbocharger Diesel Engines Turbochargers Construction

Also note the way the impeller wheels cantilever from the bushings, so that the masses of the rotating assembly are concentrated near the ends of the shaft. This “dumb-bell” configuration requires precise wheel balance and extremely accurate shaft alignment.

The bearing section is lubricated and cooled by engine oil, generally routed through external pipes or hoses. Shaft seals keep oil from entering the turbine and compressor sections.

The turbocharger is usually the last component to receive oil pressure and might continue to rotate after the engine stops. To ensure an adequate oil supply to the bearings, operators should idle the engine for at least 30 seconds upon starting and for the same period before shutdown.

Written by Ed

February 23rd, 2011 at 2:04 am

Diesel Engines Turbochargers Applications

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Depending on how it is accomplished, turbocharging can have three quite distinct effects on performance. If no or little additional fuel is supplied, power output remains static, but emissions go down. Surplus air lowers combustion temperatures and provides internal cooling. Such engines should be more durable than their naturally aspirated equivalents.

The high-boost/low-fuel approach to turbocharging is limited to large stationary and marine plants. Makers of small, high-speed engines are more concerned with maximum power or mid-range torque.

Supplying additional fuel in proportion to boost yields power, which can translate as fuel savings for engines that run under constant load. Some gains in fuel efficiency (calculated on a hp/hour basis) accrue from turbocharging, but the real advantage comes about when high supercharge pressures allow for lower piston speeds. Large marine engines, developing a 1000 hp and more per cylinder, use this approach to achieve thermal efficiencies of 50%. On a more familiar scale, the naturally aspirated International DT-414 truck engine produced 157 hp at 3000 rpm. The addition of a large turbocharger boosted output to 220 hp, for a gain of 40%. Once in the fairly narrow power band, the trucker could save fuel by selecting a higher gear.

The third approach is more characteristic of automotive and light truck engines, which operate under varying loads and rarely, if ever, develop full rated power. What is wanted is torque.

The Navistar 7.3L, developed for Ford pickups and Econoline vans, is perhaps the best demonstration of the way a turbocharger can be throttled for torque production. In its naturally aspirated form, the engine develops 185 hp at 3000 rpm and 360 ft./lb. of torque at 1400 rpm. A Garrett TC43 turbocharger boosts power output marginally to 190 hp; but torque goes up 17.8% to 385 ft/lb. In normal operation the wastegate remains closed, directing exhaust gases to the turbine, until 1400 rpm. At that point, which corresponds to the torque peak, the hydraulically actuated wastegate begins to open, shunting exhaust away from the turbine.

Serious applications of turbocharging, whether for peak power or mid-range torque, impose severe mechanical and thermal loads, which should be anticipated in the design stage. Insofar as they work as advertised, add-on turbocharger kits—which rarely involve more than a rearrangement of the plumbing—are a buyerbeware proposition. The 7.9L is considered a very rugged engine in its naturally aspirated form. But turbocharging called for hundreds of engineering changes, including shot-peened connecting rods, oversized piston pins, Inconel exhaust valves, and special Zollner pistons, with anodized crowns.

Written by Ed

February 23rd, 2011 at 1:57 am

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