The gas turbine boost and charge air cooling system, by utilising part of the energy of the exhaust gases, ensures the supply of pre-compressed and cooled air to the engine cylinders.
Boosting makes it possible to increase the density of the air charge entering the cylinders and, in the same working volume, to burn a greater quantity of fuel and to raise the engine's litre power output.
The use of boosted engines extends the operating capabilities when driving on long ascents, over rough terrain and in mountainous conditions.
The gas turbine boost system of the engine (Figure 1) consists of two interchangeable turbochargers (TKR), exhaust and intake manifolds and branch pipes, a charge air cooler (CAC) of the «air-to-air» type, and supply and discharge pipelines.
The air enters the centrifugal compressor of the turbocharger from the air cleaner, is compressed and delivered under pressure into the CAC, and then the cooled air enters the engine.
The turbochargers are installed on the exhaust branch pipes, one per cylinder bank.
The exhaust manifolds and branch pipes are made of high-strength cast iron.
The gas joints between the mounting flanges of the turbocharger turbine, the exhaust branch pipes and the manifolds are sealed by gaskets made of heat-resistant steel.
The gaskets are single-use items and must be replaced when the system is dismantled.
The gas joint between the exhaust manifold and the cylinder head is sealed by a gasket made of asbestos-steel sheet, edged with metal clad tape.
The exhaust manifolds are made as one-piece castings and are fastened to the cylinder heads by bolts, locked with lock washers.
To compensate for the angular displacements that arise during heating, special spherical washers are installed under the heads of the exhaust manifold fastening bolts.
The engine's turbo boost and charge air cooling system must be airtight. Lack of airtightness in the system leads to an increase in the thermal stress of the components and to a reduction in the engine's power and service life.
In addition, lack of airtightness in the intake tract leads to «dust» wear of the cylinder-piston group and to premature engine failure.
The lubrication of the turbocharger bearings is provided from the engine lubrication system through fluoroplastic tubes with a metal braid.
The oil drain from the turbochargers is provided through steel tubes into the engine sump.
Two TKR 7S-6 turbochargers are installed on the engine. Instead of the TKR7S-6 turbochargers, S2B/7624TAE/0,76D9 turbochargers from the company «Schwitzer» may be installed.
The technical characteristics of the turbochargers are given in the table.
The TKR 7S-6 turbocharger consists of a centripetal turbine and a centrifugal compressor, connected to each other by a bearing assembly.
The turbine with a twin-entry housing 7 (Figure 3) of high-strength cast iron converts the energy of the exhaust gases into kinetic energy of rotation of the turbocharger rotor, which is then converted in the compressor stage into air compression work.
The turbocharger rotor consists of the turbine wheel 9 with the shaft 10, the compressor wheel 20, the oil deflector 16 and the bush 15, secured on the shaft by the nut 19.
The turbine wheel is cast from a heat-resistant alloy by the investment casting method and is friction-welded to the shaft.
The compressor wheel with blades curved backwards in the direction of rotation is made of aluminium alloy and, after machining, is dynamically balanced to a value of 0.4 g·mm.
The bearing journals of the rotor shaft are high-frequency hardened to a depth of 1–1.5 mm. After machining, the rotor is dynamically balanced to a value of 0.5 g·mm.
The bush, the oil deflector and the compressor wheel are installed on the rotor shaft and tightened by the nut to a torque of 7.8–9.8 N·m (0.8–1.0 kgf·m).
After assembly, the rotor is not additionally balanced; only the radial runout of the shaft journals is checked.
When the radial runout value is not more than 0.03 mm, marks are applied to the rotor parts in the same plane and the rotor is accepted for assembly of the turbocharger.
When installing the rotor into the bearing housing, it is necessary to align the marks on the rotor parts.
The rotor rotates in bearings 5, which are floating rotating bushes.
The axial displacements of the rotor are limited by the thrust bearing 4, clamped between the bearing housing 3 and the cover 2. The bearings are made of bronze.
The turbocharger bearing housing, in order to reduce heat transfer from the turbine to the compressor, is made composite, from a cast-iron housing and a cover of aluminium alloy.
To reduce heat transfer between the turbine housing and the bearing housing, a shield 11 of heat-resistant steel is installed.
An oil slinger shield 14 is installed in the bearing housing, which, together with the elastic split rings 8, prevents oil leakage from the housing cavity.
To eliminate air leaks in the joint «compressor housing — bearing housing», a rubber sealing ring 21 is installed.
The turbine and compressor housings are fastened to the bearing housing by means of bolts 13, 18 and straps 12, 17. This design allows the housings to be installed at any angle to each other, which in turn facilitates the installation of the TKR on the engine.
Maintenance of the gas turbine boost system and charge air cooling during engine operation consists, by external inspection, in checking the airtightness of the exhaust gas pipeline route and of the air supply to the engine.
The reliability of the fastening of the parts and assemblies of the said systems is checked periodically, and if necessary the bolts, fastening nuts and clamps are retightened.
The operation of the turbocharger has a substantial influence on the parameters and the operability of the engine. A turbocharger fault can lead to engine breakdown.
Although the turbochargers do not require adjustments during operation, it is necessary to systematically comply with the engine maintenance rules established by the manufacturer and to periodically check the operation of the turbochargers by ear.
During TO-2 it is necessary to check the ease of rotation of the turbocharger rotors. To do this, the receiving pipe of the exhaust gas system must be removed.
Then check by hand how the rotor rotates in its extreme axial and radial positions.
The rotor must rotate easily, without jamming and without touching the stationary parts of the turbocharger.
The turbocharger bearings are very sensitive to the quantity and cleanliness of the oil, therefore the necessary conditions for the normal operation of the bearing assembly are the timely replacement of the oil and of the filter elements of the engine oil filter, as well as the use of the oil grades recommended by the manufacturer.
During seasonal maintenance, it is recommended to remove the turbochargers from the engine once every two years to clean the centrifugal compressor. It is advisable to remove the unit together with the exhaust manifold.
Cleaning of the centrifugal compressor must be carried out in the following sequence:
- apply alignment marks on the end surfaces of the compressor housing and the cover;
- unscrew the fastening bolts of the compressor housing;
- with light hammer blows on the bosses, remove the compressor housing;
- inspect the rubber sealing ring in the groove of the cover. If defects are found (cuts, loss of elasticity), replace the sealing ring;
- inspect the blades of the compressor wheel. If traces of contact with the compressor housing, deformation of the blades or their destruction are found, the turbocharger must be repaired at a specialised enterprise or replaced;
- wash the inner cavity of the compressor housing and the surface of the cover with a cloth moistened in diesel fuel. When cleaning the compressor wheel, it is recommended to clean the inter-blade surfaces with a bristle brush using diesel fuel;
- check the ease of rotation of the rotor; jamming of the rotor is not permitted;
- before assembly, it is necessary to lubricate the sealing ring with engine oil, align the marks, install the compressor housing on the cover disc, and tighten the bolts with a torque wrench.
Check once again the ease of rotation of the rotor. In the extreme axial and radial positions, the rotor wheels must not contact the housing parts.
Since the turbocharger rotor is balanced with high precision, the complete dismantling, repair and maintenance of the boost units must be carried out at specialised enterprises having the necessary equipment, tools, devices, instruments and qualified personnel.
During seasonal maintenance, the condensate accumulated in the CAC must be drained.
