EXPRESSOR

INTRODUCTION

In earlier locomotives we used only air brakes. For that a compressor is needed to produce compressed air. But after 1980, almost all of the locomotives have dual brake systems, i.e. air brake is used for application of brakes in the locomotive (engine) and vacuum brakes are used for the application of brakes in the wagons or compartments. For that we need compressed air for air brake and high vacuum for vacuum brake.
A simple compressor cannot provide high compressed air and high vacuum. For obtaining these two, we need a compressor and an exhauster together. For that we use a single unit system known as EXPRESSOR, in the dual brake locomotives.
Therefore expressor is simply expressed as

EXhauster + comPRESSOR = EXPRESSOR

COMMON FEATURES OE EXPRESSOR

Expressor is a reciprocating type system. It has six cylinders, which are arranged in a ‘W’ type manner i.e., two cylinders are in a vertical position and four in a V shape. It is an air-cooled system with forced feed lubrication. For cooling purpose a fan is used which is of a forced draught type, mounted on the crankshaft. The oil fill capacity is 30 litres; the oil grade being Servo press 150-IOC.


TYPES OF EXPRESSOR

Expressors are generally classified on the basis of the number of cylinders they have and out of which how many are used as compressors. The three main classifications are:

1) 6CD 4UC: - Here, the expressor totally consists of six cylinders of which two cylinders are compressors. These two provide compressed air for the actuation of air brakes and other four are the exhausters.


6CD 4UC



2) 6CD 3UC: - Here the expressor totally consists of six cylinders in which three cylinders provide compressed air for air brakes and other three help in the creation of vacuum for the actuation of vacuum brakes.


3) 3CD B: - This type of expressor consists of three cylinders, which bare used only as compressors. This type of expressor is not capable of producing exhaust though it is termed as an expressor. This system is presently not used in dual brake system and is limited only in locomotives having air brakes only

The different types of expressor are shown in the following page. As the construction and working of the two expressors are in the similar fashion, the construction and working of only 6CD 4UC is explained in this seminar.


6CD 4UC EXPRESSOR

MAIN PARTS: -

1. CRANK CASE
2. CYLINDER AND CYLINDER HEAD
3. INLET AND DISCHARGE VALVE
4. AIR INTAKE AND STRAINER ASSEMBLY
5. PISTON AND CONNECTING ROD
6. CRANK SHAFT
7. INTERCOOLER
8. UNLOADER VALVE
9. SAFETY VALVE
10. CRANK CASE VACUUM MAINTAINING VALVE

The description of the parts is as follows.

CRANK CASE

The crankcase is the main body of the expressor to which all the cylinders are connected. The crankshaft is mounted centrally in the crankcase. One side of the crankcase is fully closed and the other side fully open, through which the crankshaft extends out words, on which the drive for the expressor is given. On the closed side the fan is fitted for cooling purpose. Inside the crankcase, the lube oil is filled. The oil fill capacity is 30 liters. On the inner side of the crankcase oil pump is fitted, which helps to circulate the lube oil under pressure.


CYLINDER AND CYLINDER HEAD

The 6CD 4UC expressor consists of six cylinders in which four are uncompressors. They are used to create vacuum for the actuation of vacuum brake. They are arranged in ‘V’ type manner with a pair of cylinders on the two sides of the crankcase. The other two cylinders are the compressor used for providing the compressed air. They are mounted vertically on the top of the crankcase so that the six cylinders together form a ‘W’ pattern. Of the two compressors one is a low-pressure cylinder and the other is a high-pressure cylinder.
The low-pressure cylinder head has two inlet and two delivery valves whereas the high-pressure cylinder has an inlet valve and a delivery only. Each exhauster cylinder head consists of two inlet and two delivery valves.

INLET AND DISCHARGE VALVES

The inlet and discharge valves used in expressor are of concentric disc valve type. The assembly is shown in the figure. Care should be taken when cleaning and assembling the different parts of the inlet and discharge valves so that a mix up does not occur, as these parts are not interchangeable.
The inlet valves of the L.P. cylinder and H.P. cylinder are fitted with suction unloader valves as a measure. This control enables the compressor to be operated continuously with out stopping.









AIR INTAKE STRAINER ASSEMBLY

An air intake strainer is mounted on the inlet of the L.P. cylinder head means of stud and nut. A minimum space of 160 mm is required to remove the element from the air intake strainer.
The strainer element is made up of spirally wound steel wire mesh. The layers of dust can be cleaned by dislodging the element from the air intake strainer and by using compressed air. Thus we can prevent the impurities from entering into the compressor



PISTON AND CONNECTING ROD

The low pressure and exhauster pistons are made of aluminium alloy and the high-pressure piston is made of high quality cast iron. High quality cast iron is used so as to withstand very high pressure produced in the high-pressure cylinder. For every piston there are four rings; two compression rings and two oil control rings. The compression rings used in the piston are of two types. The first compression ring provided on the top of the piston does a stepped compression ring follow a plain compression ring. Together with theses two compression rings, a plain oil control ring is provided at the top, where as the second slotted oil ting is provided at the bottom of the piston to ensure added safety.


The connecting rods are made of forged steel, which are identical. Three of them are mounted on each crank pin of the crankshaft. The small end of the connecting rod is provided with bush bearings. Inner bearing of the big end of the connecting rod can be easily replaced in case of necessity during repair.

CRANK SHAFT

The crankshaft of the expressor is directly coupled to the diesel engine. The crankshaft is supported on two main double row ball bearings for frictionless operation. On the other end of the crankshaft is mounted a fan which is provided for cooling purposes. The crank consists of two bearing seats, which support the main double row ball bearings. The crankshaft consists of crank pins on which the connecting rods are mounted. Oil seats fixed at the outer position of the two bearings prevent the atmospheric air from entering into the crankcase and also prevent the oil from leaking out. The drive to the gear type oil pump, fixed to the inner side of the crankcase is provided from the crankshaft via the gear train

drive. For the proper working of the expressor, the crankshaft should be balanced in both static and dynamic conditions so that proper pressure can be maintained. Counter weights are provided to the crankshaft at proper positions to enable it’s balancing.

INTER COOLER

The inter cooler I provided for the cooling of compressed air. It is provided in between the low pressure and high-pressure cylinders. It consists of 24 tubes on one side and 21 on the other. The air coming from the low-pressure cylinder first enters the inter cooler through the side having 24 tubes, reaches the side having 21 tubes and then enters the high-pressure cylinder. Thus, the air which enters the inter cooler (radiator type) gets cooled before going into the high-pressure cylinder. The cooling improves the volumetric efficiency of the compressor. The fan and the finned tubes of the inter cooler increases the cooling efficiency.













UNLOADER VALVE

The unloader is used to unload the excess air sucked by the compressor after the preset pressure in the main air tank is reached. The inlet valves of the low and high-pressure cylinders are fitted with unloaders controlled by the governor. The inlet valve gets opened when the designed pressure in the tank is reached and air gets unloaded through the air filters of the compressor.





















SAFETY VALVE

The safety valve which is fitted in the inter cooler manifold is used to blow out the excess air if the pressure exceeds 3.2 bars in the inter cooler. If there is symptom of choking, found in the inter cooler or the suction valves of the high-pressure cylinder, care should be taken to ensure the tightness of the cap nut of the safety valves.














CRANK CASE VACUUM MAINTAINING VALVE

It is also known as breather valve. Partial vacuum is developed in the crank -case by connecting the crankcase to the inlet manifold of the exhauster by means of a pipe through the vacuum-maintaining valve.






The piston in the vacuum maintaining valve moves up when the pressure is built up in the crank case assisted by spring force which opens the port connecting the rank case to inlet manifold of the exhauster, there by evacuating the crank case. When the vacuum in the crankcase reaches a predetermined level as set, it is maintained at that level by the piston moving down and cutting off the connection to the vacuum manifold.


WORKING OF AN EXPRESSOR (6CD 4UC)

PROCESS OF AIR COMPRESSION

When drive is given to the expressor through the crankshaft, the piston of the low-pressure cylinder moves towards the bottom dead center (B D C). The combined action of the delivery valve return spring and the pressure differential developed between discharge manifold and the low-pressure cylinder closes the delivery valve. At the same time, inlet valves open due to the pressure differential developed between the inside and outside of the low-pressure cylinder.
The atmospheric air is sucked into the low-pressure cylinder through air filter, which is fitted to the suction side of the low-pressure cylinder head. During the upward motion of the piston, the combined action of the inlet valve spring and the pressure difference developed between the inside and outside of the low-pressure cylinder closes the inlet valves. The air, which is trapped in the cylinder, gets compressed. The compressed air forcibly opens the delivery valves due to the formation of pressure differential, which has been developed between the discharge manifold and the cylinder. The compressed air is delivered into the discharge manifold through the delivery valves. It then reaches the inter-cooler through the discharge manifold.
The air coming from the low-pressure cylinder first enters the inter-cooler through the side having 24 tubes, then passes through the side having 21 tubes and reaches the high-pressure cylinder. During compression in the low-pressure cylinder, the temperature of the compressed air is increased. Further compression of this air in the high-pressure cylinder is effective only if the temperature of this air is lowered, or else more work will have to be done on this air, which will lead to reduction in efficiency. To prevent this, we use an inter-cooler, which reduces the temperature of the compressed air coming from the low-pressure cylinder and thus increases the volumetric efficiency of the compressor. The fan and the finned tubes of the inter-cooler increase the efficiency of cooling. The cooled compressed air gets compressed to a higher pressure in the high- pressure cylinder and is delivered to the air receiver. This compressed air developed by the compressor is used for the application of brakes for the locomotive, horns and wind- screen vipers.




WORKING OF AN EXHAUSTER

When vacuum brake is applied, air is entered into the vacuum lines. For further application of this vacuum brake, the air present in the vacuum lines should be removed so that the vacuum tank is connected to the exhauster. The air present in the vacuum brake pipe is sucked into the inlet of the exhauster head, which is connected by the inlet manifold. The cylinder heads of all the exhauster cylinders
are provided with two inlet and two delivery valves. During the downward motion of the piston, i.e. moving to B D C, air is sucked through the inlet valves of the exhauster cylinder. The pressure differential developed between the exhauster cylinder and vacuum brake pipe makes the inlet valve open, during which the double action of delivery valve spring and pressure differential developed between the inside and outside of the exhauster cylinders keep the delivery valves closed. When the piston moves to T D C, the air is compressed to a pressure, which is comparatively higher than the atmospheric pressure. The delivery valves open and the air escapes through the manifold to the atmosphere. The process is continued, until the vacuum is attained to 585mm or 23” of mercury.

LUBRICATION IN EXPRESSOR

For efficient working of the expressor, the lubrication should be very good. The expressor lubrication system is piloted by gear train driven type oil pump. From the crank- case, the lubricating oil is fed to the pump through the oil strainer assembly. The oil pump, which is fixed to the inner side of the crankcase, circulates the oil under pressure. The gear train drive from the crankshaft of the expressor enables the driven gear mounted o one of the two gears of the oil pump, to pump the oil through the system. An oil strainer is provided, which is fitted with two wire meshes and which face the inner side of the crank case bottom. It filters the oil to prevent the ingress of external agents like dust and other solid particles from entering the pump and the lubricating system. The pump due to the meshing of gears takes up the filtered oil, and the delivery port takes the oil up, through the groove in the distributing ring.
Through the distributing ring, the lubrication oil flows to each crank pin in the crankshaft through the oil holes drilled in it. Axially drilled holes through each crank pin are connected to the distributing holes. The oil lubricates the inner bearings of their respective six connecting rods through the grooves provided on its top half in all the connecting rods. The hole drilled through the connecting rod carries the oil to the small end to lubricate the gudgeon pin. A relief valve fitted on the body of the oil pump, maintains the oil pressure between 4 and 6 bars. In case, the oil pressure exceeds, the oil relief valve opens and allows the oil to pass out of the pump, thus maintaining the set pressure. An oil pressure indicator valve, which is mounted on the side cover of the crankcase, helps to ensure the line pressure of the oil system. A tube is connected from the crankcase body to the oil gauge through which the gauge senses the pressure recommended oil pressure is a minimum of 4 bars at idle and 6 bars at full load. Oil seats fixed at the outer position of the two bearings prevent the atmospheric air entering inside the crankcase. Oil level can be checked visibly, and by the dipstick through the transparent oil level indicator, fitted on the side cover of the crankcase. Oil will overflow while filling, when the maximum level is reached.




TECHNICAL DATA (6CD 4UC)

01 MODEL TRC 155*615
02 TYPE RECIPROCATING,’W’TYPE,.AIR COOLED,FORCED FEED LUBRICATION
03 NORMAL WORKING PRESSURE Psi 140
04 FREE AIR DELIVERY 2.833 m3/min
05 RATED SPEED Rpm 1000
06 SPEED RANGE Rpm 400-1100
07 DISPLACEMENT Rpm AIR VACUUM
CFM LPM CFM LPM
400 61.35 1740 245.8 6959
500 76.69 2180 307.0 8693
750 115.03 3270 460.5 13039
1000 153.5 4349 614.0 17398
1025 157.4 4457 629.8 17833
08 TYPE OF CONFIGURATION VERTICAL LP & HP CYLINDERS, ‘V’ VACUUM CYLINDERS
09 COMPRESSION STAGE Nos 2
10 CYLINDERS Nos LP-1, HP-1, VACUUM-4
11 CYLINDER SIZE & STROKE mm 196.85 * 107.95 * 142.87
12 TYPE OF PISTON RING Dia (197) 5 Nos Dia (108) 1 No
a. PLAIN COMPRESSION 1 1
b. STEPPED COMPRESSION 1 1
c. SLOTTED OIL CONTROL 2 2
13 TYPE OF VALVES CONCENTRIC TYPE, DISC VALVE
14 DIRECTION OF ROTATION ANTI-CLOCKWISE FROM DRIVE END
15 TYPE IF LUBRICATION FORCED FEED
16 SHAFT HORSE POWER @ 1000 RPM
a. 140 Psi Air, 0” Vacuum BHP 115
b. 23” Vacuum BHP 60
17 TYPE OF COOLING FAN
18 TYPE OF FAN FORCED DRAUGHT, MOUNTED ON CRANKSHAFT
19 OIL FILL CAPACITY Lts 30
20 GRADE OF OIL Servo Press 150-IOC
21 LUBE OIL PRESSURE 4 to 5 min, 6 to 7 max Kgf/cm2
22 LUBE OIL PUMP DRIVE GEAR, MOUNTED ON CRANKSHAFT
23 SAFETY VALVE SET PRESSURE, HP-Inlet Psi 50
24 OVERALL DIMENSION mm 1207 * 1410 * 1000
25 NET WEIGHT Kg 865 approx












CONCLUSION

Thus, in locomotives, where high efficiency braking and dual system are required for both air and vacuum braking, the use of expressor is seen to produce the best results. Further, Its use can be extended in future, to heavy- duty automobiles, which may require the use of dual braking.






















BIBILIOGRAPHY

ELGI- EXPRESSOR MANUAL
THOMAS BELL - LOCOMOTIVE BRAKES & ALLIED SYSTEMS
KASI VISWANATHAN -ALL ABOUT LOCOMOTIVES
TRIVEDI -BRAKING SYSTEMS IN INDIAN RAILWAYS

WEB BASED
www.dias.net/compressor/exhauster
www.howstuffworks.com
www.irfca.org