CHAPTER 3 AUTOMOTIVE AIR COMPRESSOR

Transcription

1 30 CHAPTER 3 AUTOMOTIVE AIR COMPRESSOR 3.1 INTRODUCTION A machine providing air at a high pressure is called as an air compressor. Air compressors have been used in industry for well over 100 years because air as a resource is safe, flexible, clean and convenient. These machines have evolved into highly reliable pieces of equipment that are almost indispensable in many of the applications they serve. Compressors are available in a wide variety of different types and sizes. Every compressed-air system begins with a compressor - the source of air flow for all the downstream equipment and processes. The main parameters of any air compressor are capacity, pressure, power and duty cycle. It is known that capacity does the work; pressure affects the rate at which work is done. Kazutaka Suefuji and Susuma Nakayama (1980) in their study on hermetic compressor have quoted that adjusting an air compressor's discharge pressure does not change the compressor's capacity. There are a number of basic air compressor designs and variations in the market today. The three basic types of air compressors are Rotary Screw Rotary Centrifugal Reciprocating

2 31 These types are further specified by the number of compression stages cooling method (air, water, oil) drive method (motor, engine, steam, other) lubrication (oil, oil-free) packaged or custom-built 3.2 ROTARY SCREW COMPRESSORS Rotary air compressors are positive displacement compressors. The most common rotary air compressor is the single stage helical or spiral lobe oil flooded screw air compressor. These compressors consist of two rotors within a casing where the rotors compress the air internally. There are no valves. These units are basically oil cooled (with air cooled or water cooled oil coolers) where the oil seals the internal clearances. Since the cooling takes place right inside the compressor, the working parts never experience extreme operating temperatures. The rotary compressor, therefore, is a continuous duty, air cooled or water cooled compressor package. Rotary screw air compressors are easy to maintain and operate. Capacity control for these compressors is accomplished by variable speed and variable compressor displacement. For the latter control technique, a slide valve is positioned in the casing. As the compressor capacity is reduced, the slide valve opens, bypassing a portion of the compressed air back to the suction. Advantages of the rotary screw compressor include smooth, pulsefree air output in a compact size with high output volume over a long life. The oil free rotary screw air compressor utilises specially designed air ends to compress air without oil in the compression chamber yielding true oil free air. Oil free rotary screw air compressors are available as air cooled and water cooled and provide the same flexibility as oil flooded rotaries when oil free air is required.

3 CENTRIFUGAL COMPRESSORS The centrifugal air compressor is a dynamic compressor which depends on transfer of energy from a rotating impeller to the air. Centrifugal compressors produce high-pressure discharge by converting angular momentum imparted by the rotating impeller (dynamic displacement). In order to do this efficiently, centrifugal compressors rotate at higher speeds than the other types of compressors. These types of compressors are also designed for higher capacity because flow through the compressor is continuous. Adjusting the inlet guide vanes is the most common method to control the capacity of a centrifugal compressor. By closing the guide vanes, volumetric flows and capacity are reduced. The centrifugal air compressor is an oil free compressor by design. The oil lubricated running gear is separated from the air by shaft seals and atmospheric vents. 3.4 RECIPROCATING AIR COMPRESSORS Reciprocating compressors are used in commercial automotives with air brake system. They are in use for more than six decades. Development of a compressor requires an insight into the design parameters and their effects on performance, cost and life of the compressor. Reciprocating air compressors are positive displacement machines that they increase the pressure of air by reducing its volume. This means they are taking in successive volumes of air which is confined within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by a piston within a cylinder as the compressing and displacing element. Single-stage and two-stage reciprocating compressors are commercially available. Single-stage compressors are generally used for pressures in the range of 500 kpa to 900 kpa. Two-stage compressors are generally used for higher pressures in the range of 900 kpa to

4 kpa. The reciprocating air compressor is single acting when the compression is accomplished using only one side of the piston. A compressor using both sides of the piston is considered double acting. Load reduction is achieved by unloading individual cylinders. Typically, this is accomplished by throttling the suction pressure to the cylinder or bypassing air either within or outside the compressor. Capacity control is achieved by varying the speed in engine-driven units through fuel flow control. Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations, may be packaged, and provide a wide range of pressure and capacity selections. Figure 3.1 shows the schematic diagram of a single stage single acting reciprocating air compressor. A reciprocating compressor consists of a crankshaft (driven by a gas engine, electric motor, or turbine) attached to a connecting rod, which transfers the rotary motion of the crankshaft to the piston. The piston travels back and forth in a cylinder. The piston acting within the cylinder then compresses the air contained within that cylinder. Air enters the cylinder through a suction valve at suction pressure and is compressed to reach the desired discharge pressure. When the air reaches the desired pressure, it is then discharged through a discharge valve. Desired discharge pressure can be reached through utilisation of either a single or double acting cylinder. In a double acting cylinder, compression takes place both at the head end and crank end of the cylinder. The cylinder can be designed to accommodate any pressure or capacity, thus making the reciprocating compressor the most popular in the gas industry.

5 34 Air from atmosphere Valve Plate Cylinder Suction Valve Discharge Valve Discharged air to reservoir TDC Piston BDC Connecting rod Crank Crank Shaft Figure 3.1 Schematic diagram of a reciprocating air compressor 3.5 SUCTION AND DISCHARGE VALVES A compressor valve is a device that controls the inward flow of lower pressure gas at atmospheric conditions and the outward flow of higher pressure gas from a reciprocating compressor cylinder. Normally these valves open and close automatically, solely governed by the pressure differential in the cylinder and the intake or exhaust line pressure. There is atleast one suction valve and one discharge valve for every compression chamber. Each valve opens and closes in every cycle. A valve used in a compressor operating at 1200 rpm for 12 hours a day and 280 days a year, opens and closes 72,000 times per hour or 864,000 times per 12 hours in a day or 241,920,000 times per year.

6 35 There are essentially two requirements to be met by a valve, (a) the valve must be efficient, and (b) the valve must be durable and quiet in service. The above criteria can be refined and can include both the aerodynamic flow efficiency and the volumetric efficiency. Under durability, the maintenance free operation for over several thousand hours plus relative ease in servicing and repair can also be included. There are different kinds of compressor valves: plate or disc valves, ring valves, channel valves, feather valves, poppet valves, ball valves, reed and concentric valves, to name just a few. Each design has a specific criteria with regard to the sealing element and all the other components are designed accordingly. Most of the air compressors used in automotive braking system use reed, disc or ring valves. In disc valves the plate is operated by a compression ring. The ring valve is an annular disc valve operated by a spring. Figure 3.2 shows the opening of disc valve used on suction and delivery sides. Figure 3.2 Inlet and Delivery disc valve openings

7 36 When the valve is closed, part of the valve plate or valve ring is firmly set against the seat lands. The sealing element initially lifts off the seat land slowly but accelerates rapidly towards the guard once spring forces are overcome. The factors that account for the initial pressure differential between cylinder and line pressure at valve opening that is seen on all PV-diagrams are (i) the cylinder pressure exposed to the entire surface area of the sealing element (ii) the sticking effect of lubrication or condensate and (iii) the spring load force. To lift the sealing element off the seat land, a pressure differential is required across the sealing element. The difference in area of a sealing element is normally 15% to sometimes as high as 30% between exposure underneath (seat side) and exposure on top (guard side). Since there is always some leakage through the closed valve plate along the seat lands, there is a certain amount of pressure build-up in this area. Therefore, the actual pressure differential needed to induce or cause the valve open is only 5% to 15% over the line pressure. As the sealing element lifts off the seat lands, it accelerates rapidly against the spring load towards the guard. The sealing element impacts against the guard causing the opening impact, at this stage the valve is considered fully open. Piston velocity at top or bottom dead center is zero and increases gradually to a maximum at the middle of its stroke. Valve velocity follows a slower path than the piston. The flow of the gas out through the seat keeps the sealing element open. As the flow diminishes due to the decreasing piston speed, the springs or other cushioning elements force the sealing element to

8 37 return to the seat lands and close the valve on time. Preferably, the valve is completely closed when the piston is at or near dead center. A reed valve is a flow actuated one-way valve. A port in the line is covered by the free end of a thin and flexible blade whose other end is fastened so that the port is normally closed. Pressure in the port or vacuum on the far side, will lift the blade, permitting the flow. If the pressure reverses, it closes the blade, stopping the flow. Usually the reed valves use a single blade, but modern versions combine four, six or eight blades, or petals, into tent-like arrays, fastened to a multi-ported reed cage. Reed valve involves the loss of pressure, as some pressure difference is required to open the valve. Even with this limitation, they have excellent versatility. Figure 3.3 shows the inlet and delivery valves employed in a 160 cc air cooled compressor. Inlet valve Cylinder bore Valve lift Delivery valve stopper Delivery valve Valve lift Cylinder bore Figure 3.3 Inlet and Delivery Reed Valve openings

9 38 features: Modern compressors employ reed valves because of the following 1. Number of components required is less. So almost no wear takes place. 2. The number of holes in the valve plate can be increased which will increase the flow area. This will reduce the pressure required to open the valves, and hence lesser pressure drop across the valves. 3. Lesser assembly difficulties. 3.6 PERFORMANCE PARAMETERS OF COMPRESSOR The performance of the compressor can be studied by individual parameters, such as pump up time, delivery air temperature, speed and power Pump up time Pump up time is the time required to develop a delivery pressure in a reservoir of given volume connected to the compressor air outlet. Pump up time is important as it indicates the volume flow rate of air inside the compressor under given operating conditions. Mainly the clearance volume affects pump time performance in addition to the flow area available in the cylinder head. The flow area available should not be less than the adapter inside flow area Delivery air temperature It is the temperature of air after compression measured at the delivery port of the cylinder head. Delivery air temperature has two issues:

10 39 (i) the degree of heat generated by the compression process and (ii) the degree of cooling of the compressor after the compression process. The air from the compressor is led into the air drier (Air processing unit) which purges the air from most of the moisture. The temperature of the air that enters the air processing unit is limited to about 70 o C. This necessitates the use of long metallic finned pipelines (nearly 6 m long) in order to allow sufficient time for cooling of air. A long pipeline complicates assembly issues on the vehicle. Thus a reduced delivery air temperature would reduce the need for long pipelines and thereby simplify the problems. A high delivery air temperature increases oil carryover and thereby further increase in the delivery air temperature due to the formation of carbon deposits on the piston and the cylinder head. Carbon deposits on the cylinder head reduce the heat dissipation capacity of the fins on the inner cavity of the cylinder head. Cylinder head design has a vital influence on the delivery air temperature Power Power is measured under three conditions: Loaded power: Loaded power is the power consumed by the compressor while pumping against a pressure gradient. Unloaded power: Unloaded power is the power consumed while pumping to atmosphere (with ideally no pressure gradient) through the unloaded valve. The unloaded valve regulates the pressure against which the compressor is pumping. Unloaded power reflects the power losses at the unloaded valve due to flow resistance. No load power: No load power is the power consumed while the compressor s delivery is open to atmosphere. No load

11 40 power is indicative of the power losses due to the flow resistance in the cylinder head of the compressor. 3.7 COMPRESSOR TERMINOLOGY Various terms related to the compressor specification are shown in Table 5.1 and the performance analysis are discussed below Discharge and suction pressure Discharge pressure is the pressure of discharged air or theoretically the reservoir pressure. The pressure of air during suction process is called suction pressure Free air delivered (FAD) The volume of air delivered by the compressor, when the state of air is reduced to intake (p s, T s ) or atmospheric (p a, T a ) or normal (p a, T n ) or required (p, T) condition is called FAD. Let, m 1 = Initial mass of air in the reservoir in kg p 1 T 1 m 3 p 3 T 3 = Initial pressure of the reservoir in Pa = Initial temperature of the reservoir in K = Final mass of air in the reservoir in kg = Final pressure of the reservoir in Pa = Final temperature of the reservoir in K V = Volume of the reservoir in m 3 t = Time taken for the pressure to build up from p 1 to p 3 in second

12 41 m od = Mass of air discharged into the reservoir in kg p V 1 m1 (3.1) RT1 p V 3 m3 (3.2) RT3 p 2 = m 3 m 1 The mass added during the interval t at intermediate pressure p3 p 1 V (3.3) T3 T1 R Mass added per cycle at p 2 (m od ) p 3 p 1 60 V (3.4) T3 T1 R N t FAD m R T od f (3.5) p f N From Equation (3.4), the FAD is p T 3 3 p T VTf t pf (3.6) where, p f = Free air pressure in Pa T f = Free air temperature in K N = Compressor speed in rpm There will be a rise in temperature during filling process at constant volume. Therefore it is required to measure the temperature at p 1 and p 3. If free air temperature is the tank temperature, it is taken as the temperature at the intermediate pressure p 2. This intermediate temperature should be used for calculating the mass of air discharged.

13 Indicated power (IP) Work energy imparted to the air per unit time is called indicated power. This power can be obtained from the p-v diagram Power consumption The power available at the compressor shaft to run the compressor at the desired discharge pressure and speed is termed as the power consumption. The power imparted to the air in the cylinder is Indicated power (IP). All the power available at the compressor shaft will not be imparted to the air in the cylinder. The friction between the moving parts absorbs some power and it is called friction power (FP). The FP varies with compressor speed. The load (discharge pressure) on the compressor has a negligible effect on FP. As the speed increases FP increases. For power absorbing machines, like compressor, Mechanical efficiency, IP m (3.7) BP If the compressor gets power from electric motor, then the power IP required to run the compressor (3.8) m g where, g = Generator (or) motor efficiency (generally the value lies between 0.85 and 0.95) If the compressor gets power from I.C engines, it is convenient to take the power required to run the compressor equal to the brake power (BP) of the compressor. The mechanical efficiency ( m ) of any reciprocating machine will be around 0.75 to 0.8 at rated speed. For the same speed, the

14 43 power required to run the compressor decreases with decrease in mass of air handled Indicated torque Torque (or often called a moment) can be thought of as a rotational force or angular force which causes a change in rotational motion. This force is defined by linear force multiplied by a radius. If a force is allowed to act through a distance, it does mechanical work. Similarly, if moment is allowed to act through a rotational distance, it does work. Power is the work per unit time. However, the time and rotational distance are related by the angular speed where each revolution results in the circumference of the circle being travelled by the force that is generating the torque. This means that, torque causes the angular speed to increase in doing work and the generated power may be calculated as P = Torque x Angular Velocity Indicated Power (IP) at a particular crankangle can be estimated from IP = T (3.9) From the torque calculation at different crankangles, it is possible to find the maximum torque and maximum indicated power which the compressor absorbs in a cycle Volumetric efficiency Analysis of volumetric efficiency ( v ) is essential to estimate the suitability of a compressor for a particular application. The factors affecting volumetric efficiency are

15 44 Clearance volume (Increase in clearance volume decreases v ) Discharge pressure (Increase in discharge pressure decreases v ) Temperature of cylinder (Heating of the cylinder decreases v ) Compressor speed (Increase in speed decreases the increase in v ) Leakage (Leakage past the piston, decreases v, but this effect can be neglected) Actual volume of air entering thecycle during suction process v Maximum possible volume of air that can enter thecylinder during suction process v 1 / n p d 1 k k p s (3.10) where, k = Clearance ratio = V c / V s (3.11) This expression is valid only for ideal compressors. In an ideal compressor, the index of expansion and compression are the same and the discharge and suction pressures are constant throughout the discharge process and suction process. For practical compressors, the volumetric efficiency is defined in terms of mass of air or FAD. v Actualmassof air drawnin percycle Maximumpossiblemassthatcouldbedrawnin percycle Maximum possible mass = a V s (3.12) Actual mass = Mass drawn in (or) Mass delivered out per cycle where, a = Density of ambient air = p a / (R T a ) (3.13) V s = Swept volume

16 Clearance volume and stroke volume Clearance volume (V c ) is the volume that is available after the piston reaches the TDC. This volume is provided in the compressor for ensuring free movement of compressor valves. The presence of clearance volume reduces the volumetric efficiency. Stroke volume (V s ) or swept volume is the volume corresponding to stroke Working volume It is the volume of air at any crankangle and is obtained using Equation (4.21). The cylinder volume at various crankangles is shown in Figure 3.4. Figure 3.4 Cylinder volume-crankangle diagram of Compressor Valve Lift It is the vertical distance travelled by the suction or discharge valve at any crankangle. Valve lift is governed by the goal to design valves with acceptable life and uninterrupted service. Since the plate or sealing element opens and closes with every revolution of the crankshaft, factors such as rotating speed, operating pressure and molecular weight of the gas determine

17 46 the limits of allowable valve lift. The impact resilience of various materials used for valve plates (steel, polymers, etc.) also has an influence on maximum acceptable valve lift. Different valve manufacturers use more or less conservative guidelines for allowable lift for a given set of operating conditions. Excessive valve lift can have detrimental effects on valve life, due to high-velocity impact forces, valve flutter, late closing, and other lifedeteriorating developments. Once an acceptable valve lift is defined, the rest of the valve geometry can be selected to balance the ratios of seat and guard area to free lift area. The diverse applications result in a variety of valve concepts. For example, slow-speed applications favour wide-ported seats and guards and high valve lifts, while high-speed applications require narrow ports and lower lifts would be applied Back flow during discharge and suction Whenever the valve closes, there will be a flow of some discharged air into the cylinder. This phenomenon is called Back flow during discharge and this reduces the mass of air discharged. Similarly, whenever the valve closes, there will be a flow of some drawn air from the cylinder to the atmosphere. This phenomenon is called Back flow during suction and this reduces the mass of air drawn in Head Volume The volume just above the valve plate is called head volume. It is also called plenum chamber volume. There are two compartments in the head, suction and discharge plenum chambers. Discharge Head: The air is discharged into the receiver through the head volume. The pressure in the head will not be constant, because, the mass going out of head per degree of crank rotation is not equal to the mass

18 47 coming into the head from the cylinder. There will be a pressure fluctuation in the head and this will affect the discharge of air from the cylinder. Driving force for flow of air from the cylinder is proportional to (p p d ) in theoretical case and is proportional to (p p h ) in actual case, where, p is the cylinder pressure, p d is the discharge pressure and p h is the head pressure. Suction Head: The air enters the cylinder during suction through the suction head. Driving force for flow of air from the cylinder is proportional to (p a p) in theoretical case and is proportional to (p h p) in actual case, where, p is the cylinder pressure, p a is the ambient air pressure and p h is the head pressure. Flow of air through the valve resists velocity changes because of its mass. The flow in compressor manifold is intermittent. When a discharge valve opens, the gas flowing from the cylinder has to push the gas already present in the manifold. This is a problem which increases with the compressor speed. At 3600 rpm, the time available is only 1/60 s per revolution and only a small fraction of this is available for the gas mass in the cylinder to be emptied into the manifold, accelerating in turn the air already present in the manifold. The result is the development of a back-pressure against which the compressor has to work and the losses can be significant. In reality, pressure surge will be occurring in the manifold. Carl et al (1974) stated that the volume directly behind the discharge valve should be as large as possible, as a minimum it should be equal to the cylinder volume for high speed compressors, but preferably three times as large. The same is true for suction valves, since the sudden filling of the cylinder depletes the supply of gas in the suction manifold and an underpressure is created against which the valve has to work. The volume acts like a collection tank or accumulator of gas, so that an over or under supply of gas can be stored temporarily.

19 48 Figure 3.5 shows the sectional view of a reciprocating air compressor used in braking system of heavy automotive vehicles. Figure 3.5 Sectional view of a typical automotive compressor The important parts of a reciprocating compressor are piston, cylinder, connecting rod and suction and discharge valves. The valve plate accommodates both inlet and delivery valves. The cylinder block houses the piston with connecting rod. The compressor is run by the engine and receives

20 49 power through the belt drive at drive end. The compressor is mounted to the engine using mounting flanges. 3.8 VALVE DYNAMICS Each compressor valve has to open and close in every compression cycle. The timing and pattern of the opening and closing events are referred to as valve dynamics. The valve opening and closing at the right time and without flutter is important. Compressor valve dynamics are important since they influence the valve life and compression efficiency. The valve dynamics can be influenced through proper spring and/or the mass of the moving components. For proper performance, the valves must be designed for the specific operating window. Valve flutter is not only detrimental to valve life because of multi impacting, but it reduces the effective lift area and also flow efficiency. Delayed closing will especially damage the valve since it is associated with slamming of the valve against a seat; the resultant back flow lowers overall efficiency by a substantial margin. Major valve manufacturers have used valve motion studies to improve valve performance and altered the design conditions of the valve offered for a specific application to optimise the performance. This chapter explained the working and theory of air compressors used in automotive braking system. The performance parameters like, volumetric efficiency, free air delivered, power and torque were also discussed. The development of mathematical model starting from the basic ideal model is explained in Chapter 4.