Superchargers 4

DieselNet Technology Guide Air Induction for Diesel Engines

www.DieselNet.com. Copyright Ecopoint Inc. Revision 2001.07

Superchargers for Diesel Engines

Abstract: Superchargers are mechanically, electrically, or hydraulically driven devices employed to boost the charge air pressure in engines. A number of compressor and blower types have been used as superchargers, including roots blowers, sliding vane compressors, screw compressors, rotary piston pumps, spiral-type superchargers, variable displacement piston superchargers, and centrifugal compressors.

Classification of Superchargers

Roots Blower

Sliding Vane Compressor

Screw Compressor

Rotary Piston Supercharger

Spiral-Type Supercharger

Variable Displacement Piston Supercharger

Centrifugal Compressor Supercharger

Classification of Superchargers

Superchargers are mechanically, electrically, or hydraulically driven pumps, compressors, or blowers employed to boost the pressure of the charge air in diesel engines or of the mixture in spark ignited engines. Most superchargers are positive displacement devices, but aerodynamic (centrifugal) compressors are also possible. A multitude of device types can be used as superchargers, as shown in the classification chart in Figure 1.

Figure 1. Types of Superchargers

The top six devices in the chart are positive displacement, while the centrifugal compressor is classified as an aerodynamic or continuous flow device. Positive displacement devices deliver a

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specific volume of air per revolution. Since the volumetric efficiency is almost constant, air flow is usually proportional to the supercharger or engine speed. Positive displacement devices can provide high boost pressures without the need for high speed. Therefore, they are well suited for a mechanical connection with the engine, such as through a gearbox or a belt/pulley drive. Each of the particular devices has its advantages and disadvantages, that determine which supercharger is best suited for a specific application.

Centrifugal compressors are well suited to deliver high flow volumes at relatively low pressure ratios. With the boost pressure generally proportional to the square of the supercharger speed, centrifugal compressors must operate at relatively high velocities. In superchargers, they are better suited for coupling with high speed electric motors, rather than for a mechanical gearbox connection with the engine. Centrifugal devices are also the standard type of compressors that are driven by an exhaust gas turbine in the engine turbocharger.

Roots Blower

The Roots blower may have two or three rotors, as shown in Figure 2 [Heywood 1988]. Rotors are straight, but can also be helical for noise suppression. As the rotors turn, air enters the displacement volume between the rotors and the housing, as represented by line AB in Figure 3. It is then carried across to the discharge port without compression. When the discharge port opens, the relatively hot air volume is instantaneously delivered from the blower and the pressure rises to P2 as indicated by line BC. In practical packages, leakage between the rotors as well as backflow from the receiver to the inlet side of the blower take place, thus reducing its overall compression process. Therefore, the delivery of air from the blower may be better represented by line BD [Heywood 1988]. In fact, Roots blowers are used in applications where the pressure ratio is rather low, typically in the range of 1.0-1.3 [Obert 1968]. More losses would be experienced at higher pressure ratios where the use of Roots-type blowers would be questionable. However, Roots blowers are popular for their potential for high speed operation, high mechanical efficiency, simplicity, and cost.

Figure 2. Roots Blower Supercharger



Figure 3. Pressure-Volume Diagram for Roots Blower

A typical performance map of a Roots blower appears in Figure 4 [Heywood 1988]. Its performance is characterized with low pressure ratio and its flow rate at constant speed is a function of the pressure ratio. However, Roots blowers are generally noisy and their size is large. One of its more famous applications in the USA is the two-stroke Detroit Diesel engine that powers the majority of the public transit buses.

Figure 4. Performance Map of a Roots Blower

Roots blowers may have good volumetric efficiency (about 90%) as well as reasonable mechanical efficiency (85%). However, their isentropic efficiency is barely 65% and strongly contributes to the blowers low overall efficiency of about 55% [Taylor 1985][Ronzi 1995].

Sliding Vane Compressor

In the sliding vane compressor, slots in the rotor house thin silicon carbide vanes that move in a radial direction (Figure 5) [Heywood 1988]. Their motion is dictated by centrifugal force that results from the high speed rotation of the rotor. The rotor itself is mounted eccentrically in the housing. Therefore, the high speed rotation of the rotor causes the vanes to move towards the housing thus trapping air between two vanes, the housing, and the rotor.



Figure 5. Sliding Vane Compressor

Air enters through the inlet port of the sliding vane compressor and immediately occupies a crescent-like volume as shown in Figure 5. As the rotor continues its rotation which is controlled by its eccentricity relative to the housing, trapped air undergoes a volume increase followed by compression (decreasing volume) at the point of delivery. The capacity of the sliding vane compressor depends on several factors. Among them is the amount of eccentricity, the volume of air inducted (which depends on the size of the housing and rotor), number of vanes, and its speed. As in the case of the Roots blower, actual compressor performance suffers from leakage between the blades and the housing, especially at low speeds where little centrifugal force is experienced, thus reducing the sealing between the vanes and the housing.

Heating results from the friction of the rotors against the housing. Unless this heat is dissipated through cooling, it is transferred to the air thus decreasing its density and increasing its volume. This development eventually reduces the compressor efficiency and adds to the engine cooling system load. A performance map for the sliding vane compressor is shown in Figure 6 [Heywood 1988]. It is worth noting that the compressor isentropic efficiency (c) is rather low.

Figure 6. Sliding Vane Compressor Performance Map

The overall efficiency of the sliding vane compressor is only 40%. This low performance is due to a combination of low volumetric efficiency (85%), mechanical efficiency (about 65%), and an isentropic efficiency of just 60% [Taylor 1985].

Screw Compressor

At the first glance, the screw type compressor, Figure 7, may have a strong resemblance to the Roots blower [Heywood 1988]. They do indeed share similar features such as a housing and a rotor. The rotor in the screw compressor is precision machined to maintain very tight tolerances between the rotor and the housing.



Figure 7. Screw Compressor

As the rotor turns, air is inducted through ports arranged around the cylindrical housing and occupies the volume between two consecutive screws and the housing. This air is delivered through a discharge port, as shown in Figure 7. The delivery pressure is a function of the rotor speed and the discharge port flow area. Screw compressors have a rotational speed that ranges from 3,000 to 30,000 rpm, and generate substantial heat from friction between the rotor and the housing. Measures are usually taken to dissipate that heat to maintain the compressors mechanical integrity. Screw compressors enjoy high volumetric efficiencies as long as their clearances are kept extremely small. Performance of the screw compressor depends on the rotor speed as long as leakage can be kept to a minimum. This leads to a relatively flat performance characteristic over a wide speed range as seen in Figure 8 [AlliedSignal 1996].

Figure 8. Performance Map of a Screw Compressor

Rotary Piston Supercharger

The rotary piston supercharger is an internal-shaft rotary piston device where the shaft rotates eccentrically inside a cylindrical housing (item A in Figure 9) [Bosch 1986]. In the example shown, the outer rotor has three teeth (B). The driven inner rotor (C) has two teeth (lobes) and rotates eccentrically within the outer rotor (B). The rotors have a ratio of 3:2 and do not contact each other while rotating. The level of internal compression is determined by the timing of the outer rotor clearing the outlet edge (D) and allowing compressed air to escape into the delivery duct. The compressed volume of air is represented by hatched area (E) while area (F) represents the evacuated volume. Area G represents the fill volume that is exposed to the inlet duct as the rotors continue to rotate. A pair of cylindrical gears synchronize the motion of the rotors and prevent them from contacting each other.

The isentropic efficiency of rotary pistons superchargers is about 65% while its volumetric efficiency is about 90%. These superchargers are usually belt-driven and can reach speeds of 15,000 rpm. They are capable of delivering boost up to approximately 80 kPa (12 psi).



Figure 9. Rotary Piston Supercharger

Spiral-Type Supercharger

The spirals are arranged in a flat-sided casing having a shaft that rotates eccentrically. Sandwiched in between the fixed spirals are moving displacer walls attached to a disc that is connected to an eccentric pin roller bearing (Figure 10). As the drive shaft rotates, the displacer performs an oscillating circular motion of double eccentricity. Air entering the blower moves from one working chamber to the next performing filling, transporting, and expelling of the air at a hub, then delivering it through its discharge. The rotation of the eccentric and the rotor is through a toothed belt, as shown in Figure 10 [Bosch 1986].



Figure 10. Details of a Spiral (Scroll)-Type Supercharger

The overall efficiency of this supercharger is about 55%. Its isentropic efficiency is 68% and its volumetric efficiency is close to 90%. The speed range for this type of supercharger is 0-13,000 rpm, and it can deliver up to 12 psi boost pressure. The spiral-type supercharger is often referred to as the scroll-type supercharger. The casing of the supercharger is die cast aluminum and the displacer is made of die cast magnesium.

Variable Displacement Piston Supercharger

This supercharger was developed in the mid-1990s and has a design that incorporates four radially arranged rectangular pistons (Figure 11) [Ronzi 1995]. The pistons are driven within their respective chambers in a circular motion referred to as nutation. The chambers are free to move laterally to allow for the piston side movement. The chambers movement opens and closes inlet and outlet passages at the top of the cylinders. The design also incorporates a sliding pin mechanism that allows adjusting the eccentricity, thus permitting the piston stroke to be varied.



Figure 11. Conceptual Drawing of a Variable Displacement Piston Supercharger

The overall efficiency of this supercharger is greater than the scroll-type, about 65%. Its isentropic, mechanical, and volumetric efficiencies are about 80, 80, and 90%, respectively. The speed range for this type of supercharger is a relatively low 0-5,000 rpm, and it can deliver up to 100 kPa (15 psi) in boost pressure.

Centrifugal Compressor Supercharger

The centrifugal compressor is normally used with an exhaust-driven turbine [Heywood 1988]. However, it is also used independent from an exhaust-driven application. In one configuration it may be driven by a high speed electric motor, and in another it could be driven hydraulically. It consists mainly of a single stage radial compressor through which air is pulled from the ambient. The air accelerates to a high velocity and flows radially outward via a stationary diffuser stage toward a shell-shaped housing called the volute as shown in Figure 12 [Bosch 1986]. The volute is shaped in such a way that its diameter, as well as its cross-sectional flow area is always increasing. For this reason, the air velocity decreases, and its kinetic energy is converted to potential energy as manifested in increasing pressure. The centrifugal compressor is ideal for providing high mass flow rates at relatively low pressure ratio of less than 3.5 that is normally the case in internal combustion engine applications.



Figure 12. Centrifugal Compressor

Figure 13 is a performance map for a centrifugal compressor [Heywood 1988]. This map relates the compressors air flow characteristics with its pressure ratio. The topographical contour lines indicate islands of constant efficiency. The area to the left of the operating range of the compressor is an area of unstable operation. The surge line is the line that separates the stable from the unstable operating regime on the left side of the compressor. Performance at constant compressor speeds is also indicated in the compressor map. The performance is limited on the right side of the map by choking that occurs at each of the compressor speeds. Choking is the inability to flow any more air through the compressor blades or the diffuser channels. Normally one would want the compressor to operate at its highest possible mechanical efficiency at any speed. A line that would connect the most efficient points at each operating speed can be referred to as the most efficient operating line. Therefore, the resultant line that attempts to describe the optimum operation of a compressor is referred to as the operating line. However, in most cases we may settle for a point on a constant speed line that may not be the most efficient for specific design reasons.



Figure 13. Performance Map of a Centrifugal Compressor

Figure 14 is a photograph of an electrically-driven centrifugal supercharger. The motor is a brushless, permanent magnet, high speed motor capable of up to 40,000 rpm using short bursts of relatively high current to achieve good response time [Turbodyne 1995].

Figure 14. Electrically-Driven Centrifugal Compressor (Courtesy of Turbodyne Systems, Inc.)

References

AlliedSignal, 1996. "Turbocharging Systems", Sales Brochure AS6PS6

Bosch, 1986. "Automotive Handbook", Society of Automotive Engineers, Warrendale, PA, 2nd Edition

Heywood, J.B., 1988. "Internal Combustion Engine Fundamentals", McGraw-Hill, New York

Obert, E.F., 1968. "Internal Combustion Engines", International Text Book



Ronzi, W., Ericson, B., McNaughton, J. M., 1995. "Supercharger Bench Testing", General Motors Institute and Eaton Corporation, Internal Report

Taylor, C.F., 1985. "The Internal Combustion Engine in Theory and Practice", M.I.T. Press, Volume 2, Revised Edition

Turbodyne, 1995. "Performance For Your Engine And The Environment", Turbodyne Systems, Inc., Carpinteria, California

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