ROOTS SUPERCHARGER WITH A SHUNT PULSATION TRAP
A shunt pulsation trap for a Roots supercharger reduces pulsation, NVH and improves efficiency without significantly increasing overall size of the supercharger. Generally, a Roots supercharger with the shunt pulsation trap has a pair of interconnected and synchronized parallel multi-helical-lobe rotors housed in a transfer chamber with the same number of lobes for propelling flow from a suction port to a discharge port of the transfer chamber without internal compression. The shunt pulsation trap comprises an inner casing as an integral part of the transfer chamber, and an outer casing oversized surrounding the inner casing, therein housed various pulsation dampening means or pulsation energy recovery means or pulsation containment means, at least one injection port (trap inlet) branching off from the transfer chamber into the pulsation trap chamber and a feedback region (trap outlet) communicating with the supercharger outlet pressure.
This application claims priority to Provisional U.S. patent application entitled ROOTS SUPERCHARGER WITH A SHUNT PULSATION TRAP, filed Jul. 20, 2010, having application No. 61/366,140, the disclosure of which is hereby incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates generally to the field of rotary blowers or compressors used in automotive supercharging applications, and more particularly relates to a double rotor helical shaped multi-lobe type commonly known as Roots blowers or superchargers (other often used names are rotary PDs, rotary lobe or rotary piston types), and more specifically relates to a shunt pulsation trap for reducing pulsations and induced vibration, noise and harshness (NVH) from such superchargers for internal combustion engines.
2. Description of the Prior Art
Ever since German engineer Gottlieb Daimler filed the first patent in late 19th century, the Roots supercharger has been most widely used in supercharging automotive engines until turbocharging took its place. However, they are still popular for all kinds of 2-stroke and 4-stroke cycle engines either gasoline or diesel.
It has long been known that Roots blower or supercharger possesses a unique capability for generating adequate discharge pressures over a wide speed range. This unique variable pressure adaptability is attributed better to a wave Roots compression theory postulated by this author. Inside a Roots blower or a supercharger, air is not compressed “by a rapid backflow without internal compression” as the conventional Roots principle has been believed, but instead it is compressed by a series of pressure waves or shock waves generated by a sudden opening of lobes to the supercharger discharge pressure. The wave theory is based on a well studied physical phenomenon as occurs in a shock tube (invented in 1899) where a diaphragm separating a region of high-pressure gas from a region of low-pressure gas inside a closed tube. As shown in
To understand the Roots compression principle in light of the shock tube theory, let's review a cycle of a classical Roots supercharger as illustrated from
According to the conventional theory, a backflow would rush in compressing the air inside the cell at this point as shown in
However, according to the shock tube theory, the lobe opening phase as shown in
From the above Roots cycle analysis, it should be noted that energy transfers directly between two fluids by waves without using mechanical components like pistons or vaned impellers. Their major benefits are their potentials to generate large pressure changes in short time or small distance in an efficiency equivalent to those of dry screw compressors of the present times. Moreover, there is no over-compression or under-compression as in the case of conventional positive displacement compressors with a pre-determined pressure ratio, a unique ability to adapt to varying pressure demands. This makes Roots supercharger ideal for variable pressure applications such as in automotive supercharging at different speeds or different pressure boosting levels while maintaining a good efficiency throughout the process. Since the compression is achieved through faster moving waves or shock waves without hardware or the associated inertial, Roots supercharger can be build very small in size and simple in structure without complicated geometry or rotor contours.
Despite the above mentioned generally attractive features for Roots potentials, several challenges have impeded their extensive commercial applications. Among them, the number one issue is pulsation control. According to the wave Roots theory, when pressure waves or shockwaves are generated on low pressure side compressing the air inside the lobe cell, a series of expansions waves are generated simultaneously on high pressure side. Those large amplitude expansion waves combined with the reflected pressure wave or shockwaves from the lobe cells, if not blocked or treated, could travel downstream, creating huge pressure and flow pulsations and induced vibrations that could destroy downstream components, or generate noises as high as 170 dB for high pressure applications. Therefore, a large sized pulsation dampener, either in the form of a plenum or a reactive type, is usually required at the discharge stream of a Roots supercharger to dampen the air borne pulsations. It is generally very effective in pulsation control but requires large size to be effective, not suitable for mobile applications such as automobiles and trucks. At the same time, discharge dampeners used today could create high pressure losses that contribute to poor supercharger efficiency. For this reason, Roots superchargers are often cited with high pulsation, noise and low efficiencies, all of which prevent it from a wider use in spite of its unique merits due to wave compression.
Various attempts have been made to reduce Roots pulsations throughout years, but only limited successes have been achieved. The main reason for this failure is believed to be lacking an adequate Roots compression mechanism that could point to the root cause of pulsations. Traditionally, Roots compression has been regarded as a backflow mechanism instead of the wave mechanism as described above. Based on the conventional backflow principle which attributes sudden backflow as the cause of discharge pressure pulsations, most of the efforts have been focused on controlling this backflow. Among the methods, a flow feedback principle is most widely used, for example, as first disclosed in U.S. Pat. No. 4,215,977 to Weatherston, and later in U.S. Pat. No. 4,768,934 to Soeters (Eaton), U.S. Pat. No. 6,589,034 to Vorwerk (Ford) and U.S. Pat. No. 6,874,486 to Prior (GM). The idea is to feed back a portion of the outlet air through an injection port to the transfer chamber prior to discharging to the outlet, thereby gradually increasing the air pressure inside the cell and lengthening the pressure equalizing time, hence reducing discharge pressure spikes compared with a sudden opening at discharge. However, its effectiveness for pulsation attenuation is limited because it fails to recognize that the waves, not fluid flow, are the primary cause of the air-borne pulsations. In view of the wave compression theory, having a flow back earlier could reduce pulsations by elongating releasing time to discharge pressure. However, it failed to recognize hence attenuate the simultaneously generated expansion waves at the injection port that eventually travel down-stream unblocked, causing high pulsations. Moreover, the prior art failed to address the high flow losses associated with the high induced jet velocity through the injection port, resulting in a low supercharger efficiency that hampers it from being used more widely to more energy sensitive applications.
Since the amplitude of pressure pulsation in a supercharger is typically much higher than the upper limit of 140 dB set in classical acoustics, the small disturbance assumption or the resulting linear theory is inadequate to predict its behavior. Instead, the following rules can be used for large disturbances when the SPL is beyond 140 dB. These rules are based on the above discussed Shock Tube theory and can be used to judge the source of gas pulsation and quantitatively predict its amplitude and travel directions. In principle, these rules are applicable to the discharge process of any positive displacement fluid machines such as internal combustion engines, expanders and pneumatic motors, or compressors or pumps.
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- 1. Rule I: For two closed compartments (either moving or stationery) with different pressure levels p3 and p1 (
FIG. 1 a), there will be no pulsation generated if the two compartments are kept isolated with each other - 2. Rule II: If the divider between high pressure p3 and low pressure p1 is suddenly removed (
FIG. 1 b), it will trigger pulsation generation at opening as a mixture of Pressure Waves (PW) or a shock wave, Expansion Waves (EW) and an Induced Fluid Flow (IFF) with magnitudes as follows:
- 1. Rule I: For two closed compartments (either moving or stationery) with different pressure levels p3 and p1 (
PW=p2−p1 (1)
EW=p3−p2 (2)
IFF Velocity=(p2−p1/(d1×W) (3)
where d1 is the density of low pressure region and W the speed of shock wave travelling into the low pressure region, and
p2=(p3×p1)1/2 (4)
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- 3. Rule III: the generated Pressure Waves (PW) or shock wave travel at the shock wave speed W to low pressure region while Expansion Waves (EW) move at the speed of sound in a direction opposite to PW, while at the same time both waves induce an unidirectional fluid flow (IFF) moving in the same direction as the pressure waves (PW)
Pay attention to Rule #2 which gives the location of pulsation source as place of sudden opening between p3 and p1. It also indicates the sufficient conditions for gas pulsation generation as existence of pressure difference and sudden opening. Because all PD fluid machines convert energy between shaft and fluid by dividing incoming continuous fluid stream into parcels of compartment size for delivery to the discharge as indicated by its corresponding cycle, there always exists a “sudden” opening at discharge to return these discrete parcels of cavities back to a continuous stream again. So the two sufficient conditions are automatically satisfied at the moment of discharge opening if there is a pressure difference existing between the cavity and outlet it is opened to. The pulsation magnitude predicted by Rule #2 can be very high if (p3−p1) is large enough for an un-throttled (or infinitely fast) opening as in a shock tube. However, most PD type fluid machines operate with finite discharge opening speed which somehow throttles the induced fluid flow to a maximum sonic velocity that takes places at a pressure ratio of 1.89, say for a perfect gas with 1.4 specific heat ratio. In addition, a hardware (like lobe or valve disk) induced flow pulsation co-exists with pressure difference induced pulsation, but its magnitude is typically much smaller for most existing fluid machinery, and is roughly proportional to its equivalent velocity pressure.
- 3. Rule III: the generated Pressure Waves (PW) or shock wave travel at the shock wave speed W to low pressure region while Expansion Waves (EW) move at the speed of sound in a direction opposite to PW, while at the same time both waves induce an unidirectional fluid flow (IFF) moving in the same direction as the pressure waves (PW)
It should be pointed out the drastic magnitude and behavior difference between acoustic waves and pulsations discussed above. First of all, the linear acoustics is limited to pressure fluctuation level below 140 dB, equivalent to pressure level of 0.002 Bar or 0.03 psi. For fluid machinery, the measured pressure fluctuation or pulsation is often in the range of 0.3-30 psi (or even higher), equivalent to 160-200 dB. So pulsation pressures are much higher and well beyond the pressure range intended in classical acoustics. Physically, the acoustic waves are sound waves travelling at the speed of sound with no macro fluid movement with it while pulsations are a mixture of strong pressure and expansion waves that also induce an equally strong macro fluid flow travelling with speeds from a few centimeters per second up to 1.89 times of the speed of sound (Mach Number=1.89), for example. It is this large pressure forces and induced high velocity fluid flow that could directly damage a system and components on its travelling path, in addition to exciting vibrations and noises. With the above Pulsation Rules, it is hoped that more realistic pulsation prediction is made possible so that the true nature of pulsations can be realized, hence controlled.
Accordingly, it is always desirable to provide a new design and construction of a Roots supercharger that is capable of achieving high pulsation and NVH reduction at source and improving supercharger efficiency without externally connected silencers while being kept light in mass, compact in size and suitable for high efficiency, high pressure ratio applications at the same time.
SUMMARY OF THE INVENTIONAccordingly, it is an object of the present invention to provide a Roots supercharger with a shunt pulsation trap in parallel with the transfer chamber for trapping and attenuating pulsations at source.
It is a further object of the present invention to provide a Roots supercharger with a shunt pulsation trap as an integral part of the supercharger casing that does not need an externally connected pulsation dampener or silencer so that it remains light in weight and compact in size with less noise radiation surfaces.
It is a further object of the present invention to provide a Roots supercharger with a shunt pulsation trap that emits pulsation free gases downstream hence reduce fatigue failure of downstream components.
It is a further object of the present invention to provide a Roots supercharger with a shunt pulsation trap that is capable of achieving all the above objectives in a wide range of engine operating speeds and loads.
It is a further object of the present invention to provide a Roots supercharger with a shunt pulsation trap that is capable of achieving higher adiabatic efficiency in the range equivalent or close to conventional turbocharger or dry screw supercharger, say up to about 80%.
Referring particularly to the drawings for the purpose of illustration only and not limited for its alternative uses, there is illustrated:
Although specific embodiments of the present invention will now be described with reference to the drawings, it should be understood that such embodiments are examples only and merely illustrative of but a small number of the many possible specific embodiments which can represent applications of the principles of the present invention. Various changes and modifications obvious to one skilled in the art to which the present invention pertains are deemed to be within the spirit, scope and contemplation of the present invention as further defined in the appended claims.
It should also be pointed out that though drawing illustrations and description are devoted to a helical three-lobe Roots supercharger in the present invention, the principle can be applied to other types of rotary supercharger with different numbers of lobes such as four-lobed, five-lobed or six lobed, etc. as long as both rotors have the same number of lobes. The principle can also be applied to either gas or liquid media, such as helical lobe or helical gear pumps that are variations of helical Roots superchargers for liquid and the later uses involute lobe shape to allow the lobes function as gears with rolling interfacial contact. In addition, helical lobe expanders are the above variations too except being used to generate shaft power from a media pressure drop.
As a brief introduction to the principle of the present invention,
The principal difference with the conventional Roots supercharger is in the compression and dampening phase: instead of waiting and delaying the compression and attenuation action until the lobe tip reaches the outlet by using a serially-connected dampener silencer, the shunt pulsation trap would start compression and induce pulsations into the trap as soon as the trap inlet is exposed to the trapped flow cell after it is sealed from the inlet. It then dampens the pulsations within the trap simultaneously as the cell flow is being compressed before reaching the outlet. In this process, the flow cell being compressed and pulsations being attenuated are happening in parallel with each other instead of in series as in the conventional Roots supercharger. Or in another word, compression and pulsation dampening are conducted at the same time (hence the name parallel or shunt), not one after the other (in series).
There are several advantages associated with the parallel pulsation trap compared with the traditional serially connected dampener silencer. First of all, pulsating wave attenuation is separated from the main cell flow so that an effective attenuation will not affect the main flow cell, resulting in both higher compression efficiency and attenuation efficiency. In a traditional serially connected silencer, both pulsating waves and fluid flow travel together through the dampening elements inside the silencer where a better attenuation always comes at a cost of higher static pressure drop. So a compromise is often made in order to reduce pressure loss by sacrificing the degree of pulsation dampening or use a very large volume silencer in a serial setup.
Secondly, the parallel pulsation trap attenuates pulsation much closer to the pulsation source than a serial one and is capable of using a more effective pulsation dampening means without affecting main flow efficiency. It can be built as an integral part and conforming shape of the supercharger casing with a much smaller size and footprint; hence less weight and cost. By replacing the traditional serially connected silencer with an integral paralleled pulsation trap, it will be light in weight and compact in size which also reduces noise radiation surfaces and is more suitable for mobile applications.
Moreover, the pulsation trap is so constructed that its inner casing is an integral part of the outer casing of the transfer chamber, and the outer casing are oversized surrounding the inner casing, resulting in a double-walled structure enclosing the noise source deeply inside the core with much less noise radiation surface area. The casings could be made from a casting that would be more wave absorptive, thicker and more rigid than a conventional sheet-metal silencer casing, hence less noise radiation.
With an integral pulsation trap, the supercharger outer casing would be structurally more rigid and resistant to stress or thermal related deformations. At the same time, the double-wall casing tends to have a more uniform temperature distribution inside the pulsation trap so that the traditional “banana shaped” casing distortion would be kept to minimum, thus reducing internal clearances and leakages, resulting in higher supercharger efficiency.
Referring to
As an important novel and unique feature of the present invention, a shunt pulsation trap apparatus 50 is conformingly surrounding the Roots supercharger 10 of the conventional design shown in
When a Roots supercharger 10 is equipped with the shunt pulsation trap apparatus 50 of the present invention, there exist both a reduction in pulsation discharged from Roots supercharger to supercharger downstream flow as well as an improvement in internal flow field (hence its adiabatic efficiency) so that it is compactly suitable for mobile applications, and efficiently suitable for applications typically reserved for conventional turbochargers or dry screw superchargers.
The theory of operation underlying the shunt pulsation trap apparatus 50 of the present invention is as follows. As illustrated in
Moreover, the hot feedback flow 53 sandwiched between the cored and integrated inner casing 20 and outer casing 28 acts like a water jacket in a piston cylinder of an internal combustion engine, tending to equalize temperature difference between the cool inlet port 36 and hot outlet port 38. This would lead to less thermal distortion of the inner casing 20, which in turn would decrease the internal end clearance and tip clearance. In addition, by getting rid of the serially connected silencer, the associated discharge dampening losses are eliminated for the main cell flow At the trap inlet 41, the induced injection flow could be “choked” as pressure ratio across reaches 1.89, seriously limiting injection flow capacity and creating pressure losses. So using a flow nozzle 63 or de Laval nozzle 65, as shown in
It is apparent that there has been provided in accordance with the present invention a Roots supercharger with a shunt pulsation trap for effectively reducing the high pulsations caused by wave compression without increasing overall size of the supercharger. While the present invention has been described in context of the specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.
Claims
1. A Roots supercharger with a shunt pulsation trap apparatus, comprising:
- a. a housing structure having an inner casing with a flow suction port, a flow discharge port and a transfer chamber there-between, and at least one injection port located at least one lobe span away from said flow suction port communicating with said transfer chamber and at least one feedback region communicating with said flow discharge port, and an outer casing enclosing said inner casing;
- b. two parallel multi-helical-lobe rotors having same number of lobes and rotatably mounted on two parallel rotor shafts respectively inside said inner casing and interconnected through a set of timing gears to rotate in synchronization for propelling flow from said suction port to said discharge port:
- c. a shunt pulsation trap apparatus comprising said inner casing as an integral part of said transfer chamber, and said outer casing oversized surrounding said inner casing, therein housed various pulsation dampening means or pulsation energy recovery means or pulsation containment means, at least one trap inlet (said injection port) branching off from said transfer chamber into said pulsation trap and at least one trap outlet (said feedback region) communicating with said supercharger discharge port;
- d. whereby said Roots supercharger is capable of achieving high pulsation and NVH reduction at source and improving supercharger efficiency while being kept light in mass, compact in size and suitable for both mobile and stationary applications at the same time.
2. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said multi-helical-lobe rotor is of twisted shape in its axial direction and having at least three or more lobes per rotor.
3. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said injection port (trap inlet) is at least one lobe span away from said supercharger suction opening and has a converging cross-sectional shape or a converging-diverging cross-sectional (De Laval nozzle) shape in feedback flow direction.
4. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means comprises at least one layer of perforated plate or acoustical absorption materials or other similar types for turning pulsation into heat.
5. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means comprises at least one layer of perforated plate on which there is at least one synchronized valve that is closed and opened as said each lobe passes said trap inlet.
6. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means comprises at least one Helmholtz resonator.
7. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means comprises at least one Helmholtz resonator in parallel with at least one layer of perforated plate or acoustical absorption materials or other similar types for turning pulsation into heat.
8. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means comprises at least one Helmholtz resonator in parallel with at least one synchronized valve that is closed and opened as each said lobe passes said trap inlet.
9. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means and said energy recovery means comprise at least a diaphragm or a piston or other similar types in parallel with at least one layer of perforated plate or acoustical absorption materials or other similar types for partially turning pulsation into heat and partially absorbing pulsation energy and turning that energy into pumping air from said trap outlet through said perforated plate into said trap.
10. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means and energy recovery means comprise at least a diaphragm or a piston or other similar types in parallel with an opening for absorbing pulsation energy and turning that energy into pumping air from said trap outlet through said opening into said trap.
11. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means and energy recovery means comprise at least a diaphragm or a piston or other similar types synchronized with at least one valve for absorbing pulsation energy and turning that energy into pumping air from said trap outlet through said valve into said trap.
12. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means and energy recovery means comprise at least a diaphragm or a piston or other similar types synchronized with at least one valve for absorbing pulsation energy and turning that energy into driving an externally connected load.
13. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means and energy recovery means comprise at least a diaphragm or a piston or other similar types synchronized with at least two valves, one at trap inlet, the other at trap outlet, for absorbing pulsation energy and turning that energy into driving an externally connected load.
14. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation trap further comprises at least one perforated plate located at said suction port or at least one perforated plate located at said discharge port or both either before or alternatively after said trap outlet
15. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation containment means comprises at least one control valve located at said trap outlet.
16. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation containment means comprises at least one layer of perforated plate or acoustical absorption materials or other similar types for turning pulsation into heat, in series with at least one control valve located at said trap outlet.
17. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 15 or 16, wherein said control valve in said pulsation containment means is a one way valve, like a reed valve.
18. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 15 or 16, wherein said control valve in said pulsation containment means is a rotary valve that is timed to close and open as each said lobe passes said trap inlet.
19. The perforated plate as claimed in claim 14 has holes with a cross-sectional shape of either constant area or converging shape or a converging-diverging (De Laval nozzle) shape in flow direction.
Type: Application
Filed: Jul 3, 2011
Publication Date: Jan 26, 2012
Inventors: Paul Xiubao Huang (Fayetteville, GA), Sean William Yonkers (Peachtree City, GA)
Application Number: 13/175,875
International Classification: F01C 21/00 (20060101);