A commercially successful supercharger arrangement for diesel engines to date is the type generally referred to as an aerodynamic wave machine and described in German Pat. No. 485,386 (1929) and Swiss Pat. No. 225,426 (1946). The basic principle of that supercharger is the transfer of pressure energy from the high pressure engine exhaust gas to low pressure air which is then delivered at high pressure back to the engine air intake manifold. The wave machine includes a cylindrical chamber in which there is located a rotor with multiple radial vanes, the rotor being driven from the engine by means of a belt drive arrangement. Openings are provided at the cylinder ends at appropriate locations to effect ingress and egress of fluid to and from the chamber. In operation, atmospheric air, present in each space between adjacent rotating vanes, experiences a pressure wave when that space passes an inlet opening at one of the chambers to which the high pressure engine exhaust gas is directed. The compression wave, moving at the speed of sound, compresses the air in the space as the wave passes by. The compressed air behind the wave occupies less space than before, permitting the engine exhaust gas to enter into the space between the vanes. As the compression wave reaches the opposite end of the cylinder, the vanes in question pass an outlet opening in that end which permits the compressed air to egress to the engine intake. By the time the compression wave reflects back toward the exhaust gas supply end of the chamber, the vanes have moved past the exhaust gas supply opening. Eventually the still slightly pressurized exhaust gas is permitted to egress from another chamber opening to ambient as the vanes pass the latter opening. A still further opening admits air into the space between the vanes before the rotation cycle is completed.
The aforementioned wave machine thus serves as a supercharger for the engine by utilizing the pressurized hot engine exhaust gases to pressurize the cold air delivered to the engine. The system is relatively efficient and has been used for a number of years. Nevertheless, there are some problems associated with the wave machine which have served as limiting factors on its increased utilization. Specifically, the wave machine requires additional controls during cold engine start up conditions because the exhaust gas flows straight through the machine to the engine intake under low engine rpm conditions. These controls usually take the form of a manually-operated choke-like arrangement which moves a butterfly valve or the like in position to admit air into the engine from a source other than the wave machine. In addition, the wave machine has a rotor arrangement which is subject to temperature variations on the order of 800.degree. C., thereby limiting the choice of materials that can be used while maintaining the critical dimensional tolerances necessary to keep the engine operating. Further, because of the complex moving parts and their critical tolerances, wear and tear and a continuing need for lubrication present significant problems. Still further, because of the need for special materials and the relative complexity of the machine, cost becomes a problem, particularly when the machine is used in conjunction with a passenger vehicle. Another problem relates to the fact that the wave machine must be mechanically driven by the engine, resulting in operation over a wide range of rotating frequencies, some of which result in highly inefficient operation and power loss. The need to drive the wave machine from the engine also subjects the engine to being smothered by its exhaust gas if a drive belt breaks and also requires the wave machine to be located near the engine which often presents a severe space problem to a vehicle designer.
There have been attempts in the prior art to provide gas energy exchangers which eliminate the aforesaid problems of the aerodynamic wave machine. For example, in German Pat. OS No. 1,628,430 an energy exchange conduit is provided with externally-controlled valving at each end, the valving being timed to effect a specified sequence of gaseous ingress and egress into the conduit. Initially, gas in the conduit is compressed by admitting high pressure steam into one end. The opposite end is then opened to permit egress of the compressed gas, followed by opening of the first end to permit egress of the steam, now at a reduced pressure. The lower pressure steam still in the conduit expands permitting low pressure gas to be admitted through the opposite end, whereupon the cycle is repeated. This arrangement, while in some respects more advantageous than the aforesaid aerodynamic wave machine, introduces certain complexities which severely limit its practical utility, particularly in internal combustion supercharging applications. Specifically, this arrangement requires controlled valving at both ends of the energy exchange conduit and also requires some sort of accurate timing control external to the energy exchanger per se. The multiple valves, which must operate at high repetition rates and are subjected to wide temperature variations, are prone to failure, particularly when used in a supercharger application. The timing control is complex and also not well suited for automotive applications.
Another prior art gas energy exchanger is found in German Pat. No. AS 2,328,692. This arrangement utilizes one or more pairs of engine cylinders for each energy exchanger conduit. The pressurized exhaust gas from each cylinder, which flows as pulses in accordance with the combustion cycle of that cylinder, is split into two flows. One portion is fed to an energy exchange conduit wherein the pulse pressurizes air contained in the conduit. The other flow portion is directed through an ejector nozzle from which the flow aspirates spent exhauster gas from another energy exchange conduit associated with the other cylinder of the cylinder pair. The compressed air is forced through a check valve into a common inlet conduit for the cylinder pair. This inlet conduit has a check valve at either end, the valves being operated at different phases to permit alternate entry of pressurized air into the inlet conduit from respective energy exchange conduits of the cylinder pair. This arrangement also effects certain simplifications relative to the aforesaid aerodynamic wave machine; however, it, too, suffers from practical disadvantages which limit its commercial usefulness. In particular, the arrangement is workable only with an even number of engine cylinders, with each pair operating 180.degree. out of phase with one another, thereby eliminating its application with three, five or seven cylinder diesel engines, or with engines of even numbered cylinders not having the desired phase relationship in cylinder operation. Further, there must be at least part of an energy exchange system provided for each pair of cylinders, thereby requiring duplication (or triplication or quadruplication) of valves, tubing, etc. for a given multi-cylinder engine. Still further, the high pressure exhaust gas pulses are derived directly from combustion in each individual cylinder. This presents wavelength matching problems since the particular conduit length is properly matched to the cylinder for only on engine speed. The wavelength mismatch results in loss of available energy and poor supercharging efficiency.
A still further approach toward solving the problems of the aerodynamic wave machine is found in Swiss Pat. No. 450613. In this arrangement the total pressurized exhaust gas flow from the engine is delivered to a fluidic oscillator wherein the exhaust gas flow is cyclically deflected so that it alternately flows into two energy exchange conduits. The pulse of exhaust gas entering each energy exchange conduit produces a pressure wave which traverses the air in that conduit, compressing the air as it passes. The compressed air is then forced into a supply tube for the engine air intake. An air aspiration inlet in the conduit permits air to be aspirated into the conduit behind the flowing exhaust gas slug passing the aspiration inlet. Thus, each energy exchange conduit receives air, has that air compressed by the pressure wave which runs ahead of the exhaust gas pulse, flows the compressed air back to the engine, and then causes the exhaust gas to egress via a separate path while aspirating fresh air into the conduit. The timing of these events is repetitive with the cycle in the two conduits being oppositely phased. This arrangement turns out to be totally impractical. For one thing, in order to divert the egressing compressed air into one outlet (i.e. the engine air intake) and the spent exhaust gas into another outlet (i.e. ambient), a second oscillator or other diverter is required in each of the two energy exchange conduits. This requires a large number of components, such as piping, tubing, etc., and also results in very large pressure losses. The large pressure losses occur because the flow experiences a pressure drop of approximately 50% through the fluidic element employed. Two fluidic oscillators in series, as used in this arrangement, results in a pressure loss of approximately 75%. Moreover, the air aspiration junction (connected in series), the forward control flow junctions or pick-offs (in series), and the open-to-ambient compressed air recombination junction for the two conduits all cause further pressure losses, of approximately 75%. As a result, the pressure recovery of the arrangement is only on the order of 1/16th of the exhaust gas energy applied to the system. In addition, further pressure losses are experienced due to shock waves created at supersonic flow regimes at junctions in the system.
Apart from these serious drawbacks to the system of Swiss Pat. No. 450,612, there are still other drawbacks. For example, the air aspiration junctions can be configured for effectiveness over only a narrow range of exhaust gas flow conditions; this presents a problem since the exhaust gas outflow from the engine and into the system varies a great deal. Moreover, there are extremely critical timing problems involved in properly operating the system, which problems are not considered in the patent. Specifically, the diverters must be synchronized to the oscillator in a way which eliminates mixing of the compressed air and exhaust gas. This synchronization problem is exacerbated in the supersonic flow regime which usually results in irregular flow conditions, particularly at flow passage discontinuities such as branches, junctions, etc. Since there are two interfaces between the exhaust gas and air for each pulse, the timing problems become all the more crucial if one is to avoid mixing exhaust gas with the air delivered to the engine air intake. In general then, this system is nothing more than a paper design having no practical application.
It is therefore an object of the present invention to provide a gas energy exchanger which avoids the cost and complexity factors of the aforesaid aerodynamic wave machine yet which is practical and useful over a wide range of pressurized gas flows. It is another object to provide such an energy exchanger which has a minimum number of parts and does not require critical material or tolerances. A still further object of the present invention is to provide a gas energy exchanger in which the disadvantages of the aforementioned prior art are avoided.