Abstract:
An internal combustion engine is provided with an expansion cylinder and at least one combustion cylinder, preferably two or four combustion cylinders per expansion cylinder. An air-fuel mixture is ignited within the combustion cylinders to drive a combustion piston which, in turn, drives an engine crankshaft. The gaseous products of combustion are exhausted at a pressure substantially above atmospheric to an expansion cylinder wherein they are allowed to further expand against an expander piston to drive an expander crankshaft. Torque produced at the engine crankshaft and torque produced at the expander crankshaft are combined to drive vehicle wheels.

Description:
This application claims benefit to U.S. provisional 60/096,403 filed Aug. 13, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the invention is internal-combustion engines for motor vehicles. 
     2. Related Art 
     The growing utilization of automobiles greatly adds to the atmospheric presence of various pollutants including oxides of nitrogen and greenhouse gases such as carbon dioxide. Accordingly, a need exists for a new approach which can significantly improve the efficiency of fuel utilization for automotive powertrains while still achieving low levels of NOx emissions. 
     Internal combustion engines create mechanical work from fuel energy by combusting the fuel over a thermodynamic cycle consisting (in part) of compression, ignition, and expansion. The efficiency with which mechanical work is converted from the available fuel energy is determined by the thermodynamic efficiency of the cycle. Thermodynamic efficiency, in turn, is determined in part by (a) the degree to which the fuel/air mixture is compressed prior to ignition (compression ratio), and (b) the final pressure to which the combusted mixture can be expanded while performing useful work on the piston which is related to the expansion ratio of the power or expansion stroke. Generally speaking, the lower the final pressure achieved during expansion against the piston, the greater the amount of work extracted. The pressure drop is limited by the fixed maximum volume of the cylinder, since there is only a finite volume available in which combusting gases may expand and still perform work on the piston. At some point the piston will reach bottom dead center, after which the gases, still at a high enough pressure to perform work, must be exhausted from the cylinder as the piston begins to rise again. 
     To fully utilize the pressure of the combustion gases, it would be necessary to expand the gases to ambient pressure while pushing against the piston. The phenomenon is illustrated in FIG.  1 . Normally, gases are exhausted to the atmosphere when the expansion of the combustion cylinder stops. Some of the work extracted is represented by the unshaded area under the curve. The pressure of this exhausted gas is still higher than ambient pressure. If this residual pressure were expanded against another piston to ambient pressure, the additional work would equal the area represented by the shaded area under the curve. Some of this additional work (“A”) would go toward operating the engine itself, but a significant amount (“B”) would remain to create a net increase in work extracted. 
     Reaching such a low pressure would require a larger volume in which to expand the products of combustion, suggesting that the stroke of the piston or the maximum volume of the cylinder should be increased during the expansion stroke. Of course, the compression ratio would then increase in the same manner because the compression ratio is also governed by maximum cylinder volume. The result would be simply a larger engine cylinder, or an unacceptably large compression ratio. 
     Conventional engines are limited to having an expansion ratio roughly equal to the compression ratio. This is because compression and expansion both take place in a single cylinder that has a fixed maximum and minimum volume. It is possible to effectively change the two ratios relative to one another by manipulating the characteristics of the fuel-air mixture. For example, turbocharging and supercharging are used to increase the effective compression ratio relative to the expansion ratio. This is done by forcing a greater mass of air (and ultimately fuel/air mixture) into the combustion chamber without changing the actual volumetric compression ratio. This leads to increased power for a given engine displacement. But this approach does not affect the actual volumes involved and cannot provide a way to improve the expansion ratio relative to the compression ratio. Similarly, by restricting the flow of air into the cylinder during the intake stroke, or by other manipulation of exhaust or intake valves, it would be possible to reduce the effective compression ratio relative to the expansion ratio. However, this would introduce fluid-mechanical problems due to air flow and cylinder pressures that would probably require sophisticated timing strategies and detrimentally affect the efficiency of the thermodynamic cycle. 
     An engine design for increasing the expansion ratio relative to the compression ratio by means of dual cylinder expansion, is disclosed in a 1993 paper published by the Society of Automotive Engineers (SAE number 930986). The disclosed design includes an auxiliary cylinder dedicated to further expansion of gases against a piston after they have been exhausted from the main combustion cylinders. The system also includes a compression cylinder to provide supercharging capability. However, the valving arrangements of this system would require two additional valves per cylinder, one for supercharging and one for expanding, for a total of four valves per combustion cylinder. In addition, the design disclosed in this SAE paper utilizes two valves each, for the separate expansion and companion cylinders. The configuration as shown requires long runners between the combustion cylinders and the auxiliary cylinders, which runners would increase the effective expansion volume, introduce pressure losses, and possibly introduce back-pressure problems that would require complex valving and control to overcome. Its main purpose seems to be to improve power output rather than reduce NOx emissions and improve energy conversion efficiency, as indicated by an integrated supercharging device. 
     SUMMARY OF THE INVENTION 
     The present invention is a unique mechanism, with a simplified valve arrangement and/or drive output, for increasing the expansion ratio relative to the compression ratio, thereby allowing the additional pressure of expanding gases to be brought closer to ambient pressure while performing useful work. The engine combustion cylinders (hereafter called engine cylinders) are connected to expansion cylinders which can be arranged to minimize or eliminate runner length. Valving is simplified by elimination of all but a single exhaust valve between the expansion cylinder and the combustion cylinders. In at least one embodiment, there is one complete cycle of the expansion cylinder for every stroke of the connected 4-stroke combustion cylinder. Thus, up to four combustion cylinders of a four-stroke engine could be served by a single expansion cylinder. 
     In at least one embodiment, gases are not delivered to the expansion cylinder(s) until the gases in the engine cylinder have reached their maximum expansion, so that all of the energy produced by the expansion within the expansion cylinder is energy that would otherwise have been discarded. The invention is dedicated to improving the thermodynamic efficiency of the cycle, and does not require additional energy for supercharging or other means of power improvement, although same could be added very efficiently. 
     Using the apparatus of the present invention, the combustion cylinder can be operated with late fuel ignition to minimize NOx formation, while the expansion chamber allows full expansion of the combustion gases. 
     Accordingly, the present invention provides an internal combustion engine which includes at least one combustion cylinder with a combustion piston reciprocably mounted therein and an expansion cylinder with an expansion piston reciprocably mounted therein. Each combustion cylinder has at least one intake port for intake of combustion air and at least one exhaust port for exhausting the gaseous products of combustion, as well as ignition means for igniting an air-fuel mixture therein to produce the gaseous products of combustion. The one or more combustion pistons are linked to an engine crankshaft whereby the crankshaft is driven responsive to combustion within the one or more combustion cylinders. The expansion cylinder is provided with a gas inlet port for receiving the gaseous products of combustion exiting the combustion cylinder or cylinders at a pressure above atmospheric and a gas outlet port for exhausting the exhaust gases to the ambient atmosphere after having undergone further expansion to drive the expander piston. The expander piston is linked to an expander crankshaft, whereby the expander crankshaft is driven and its output is combined with the output of the engine crankshaft at a drive shaft to drive the wheels of the vehicle. The flow of exhausted combustion gases out of the combustion cylinder and into the expansion cylinder, as well as the intake of combustion air into the combustion cylinder may be controlled by poppet valves mounted in the cylinder head closing the combustion cylinder. Alternatively, a combustion cylinder may be operated in conjunction with an expander cylinder using only two valves located, respectively, at an air intake duct for the combustion air and in a gas passage connecting the exhaust port of the combustion cylinder with the gas inlet port of the expansion cylinder. In this latter embodiment the gas inlet port is located above top dead center in the expansion cylinder and the gas outlet port is located adjacent bottom dead center, but between top dead center and bottom dead center so that the expander piston serves to open and close the gas outlet valve in the course of its reciprocating motion. 
     The present invention also provides a method of powering an engine vehicle with two expansion strokes per cycle of a combustion cylinder An air-fuel mixture is ignited within a combustion cylinder and the gaseous products of combustion are allowed to expand against a combustion piston to drive an engine crankshaft with a first amount of torque. The gaseous products of combustion are transferred from the combustion cylinder to an expansion cylinder at a pressure substantially above atmospheric pressure, and allowed to expand within the expansion cylinder against an expander piston, to drive an expander crankshaft with a second increment of torque. The two amounts of torque are then combined to drive wheels of the vehicle. 
     This invention also allows for operation of an internal combustion engine in a manner that reduces NOx formation without sacrificing efficiency. NOx formation in an internal combustion engine is strongly related to and increases with increasing peak combustion temperature. A common means of reducing peak combustion temperature, and thus NOx formation, is ignition of the fuel late in the compression stroke or early in the expansion stroke so that peak combustion temperature occurs after the engine has begun its expansion stroke, and the expansion process imparts a cooling effect on the combustion gases, thereby resulting in a lower peak combustion temperature. Unfortunately, such late combustion in conventional engines results in reduced fuel efficiency because the pressure resulting from combustion is occurring after the expansion process has begun, and the remaining effective expansion ratio is less than the compression ratio. The result is that the combustion pressure is not as fully expanded as it would have been had the ignition and pressure release occurred before the expansion process began. When the exhaust valve opens, the higher pressure gas is exhausted and its remaining energy is wasted. In contrast, this invention allows operation with late ignition and low NOx formation, but without the fuel economy penalty associated with such operation in conventional engines. This combination is possible because the second expander cylinder is still capable of full expansion of the combustion gas pressure. 
     The unique features of the invention provide the following advantages over conventional engines and over prior methods of increasing the expansion ratio relative to the compression ratio. 
     Firstly, compared to conventional engines, the present invention increases the actual volumetric expansion ratio relative to the actual compression ratio, and leads to greater utilization of the chemical energy contained in the fuel. 
     Secondly, compared to prior approaches to increasing expansion ratio relative to the compression ratio, the present invention provides simplification of necessary valving (to the point of eliminating the need for additional valving), minimization of passage volume and the associated back-pressure problems, and minimization of wasted expansion volume contained in passageways. 
     Thirdly, the present invention utilizes dual cylinder expansion to achieve a greater expansion ratio than compression ratio without increasing the number of combustion cylinder valves. 
     Fourthly, the present invention allows one expander cylinder/piston to serve multiple (i.e., two or four) primary engine cylinders/pistons. 
     Fifthly, in a preferred embodiment the present invention provides an expander design which operates without intake or exhaust valves, wherein exhaust gas is expelled through lower cylinder exhaust ports. 
     Sixthly, in yet another preferred embodiment the present invention provides an expander design which utilizes a unique double-piston crank loop mechanism. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a graph of pressure versus volume in a combustion cylinder, illustrating extraction of work from the pressure generated by combustion; 
     FIG. 2 is a schematic view of a first embodiment of the present invention; 
     FIG. 3 is a schematic view of a second embodiment of the present invention; 
     FIG. 4 is a schematic view of a third embodiment of the present invention; 
     FIG. 5 is a graph of pressures within two combustion cylinders and within a single expander cylinder, receiving exhaust gas from both of the combustion cylinders, versus crank angles and of expander work versus the same crank angles; 
     FIG. 6 is a graph of volume within a single combustion chamber and a connected expander cylinder versus crank angles and flow areas of exhaust ports versus the same crank angles; 
     FIG. 7 is a schematic view of paired expansion cylinders in a third embodiment of the invention; and 
     FIG. 8 is a schematic view of gearing connecting the engine crankshaft with the expander crankshaft. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 shows an embodiment of the invention as consisting of at least two cylinders (or rotors for a rotary engine), one of which is a cylinder  10  of an internal-combustion engine and the other a dedicated expansion cylinder  20 . Cylinder  10  is provided with a spark plug  49  but the expansion cylinder  20  is devoid of any spark plug, glow plug or other ignition device. The cylinders are united by a short passage or port  30 , governed by one-way valve  33  which allows gases to flow from the combustion cylinder  10  to the expansion cylinder  20 . There is also a conventional intake passage and valve  32  on the combustion cylinder  10 , and a final exhaust passage  34  on the expansion cylinder  20 . Both cylinders have a piston  13 ,  28  against which expanding gases may perform useful work and deliver the work to a rotating crankshaft  38 ,  40 . The expander piston  28  powers a crankshaft  40  separate from the engine crankshaft  38 . Both crankshafts  38 ,  40  are connected although they may be timed differently or have different rotational speeds (depending on the number of power cylinders served by a single expander piston). 
     Together the two cylinder assemblies  10 ,  20  perform a role similar to a single conventional engine cylinder. Combustion, ignition, and expansion take place in the engine cylinder  10  in the usual manner. The expansion cylinder  20  provides the means for a second stage of expansion to take place instead of exhausting the gases from the engine cylinder directly to the atmosphere. Thus, the expansion ratio is effectively increased relative to the compression ratio by adding a second expansion volume that is separate from the engine cylinder  10 . Since the compression process still takes place entirely within the engine cylinder  10 , it remains unchanged. 
     The expansion cylinder  20  has a piston  28  on which expanding gases from the engine cylinder, having already performed work on the engine piston  13 , can continue to perform useful work. Considering both cylinders and the expansion/work therein, the pressure of the exhaust finally exiting from the expander exhaust port  34  is lower than if exhausted from the engine cylinder alone without the further expansion, indicating that additional work was extracted in expansion cylinder  20 . The expansion cylinder  20  allows the relatively high-pressure gases that would normally be discarded at the end of the power stroke of the engine cylinder  10  to be used for another power stroke in the expansion cylinder before finally exhausted to the atmosphere. 
     In another preferred embodiment as shown in FIG. 3 a full cycle takes place as follows. During the intake cycle, initiated at or near point (A) (top dead center or “TDC”), the intake valve  51  opens while a one-way valve  54  remains closed. The engine piston  53  travels downward, causing air or air/fuel mixture to be taken into the combustion cylinder  50  as in a typical Diesel or Otto cycle engine. At point (B) (bottom dead center or compression begins as the piston  53  travels upward and intake valve  51  closes (the actual point at which compression begins may vary depending on valve timing). Upon returning to position (A), compression of the air/fuel mixture is complete and combustion begins. The expanding combustion products perform work on the piston  53  as it travels downward, delivering mechanical energy to crankshaft  55 . Upon reaching position (B), the expansion within cylinder  50  has reached its maximum and work can no longer be performed on piston  53 . At this point, valve  54  opens, allowing the spent gases to be exhausted through connecting passage  56  to expander  52 . As gases begin to enter expander  52 , piston  53  begins to leave position (B), and the expander piston  57  is positioned at point (D) near the top of its stroke (actual location may vary with relative crank angle timing). While engine piston  53  travels from point (B) to point (C), expander piston  57  travels from point (D) to point (E) at the bottom of its stroke, during which time the spent gases from combustion cylinder  50  perform additional work on expander piston  57 . In this embodiment, the speed of the expander crankshaft  59  is twice that of the engine crankshaft  55 , allowing one full cycle of the expander  52  to take place for each exhaust cycle of the combustion cylinder  50 . This work powers expander crankshaft  59 . While the engine piston  53  completes the final portion of its exhaust stroke by traveling from point (C) to point (A), the exhaust of expander  52  takes place through valve  58  as expander piston  57  approaches position “D”. 
     One salient feature of the embodiment of FIG. 4 is that the expander has no valves. In the embodiment of FIG. 4, for example, the expander inlet gas flow through passage  66  is controlled by the opening and closing of the engine exhaust valve  62 . The exhaust of gas from the expander  70  is controlled by the expander piston  57  uncovering openings (exhaust ports  59 ) in the expander cylinder as it approaches its bottom dead center (BDC) (position “E” in FIG.  3 ). The timings of the engine crankshaft  65  and the expander crankshaft  69  must be significantly offset to provide proper functioning. For example, in a configuration where the speeds of the engine  60  and expander  70  are equal, the engine has two cylinders operating on a four-stroke cycle, the expander  70  has one cylinder and the swept volume of the expander piston  68  is two and one half times the swept volume of an engine piston  53 . As an engine piston  63  is completing its expansion stroke, the expander piston  57  is completing its upward stroke compressing the residual exhaust gas from the previous cycle. At that point where the pressure within the engine cylinder  60  and the pressure within the expander  70  are equal, the engine exhaust valve  62  begins opening. As the engine piston  63  crosses BDC on its expansion stroke and begins the upward motion of its “exhaust” stroke, the expander piston  57  crosses top dead center (TDC) and begins its downward or expansion stroke. Since the swept volume of the expander piston  57  is greater than that of an engine piston  53 , the combustion gases experience a greater expansion than what would have been experienced in the engine alone. As the engine piston  53  approaches TDC, its exhaust valve  54  begins shutting, and the expander piston  57  approaches BDC (position “E”). The expander exhaust ports  58  must be open for a sufficient period (i.e., number of crank angle degrees) for exhaust gases to be expelled equivalent to the last engine cycle exhaust gas mass. As the expander piston  57  crosses BDC and begins its upward “compression” stroke, the piston from the other engine cylinder is beginning its expansion stroke, and the expander cycle repeats. FIGS. 5 and 6 show engine cylinder and expander volumes, valve and port flow areas (i.e., valve opening and closing timings), engine cylinder and expander pressures, and expander piston work as a function of crank angle, for the case where the crank angle offset is 120° and the expander exhaust port “event” is 184° crank angle. 
     In many embodiments, the speed of the expander crankshaft  59  will be greater than that of the engine crankshaft  55 , and the crank angles will differ, but these relationships need not hold for all embodiments. In the embodiment of FIG. 4, the expander  70  operates at twice the speed of the engine, so that one complete expansion and exhaust cycle in the expander  70  takes place for each exhaust stroke of an engine cylinder  60 . In this manner, up to four engine cylinders can be served by a single expander. 
     As shown in FIG. 6, the expansion ratio for a combustion cylinder operated in accordance with the present invention is typically about 1:18, ranging from about 1:10 to above 1:25, and the expansion ratio for the expansion cylinder is typically about 1:10, ranging from about 1:8 to about 1:12. As seen in FIG. 5 the exhaust from the combustion cylinder is typically received by the expansion cylinder at 3.5-4.0 bars and exhausted at 1 bar (ambient). The relationship between crank angles is also shown in FIGS. 5 and 6. In order to minimize NOx formation ignition is started within the interval of from 10° before top dead center in the compression stroke to 5° after top dead center in the expansion stroke. 
     In order to produce net positive work in an expander, from the further expansion of an engine&#39;s residual exhaust gas pressure, the expander&#39;s frictional losses must be less than the potential work extractable by the expander. FIG. 7 shows a unique double piston crank loop expander design. While single-piston crank loop designs are well known, as are their low friction characteristics, utilizing pistons on each end of a single crank loop mechanism provides a doubling of the expander capacity with only a modest increase in cost as compared to utilizing two separate single-piston crank loop mechanisms. As shown in FIG. 7, first and second expander cylinders  72 ,  73  are aligned on opposite sides of an expander crankshaft  74  with cam  76  engaging a continuous camming surface  79  of cam follower  68 . Piston  82  of expander cylinder  72  is connected to the cam follower  80  through a piston shaft  84  for reciprocating motion between TDC and BDC, the linearity of which is ensured by bushing  85 , surrounding piston shaft  84 . Likewise, piston  83  within expander cylinder  73  is connected to cam follower  80  through a second piston shaft  86 . The linearity of the reciprocating motion of piston  83  and piston shaft  86  is likewise ensured by bushing  87 . In the embodiment shown in FIG. 7 piston shafts  84  and  86  are integral with cam follower  80 . 
     FIG. 8 shows gearing connecting the outputs of engine crankshaft  38  and expander crankshaft  40  at a single drive shaft  48  which connects with a conventional differential and, through that differential, left-hand and right-hand wheel shafts. At  18  is a schematic representation of gearing for combining the outputs of the two crankshafts  40 ,  46 . In the embodiment shown in FIG. 8, the single expansion cylinder  20  completes one cycle (a compression stroke and an expansion stroke) for each exhaust stroke of a combustion cylinder  10  and receives exhaust gas from four combustion cylinders  10 . 
     Preliminary studies suggest that the efficiency of the invention may be optimized by varying many of the parameters mentioned above. For instance, it was found that there are benefits to having the flow area of the expander exhaust be significantly larger than the flow area of the engine exhaust port, to have the expander crankshaft operate at the same speed as the engine crankshaft, to have two engine cylinders for each expander cylinder, and an expander displacement about 2.5 times that of the engine cylinder displacement. None of these specific variations are considered to be a departure from the basic design or operating principles of the invention. Naturally, optimization of the design or specific purposes or for maximum efficiency may call for variation of parameters such as the timing of the relative crank angles of engine and expander, relative crankshaft speeds, valve timing, valve types, presence of valves between the combustion cylinder(s) and expander(s), relative flow areas of engine exhaust and expander exhaust, relative displacement of engine cylinder(s) and expander cylinder(s), expander volumetric expansion ratio, and the number of combustion cylinders served by each expander. Such variations are considered to be consistent with the spirit of the invention and within the scope of the claims. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.