Patent Publication Number: US-9897000-B2

Title: Exhaust compound internal combustion engine with controlled expansion

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 14/050,089, titled PISTON COMPOUND INTERNAL COMBUSTION ENGINE WITH EXPANDER DEACTIVATION, filed Oct. 9, 2013, which claimed the benefit of the priority date of U.S. Provisional Patent Application Ser. No. 61/721,958, titled PISTON COMPOUND INTERNAL COMBUSTION ENGINE WITH EXPANDER DEACTIVATION, filed Nov. 2, 2012. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates generally to a compound internal combustion piston engine and, more particularly, to a compound internal combustion piston engine with a secondary expander piston for improved efficiency at medium and high loads, where the secondary expander piston can be deactivated and made stationary under low load conditions in order to reduce parasitic losses and over-expansion, and where groups of two power pistons and one expander piston are replicated to define various six-cylinder configurations. 
     Discussion of the Related Art 
     Internal combustion engines are a proven, effective source of power for many applications, both stationary and mobile. Of the different types of internal combustion engines, the piston engine is by far the most common in automobiles and other land-based forms of transportation. While engine manufacturers have made great strides in improving the fuel efficiency of piston engines, further improvements must be made in order to conserve limited supplies of fossil fuels, reduce environmental pollution, and reduce operating costs for vehicle owners. 
     One technique for improving the efficiency of piston engines is to employ a secondary expander piston to extract additional energy from exhaust gases before the exhaust gases are expelled to the environment. Secondary expander pistons can be effective at improving efficiency under relatively high loads, where exhaust gases still have a considerable amount of energy. However, secondary expander pistons are not very effective, and in fact can be counter-productive, under low load conditions, where parasitic losses can outweigh the benefit of any additional extracted energy. Because automobile engines inherently operate under widely varying conditions, including a substantial amount of low-load operation, traditional secondary expander piston engine designs have not proven beneficial. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a piston compound internal combustion engine is disclosed with an expander piston deactivation feature. A piston internal combustion engine is compounded with a secondary expander piston, where the expander piston extracts energy from the exhaust gases being expelled from the primary power pistons. The secondary expander piston can be deactivated and immobilized, or its stroke can be reduced, under low load conditions in order to reduce parasitic losses and over-expansion. Two mechanizations are disclosed for the secondary expander piston&#39;s coupling with the power pistons and crankshaft. Control strategies for activation and deactivation of the secondary expander piston are also disclosed. In addition, six-cylinder engine configurations are defined by replicating groups of two power pistons and one expander piston. 
     Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view illustration of a piston engine which is compounded with a secondary expander piston; 
         FIG. 2  is a side view illustration of a first mechanization for coupling the secondary expander piston to the engine&#39;s power pistons and crankshaft, while allowing deactivation or reduced stroke of the expander piston; 
         FIG. 3  is a side view illustration of a second mechanization for coupling the secondary expander piston to the engine&#39;s power pistons and crankshaft, while allowing deactivation of the expander piston; 
         FIG. 4  is a flowchart diagram of a first method for activating and deactivating the secondary expander piston in order to optimize engine efficiency; 
         FIG. 5  is a top view illustration of a piston engine which is compounded with secondary expander pistons, in a straight six cylinder configuration; 
         FIG. 6  is an end view illustration of a piston engine which is compounded with secondary expander pistons, in a V-six cylinder configuration; 
         FIG. 7  is an end view illustration of a piston engine which is compounded with secondary expander pistons, in a horizontally opposed six cylinder configuration; and 
         FIG. 8  is a graph showing how expander piston desired stroke can be controlled as a function of engine load or temperature. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to an exhaust compound internal combustion engine with controlled expansion is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
     Obtaining the maximum fuel efficiency from internal combustion engines has long been an objective of engine designers. One technique which has been employed in the past is to incorporate a secondary expander piston into an engine, where the expander piston extracts additional energy from the engine&#39;s exhaust gases. 
       FIG. 1  is a top view illustration of a piston engine which is compounded with a secondary expander piston. The engine  10  includes two power pistons  12 , which are the pistons normally found in an internal combustion engine. The power pistons  12 , in their respective cylinders, receive a charge of fuel and air through an inlet port  13 , which is then compressed, ignited, and expanded. After the combustion gases are expanded on the power stroke, the gases are exhausted from the power pistons&#39; cylinders. In the compound engine  10 , instead of exhausting the gases from the power pistons  12  through an exhaust system to the environment, the exhaust gases are routed through a transfer port  15  to a secondary expander piston  14 , which extracts additional energy from the exhaust gases on its power stroke, then exhausts the gases to the environment through an exhaust port  17 . Because the gases have already been expanded once by the power pistons  12 , gas pressures are lower on the expander piston  14 . Therefore, the expander piston  14  has a considerably larger bore than the power pistons  12 . 
     A ratio of two of the power pistons  12  to one of the expander pistons  14  is ideal in a 4-stroke-per-cycle engine. This is because the two power pistons  12 , which are mechanically in phase (both at Top Dead Center (TDC) at the same time, etc.), are 360 degrees out of phase relative to their combustion cycles (one of the power pistons  12  is beginning an intake stroke when the other is beginning a power stroke, etc.). Therefore, each time the expander piston  14  reaches TDC, one of the power pistons  12  has reached Bottom Dead Center (BDC) on its power stroke and is ready to discharge its gases to the expander piston  14  through its respective transfer port  15 . Thus, the expander piston  14  operates in a 2-stroke mode, with a power stroke and an exhaust stroke on each crankshaft revolution. 
     The engine  10  could operate on diesel fuel (compression ignition), or it could operate on gasoline or a variety of other fuels (spark ignition). The engine  10  could include only the two power pistons  12  and the one expander piston  14 , or the engine  10  could be scaled up to four or eight of the power pistons  12 , with one expander piston  14  for every two power pistons  12 . In automotive applications, the engine  10  could directly power the vehicle via a transmission and driveline, or the engine  10  could serve as an auxiliary power unit to provide electrical energy via a generator. The engine  10  could also be used in a wide variety of non-automotive applications, including primary or backup electrical generation, pumping, etc. 
     Although secondary expander piston engine designs have been known for some time, the concept has not proven viable for most engine applications, largely because the parasitic losses associated with the secondary expander piston  14  outweigh the additional energy extracted under low load conditions. Specifically, in situations where there is little energy remaining in the exhaust gases after the primary expansion by the power pistons  12 , the energy extracted from a secondary expansion of the exhaust gases is not enough to overcome the friction of the expander piston  14  in its cylinder. Because engines in automobiles—and most other applications—frequently operate at low load, little or no overall fuel efficiency improvement has been realized by secondary expander piston engines. However, if the expander piston  14  could be deactivated and made stationary at low loads, the parasitic losses associated with the expander piston  14  would be eliminated, and the engine&#39;s overall fuel efficiency would be significantly increased. 
       FIG. 2  is a side view illustration of a first mechanization for coupling the secondary expander piston  14  to the engine&#39;s power pistons  12  and crankshaft, while allowing deactivation or reduced stroke of the expander piston  14 . The power pistons  12  (one shown) are coupled to a crankshaft  16  via a connecting rod  18 , in an arrangement typical of any piston engine. The crankshaft  16  is then coupled to a stroke adjustment link  20  via a connecting link  22 . The stroke adjustment link  20  includes a slot  24  which allows the position of the stroke adjustment link  20  to be adjusted relative to a pivot pin  26 . The pivot pin  26  is a “ground” point—that is, it is attached to the block of the engine  10 . A connecting rod  28  is connected at one end to the expander piston  14 , and at the other end to the stroke adjustment link  20  at a pivot point  30 . 
     By adjusting the position of the stroke adjustment link  20  relative to the pivot pin  26 , the stroke of the expander piston  14  can be increased or decreased. As shown in  FIG. 2 , with the pivot pin  26  approximately centered along the length of the stroke adjustment link  20 , the expander piston  14  will have approximately the same stroke as the power piston  12 . However, if the stroke adjustment link  20  is positioned such that the pivot pin  26  is at the far (right) end of the slot  24 , then the expander piston  14  will have a very short stroke. In practice, a design can be realized which allows the pivot point  30  to be positioned along the axis of the pivot pin  26 , thus resulting in no motion of the expander piston  14 . Under low load engine conditions, it may be desirable to completely deactivate and immobilize the expander piston  14 . However, as will be discussed below, under certain conditions it may be desirable to reduce the stroke of the expander piston  14 , but not completely immobilize it. 
       FIG. 3  is a side view illustration of a second mechanization for coupling the secondary expander piston  14  to the engine&#39;s power pistons  12  and crankshaft  16 , while allowing deactivation of the expander piston  14 . In this embodiment, the secondary expander piston  14  is coupled to a secondary crankshaft  32  via a connecting rod  34 . The rotation of the secondary crankshaft  32  is coupled to the rotation of the crankshaft  16  via a clutch  36 . The clutch  36  must be a dog clutch or other such design that provides a positive mechanical engagement between the secondary crankshaft  32  and the crankshaft  16 —such that the rotational speeds of the two shafts are the same, and the required relative position is maintained. In this embodiment, the expander piston  14  can easily be deactivated and immobilized by disengaging the clutch  36 . A reduced stroke mode of operation is not inherently enabled in this embodiment, although a reduced stroke feature could be added to the secondary crankshaft  32 . 
     In both of the embodiments discussed above, which may collectively be referred to as de-stroking mechanisms, a controller  38  monitors engine conditions and establishes the desired stroke, or activation/deactivation, of the expander piston  14 . The controller  38  then actuates the link  20  or the clutch  36  to control the actual stroke of the expander piston  14  based on the desired stroke. 
     The controller  38  is a device typical of any electronic control unit (ECU) in an automobile, including at least a microprocessor and a memory module. The microprocessor is configured with a particularly programmed algorithm based on the logic described herein, using data from sensors—such as exhaust gas temperature sensors, an engine torque sensor, a throttle position sensor, etc.—as input. 
     In both design embodiments, the proper geometric relationship between the power pistons  12  and the expander piston  14  is maintained. That is, when the power piston  12  is at TDC, the expander piston  14  is at BDC, and vice versa. This relationship is inherently maintained by the linkage of the first embodiment ( FIG. 2 ), and maintained by way of the design of the clutch  36  in the second embodiment ( FIG. 3 ). 
     In  FIG. 3 , it is even conceivable to allow the expander piston  14  and the secondary crankshaft  32  to operate independent of any mechanical coupling to the crankshaft  16 . For example, in an electrical power generation application, the secondary crankshaft  32  could drive a small secondary generator. The valving of the exhaust gases from the power pistons  12  to the expander piston  14  would inherently tend to drive the secondary crankshaft  32  at the same speed as, and at the correct phase relationship to, the crankshaft  16 . 
     A variety of control strategies can be envisioned which take advantage of the piston compound internal combustion engine with expander deactivation or stroke adjustment. As discussed above, it is known that expander deactivation is desirable at low load conditions. Other factors also come into consideration. For example, exhaust gas after-treatment devices, such as catalytic converters, are only effective when they reach a certain minimum temperature. In a real world automotive application, it would not be desirable to extract so much energy from the exhaust gases that the exhaust after-treatment system drops below its minimum effective temperature. This criterion can be incorporated into a control strategy. Also, in practice, it may be desirable to add a hysteresis effect to the control of the expander piston  14 , such that it is not repeatedly activated and deactivated at high frequency. 
       FIG. 4  is a flowchart diagram  40  of a method for activating and deactivating the secondary expander piston  14  in order to optimize engine performance and efficiency. The controller  38  would be configured to follow the method steps of the flowchart diagram  40 . At start box  42 , the engine  10  is started. When the engine  10  is started, the expander piston  14  is deactivated and immobilized. At box  44 , exhaust system temperature is measured. At decision diamond  46 , the exhaust system temperature is compared to a first threshold temperature. If the exhaust system temperature is below the first threshold, which is the minimum effective temperature of the exhaust after-treatment devices, then the expander piston remains deactivated and immobilized, and the process loops back to again measure the exhaust system temperature at the box  44  after some time delay. 
     If the exhaust system temperature is above the first threshold temperature at the decision diamond  46 , then engine output torque is measured at box  48 . Engine output torque is considered to be a good indicator of whether engine load is high enough to warrant the engagement of the secondary expander piston  14 . It is certainly conceivable to use other measurements, individually or in combination, as an indication of engine load level. Such other measurements could include fuel flow rate, cylinder head temperature (for the power piston  12 ), cylinder pressure (for the power piston  12 ), etc. In any case, some reliable indication of engine load is needed, and is obtained at the box  48 , for control of the expander piston  14 . 
     At box  50 , exhaust system temperature is again measured. At box  52 , a control algorithm is used to determine the desired stroke of the expander piston  14 , and the process loops back to again measure engine output torque. The control algorithm can be adapted to handle variable stroke engine designs, where the stroke of the expander piston  14  may be normalized to vary from zero (immobilized) to one (full or maximum stroke possible for the engine mechanization). The algorithm can also be adapted to allow only full activation and deactivation of the expander piston  14 , but not variable stroke. 
     The control algorithm may advantageously use a strategy which considers both engine load (torque) and exhaust system temperature, while including a hysteresis effect to avoid rapid repeated activation and deactivation of the expander piston  14 . For example, if engine torque is below a first torque threshold or exhaust system temperature is below the first temperature threshold, the expander piston  14  would be deactivated. If engine torque is above a second torque threshold and exhaust system temperature is above a second temperature threshold, the expander piston  14  would be activated at full stroke. If the engine  10  supports variable stroke of the expander piston  14 , then the stroke can be adjusted between the values of zero and one as a function of the engine torque and the exhaust system temperature relative to their respective thresholds. If the engine  10  supports only full activation and deactivation of the expander piston  14 , only one temperature threshold and one torque threshold may be used, where the expander piston  14  is activated when both thresholds are exceeded. Hysteresis can be added, for example by requiring several consecutive measurement cycles at a certain condition before changing the stroke of the expander piston  14 . 
     By adding a deactivation feature or a variable stroke feature to a piston compound internal combustion engine as described above, the fuel efficiency improvement of a secondary expander piston can be realized when an engine is operating at medium or high load, but the parasitic losses of the expander piston can be eliminated when the engine is operating at low load. This selective expander piston de-stroking offers another approach to increasing fuel efficiency, which is so important to both automakers and consumers. 
     As mentioned briefly above, it is possible to scale up the engine  10  to include more than just the three cylinders (two power pistons and one expander piston) shown in  FIG. 1 . 
       FIG. 5  is a top view illustration of a piston engine  100  which is compounded with secondary expander pistons, in a “straight six” cylinder configuration. The engine  100  shows how the concept of exhaust compounding with expander de-stroking or deactivation can be scaled up to a larger engine size capable of powering a full-size car or truck. 
     The engine  100  includes power pistons  102  and secondary expander pistons  104  in a cylinder block  106 , where the power pistons  102  and the expander pistons  104  are arranged in groups of three. That is, a first group  110  is comprised of two of the power pistons  102  and one of the expander pistons  104 . Likewise for a second group  112 . The advantage of grouping two of the power pistons  102  with one of the expander pistons  104  was explained in detail previously, where the two power pistons  102  operate in a 4 stroke/cycle mode and are 360 out of phase with each other, and the expander piston  104  operates in a 2 stroke/cycle mode and receives exhaust gas from one of the power pistons  102  on every stroke at TDC. 
     Although the centerlines of all six cylinders in the engine  100  are not in a single plane, the engine  100  generally resembles a “straight six” engine in that all six cylinders are contained in a single block or bank of cylinders, and all six cylinders have the same orientation (for example, pistons at the top and crankshaft at the bottom). 
     In the preferred design of the straight six cylinder engine  100 , all four of the power pistons  102  share the same crankshaft. The phasing of the four power pistons  102  could be handled in at least two different manners. The simplest approach is to have all four of the power pistons  102  in phase (such as all at TDC at the same time), with each of the power pistons  102  feeding exhaust gas to the nearest of the expander pistons  104  as shown in  FIG. 5 . Another approach would be akin to a typical four cylinder engine where, in order to optimize mechanical balance, the inboard two pistons are in phase (such as at BDC) while the outboard two pistons are in phase (such as at TDC). This piston/crankshaft arrangement would require a different exhaust porting configuration, where the inboard two pistons would feed one of the expander pistons  104  while the outboard two pistons would feed the other of the expander pistons  104 . 
     The engine  100  can be designed to employ either of the expander piston de-stroking/deactivation mechanisms shown in  FIGS. 2 and 3  and discussed previously. Using the variable stroke slider mechanism of  FIG. 2 , both of the expander pistons  104  would be set to the same stroke. Using the dual crankshaft and clutch mechanism of  FIG. 3 , both of the expander pistons  104  would share the same secondary crankshaft, and both would be either engaged or disengaged based on the status of the clutch. 
     The engine  100  may advantageously be supercharged or turbocharged, thereby increasing the power density from the power pistons  102 , and also making additional exhaust energy (temperature and pressure) available for secondary expansion under many circumstances. Other six cylinder engine arrangements employing exhaust compounding with expander de-stroking or deactivation can also be devised. Two of these are discussed below. 
       FIG. 6  is an end view illustration of a piston engine  120  which is compounded with secondary expander pistons, with two banks of cylinders in a V-six cylinder configuration. The engine  120  includes two groups of three cylinders each, as discussed above for the engine  100 , however the groups are configured differently. A first group  122  includes two power pistons operating in cylinders with a centerline  126 , along with one expander piston operating in a cylinder with a centerline  128 . Similarly, a second group  124  includes two power pistons operating in cylinders with a centerline  130 , along with one expander piston operating in a cylinder with a centerline  132 . It is readily apparent in  FIG. 6  how the two power pistons operating along the centerline  126  and the two power pistons operating along the centerline  130  can share a crankshaft, as in any V-block engine configuration. Likewise, the expander piston operating along the centerline  128  and the expander piston operating along the centerline  132  can share a secondary crankshaft (in the  FIG. 3  embodiment), or each cylinder bank could have its own secondary crankshaft, as determined to best optimize packaging and mass. 
       FIG. 7  is an end view illustration of a piston engine  140  which is compounded with secondary expander pistons, with two banks of cylinders in a horizontally opposed six cylinder configuration. The engine  140  includes two groups of three cylinders each, as discussed above for the engine  120 , with the only difference being that the two cylinder banks are horizontally opposed rather than in a V-block configuration. A first group  142  includes two power pistons operating in cylinders with a centerline  146 , along with one expander piston operating in a cylinder with a centerline  148 . Similarly, a second group  144  includes two power pistons operating in cylinders with a centerline  150 , along with one expander piston operating in a cylinder with a centerline  152 . Crankshaft sharing in the horizontally opposed engine  140  can be handled in a manner analogous to the V-six engine  120  discussed above. 
       FIG. 8  is a graph showing how expander piston desired stroke can be controlled as a function of engine load or exhaust gas temperature. Horizontal axis  182  represents engine load (which may be represented by torque, throttle position or other appropriate value, as discussed previously) or exhaust gas temperature. Vertical axis  184  represents expander piston desired stroke. Line  186  defines the desired expander piston stroke as a function of engine load or exhaust gas temperature, as described above in reference to the flowchart diagram  40  of  FIG. 4 . 
     A first threshold  190  represents a value (of engine load or exhaust gas temperature) below which the expander piston stroke should be set to zero, or to the minimum stroke value possible with the variable stroke mechanism of  FIG. 2 . A second threshold  192  represents a value above which the expander piston stroke should be set to full-stroke. In between the first threshold  190  and the second threshold  192 , the expander piston stroke can be controlled according to the linear ramp function of the line  186 . The line  186  could also have some shape other than a straight line ramp, such as a ¼ sine wave which provides a smooth transition at the thresholds  190  and  192 . 
     As described above, engine load and exhaust gas temperature may be used as control parameters for expander piston stroke. This is because it is desirable to run the expander piston only when there is sufficient energy (pressure and temperature) in the exhaust gas. It is also desirable to ensure exhaust gas temperature (after the secondary expansion) is sufficiently high for exhaust after-treatment. A combination of engine load and exhaust gas temperature may be used in a two-step decision process. An example of a two-step decision process would be to first evaluate exhaust gas temperature and, if exhaust gas temperature is above a temperature threshold, continue to evaluate engine load and thereby establish expander piston stroke according to  FIG. 8 , and as described above in reference to the flowchart diagram  40  of  FIG. 4 . 
     The graph shown in  FIG. 8  is applicable to the variable stroke mechanization shown in  FIG. 2 , where the stroke of the expander piston  14  can be continuously controlled from 0-100% of its maximum value, or from a minimum stroke value to a full-stroke value. A similar control strategy to that shown in  FIG. 8  can also be applied to the clutch-based mechanization shown in  FIG. 3 , where the stroke of the expander piston  14  would be set to 0% (disengaged) if the control parameter (engine load or exhaust gas temperature, or combination) is below a threshold value, and the stroke would be set to 100% (engaged) if the control parameter is above the threshold value. The single threshold value in the case of the clutch-based mechanization would be in between the thresholds  190  and  192  shown on  FIG. 8 . A hysteresis effect may be added to the control of the expander piston  14 , such that it is not rapidly activated and deactivated, as discussed previously. 
     Based upon the discussion above, it should be apparent to those skilled in the art of engine design that exhaust compounding with expander de-stroking or deactivation could be further scaled up to even larger engine sizes, such as a straight nine cylinder or a V-12 cylinder. These six cylinder and larger engines can deliver all of the efficiency advantages of variable stroke exhaust compounding, while also delivering enough power for larger vehicle applications. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.