Abstract:
A Rankine cycle system includes: an evaporator for heating water with thermal energy of exhaust gas of an engine so as to generate steam; an expander for converting the thermal energy of the steam generated by the evaporator into mechanical energy; and a distribution device for manipulating the amount of water supplied to the evaporator in order to make the temperature of the steam supplied from the evaporator to the expander coincide with a target temperature. The distribution device controls a distribution ratio between the amount of water supplied to the entrance of the evaporator and the amount of water supplied to a portion partway along the evaporator, thereby suppressing an overshoot in the temperature of the gas-phase working medium due to a sudden increase in the thermal energy of the exhaust gas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2005-69366 filed on Mar. 11, 2005 the entire contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a Rankine cycle system that includes an evaporator for heating a liquid-phase working medium with thermal energy of exhaust gas of an engine so as to generate a gas-phase working medium, an expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy, and temperature control means for manipulating the amount of liquid-phase working medium supplied to the evaporator in order to make the temperature of the gas-phase working medium supplied from the evaporator to the expander coincide with a target temperature.  
         [0004]     2. Description of Background Art  
         [0005]     Japanese Utility Model Registration Publication No. 2-38162 discloses an arrangement in which the temperature of steam generated by a waste heat once-through boiler using, as a heat source, exhaust gas of an engine rotating at a constant speed is compared with a target temperature. When a water supply signal obtained from this deviation is used in a feedback control of the amount of water supplied to the waste heat once-through boiler, a feedforward signal, that is obtained by correcting with steam pressure a degree of throttle opening signal of the engine, is added to the above-mentioned feedback signal, thus compensating for variation in the load of the engine to improve the precision of control.  
         [0006]     In the above-mentioned conventional arrangement, since the steam temperature is controlled only by manipulating the amount of water supplied to the evaporator, in the case where the load of the engine changes suddenly and the thermal energy of the exhaust gas increases rapidly, there is a possibility that a response lag might occur in the steam temperature due to the length of a water supply pipe or the heat capacity of the evaporator. Thus, the steam temperature might overshoot the target temperature to deteriorate the operating efficiency of the expander.  
         [0007]     As another method for preventing the steam temperature from overshooting the target temperature when the load of the engine changes suddenly, cylinder shut-off in the engine could be considered. However, if cylinder shut-off is carried out, since the engine output itself changes, there is a problem that this Rankine cycle system mounted in an automobile gives an uncomfortable feeling to the driver.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been accomplished under the above-mentioned circumstances, and it is an object thereof to carry out control with good responsiveness so that the temperature of steam generated in an evaporator does not overshoot a target temperature even when the operating conditions of the engine change and the energy of the exhaust gas increases rapidly.  
         [0009]     In order to achieve the above object, according to a first feature of the present invention, there is provided a Rankine cycle system comprising: an evaporator for heating a liquid-phase working medium with thermal energy of exhaust gas of an engine so as to generate a gas-phase working medium with an expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy. Temperature control means are provided for manipulating the amount of liquid-phase working medium supplied to the evaporator in order to make the temperature of the gas-phase working medium supplied from the evaporator to the expander coincide with a target temperature, wherein the temperature control means controls a distribution ratio between the amount of liquid-phase working medium supplied to the entrance of the evaporator and the amount of liquid-phase working medium supplied to a portion partway along the evaporator.  
         [0010]     With the first feature, the temperature control means for manipulating the amount of liquid-phase working medium supplied to the evaporator controls the distribution ratio of the amount of liquid-phase working medium supplied to the entrance of the evaporator and the amount of liquid-phase working medium supplied to the portion partway along the evaporator in order to make the temperature of the gas-phase working medium supplied from the evaporator to the expander of the Rankine cycle system coincide with the target temperature. Therefore, it is possible to suppress an overshoot in the temperature of the gas-phase working medium due to a sudden increase in the thermal energy of the exhaust gas by supplying the liquid- phase working medium to a portion partway along the evaporator.  
         [0011]     According to a second feature of the present invention, the temperature control means supplies the liquid-phase working medium to a portion partway along the evaporator at a predetermined distribution ratio when the thermal energy of the exhaust gas changes suddenly accompanying a change in load of the engine and the temperature of the gas-phase working medium cannot be controlled at the target temperature by supplying the liquid-phase working medium only from the entrance of the evaporator.  
         [0012]     With the second feature, when the temperature of the gas-phase working medium cannot be controlled at the target temperature by supplying the liquid-phase working medium only from the entrance of the evaporator due to a sudden change in the thermal energy of the exhaust gas, part of the liquid-phase working medium that has been supplied to the entrance of the evaporator until then is supplied to the portion partway along the evaporator. Therefore, it is possible to decrease the temperature of the gas-phase working medium and reliably prevent the occurrence of overshooting.  
         [0013]     According to a third feature of the present invention, the temperature control means supplies the liquid-phase working medium to a portion partway along the evaporator at a predetermined distribution ratio when the temperature of the gas-phase working medium supplied from the evaporator to the expander is higher than the target temperature.  
         [0014]     With the third feature, when the temperature of the gas-phase working medium supplied from the evaporator to the expander is higher than the target temperature, part of the liquid-phase working medium that has been supplied to the entrance of the evaporator until then is supplied to the portion partway along the evaporator. Therefore, it is possible to decrease the temperature of the gas-phase working medium and reliably prevent the occurrence of overshooting.  
         [0015]     According to a fourth feature of the present invention, the temperature control means supplies the liquid-phase working medium to a portion partway along the evaporator at a predetermined distribution ratio according to an air/fuel ratio.  
         [0016]     According to a fifth feature of the present invention, at least when the air/fuel ratio is stoichiometric, the temperature control means increases the distribution ratio of the liquid-phase working medium to a portion partway along the evaporator as compared with the case of another air/fuel ratio.  
         [0017]     With the fourth and fifth features, when the air/fuel ratio is stoichiometric the temperature of exhaust gas rises and the thermal energy increases as compared with when it is rich or lean, but in this case the liquid-phase working medium is supplied to the portion partway along the evaporator at the predetermined distribution ratio according to the air/fuel ratio, that is, the distribution ratio of the liquid-phase working medium supplied to the portion partway along the evaporator is increased at least when the air/fuel ratio is stoichiometric as compared with when it is another air/fuel ratio. Therefore, it is possible to suppress an excessive increase in the temperature of the gas-phase working medium supplied from the evaporator to the expander, and it is also possible to suppress an excessive decrease in the temperature of the gas-phase working medium supplied from the evaporator to the expander when the air/fuel ratio is rich or lean, thereby making the temperature of the gas-phase working medium coincide with the target temperature with good precision.  
         [0018]     The above-mentioned object, other objects, characteristics, and advantages of the present invention will become apparent from a preferred embodiment that will be described in detail below by reference to the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0020]      FIG. 1  is a diagram showing the overall arrangement of a Rankine cycle system;  
         [0021]      FIG. 2  is a control block diagram of temperature control means;  
         [0022]      FIG. 3  is a graph showing the relationship between optimum steam temperature and maximum efficiency of an evaporator and an expander;  
         [0023]      FIG. 4  is a flowchart of steam temperature control;  
         [0024]      FIG. 5  is a map in which the total water supply amount is looked up from exhaust gas energy;  
         [0025]      FIG. 6  is a map in which an intermediate position water supply amount distribution ratio is looked up from an air/fuel ratio;  
         [0026]      FIG. 7  is a graph showing the relationship between exhaust gas energy, intermediate position water supply amount distribution ratio, and air/fuel ratio;  
         [0027]      FIGS. 8A and 8B  are time charts for explaining the effect of the intermediate position water supply;  
         [0028]      FIG. 9  is a graph showing temperature distribution in the direction of steam flow of the evaporator; and  
         [0029]      FIG. 10  is a graph showing parameter changes when engine operating conditions change.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENT  
       [0030]      FIG. 1  shows the overall arrangement of a Rankine cycle system R to which the present invention is applied. The Rankine cycle system R recovers thermal energy of exhaust gas of an engine E and converts it into mechanical energy. The Rankine cycle system R includes: an evaporator  11 ; an expander  12 ; a condenser  13 ; a water supply pump  14 ; and a distribution device  15  for water supplied from the pump  14  to the evaporator  11 . The evaporator  11  heats water with the exhaust gas discharged by the engine E so as to generate high temperature, high pressure steam. The expander  12  is operated by the high temperature, high pressure steam generated by the evaporator  1   1  so as to generate mechanical energy. The condenser  13  cools decreased temperature, decreased pressure steam that has completed work in the expander  12  so as to turn it back into water. The water supply pump  14  pressurizes water discharged from the condenser  13  and supplies it to the evaporator  11  again. Supply of water to the evaporator  11  can be carried out not only via main water supply from the upstream end of the evaporator  11 , but also via intermediate position water supply from a portion partway along that is close to the downstream end of the evaporator  11 . The distribution device  15  can freely control the ratio of the amount of water of the main water supply and the amount of water of the intermediate position water supply by means of a distribution valve that is duty-controlled.  
         [0031]     The intermediate position water supply may be independent from the main water supply, and may optionally involve supplying water via another route and pump, etc.  
         [0032]      FIG. 2  shows the arrangement of temperature control means  21  included in the Rankine cycle system R. The temperature control means  21  includes: feedforward water supply amount calculation means  22 ; feedback water supply amount calculation means  23 ; comparison means  24 ; and intermediate position water supply amount calculation means  25 . The feedforward water supply amount calculation means  22  calculates a feedforward water supply amount for the evaporator  11  based on a parameter such as engine rotational speed, intake negative pressure, fuel injection quantity, or exhaust gas temperature. The feedback water supply amount calculation means  23  calculates a feedback water supply amount by multiplying a deviation, from a target temperature for steam at the entrance of the expander  12 , of the steam temperature at the exit of the evaporator  11  by a predetermined gain. A total water supply amount, which is the sum of the main water supply amount and the intermediate position water supply amount, is calculated by subtracting the feedback water supply amount calculated by the feedback water supply amount calculation means  23  from the feedforward water supply amount calculated by the feedforward water supply amount calculation means  22 .  
         [0033]     The target steam temperature is determined as follows. That is, as shown in  FIG. 3 , the efficiency of the evaporator  11  and of the expander  12  of the Rankine cycle system R, which are represented by the efficiency of each element on the left-hand ordinate, change according to the steam temperature; when the steam temperature increases, the efficiency of the evaporator decreases and the efficiency of the expander increases, whereas when the steam temperature decreases, the efficiency of the evaporator increases and the efficiency of the expander decreases. Therefore, there is an optimum steam temperature (a target temperature) at which the overall efficiency of the two, which is represented by the overall efficiency on the right-hand ordinate, becomes a maximum.  
         [0034]     Returning to  FIG. 2 , the comparison means  24  compares the steam temperature at the exit of the evaporator  11  with the target temperature for steam at the entrance of the expander  12 , and if the result is that the steam temperature at the exit of the evaporator  11  is higher than the target temperature for steam at the entrance of the expander  12 , the intermediate position water supply amount calculation means  25  calculates an intermediate position water supply amount by map lookup. When the intermediate position water supply amount is calculated in this way, the main water supply amount is calculated by subtracting the intermediate position water supply amount from the total water supply amount. While maintaining the total water supply amount, the distribution valve of the distribution device  15  is duty-controlled so that the main water supply amount and the intermediate position water supply amount satisfy the predetermined ratio.  
         [0035]     The above-mentioned operation is now explained in further detail by reference to the flowchart of  FIG. 4 .  
         [0036]     In step S 1 , the main water supply amount, the intermediate position water supply amount, and the total water supply amount are all set at 0. In the subsequent step S 2 , the engine rotational speed, intake negative pressure, fuel injection quantity, and exhaust gas temperature are detected, in step S 3  an air/fuel ratio A/F is calculated from the engine rotational speed, the intake negative pressure, and the fuel injection quantity, and in step S 4  the exhaust gas energy is estimated. Subsequently, in step S 5 , the total water supply amount (a feedforward value) is looked up in the map of  FIG. 5  from the exhaust gas energy. The total water supply amount is set so as to increase in response to an increase in the exhaust gas energy.  
         [0037]     In the subsequent step S 6 , the steam temperature at the exit of the evaporator  11  is measured; if in step S 7  the exit steam temperature is higher than the target steam temperature, then in step S 8  a distribution ratio (intermediate position water supply amount/total water supply amount) of the intermediate position water supply amount is looked up in the map of  FIG. 6  from the air/fuel ratio A/F. Switchover of the distribution ratio shown in  FIG. 6  is not limited to a stepwise form (ref. the solid line), and it may be in a curved form (ref. the dashed line) in order to moderate a sudden change in steam temperature.  
         [0038]     In the case where the air/fuel ratio A/F is rich, since the temperature of the exhaust gas decreases as compared with the case where it is stoichiometric (theoretical air/fuel ratio), and the temperature of steam at the exit of the evaporator  11  also decreases, the proportion of the intermediate position water supply amount (intermediate position water supply amount distribution ratio) for decreasing the exit steam temperature is set low. Also in the case where it is lean, in the same manner as in the case where it is rich, the exhaust gas temperature decreases as compared with the case where it is stoichiometric. Therefore, the proportion of the intermediate position water supply amount is set low as in the case of rich. Thus, in the case where it is stoichiometric, the proportion of the intermediate position water supply amount is set high. In step S 9  an intermediate position water supply amount (feedforward value) is calculated by multiplying the total water supply amount by the intermediate position water supply amount distribution ratio.  
         [0039]     The reason why the intermediate position water supply amount distribution ratio is set based on the air/fuel ratio is explained below. As shown in  FIG. 7 , there is no correlation found between exhaust gas energy and intermediate position water supply amount distribution ratio, but instead the intermediate position water supply amount distribution ratio becomes substantially constant depending on the air/fuel ratio being stoichiometric or rich (or lean, although it is not illustrated). If the intermediate position water supply amount distribution ratio is set according to the air/fuel ratio, the intermediate position water supply amount distribution ratio can be calculated instantaneously from the fuel injection quantity and the intake air amount. Thus, an advantage is obtained in that the responsiveness is improved as compared with the case where the intermediate position water supply amount distribution ratio is calculated using the exhaust gas temperature and the steam temperature.  
         [0040]     Returning to the flowchart of  FIG. 4 , in step S  10  a PID control amount (feedback value) is calculated by multiplying a deviation, from a target steam temperature, of the exit steam temperature by a gain, in step S 1  a total water supply amount is calculated by subtracting the feedback value from the feedforward value, in step S 12  the main water supply amount is calculated by subtracting the intermediate position water supply amount from the total water supply amount, in step S 13  the water supply amount of the water supply pump  14  is controlled based on the total water supply amount, and the operation of the distribution valve of the distribution device  15  is controlled based on the main water supply amount and the intermediate position water supply amount.  
         [0041]     As shown in  FIG. 8A , in the case where no intermediate position water supply is carried out, when a driver depresses an accelerator pedal and the exhaust gas energy increases, only the main water supply amount calculated from the exhaust gas energy is controlled. Therefore, the steam temperature overshoots and it becomes difficult to converge it on the target temperature. On the other hand, as shown in  FIG. 8B , when both the main water supply amount and the intermediate position water supply amount are controlled, it is possible to suppress the overshooting of the steam temperature and quickly converge it on the target temperature. In this process, since the output of the engine E is not changed unlike the case of cylinder shut-off, the Rankine cycle system R mounted on an automobile gives no disagreeable sensation to the driver.  
         [0042]      FIG. 9  shows changes in the steam (water) temperature corresponding to each position from the upstream end to the downstream end in the direction of steam (water) supply for the evaporator  11 , and it can be seen that carrying out intermediate position water supply makes the steam temperature converge on the target temperature at the exit of the evaporator  11 .  
         [0043]      FIG. 10  shows changes in the engine rotational speed, the total water supply amount, the intermediate position water supply amount distribution ratio, and the steam temperature when the operating conditions of the engine E change from an idling state to a high load state, and then to a fuel-cut state, and it can be seen that variation in the steam temperature can be suppressed to a low level by increasing the intermediate position water supply amount distribution ratio in the high load state.  
         [0044]     Although one embodiment of the present invention is explained above, the present invention can be modified in a variety of ways as long as the modifications do not depart from the spirit and scope of the present invention.