Abstract:
A Rankine cycle system includes an evaporator for heating water with thermal energy of exhaust gas of an engine for generating steam with a displacement type expander for converting thermal energy into mechanical energy. A temperature controller manipulates the amount of water supplied to the evaporator so that the temperature of the steam supplied from the evaporator to the expander coincides with a target temperature. A pressure controller manipulates the rotational speed of the expander by changing a load of the expander so that the pressure of the steam supplied from the evaporator to the expander coincides with a target pressure. The temperature controller and/or the pressure controller continue to control the amount of water supplied to the evaporator and/or the rotational speed of the expander in set ranges at least in a state in which the engine has stopped and the thermal energy of the exhaust gas has disappeared.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2005-69367 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 an exhaust gas of an engine so as to generate a gas-phase working medium, and a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy.  
         [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 waste heat from a boiler using exhaust gas of an engine rotating at a constant speed as a heat source is compared with a target temperature. When a water supply signal obtained from this deviation is used in feedback control of the amount of water supplied to the waste heat once-through boiler, a feedforward signal 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 and thereby improving the precision of control.  
         [0006]     The arrangement provided in WO03/031775 discloses steam temperature that is controlled by manipulating the amount of water supplied to an evaporator of a Rankine cycle system. The steam pressure is controlled by manipulating the rotational speed of a displacement type expander into which steam flows.  
         [0007]     The steam temperature and the steam pressure can be controlled by a conventional technique to a degree corresponding to load variation accompanying normal acceleration/deceleration after an engine and a Rankine cycle system are warmed up. However, in the process of operations from starting the engine in a low temperature state to completing warm-up of the Rankine cycle system, there are unstable states involving the effect of phase changes of a working medium within a system in going from water to saturated steam and then to superheated steam, and control of the amount of water supplied until the temperature gradient of the interior of the evaporator becomes stable. Furthermore, when the engine is stopped by a driver&#39;s intention by means of, for example, an ignition switch ON/OFF (hereinafter, ‘when the engine is stopped’ means this state), and not by means of transitional operating conditions such as fuel cut or idle stop when driving a vehicle, high temperature, high pressure steam remains in the interior of the evaporator. If the Rankine cycle system is also stopped at the same time there is a loss from the viewpoint of the efficiency of energy recovery. Moreover, if the expander is made to freely rotate (freely run) without load by the high temperature, high pressure steam remaining in the interior of the evaporator at the same time as the engine stops, there are problems that the rotational speed of the expander increases rapidly and the high temperature, high pressure steam remaining in the interior of the evaporator causes the temperature of an engine compartment to become high.  
         [0008]     For example, as shown in  FIG. 14 , when the supply of water to the evaporator is stopped at the same time as the engine stops and the energy of the exhaust gas disappears, the steam temperature slowly decreases and a high temperature state is maintained (ref. region a). In this process, water continues to evaporate in the interior of the evaporator as the pressure decreases, the internal density decreases excessively, and as a result the next time the engine is started the evaporator is heated while empty, leading to a possibility that the steam temperature might overshoot (ref. region b). If the load of the motor/generator is made 0 at the same time as the engine stops, not only is it impossible to regeneratively recover energy by means of the motor/generator, but also the remaining steam pressure brings the expander in a free operating state and the rotational speed increases excessively, leading to a possibility that the expander might be damaged (ref. region c). In this process, even if rotation of the expander is stopped at the same time as the engine stops to avoid damage due to excessive rotation, there is still a problem that the energy of the remaining high temperature, high pressure steam cannot be recovered.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention has been accomplished under the above-mentioned circumstances, and it is an object of an embodiment thereof to effectively utilize thermal energy remaining in the interior of an evaporator when an engine stops and to allow a Rankine cycle system to make a transition to a stable stopped state.  
         [0010]     In order to achieve the above-mentioned object, according to a first feature of the invention, there is provided a Rankine cycle system including 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 a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy. A temperature control means is provided for manipulating the amount of liquid-phase working medium supplied to the evaporator so that the temperature of the gas-phase working medium supplied from the evaporator to the expander coincides with a target temperature. Pressure control means are provided for manipulating the rotational speed of the expander by changing a load of the expander so that the pressure of the gas-phase working medium supplied from the evaporator to the expander coincides with a target pressure. Thus, the temperature control means and/or the pressure control means continues to control the amount of liquid-phase working medium supplied to the evaporator and/or the rotational speed of the expander in set ranges at least in a state in which the engine has stopped and the thermal energy of the exhaust gas has disappeared.  
         [0011]     With the first feature, in an arrangement in which the temperature control means manipulates the amount of liquid-phase working medium supplied to the evaporator in order to make the temperature of the gas-phase working medium coincide with the target temperature, and the pressure control means manipulates the rotational speed by changing the load of the expander in order to make the pressure of the gas-phase working medium coincide with the target pressure, a control of the amount of liquid-phase working medium supplied to the evaporator and/or control of the rotational speed of the expander are continued so as to be in the set ranges even after the engine has stopped and the thermal energy of the exhaust gas has disappeared. Therefore, it is possible to effectively recover the thermal energy remaining in the interior of the evaporator while making a transition to a stable stopped state by inhibiting a rapid increase in the rotational speed of the expander after the engine stops. Moreover, it is possible to prevent, by converting the thermal energy into mechanical energy, the temperature of the interior of an engine compartment from increasing.  
         [0012]     According to a second feature of the present invention, the temperature control means continues to supply the liquid-phase working medium to the evaporator until the temperature of the gas-phase working medium decreases to at least a temperature at which the expander does not generate an output.  
         [0013]     With the second feature, after the engine has stopped, the temperature control means continues to supply the liquid-phase working medium to the evaporator until the temperature of the gas-phase working medium decreases to the temperature at which the expander does not generate an output. Therefore, it is possible to use the thermal energy remaining in the evaporator efficiently to the very end.  
         [0014]     According to a third feature of the present invention, the pressure control means continues to control the rotational speed of the expander until the pressure of the gas-phase working medium decreases to at least a pressure at which the expander does not generate an output.  
         [0015]     With the third feature, after the engine has stopped, the pressure control means continues to control the rotational speed of the expander until the pressure of the gas-phase working medium decreases to the pressure at which the expander does not generate an output. Therefore, it is possible to use the thermal energy remaining in the evaporator efficiently to the very end.  
         [0016]     According to a fourth feature of the present invention, when the rotational speed of the expander decreases to a set rotational speed, the pressure control means maintains the set rotational speed; and when the expander attains a state in which no output is generated, the pressure control means stops controlling the rotational speed of the expander and allows the expander to rotate freely in a non-load state.  
         [0017]     With the fourth feature, when the rotational speed of the expander decreases to the set rotational speed, the pressure control means maintains this set rotational speed; and when the expander attains a state in which no output is generated the pressure control means stops controlling the rotational speed of the expander and allows it to rotate freely in a non-load state. Therefore, it is possible to recover energy by allowing the expander to rotate at a stable rotational speed while inhibiting a rapid increase in the rotational speed of the expander due to the thermal energy remaining in the evaporator, and to allow the Rankine cycle system to make a smooth transition to a stable stopped state while inhibiting a rapid increase in the rotational speed of the expander due to the thermal energy remaining in the evaporator.  
         [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.  
         [0019]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     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:  
         [0021]      FIG. 1  is a diagram showing the overall arrangement of a Rankine cycle system;  
         [0022]      FIG. 2  is a layout diagram of the Rankine cycle system;  
         [0023]      FIG. 3  is a control block diagram of temperature control means;  
         [0024]      FIG. 4  is a detail of part A in  FIG. 3 ;  
         [0025]      FIG. 5  is a control block diagram of pressure control means;  
         [0026]      FIG. 6  is a detail of part B in  FIG. 5 ;  
         [0027]      FIG. 7  is a diagram for explaining a method for estimating the internal density of an evaporator;  
         [0028]      FIG. 8  is a graph showing the relationship between optimum steam temperature and maximum efficiency of an evaporator and an expander;  
         [0029]      FIG. 9  is a diagram showing a map in which a target steam pressure is looked up from steam energy and steam temperature;  
         [0030]      FIG. 10  is a time chart for explaining control in a case in which the internal density of the evaporator is normal when an ignition switch is turned ON;  
         [0031]      FIG. 11  is a time chart for explaining control in a case in which the interior of the evaporator is empty when the ignition switch is turned ON;  
         [0032]      FIG. 12  is a time chart for explaining control in a case in which the interior of the evaporator is full of water when the ignition switch is turned ON;  
         [0033]      FIG. 13  is a time chart for explaining control when the ignition switch is turned OFF; and  
         [0034]      FIG. 14  is a time chart for explaining conventional control when an ignition switch is turned OFF.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENT  
       [0035]      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 , and a water supply pump  14 . 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  11  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.  
         [0036]     As shown in  FIG. 2 , an open/close valve  15  for cutting off the supply of water is disposed between the evaporator  11  and the water supply pump  14 , and an open/close valve  16  for cutting off the supply of steam is disposed between the evaporator  11  and the expander  12 . Furthermore, a motor/generator  17  is connected to the expander  12 , and the rotational speed of the expander  12  is controlled by regulating a load of the motor/generator  17 . A Rankine controller Cr controls, based on a signal such as ON/OFF of an ignition switch, a fuel injection quantity Ti, or an engine rotational speed Ne, the rotational speed of a motor  18  for driving the water supply pump  14 , the load of the motor/generator  17 , and opening/closing of the two open/close valves  15  and  16 .  
         [0037]      FIG. 3  shows the arrangement of temperature control means  21  included in the Rankine controller Cr. The temperature control means  21  includes feedforward water supply amount calculation means  22 , feedback water supply amount calculation means  23 , water supply amount control changeover means  24 , and rotational speed calculation means  25 . The feedforward water supply amount calculation means  22  calculates a feedforward water supply amount for the evaporator  11  from the engine rotational speed Ne, the fuel injection quantity Ti, and the exhaust gas temperature of the engine E. The feedback water supply amount calculation means  23  calculates a feedback water supply amount by multiplying a deviation of the steam temperature at the exit of the evaporator  11  from a target steam temperature at the entrance of the expander  12  by a predetermined gain. The water supply amount control changeover means  24  changes the control of the water supply amount for the evaporator  11  according to the internal density of the evaporator  11  when the ignition switch of the engine E is turned ON or the internal energy of the evaporator  11  when the ignition switch is turned OFF. The rotational speed calculation means  25  calculates a target rotational speed for the water supply pump  14  from a target water supply amount outputted by the water supply amount control changeover means  24  and a steam pressure at the exit of the evaporator  11 , and controls the rotational speed of the motor  18  for driving the water supply pump  14  so that the rotational speed coincides with the target rotational speed.  
         [0038]     The target steam temperature is determined as follows: as shown in  FIG. 8 , the efficiency of the evaporator  11  and the efficiency of the expander  12  of the Rankine cycle system 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 becomes a maximum.  
         [0039]     The internal density of the evaporator  11  is obtained as follows: as shown in  FIG. 9 , a flow rate Qin of water supplied from the water supply pump  14  to the evaporator  11  and a flow rate Qout of steam supplied from the evaporator  11  to the expander  12  are measured using a flowmeter and an internal density □ of steam in the interior of the evaporator  11  is calculated from
 
ρ=∫{ Q in ( t )− Q out ( t )} dt/V. 
 
         [0040]      FIG. 5  shows the arrangement of pressure control means  26  included in the Rankine controller Cr. The pressure control means  26  includes feedforward rotational speed calculation means  27 , feedback rotational speed calculation means  28 , rotational speed control changeover means  29 , and PI feedback term calculation means  30 . The feedforward rotational speed calculation means  27  calculates a feedforward rotational speed based on a target pressure of steam supplied to the expander  12 , a commanded water supply amount, and a steam temperature at the entrance of the expander  12 . The feedback rotational speed calculation means  28  calculates a feedback rotational speed by multiplying a deviation of the steam pressure at the entrance of the expander  12  from the target pressure for steam at the entrance of the expander  12  by a predetermined gain.  
         [0041]     The target pressure is set by applying the energy (flow rate) and temperature of steam supplied from the evaporator  11  to the expander  12  to the map of  FIG. 9 . This target pressure corresponds to a steam pressure at which the expander  12  is operated at maximum efficiency.  
         [0042]     The rotational speed control changeover means  29  controls the entrance steam pressure of the expander  12  by changing, based on an ON/OFF signal of the ignition switch, a positive torque (a torque in a direction that assists rotation of the expander  12 ) or a negative torque (a torque in a direction that inhibits rotation of the expander  12 ) generated by the motor/generator  17 .  
         [0043]     The PI feedback term calculation means  30  calculates a target torque for the motor/generator  17  from a deviation of the rotational speed of the motor/generator  17  (that is, the rotational speed of the expander  12 ) from a target rotational speed outputted by the rotational speed control changeover means  29 . The rotational speed of the expander  12  is feedback-controlled at the target rotational speed by generating the above target torque in the motor/generator  17 .  
         [0044]     Functions of the temperature control means  21  and the pressure control means  26  when the ignition switch is turned ON are now explained.  
         [0045]     As shown in  FIG. 4 ,  FIG. 6 , and  FIG. 10 , in the case where the internal density of the evaporator  11  is normal when the ignition switch is turned ON, a smaller amount of water is supplied than when there is normal temperature control (ref. region d) so that the interior of the evaporator  11  does not become empty simultaneously with an increase in the exhaust gas energy, and when the steam temperature becomes close to the target temperature, the operation shifts to water supplied by normal feedback control (ref. region e). Until the steam pressure attains a starting pressure for the expander  12 , a torque in the direction opposite to the rotational direction of the expander  12  is generated in the motor/generator  17  (ref. region f), thereby braking the expander  12  so that it does not rotate spontaneously. When the steam pressure attains the starting pressure (ref. region g), a torque in the rotational direction of the expander  12  is generated in the motor/generator  17  for a moment (ref. region h) to thus start rotation of the expander  12  at the lowest rotational speed that allows stable rotation (ref. region i), thereby smoothly starting the expander  12 .  
         [0046]     As shown in  FIG. 4 ,  FIG. 6  and  FIG. 11 , in the case where the interior of the evaporator  11  is empty when the ignition switch is turned ON, the amount of water supplied to the evaporator  11  is temporarily increased simultaneously with an increase in the exhaust gas energy (ref. region j), thus preventing any response lag in the steam temperature. In this process, the amount of water supplied is not an amount that would make the evaporator  11  full of water, but is somewhat larger than when normal in order to make an easy transition to a stable control state, and the amount of water supplied is decreased accompanying an increase in the internal density of the evaporator  11 . Torque control of the motor/generator  17  is carried out in the same manner as for the above-mentioned case where the internal density of the evaporator  11  is normal, and starting rotation of the expander  12  at the lowest rotational speed allowing stable rotation enables a smooth start.  
         [0047]     As shown in  FIG. 4 ,  FIG. 6  and  FIG. 12 , in the case where the interior of the evaporator  11  is full of water when the ignition switch is turned ON, even if the exhaust gas energy increases, water supply to the evaporator  11  is maintained in a suspended state (ref. region k), and water supply is started after the internal density of the evaporator  11  has become appropriate. When the steam temperature becomes close to the target temperature, the operation shifts to normal feedback temperature control. The motor/generator  17  generates a positive torque to rotate the expander  12  at a low speed (ref. region m) before the steam pressure starts rising, thereby discharging water in a passage that is downstream of the evaporator  11 , particularly in a portion between the evaporator  11  and the expander  12 , and that is not heated by exhaust gas.  
         [0048]     When any one of the above-mentioned three types of control when starting the engine E is completed, normal water supply control for the evaporator  11  is started based on a value obtained by adding the feedforward water supply amount and the feedback water supply amount, and normal rotational speed control is started based on a value obtained by adding the feedforward rotational speed and the feedback rotational speed.  
         [0049]     Functions of the temperature control means  21  and the pressure control means  26  when the ignition switch of the engine E is turned OFF are now explained by reference to  FIG. 4 ,  FIG. 6 , and  FIG. 13 .  
         [0050]     In the case where there is a lot of thermal energy remaining in the interior of the evaporator  11  when the ignition switch of the engine E is turned OFF, if the Rankine cycle system R is stopped immediately, the thermal energy is wasted. Therefore, when the ignition switch is turned OFF, water supply to the evaporator  11  is not stopped immediately and additional water supply is carried out, thus continuing the generation of steam (ref. region n). The amount of water supplied in this process is decreased in response to a decrease in the internal energy of the evaporator  11 . When the steam temperature attains a temperature at which the expander  12  does not generate an output (for example, the saturated steam temperature), the water supply is suspended.  
         [0051]     As a result, the steam pressure is maintained at the target pressure for a predetermined period of time after the ignition switch is turned OFF, the expander  12  is rotated efficiently, and energy can be recovered. When the steam pressure decreases, the expander  12  is rotated at the lowest rotational speed allowing stable rotation, thus further recovering energy (ref. region o). When the regenerative torque of the motor/generator  17  becomes 0, rotation of the expander  12  is stopped, and recovery of energy is completed (ref. region p).  
         [0052]     In this way, by continuously supplying water and operating the expander  12  for the predetermined period of time after the ignition switch is turned OFF, not only can the thermal energy remaining in the evaporator  11  be recovered without waste, but also the Rankine cycle system R can be shifted to a stable stopped state while preventing over-rotation of the expander  12  by slowly decreasing the steam pressure. In addition, it is possible to prevent the temperature of the interior of the engine compartment from increasing due to thermal energy remaining in the evaporator  11 .  
         [0053]     Although one embodiment of the present invention has been described 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.  
         [0054]     For example, in the embodiment the amount of water supplied to the evaporator  11  is controlled based on the rotational speed of the water supply pump  14 , but it may be controlled by the degree of opening of the open/close valve  15  shown in  FIG. 2 .  
         [0055]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.