Rankine cycle system

Abstract

A Rankine cycle system 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; a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy; a target pressure setter for setting a target pressure for the gas-phase working medium based on actual flow rate and temperature of the gas-phase working medium supplied to the expander; an estimated flow rate calculator for calculating an estimated flow rate of the gas-phase working medium supplied to the expander based on an output and a rotational speed of the engine; and a target rotational speed calculator for calculating a target rotational speed for the expander based on the estimated flow rate calculated by the estimated flow rate calculator and the target pressure set by the target pressure setter.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2005-14995 filed on Jan. 24, 2005 the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

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, and a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy.

2. Description of the Related Art

Japanese Patent Application Laid-open No. 2004-60462 discloses a Rankine cycle system in which, in order to control the steam pressure at the entrance of an expander at a target pressure with good precision without changing the amount of steam supplied to an evaporator, a feedforward value is calculated based on the target pressure and a steam flow rate at the exit of the evaporator, a feedback value is calculated by multiplying a deviation of the steam pressure at the entrance of the expander from the target pressure by a feedback gain calculated based on the steam flow rate, and the rotational speed of the expander is controlled based on the sum/difference of the feedforward value and feedback value.

In order to maintain the Rankine cycle system in a highly efficient state, it is necessary to control the temperature and pressure of steam supplied to the expander at optimum values with respect to variations in the load of the engine; in conventional general control the steam temperature is controlled by changing the amount of water supplied to the evaporator, and the steam pressure is controlled by changing the rotational speed of the expander.

In accordance with this control, as shown in FIG. 12, when the accelerator opening degree is increased stepwise by depressing an accelerator pedal, the throttle opening degree increases stepwise and the engine output also increases stepwise. When the exhaust gas energy increases accompanying the increase in engine output, the temperature of steam generated in the evaporator increases beyond a target steam temperature (ref. region a), but due to the heat capacity of the evaporator the increase in steam temperature has a time lag relative to the increase in the engine output. When the steam temperature increases in this way, feedback control is carried out so as to increase the amount of water supplied to the evaporator in order to suppress the increase in steam temperature (ref. region b). Since, due to the increase in the amount of water supplied, the amount of steam supplied from the evaporator to the expander increases and the steam pressure increases, feedback control is carried out so as to increase the rotational speed of the expander in order to decrease the steam pressure. However, if the increase in steam pressure is rapid, the rotational speed of the expander reaches a maximum rotational speed and the steam pressure cannot be decreased sufficiently (ref. region c), and there is a possibility that the steam pressure might overshoot an upper limit pressure (ref. region d) and the operating efficiency of the expander might be degraded or the durability might be adversely affected.

In this way, in the case in which the steam pressure is controlled by changing the rotational speed of the expander, even if an attempt is made to decrease the steam pressure by increasing the rotational speed of the expander after detecting a steam flow rate or a steam pressure at the entrance of the expander, phase changes (liquid phase→saturation, saturation→gas phase) are accelerated accompanying a decrease in the pressure in the interior of the evaporator, and as a result the steam flow rate increases, thus causing the problem that a time lag occurs before the steam pressure decreases.

SUMMARY OF THE INVENTION

The present invention has been accomplished under the above-mentioned circumstances, and it is an object thereof to control, in a Rankine cycle system, the pressure of a gas-phase working medium supplied from an evaporator to an expander at a target pressure with good responsiveness.

In order to achieve the above-mentioned object, according to a first feature of the 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; a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy; target pressure setting means for setting a target pressure for the gas-phase working medium based on actual flow rate and temperature of the gas-phase working medium supplied to the expander; estimated flow rate calculation means for calculating an estimated flow rate of the gas-phase working medium supplied to the expander, based on an output and a rotational speed of the engine; and target rotational speed calculation means for calculating a target rotational speed for the expander based on the estimated flow rate calculated by the estimated flow rate calculation means and the target pressure set by the target pressure setting means.

With the first feature, since the target pressure setting means sets the target pressure for the gas-phase working medium supplied to the expander based on the actual flow rate and temperature of the gas-phase working medium, the estimated flow rate calculation means calculates the estimated flow rate of the gas-phase working medium supplied to the expander based on the output and the rotational speed of the engine, and the target rotational speed calculation means calculates the target rotational speed for the expander based on the estimated flow rate and the target pressure, the pressure of the gas-phase working medium can be controlled at the target pressure with good responsiveness using the estimated flow rate of the gas-phase working medium, which responds instantaneously to a change in the output of the engine, without being affected by a response lag in the actual flow rate of the gas-phase working medium supplied to the expander.

According to a second 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; a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy; target pressure setting means for setting a target pressure for the gas-phase working medium based on actual flow rate and temperature of the gas-phase working medium supplied to the expander; estimated flow rate calculation means for calculating an estimated flow rate of the gas-phase working medium supplied to the expander, based on a throttle opening degree or an accelerator opening degree of the engine, and a rotational speed of the engine; and target rotational speed calculation means for calculating a target rotational speed for the expander based on the estimated flow rate calculated by the estimated flow rate calculation means and the target pressure set by the target pressure setting means.

With the second feature, since the target pressure setting means sets the target pressure for the gas-phase working medium supplied to the expander based on the actual flow rate and temperature of the gas-phase working medium, the estimated flow rate calculation means calculates the estimated flow rate of the gas-phase working medium supplied to the expander based on the throttle opening degree or the accelerator opening degree and the rotational speed of the engine, and the target rotational speed calculation means calculates the target rotational speed for the expander based on the estimated flow rate and the target pressure, the pressure of the gas-phase working medium can be controlled at the target pressure with good responsiveness using the estimated flow rate of the gas-phase working medium, which responds instantaneously to a change in the throttle opening degree or the accelerator opening degree of the engine, without being affected by a response lag in the actual flow rate of the gas-phase working medium supplied to the expander.

According to a third feature of the present invention, in addition to the first or second feature, the target rotational speed calculation means calculates a target rotational speed for the expander based on the estimated flow rate calculated by the estimated flow rate calculation means, the target pressure set by the target pressure setting means, and the temperature of the gas-phase working medium supplied to the expander.

With the third feature, since the target rotational speed calculation means calculates the target rotational speed for the expander from, in addition to the estimated flow rate and the target pressure, the temperature of the gas-phase working medium supplied to the expander, the target rotational speed for the expander can be calculated with better precision.

According to a fourth feature of the present invention, in addition to the third feature, the estimated flow rate calculation means calculates an exhaust gas flow rate based on the throttle opening degree and the rotational speed of the engine, calculates an exhaust gas energy from the exhaust gas flow rate and an exhaust gas temperature, and calculates the estimated flow rate from the exhaust gas energy and a target temperature of the gas-phase working medium supplied to the expander.

With the fourth feature, since the estimated flow rate calculation means calculates the exhaust gas energy from, in addition to the exhaust gas flow rate, the exhaust gas temperature, and calculates the estimated flow rate of the gas-phase working medium from, in addition to the exhaust gas energy, the target temperature for the gas-phase working medium supplied to the expander, the estimated flow rate can be calculated with yet better precision.

According to a fifth feature of the present invention, in addition to the third feature, the estimated flow rate calculation means calculates an exhaust gas energy based on the throttle opening degree and the rotational speed of the engine, and calculates the estimated flow rate from the exhaust gas energy and a target temperature of the gas-phase working medium supplied to the expander.

With the fifth feature, since the estimated flow rate calculation means calculates the estimated flow rate of the gas-phase working medium from, in addition to the exhaust gas energy, the target temperature for the gas-phase working medium supplied to the expander, the estimated flow rate can be calculated with yet better precision.

The above-mentioned object, other objects, characteristics, and advantages of the present invention will become apparent from preferred embodiments that will be described in detail below by reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 to FIG. 5 show a first embodiment of the present invention;

FIG. 1 is a diagram showing the overall arrangement of a Rankine cycle system,

FIG. 2 is a block diagram of a control system for the rotational speed of an expander,

FIG. 3 is a diagram showing a map used for control of the rotational speed of the expander,

FIG. 4 is a diagram showing a map in which a target steam pressure is looked up from steam energy and steam temperature, and

FIG. 5 is a time chart of the control system for the rotational speed of the expander.

FIG. 6 is a block diagram of a control system for the rotational speed of an expander related to a second embodiment.

FIG. 7 is a diagram showing a map in which exhaust gas energy is looked up from an accelerator opening degree and an engine rotational speed.

FIG. 8 is a block diagram of a control system for the rotational speed of an expander related to a third embodiment.

FIG. 9 is a diagram showing a map in which an estimated steam flow rate is looked up from an accelerator opening degree and an engine rotational speed.

FIG. 10 is a flowchart of a changeover routine of control for the rotational speed of an expander related to a fourth embodiment.

FIG. 11 is a graph explaining the flowchart of FIG. 10.

FIG. 12 is a time chart of a conventional control system for the rotational speed of an expander.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows the overall arrangement of a Rankine cycle system R to which the present invention is applied. The Rankine cycle system R, which recovers thermal energy of exhaust gas of an engine E and converts it into mechanical energy, includes an evaporator 11, an expander 12, a condenser 13, and a water supply pump 14, the evaporator 11 heating water with the exhaust gas discharged by the engine E so as to generate high temperature, high pressure steam, the expander 12 being operated by the high temperature, high pressure steam generated by the evaporator 11 so as to generate mechanical energy, the condenser 13 cooling decreased temperature, decreased pressure steam that has completed work in the expander 12 so as to turn it back into water, and the water supply pump 14 pressurizing water discharged from the condenser 13 and re-supplying it to the evaporator 11.

As shown in FIG. 2, estimated flow rate calculation means M1 estimates, without a response lag, a steam flow rate, which is conventionally obtained by directly detecting a flow rate of steam supplied from the evaporator 11 to the expander 12 or by estimation from an amount of water supplied to the evaporator 11, and includes exhaust gas flow rate calculation means M2, exhaust gas energy calculation means M3, and steam flow rate calculation means M4.

The exhaust gas flow rate calculation means M2 calculates an exhaust gas flow rate Qg by applying an accelerator opening degree AP (that is, a throttle opening degree TH) and an engine rotational speed Ne to the map shown in FIG. 3A. The reason for the rate of increase in the exhaust gas flow rate Qg in response to an increase in the accelerator opening degree AP decreasing in region i of FIG. 3A is because when the engine rotational speed Ne is high, a fuel injection quantity increases and an air/fuel ratio A/F decreases.

The exhaust gas energy calculation means M3 calculates an enthalpy Hg of the exhaust gas by applying an exhaust gas temperature Tg and the air/fuel ratio A/F to the map shown in FIG. 3B, and calculates an exhaust gas energy Eg by multiplying the enthalpy Hg by the exhaust gas flow rate Qg calculated by the exhaust gas flow rate calculation means M2. The steam flow rate calculation means M4 calculates an estimated steam flow rate Qs by applying a target steam temperature Tobj and the exhaust gas energy Eg calculated in FIG. 3B to the map shown in FIG. 3C.

Target pressure setting means M5 sets a target steam pressure by applying an actual flow rate and a temperature of steam supplied from the evaporator 11 to the expander 12 to the map of FIG. 4. This target pressure corresponds to a steam pressure at which the expander 12 is operated with maximum efficiency.

Target rotational speed calculation means M6 includes feedforward rotational speed calculation means M7 and feedback rotational speed calculation means M8. The feedforward rotational speed calculation means M7 calculates a feedforward rotational speed of the expander 12 by applying the estimated steam flow rate Qs calculated by the estimated flow rate calculation means M1 and the target steam pressure set by the target pressure setting means M5 to the map shown in FIG. 3D. The feedback rotational speed calculation means M8, into which a deviation of the actual steam pressure from the target steam pressure is inputted, calculates a feedback rotational speed by multiplying the deviation by a predetermined gain. The target rotational speed calculation means M6 outputs as a target rotational speed for the expander 12 a value obtained by subtracting the feedback rotational speed from the feedforward rotational speed.

A motor/generator 15 is connected to the expander 12; since when the generator load of the motor/generator 15 decreases, the rotational speed of the expander 12 increases, and when the generator load increases, the rotational speed of the expander 12 decreases, the rotational speed of the expander 12 can be controlled freely. Increasing the rotational speed of the expander 12 allows the steam pressure at the entrance of the expander 12 to be decreased, whereas decreasing the rotational speed of the expander 12 allows the steam pressure at the entrance of the expander 12 to be increased.

A deviation of the actual rotational speed (feedback rotational speed) of the expander 12 from the target rotational speed of the expander 12 outputted by the target rotational speed calculation means M6 is inputted into PI feedback term calculation means M9, and by making the motor/generator 15 generate a target torque calculated in the PI feedback term calculation means M9 the rotational speed of the expander 12 can be feedback controlled at the target rotational speed.

The above-mentioned control results are summarized by reference to the time chart of FIG. 5. When the accelerator opening degree is increased stepwise by depressing an accelerator pedal, the throttle opening degree increases stepwise and the engine output also increases stepwise. The future estimated flow rate Qs of steam supplied from the evaporator 11 to the expander 12 is calculated based on this increase in engine output (that is, the increase in the accelerator opening degree or the throttle opening degree), the target rotational speed for the expander 12 is calculated based on this estimated flow rate Qs, and it is therefore possible to control the rotational speed of the expander 12 at the target rotational speed simultaneously with an increase in the engine output without being affected by a time lag between when the engine output increases and when the steam flow rate actually increases (ref. region g).

As a result, even if the temperature of steam generated by the evaporator 12 increases beyond the target temperature accompanying an increase in the engine output (ref. region e), the amount of increase can be made smaller than that with conventional control (ref. region a in FIG. 12). Furthermore, feedback control is carried out so that the amount of water supplied to the evaporator 11 increases in order to suppress an increase in steam temperature (ref. region f), but the increase in the amount of water supplied can be made smaller than that with conventional control (ref. region b in FIG. 12).

Because of this, the steam pressure hardly deviates from the target pressure (ref. region h), and a situation in which the steam pressure overshoots an upper limit pressure is reliably avoided, thus preventing the operating efficiency of the expander 12 from deteriorating or the durability from being adversely affected.

FIG. 6 and FIG. 7 show a second embodiment of the present invention; FIG. 6 is a block diagram of a control system for the rotational speed of an expander, and FIG. 7 is a diagram showing a map in which an exhaust gas energy is looked up from an accelerator opening degree and an engine rotational speed.

In the second embodiment, the arrangement of the estimated flow rate calculation means M1 of the first embodiment shown in FIG. 2 is simplified. The estimated flow rate calculation means M1 of the first embodiment includes the exhaust gas flow rate calculation means M2, the exhaust gas energy calculation means M3, and the steam flow rate calculation means M4, but estimated flow rate calculation means M1 of the second embodiment does not include exhaust gas flow rate calculation means M2, and only includes exhaust gas energy calculation means M3 and steam flow rate calculation means M4.

The exhaust gas energy calculation means M3 of the second embodiment calculates an exhaust gas energy Eg by applying an accelerator opening degree AP (a throttle opening degree TH) and an engine rotational speed Ne to the map shown in FIG. 7. In the map shown in FIG. 7, when the accelerator opening degree AP is increased at the same engine rotational speed Ne, due to an increase in the exhaust gas temperature and an increase in the exhaust gas flow rate, the exhaust gas energy Eg increases quadratically (ref. region j). When the engine rotational speed Ne is high, the fuel injection quantity increases, the air/fuel ratio decreases, and the exhaust gas flow rate decreases (ref. region k)

In accordance with the second embodiment, since the exhaust gas energy Eg is calculated only from the accelerator opening degree AP (the throttle opening degree TH) and the engine rotational speed Ne without using the exhaust gas temperature Tg, a future estimated steam flow rate Qs can be calculated yet more quickly without waiting for a change in the exhaust gas temperature Tg. Since the air/fuel ratio A/F changes instantaneously based on a fuel injection command, even if it is excluded from calculation of the exhaust gas energy Eg, the responsiveness is not affected.

FIG. 8 and FIG. 9 show a third embodiment of the present invention; FIG. 8 is a block diagram of a control system for the rotational speed of an expander, and FIG. 9 is a diagram showing a map in which an estimated steam flow rate is looked up from an accelerator opening degree and an engine rotational speed.

In the third embodiment, the arrangement of the estimated flow rate calculation means M1 of the second embodiment shown in FIG. 6 is simplified. The estimated flow rate calculation means M1 of the second embodiment includes the exhaust gas energy calculation means M3 and the steam flow rate calculation means M4, but estimated flow rate calculation means M1 of the third embodiment does not include exhaust gas energy calculation means M3, and only includes steam flow rate calculation means M4.

The steam flow rate calculation means M4 of the third embodiment calculates an estimated steam flow rate Qs by applying an accelerator opening degree AP (a throttle opening degree TH) and an engine rotational speed Ne to the map shown in FIG. 9. This estimated flow rate Qs is a steam flow rate when the steam temperature can be controlled at an optimum temperature, and since a target rotational speed for the expander 12 that corresponds to the estimated flow rate Qs is set directly, a final estimated flow rate Qs can be calculated without depending on the amount of water supplied to the evaporator 11.

In the first to third embodiments, when the range in engine output is small, if the rotational speed of the expander 12 is increased by estimating an increase in the steam pressure, there is a possibility that the steam pressure might instead decrease excessively. In a fourth embodiment, whether or not control by estimating a steam flow rate is determined based on the flowchart of FIG. 10 is carried out.

That is, if in step S1 an engine output Pse exceeds a threshold value PSESW and there is a possibility that the steam pressure might exceed an allowed maximum pressure (ref. the solid line in FIG. 11), then in step S2 control by estimating a steam flow rate of the first to third embodiments is carried out, whereas if in step S1 the engine output Pse does not exceed the threshold value PSESW and there is no possibility that the steam pressure might exceed the allowed maximum pressure (ref. the dotted-dashed line in FIG. 11), then in step S3 control by estimating a steam flow rate of the first to third embodiments is not carried out, but conventional normal control is carried out. This enables a decrease in the output of the expander 12 due to an excessive decrease in the steam pressure to be avoided effectively.

Although embodiments of the present invention have been explained above, the present invention can be modified in a variety of ways as long as the modifications do not depart from the subject matter of the present invention.

Claims

1. 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;

a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy;

target pressure setting means for setting a target pressure for the gas-phase working medium based on actual flow rate and temperature of the gas-phase working medium supplied to the expander;

estimated flow rate calculation means for calculating an estimated flow rate of the gas-phase working medium supplied to the expander, based on an output and a rotational speed of the engine; and

target rotational speed calculation means for calculating a target rotational speed for the expander based on the estimated flow rate calculated by the estimated flow rate calculation means and the target pressure set by the target pressure setting means.

2. 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;

a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy;

target pressure setting means for setting a target pressure for the gas-phase working medium based on actual flow rate and temperature of the gas-phase working medium supplied to the expander;

estimated flow rate calculation means for calculating an estimated flow rate of the gas-phase working medium supplied to the expander, based on a throttle opening degree or an accelerator opening degree of the engine, and a rotational speed of the engine; and

target rotational speed calculation means for calculating a target rotational speed for the expander based on the estimated flow rate calculated by the estimated flow rate calculation means and the target pressure set by the target pressure setting means.

3. The Rankine cycle system according to claim 1, wherein the target rotational speed calculation means calculates a target rotational speed for the expander based on the estimated flow rate calculated by the estimated flow rate calculation means, the target pressure set by the target pressure setting means, and the temperature of the gas-phase working medium supplied to the expander.

4. The Rankine cycle system according to claim 3, wherein the estimated flow rate calculation means calculates an exhaust gas flow rate based on the throttle opening degree and the rotational speed of the engine, calculates an exhaust gas energy from the exhaust gas flow rate and an exhaust gas temperature, and calculates the estimated flow rate from the exhaust gas energy and a target temperature of the gas-phase working medium supplied to the expander.

5. The Rankine cycle system according to claim 3, wherein the estimated flow rate calculation means calculates an exhaust gas energy based on the throttle opening degree and the rotational speed of the engine, and calculates the estimated flow rate from the exhaust gas energy and a target temperature of the gas-phase working medium supplied to the expander.

6. The Rankine cycle system according to claim 2, wherein the target rotational speed calculation means calculates a target rotational speed for the expander based on the estimated flow rate calculated by the estimated flow rate calculation means, the target pressure set by the target pressure setting means, and the temperature of the gas-phase working medium supplied to the expander.