Rankine cycle apparatus
A Rankine cycle system is provided in which the amount of water supplied to an evaporator (12) and the rotational speed of an expander (13) are controlled so as to make the temperature of steam at the outlet of the evaporator (12) coincide with a target steam temperature. When the amount of water supplied to the evaporator (12) is decreased stepwise, the temperature of the steam at the outlet of the evaporator (12) increases slowly and converges to a predetermined temperature. When the rotational speed of the expander (13) is decreased stepwise the steam temperature increases quickly, although the increase is temporary. It is therefore possible, by the combined use of control of the amount of water supplied to the evaporator (12) and control of the rotational speed of the expander (13), to make the steam temperature at the outlet of the evaporator (12) coincide with the target steam temperature with good responsiveness and high precision, thereby maximizing the total efficiency, which is the sum of the efficiency of the evaporator (12) and the efficiency of the expander (13). In this way, the temperature of a gas-phase working medium generated in the evaporator (12) can be controlled at a target temperature with good responsiveness and high precision.
The present invention relates to a Rankine cycle system that includes an evaporator for heating a liquid-phase working medium with 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 in the evaporator into mechanical energy.
BACKGROUND ARTJapanese Utility Model Registration Publication No. 2-38161 discloses an arrangement in which the steam temperature at the outlet of 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 steam temperature, and the amount of water supplied to the waste heat once-through boiler is feedback-controlled so as to make the steam temperature coincide with the target steam temperature, precision in control of the steam temperature being improved by adding, to the feedback signal, a feedforward signal calculated on the basis of the steam pressure at the outlet of the waste heat once-through boiler so as to compensate for fluctuations in the engine load.
As shown in
In order to control the steam temperature with good responsiveness by changing the amount of water supplied, it is necessary to reduce the heat capacity of the evaporator, and it is accordingly necessary for the evaporator to have a small casing and a short heat transfer pipe length, but this gives rise to the problems that the evaporator cannot generate a sufficient amount of steam or the efficiency of the evaporator is degraded.
DISCLOSURE OF THE INVENTIONThe present invention has been achieved under the above-mentioned circumstances, and it is an object thereof to control the temperature of a gas-phase working medium generated in an evaporator of a Rankine cycle system at a target temperature with good responsiveness and high precision.
In order to attain this object, in accordance with the present invention, there is proposed a Rankine cycle system that includes an evaporator for heating a liquid-phase working medium with 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 in the evaporator into mechanical energy, characterized in that the system includes control means for controlling the amount of liquid-phase working medium supplied to the evaporator and the rotational speed of the expander so as to make the temperature of the gas-phase working medium at the outlet of the evaporator coincide with a target temperature.
In accordance with this arrangement, by controlling the amount of liquid-phase working medium supplied to the evaporator, which heats the liquid-phase working medium with the exhaust gas of the engine and generates the gas-phase working medium, and controlling the rotational speed of the displacement type expander, which converts the thermal energy of the gas-phase working medium generated in the evaporator into mechanical energy, it is possible to make the temperature of the gas-phase working medium generated in the evaporator coincide with the target temperature with good responsiveness and high precision, thereby maximizing the total efficiency, which is the sum of the efficiency of the evaporator and the efficiency of the expander.
A controller 20 of embodiments corresponds to the control means of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Modes for carrying out the present invention are now explained with reference to embodiments of the present invention shown in the attached drawings.
As shown in
A motor/generator 18 connected to the expander 13 is disposed, for example, between the engine 11 and driven wheels; the motor/generator 18 can be made to function as a motor so as to assist the output of the engine 11, and when the vehicle is being decelerated the motor/generator 18 can be made to function as a generator so as to recover the kinetic energy of the vehicle as electrical energy. The motor/generator 18 may be connected to the expander 13 alone, and then exhibits only the function of generating electrical energy. In the present invention, the rotational speed of the expander 13 is controlled by regulating the load (amount of electric power generated) of the motor/generator 18 so as to regulate the load imposed on the expander 13 by the motor/generator 18. A controller 20, into which are input running conditions of the engine 11, that is, an engine rotational speed Ne, an intake negative pressure Pb, an exhaust gas temperature Tg, and an air fuel ratio A/F, together with a steam temperature T at the outlet of the evaporator 12 detected by a steam temperature sensor 19, controls the amount of water supplied from the injector 17 (or the rotational speed of the water supply pump 16) and the load generated by the motor/generator 18, that is, the rotational speed of the expander 13.
The reason why the steam temperature at the outlet of the evaporator 12 can be controlled by regulating the rotational speed of the expander 13 is now explained.
As shown in
As described above, since the change in steam temperature due to the change in rotational speed of the expander 13 is temporary, and the steam temperature returns to its original temperature as time elapses, the amount of water supplied from the injector 17 to the evaporator 12 is controlled at the same time as the rotational speed of the expander 13 is changed. When, for example, in order to increase the steam temperature at the outlet of the evaporator 12, the amount of water supplied to the evaporator 12 is decreased stepwise as shown in
The above-mentioned operation is now explained further with reference to flowcharts of
Firstly, in step S1 the steam temperature T at the outlet of the evaporator 12 is detected by the steam temperature sensor 19, in step S2 the running conditions of the engine 11, that is, the engine rotational speed Ne, the intake negative pressure Pb, the exhaust gas temperature Tg, and the air fuel ratio A/F are detected, and in step S3 a water supply amount feedforward value QFF is calculated on the basis of Ne, Pb, Tg, and A/F.
When the water supply amount feedforward value QFF is calculated in this way, the procedure returns to the flowchart of
In the subsequent step S5 a target rotational speed NEXP for the expander 13 in order to control the steam temperature T at a target steam temperature T0 is calculated.
A second embodiment of the present invention is now explained with reference to
When a target expander rotational speed NEXP is calculated in step S5 (see
As hereinbefore described, in accordance with the second embodiment, the combined use of feedforward control and feedback control enables the expander rotational speed to be converged to the target expander rotational speed NEXP yet more precisely.
Although embodiments of the present invention are explained in detail above, the present invention can be modified in a variety of ways without departing from the spirit and scope thereof.
For example, in the flowchart of
Furthermore, in step S11 of the flowchart of
Moreover, the working medium is not limited to water (steam), and another appropriate working medium may be employed.
Claims
1. A Rankine cycle system comprising an evaporator (12) for heating a liquid-phase working medium with exhaust gas of an engine (11) so as to generate a gas-phase working medium, and a displacement type expander (13) for converting the thermal energy of the gas-phase working medium generated in the evaporator (12) into mechanical energy,
- characterized in that the system comprises control means (20) for controlling the amount of liquid-phase working medium supplied to the evaporator (12) and the rotational speed of the expander (13) so as to make the temperature of the gas-phase working medium at the outlet of the evaporator (12) coincide with a target temperature.
Type: Application
Filed: Jul 22, 2003
Publication Date: Apr 27, 2006
Inventors: Akihisa Sato (Saitama), Shigeru Ibaraki (Saitama)
Application Number: 10/521,960
International Classification: F01K 23/06 (20060101);