Waste heat utilization device for internal combustion engine
A waste heat utilization device ( 2 ) for an internal combustion engine has a Rankine cycle ( 8 ) that recovers waste heat from an internal combustion engine ( 4 ), a generator ( 30 ) that is rotationally driven by an expander ( 14 ) and converts a rotational drive force into electric power, a converter ( 32 ) that controls the rotational speed of the expander ( 14 ) through the generator ( 30 ), refrigerant-condition detecting means ( 22, 24, 26, 28 ) that detects the pressure and temperature of a refrigerant passing through the expander ( 14 ), and a controller ( 34 ) that calculates pressure ratio Rp of the refrigerant in the immediate upstream and downstream of the expander ( 14 ) and specific heat ratio K of the refrigerant passing through the expander ( 14 ) on the basis of the pressure and temperature of the refrigerant, which have been detected by the refrigerant-condition detecting means ( 22, 24, 26, 28 ), calculates a preset pressure ratio Rps of the pressure ratio Rp by multiplying predetermined volume ratio Rv of the expander ( 14 ) by the specific heat ratio K, and specifies rotational speed N of the expander ( 14 ) to the converter ( 32 ) on the basis of the pressure ratio Rp and the preset pressure ratio Rps.
1. A waste heat utilization device for an internal combustion engine, comprising:
a Rankine cycle provided in a refrigerant channel with an evaporator that heats a refrigerant by using waste heat recovered from an internal combustion engine and thus secures predetermined heat absorption amount from the internal combustion engine, an expander that expands the refrigerant that has passed through the evaporator at a predetermined volume ratio and thus produces a rotational drive force, a condenser that condenses the refrigerant that has passed through the expander, and a pump that pumps the refrigerant that has passed through the condenser into the evaporator, in the order named;
a generator that is rotationally driven by the expander and converts the rotational drive force into electric power;
a converter that controls the rotational speed of the expander through the generator;
refrigerant-condition detecting means that detects pressure and temperature of the refrigerant passing through the expander; and
a controller programmed to calculate a ratio between refrigerant pressures in the immediate upstream and downstream of the expander, and a specific heat ratio of the refrigerant passing through the expander on the basis of the pressure and temperature of the refrigerant, which have been detected by the refrigerant-condition detecting means, calculates a preset pressure ratio of the pressure ratio by multiplying the predetermined volume ratio by the specific heat ratio, and specifies the rotational speed of the expander to the converter on the basis of the pressure ratio and the preset pressure ratio,
wherein the controller calculates a ratio of the pressure ratio to the preset pressure ratio, and previously sets the preset pressure ratio so that the calculated ratio falls in a predetermined range in consideration of heat entry into and heat exit from the expander.
2. The waste heat utilization device for an internal combustion engine according to claim 1 , wherein the controller decreases the rotational speed of the expander when the pressure ratio is lower than the preset pressure ratio, and increases the rotational speed of the expander when the pressure ratio is higher than the preset pressure ratio.
3. The waste heat utilization device for an internal combustion engine according to claim 1 , wherein the controller has a database in which the specific heat ratio is referenced on the basis of the pressure and temperature of the refrigerant, which have been detected by the refrigerant-condition detecting means.
4. The waste heat utilization device for an internal combustion engine according to claim 1 , wherein the controller sets the rotational speed of the expander back to the rotational speed before being changed when the pressure ratio is not changed by variation of the rotational speed of the expander.
5. The waste heat utilization device for an internal combustion engine according to claim 1 , wherein the controller varies the rotational speed of the expander within a range from minimum rotational speed that makes the efficiency of the expander equal to or higher than a predetermined value to maximum rotational speed based upon a safety coefficient relative to the rotation of the expander.
6. The waste heat utilization device for an internal combustion engine according to claim 5 , wherein the controller sets the rotational speed of the expander to secure the predetermined heat absorption amount.
7. The waste heat utilization device for an internal combustion engine according to claim 1 , wherein the controller previously sets the preset pressure ratio so that the ratio falls in a range from 1.0 to 1.25.
8. The waste heat utilization device for an internal combustion engine according to claim 1 , wherein the controller previously estimates a preset ratio of the ratio at which the power generation efficiency of the generator becomes the highest, and previously sets the preset pressure ratio so that the ratio falls in a range of 0.05 either side of the preset ratio.