Waste heat utilization device for internal combustion engine

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.