Turbo rotational frequency detection device

A turbo rotational frequency detection device, comprising:

• first and second pressure sensors for detecting entry and exit pressures of a compressor, respectively, intake flow rate and temperature sensors for detecting a mass flow rate and an entry temperature of intake air, respectively, and an arithmetic device for digitally storing an operating characteristic diagram showing equal rotational frequency lines of the compressor in a turbocharger based on a volumetric flow rate of the intake air and a pressure ratio under a standard environmental condition where the entry pressure of the compressor and the entry temperature of the intake air are predetermined standard values,
• said arithmetic device calculating a pressure ratio of said compressor on the basis of the entry and exit pressures of the compressor detected by the first and second pressure sensors, respectively, and calculating a volumetric flow rate of the intake air under an environmental condition at a time of detection using a gas state equation on the basis of the mass flow rate and the entry temperature of the intake air detected by the intake flow rate and temperature sensors, respectively, and said entry pressure of the compressor, correcting the calculated volumetric flow rate into a volumetric flow rate under said standard environmental condition through multiplication by a corrective coefficient based on said entry temperature, reading out a rotational frequency under the standard environmental condition on the basis of the corrected volumetric flow rate and said calculated pressure ratio and in light of said operating characteristic diagram, correcting the read-out rotational frequency into an actual rotational frequency of the compressor through multiplication by the corrective coefficient based on the entry temperature of the intake air and determine the actual rotational frequency as a rotational frequency of the turbocharger,
• wherein said arithmetic device determines overrun of the turbocharger when the rotational frequency of the turbocharger is detected to exceed a predetermined threshold over a predetermined time period.