A control device calculates an estimate of negative intake pressure based on the relationship between the rotation speed of a crankshaft and a throttle opening degree (Step S24). Then, the control device sets the estimate PE of the negative intake pressure, which is calculated in Step S24, to a greater value as combustion efficiency of CNG used in engine operation becomes higher (Step S25). When the corrected estimate PE of the negative intake pressure becomes smaller than or equal to a reference value PTh (Step S26: YES), the control device starts a negative pressure recovery procedure (Step S27).

Systems and methods for improving generation of vacuum for a vehicle are disclosed. In one example, an air conditioning compressor bypass valve is selectively activated to improve generation of vacuum by an engine. The systems and methods may reduce the possibility of compressor clutch degradation.