Control system for hybrid vehicles

A control system for a hybrid vehicle including an internal combustion engine, a drive shaft driven by said engine, a motor operable on electrical energy for driving said drive shaft, and electrical storage means for supplying electrical energy to said motor, the control system comprising:

• driving assistance amount-calculating means for calculating an amount of assistance of said motor to said engine by driving said drive shaft, based at least on load on said engine;
• motor output control means for controlling an output from said motor, based on said amount of assistance of said motor to said engine calculated by said driving assistance amount-calculating means;
• lean operating condition-determining means for determining whether conditions are-satisfied for a lean operating mode of said engine in which an air-fuel ratio of a mixture supplied to said engine is controlled to a leaner value than a stoichiometric value;
• and
• air-fuel ratio-changing means for carrying out changeover of an operating mode of said engine between a stoichiometric operating mode in which said air-fuel ratio of said mixture supplied to said engine is controlled to a value at or close to said stoichiometric value and said lean operating mode, based on said amount of assistance of said motor to said engine calculated by said driving assistance amount-calculating means and results of said determination of said lean operating condition-determining means.