Air supply system preventing backflow

An air supply system for preventing backflow in an internal combustion engine, wherein the engine includes an intake passage and a throttle disposed within the intake passage, the fuel injection system comprising:

• a fuel injector arranged on the intake passage downstream of the throttle valve, wherein the fuel injector includes a fuel injection hole;
• an air supply passage extending from an upstream portion connected to said intake passage upstream of said throttle valve to a downstream portion connected to a point close to the fuel injection hole of said fuel injector;
• a control valve arranged in said air supply passage and controlling an amount of air passing therethrough;
• an opening passage for connecting said air supply passage downstream of said control valve to the atmosphere;
• a normally closed valve arranged in said opening passage;
• and
• a first control means for controlling the opening of said normally closed valve, wherein the first control means operates to open the normally closed valve when a pressure increase in said intake passage downstream of said throttle valve during predetermined period becomes larger than a predetermined pressure increase value.