FUEL INJECTION SYSTEM OF AN INTERNAL COMBUSTION ENGINE

A fuel injecting amount of an internal combustion engine is calculated utilizing equations determined from a physical model describing a behavior of fuel in the engine. The fuel injection system includes estimation means in which estimation values fw and fv of the adhering fuel amount and the vapor fuel amount respectively are calculated based on: a product .lambda.r.multidot.m of the detected fuel/air ratio and the detected air amount; a division Vf/.omega. of fuel evaporating amount by the engine speed; and a fuel injecting amount q. The fuel injecting amount is calculated in the system based on the division Vf/.omega., the estimated values fw and fv, the product .lambda.r.multidot.m, and a summed up deviation from a target ratio. The coefficients of respective terms are determined by analyzing the physical model by modern control theory. A variation of the invention does not use an air/fuel ratio sensor.