The accuracy of HEV (hybrid electrical vehicle) engine control cannot be guaranteed because of the uncertain fuel consumption characteristic is a difficult problem. It can be resolved by the efficiency model of engine used by HEV. The theory that using the air exchange equivalent loss and the air exchange experience equivalent loss as the general efficiency model of naturally aspirated port fuel injection gasoline engine used by HEV was presented in order to determine the dynamically varied high efficient working region by theoretical analysis and by a lot of experimental investigation to a LJ491QE1 gasoline engine and a DA4G18—AA gasoline engine. The air exchange experience equivalent loss which only used two parameters of an intake pressure and an exhaust pressure could determine the dynamically varied high efficient working region of the engines and it has better responsiveness than the air exchange equivalent loss.