A digital computer for calculating the optimal air/fuel mixture of a carburettor-type internal combustion engine by injecting a variable amount of air into the main air induction system, comprising an address computer disposed between the contract breaker and the memories of the computer; a first memory containing the data concerning the characteristics of the engine, said data being stored as a function of the rotational velocity of the engine; a second memory containing the data concerning the subservience velocity as a function of the engine rotational velocity; and an injection time computer adapted to sum up the data received by said first memory and the data received by an up-and-down-counter responsive both to a chemical analysis probe and to the second memory, said injection time computer being connected to a circuit for triggering said solenoid operated valve.