Abstract: A process for controlling the supply of gaseous secondary fuel to the intake air stream of an I.C. engine normally supplied with liquid primary fuel includes the steps of supplying the gaseous secondary fuel to the engine air intake stream according to a desired air-to-fuel ratio during normal engine operation by utilizing a movable air flow responsive member in the path of the air intake stream to control the position of a gas valve in the gas fuel supply system, and effecting closure of the gas valve as the engine speed controller approaches its idle or off position without positively moving the air flow responsive member. The flow of secondary gas fuel to the engine is thus reduced or stopped without altering the position of the air flow responsive member which continues to move in response to intake air flow. The gaseous fuel, if supplied to a supercharged liquid fuel burning engine, is pressurized by the supercharger system upstream of the engine intake air ports.

Abstract: A liquid and supplemental gas fuel supply system for an I.C. engine, and in particular a diesel cycle engine, includes a gas fuel control valve that meters supplemental gas fuel into the engine intake air stream in response to engine air flow using a direct air flow responsive member to control a gas valve through a lost motion connection. The position of the gas valve is moreover controllable by a separate throttle position responsive control system that moves the gas valve towards closed position as the throttle approaches the closed or idle position, and holds the gas valve closed until the throttle has moved away from the closed or idle position. The separate control motion of the gas valve is independent from the motion of the air flow responsive member and does not restrict or interfere with the movement of the latter.