Fuel blending system

In a system for blending at least two fuel substances, a signal, representing the difference between measured octane number of the blend and a desired octane number is utilised to control the addition of one of the substances and thereby reduce the difference to zero automatically. As described, a blended petrol is produced by supplying several different constituents from separate tanks to a mixing tank from which the mixture is withdrawn and returned by a circulating pump to effect mixing. Signals corresponding to measurements of octane number of the mixture, the quantity of mixture, and the quantity of anti-knock agent supplied to the mixture are fed to an electrical computer which controls the addition of anti-knock agent intermittently until a mixture having a desired octane number is produced ready for shipment. After each addition of anti-knock agent, which is computed so that it is slight less than the required to bring the blend to the desired octane number, the blend is mixed and the next addition delayed until the octane number has been substantially constant for a predetermined interval. A signal is given if anti-knock agent concentration exceeds an allowable maximum. The computer comprises potentiometers, memories and multivibrator circuits.