a number that provides a measure of the ability of a fuel to resist knocking when it is burnt in a spark-ignition engine. It is the percentage by volume of iso-octane (C8H18; 2,2,4-trimethylpentane) in a blend with normal heptane (C7H16) that matches the knocking behaviour of the fuel being tested in a single cylinder four-stroke engine of standard design., sädesüütega sisepõlemismootorite kütuste (nt bensiini) detonatsioonikindlust iseloomustav suurus