The incipient knock limited compression ratio and the time of knock were measured for catalytically reformed naphtha, cracked naphtha and primary reference fuels in a CFR engine coupled with a direct current dynamometer at air-fuel ratio between 10 and 17.
There were appreciable differences between fuels of different type of hydrocarbon compositions not only in the knock limited compression ratio but in the time of knock occurrence. It was observed that when fuels were rated to have the same antiknock qualities under some conditions, they did not necessarily coincide with the time of knock. This was interpreted as the relations between the flame propagation time of the charge in the combustion chamber and the ignition delay of the end gas were different between fuel types.
By measuring the flame arrival time at the ionization gap, it was shown that the time of knock corresponded to the flame propagation time at the knock limited compression ratio. In view of the adove fact, study was made on the change of the ignition delay of the end gas for air-fuel ratio.