The ship had three Parsons geared steam turbines, each driving one shaft, using steam supplied by six Admiralty 3-drum boilers. The turbines were designed to produce a total of , enough to give a maximum speed of at deep load. On 24 May 1940 "Illustrious" ran her sea trials and her engines reached . Her exact speeds were not recorded as she had her paravanes streamed, but it was estimated that she could have made about under full power. She carried a maximum of of fuel oil which gave her a range of at or at or at .