Diarmuid Downs was a one-company man. He started work at Ricardo in 1942, after graduation from the University of London, Northampton Polytechnic (now City University) with a first-class honours degree in engineering, and retired 45 years later. His early meticulously executed research concentrated on spark-ignition combustion phenomena—essentially knock, pre-ignition and the effects of fuel additives— an important understanding in those early days for the oil and additive companies. Later, in the 1970s, his attention moved to engine and vehicle exhaust emission control, a key emerging technology at that time. In his more senior years, he took a broader picture of the industry and technology, building on his detailed pioneering research as he continued to develop his vision and balanced technical judgement. Recognized and encouraged initially by Harry Ricardo, he rose quickly to senior positions in the company, becoming a board member at the age of 35 and managing director 10 years later. In 1976 he became joint chairman and managing director, ultimately to become chairman and finally to retire in 1987. He authored or co-authored some 46 technical papers in his working career. He has been described as a ‘gentleman engineer’ with strong support for his staff at all levels. In the broad spectrum of engineering disciplines, Diarmuid leaned more towards the intellectual/scientific end. He was a deep thinker, with a prodigious memory and love of the arts as well as biographical and historical literature. He inspired respect through his vision, balanced judgement and supreme confidence and was an articulate orator. He was awarded many honours in his lifetime, including a CBE in 1979, a knighthood in 1985 and in the same year Fellowship of the Royal Society. He held appointments in over 30 professional organizations, including four charities, to which he and his wife, Carmel, were dedicated, helping the vulnerable and homeless. He was a lifelong devout Catholic and active supporter of the church and related charities, recognized by a Papal knighthood in 1993.