On the rare occasions we encounter one, we refer to a steam locomotive as an engine, the same word that we give to the motive power of an aircraft. But all electrical devices are driven by motors. In Britain at least, one’s personal transport is a motor car (with compounds such as motor trade, motor vehicle and motor sport), even though it’s always powered by an engine. Small boats may have outboard motors and then are often called motor boats.
However, the propulsion device of a rocket can be called either a rocket motor or a rocket engine, and usage here seems not to have settled on one or the other. The IEEE Spectrum magazine for June 1998 (which Ron Jeffries has thoughtfully sent me) reports that the debate has been so intense, and yet so inconclusive, that some rocket scientist has coined the phrase whoosh generator as “the humorous, genderless, politically correct way to refer to the propulsion device in a hobby rocket, thus avoiding the great motor/engine debate”.
In everyday, non-technical usage the words have much the same meaning. But they have such clearly defined and fixed compounds (except in the rocket case) that they can’t be thought of as entirely interchangeable. The magazine article argues that the difference is that engines contain their own fuel or are part of a highly integrated engine-fuel system, whereas a motor draws on externally supplied energy. That’s the rule given in the Oxford English Dictionary, but on reflection it seems not wholly satisfactory. It doesn’t work for outboard motor or rocket motor for example. And it doesn’t explain why the two words should have been applied in this way. For that we have to look into their history.
Engine is from the Latin ingenium, which referred to one’s ability to create things, one’s native genius; it comes from a root meaning ‘create; beget’ from which we get words like genetic, and is also the source of ingenious and ingenuity (engineer derives from a related word). Its first meaning in English, from about the fourteenth century, was very much this one of mother wit or genius, a skill in devising things. It could also, by obvious extension, refer to the result of such ingenuity, a contrivance or device, particularly any mechanical apparatus. The term was very general; a sixteenth-century text directs that a person should be “put in the stocks or other such engine”; pulleys and their like were also engines (as in Swift’s Gulliver’s Travels of 1727: “With ropes and engines, I made a shift to turn it”). The British pub served its drink by means of beer engines, hand pumps which drew the brew up from casks in the cellar; blazes were extinguished with the help of fire engines, originally hand-operated pumps. In the 1670s one John Worlidge invented a machine for pulping apples in cidermaking; he termed his device the Ingenio, an obvious reference to the Latin original. Engine was commonly applied also to weapons of war, such as the siege engine and to devices such as snares for catching game (hence gin trap, where gin is a short form of engine that also turned up in compounds such as horse gin for a horse-powered windlass).
It was an obvious enough extension to apply the word to the new devices that created power through steam. At first these were static units designed to pump water from mines, hugely complicated even in their early Newcomen incarnations. The steam engine was such an important machine, being one of the crucial developments of the Industrial Revolution, in particular making possible deep mines, that engine soon came to apply almost exclusively to it (being replaced in most other cases by machine, a word that earlier had meant almost the same as engine). And that usage came to influence later extensions of it, as in petrol and diesel engines: no longer just a contrivance, but a system for producing propulsive power.
Incidentally, computer science has several terms that include engine, such as search engine, database engine and recognition engine. In all of them engine has the sense of a central part or kernel of a software application, hidden from the user, which does intensive ‘number-crunching’ work on data, only the results of which are made available. Charles Babbage named his Victorian mechanical computer the Analytical Engine, using the pre-Industrial-Revolution sense of the word. I would guess, in view of the almost iconic regard many computer scientists have for Babbage’s work, that the modern terms derive directly from this phrase.
Motor had quite different origins, coming from the Latin movere, ‘to move’. It was first employed in English in the sense of ‘instigator’, or something that causes motion, often in a figurative sense, as of God as being the cause of the motion of the heavens. Even by the nineteenth century, it was still applied generally to the idea of something that caused change, without necessarily implying a mechanical device; for example the Civil Engineering Journal in 1839 said “The true motor of the system would ... be the weight of the atmosphere”, in which motor here is an agent or force (a sense which is still current). It was also applied early in the same century to classes of muscles or nerves whose job was to cause parts of the body to move. Only in the 1850s did it begin to be applied to a device that employed some source of energy to create movement, being applied first to the electric motor and to hydraulic devices.
When the electric motor appeared, people saw a key difference between it and the steam engine. The latter had an obvious source of energy in its fuel; the source of energy of the former was less clear, being supplied mysteriously from a battery or generator by means of wires. Steam engines obviously consumed their fuel, but electrical and hydraulic devices extracted energy from some source without obviously consuming it. Perhaps this reminded people of the original sense of motor that referred to some intangible or spiritual force, and persuaded them to apply it to these new sources of power.
By the time that vehicles driven by internal combustion engines had begun to appear in any numbers, at the very end of the century, both words had become well established in common usage. The driving force was obviously an engine, which consumed fuel to provide motive power. But why the conveyance as a whole was termed a motor vehicle is less obvious. The mere fact of it moving was obviously not sufficient; that was hardly a new idea, after all. It may be that it was a more elegant word, and also helpfully distinguished the automobile as a system from anything that belched steam and soot, such as the early steam-driven road vehicles like the Stanley Steamer.
By the beginning of the twentieth century, the two words had moved together from very different origins, even though as we’ve seen, for historical reasons they were mostly employed in set compounds. But when new forms of propulsive device came along, analogy (or sometimes chance) decided which was to be applied in a particular case. Aeroplanes were obviously powered by engines, since the earliest ones were taken over directly from petrol engines of the kind that drove cars and lorries. Though there seems to be no clear evidence for the choice, perhaps outboard motor was so termed because it was a compact device that reminded its namers of electric motors.
The confusion between rocket engine and rocket motor is less obvious. By analogy with other devices that consume fuel, it ought to be an engine, but perhaps by the time it came to be named motor had become so close a synonym that either felt right.