In a simplified view, there's a motor and a gearbox and when they both are connected the gearbox makes the motor work. You only can shift the gear when both motor and gearbox are disconnected.
For this part, there's the clutch. The clutch is the connection between motor and gearbox.

To engage means to fit two parts of an machine together and the opposite to disengage is then to free those parts.

From a technical point of view, when you engage the clutch, the motor and gearbox are connected and you are able to drive and when you disengage the clutch, the motor and gearbox are not connected.
Thus, if the clutch is engaged and motor and gearbox are connected, it's not possible to change the gear.

But I found this example in OALD:

Engage the clutch before selecting a gear.

Technically, this means you put motor and gearbox together and then change the gear. That's not possible as discussed above.

And this is taken from Merriam-Webster's Learners Dictionary:

I have to learn how to let the clutch out smoothly.

Actually, the part you need to do smoothly is to lessen the pressure onto the pedal and eventually to take the foot away. But by taking the foot away, you rather put the clutch between motor and gearbox; hence, technically you let it in and not out.

And in Collins Learner's Dictionary, they say:

Laura let out the clutch and pulled slowly away down the drive.

You can only drive when the clutch is between motor and gearbox, so actually you can't drive when the clutch is off.

In my mother-tongue we literally say to clutch out when we put our foot onto the pedal and to clutch in when removing our foot from the pedal. This corresponds to the technical function.

I wonder why in English the terms seem to be used opposite to what they actually represent. Why do you let the the clutch out when actually putting it in, and why you engage the clutch when actually take it out?

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    Because it's easier to refer to what's going on from the human interface point of view, as opposed to from the inner system's perspective. In this context, clutch refers to the clutch pedal. – J.R. Jan 9 '14 at 13:46
  • @J.R. You nailed it. – Kris Jan 9 '14 at 14:17

The expressions need to be considered in context here:

The first example "Engage the clutch before selecting a gear." refers to pushing the clutch pedal inwards from its normal rest position, thereby engaging it.

Releasing the clutch to move the vehicle forward has to be a smooth operation (not necessarily slow) to avoid stalling the vehicle.

The second example "Laura let out the clutch and pulled slowly away down the drive." and "I have to learn how to let the clutch out smoothly." refer to releasing the clutch from its engaged position to move the vehicle forward.

So the right way to look at these expressions is from the perspective of the user (the driver) and not the mechanical operation of the clutch.

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There is no question but that engage the clutch is a more descriptive phrase that let out the clutch for the actual mechanical activity. As this ngram shows, engaging was a more commonly used term in literature than letting out for many years.

The use of let out the clutch referring to an automobile goes back to at least 1908.

The logic of the phrase is based on the physical setup of a clutch pedal on cars. The pedal was often at the end of a spring loaded metal rod coming through the floor of the driver's area. When the pedal was fully up, the clutch was engaged. Pushing the pedal and the rod down disengaged the clutch. Later versions used a pedal on a lever. The pedal was pushed down or in (away from the driver) into the footwell to disengage, and allowed to come back or outward (with regard to the floor or footwell) to engage the clutch.

What a driver is actually doing is letting out the clutch pedal, which causes the clutch to engage.

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Under its entry for the verb to let in, the Oxford English Dictionary has, as its seventh definition:

Motoring. To engage (the clutch) by releasing one's pressure on the clutch pedal.

This shows that engaging the clutch means connecting the engine to the gearbox. However, the first definition of to let out includes ‘to release (the clutch of a motor vehicle)’ and this citation shows that it has the same meaning as to let in or engage:

I . . . let out the clutch and we were off.

There’s clearly a risk of confusion in using to let in and to let out in the context of engine transmission. This can best be avoided by using engage and disengage instead.

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  • Well, actually from the first quoted example above, engage seems to be used 'incorrectly' as well. "Engage the clutch before selecting a gear." Imho I need to disengage the clutch in order to select a gear. - And are you saying with "to let in" == "to release" and "to release" == "to let out" that "let in" equals "let out"? – Em1 Jan 9 '14 at 14:02

Both terms / phrases are poor, which is why the question comes up. Language should be (and should remain) unambiguous. So we should choose terms that don't rely on physical situations that could change (hang up the phone, tape a TV show) and use terms that will never change (end the call, record the show) An album is still an album (a collection of items), even when it is only on a web site, but "a record" came to refer to a vinyl LP and so it became tainted. "Track" is similar for a piece of music: when it is not on a disc (or disk) it is not a track anymore. "Song" doesn't work, because many pieces have or are edited to have no lyrics. Eventually you have fewer words and meaning is lost entirely, like the change to using periods to delimit all forms of numbers.

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There is an obvious reason for the confusion, to which you allude.

Of paramount consideration is clarity, for reasons of road safety. I haven't checked but I think the UK Highway Code refers to 'pressing the clutch down' and 'lifting the clutch up'. It may not be descriptive of what is happening mechanically to the car, but clarity of understanding between driving instructor and pupil is the important issue governing this.

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