Okay, that makes sense. You’re using ‘speed’ to meen ‘rotational speed’, in revolutions per unit time, not the speed of the car over the road, right?
That’s a very clear explanation.
I have an idea of a traditional manual transmission: two sets of gears with a clutch that lets you disconnect the input from the power source, so you can move a different output gear against the input gear. What’s a transaxle, then, if it’s different than a traditional manual transmission?
I thought a transaxle just combined the gearchange unit of a regular transmission with axles for the drive wheels, and was used to save space in front-wheel-drive front-engine cars, but it didn’t actually function any differently?
Yes 100 RPM in 100 RPM out = direct drive. 100 RPM in 120 RPM out = overdrive
You got it, combine a transmission with the axle unit and you have a transaxle. Function is the same as a RWD manual transmission. The design is different due to the difference in packaging.
To clarify, it uses a gearset, as does a regular (RWD) transmission, but the gearset is not just like, or “of,” a regular transmission. As Rick mentioned, it requires a different layout and thus is designed differently, one difference being it doesn’t have direct drive - input shaft directly coupled to output shaft - like a regular transmission.
The “-axle” part of “transaxle” is a reference to the final drive axle of a traditional RWD set-up, which has the ring and pinion and the differential, and usually (but not always) the actual rear axles. It does not refer to the unit containing the actual axles, which are external to it.
::: sigh:::
I was afraid somebody was going to ask me that.
To explain the difference I am going to have to give you a short course in how an auto trans works.
What makes an automatic transmission work is planetary gears . Planetary gears can give different ratios depending on which gear is held stationary, which one gets power and which one is the output. hydraulic pressure is directed to clutches and brakes and freewheels inside the transmission. Clutches lock rotating members together, brakes stop a rotating member, and freewheels (sometimes called sprag clutches, or one way clutches) allow free movement in one direction, but lock up in the other. Think of a ratcheting screwdriver.
Locking any two parts of the planetary gear (clutch) set together makes the entire unit rotate as if it were welded into a solid piece. Brakes (band or disc) stop a shaft from rotating, and freewheels back up the clutches so that the trans does not slip under load.
When the engine is running it spins the torque converter and the oil pump inside the transmission. The torque converter is a fluid coupling takes the place of a clutch on a manual transmission car. (it does some other stuff also, but this is good enough for now) When you are in neutral there is oil pressure in the transmission, but the different solenoids are not energized this means that the clutches, brakes, and freewheels are not applied so the transmission’s output shaft is not coupled to the input shaft. The same is true in park, except there is a mechanical lock on the output shaft that is engaged.
When you shift to drive (I am not going to explain the power flow for each gear. That will be left as an exercise for the reader.) the clutches brakes and freewheels for 1st gear are engaged. As you accelerate the transmission shifts from 1 to 2 and so on until you are in your top gear (3, 4, 5, 6 or I think BMW has a 7 speed)
Anyway you hit a downhill grade. It does not matter if your cruise is engaged, or how long you have been driving. We are going to assume a rather steep grade for the sake of this discussion. When you start down this hill, gravity, not only being a good idea, but the law acts on your car and pulls it to the bottom of the hill. Being a steep hill, your car actually speeds up somewhat. If you look closely at the tachometer you will see that your engine is running at say 1700 RPM. Why is that? Simple, the brakes, clutches and freewheels for whatever gear you were in before you got to the hill are still applied. The output shaft of the trans in still coupled to the input shaft. The fluid coupling (torque converter) is working backwards, and now your wheels are driving the engine rather than the engine driving the wheels. Freewheeling is when the engine and the drive wheels are not coupled. Since the drive wheels and the engine are still coupled, this is not freewheeling.
Still not convinced? OK try this on:
Now you are on a very shallow grade. Say 1%. You let off on the gas in gear, and the car does not speed up. Maybe it actually slows down. Why is this? We know gravity is the law and not subject to repeal so that isn’t it. The reason is that the output shaft of the trans is still coupled to the input, and the slope is not steep enough to overcome the drag of the engine. If you were to shift to neutral, the car would coast down the hill normally.
If you shift an automatic trans to neutral it will freewheel, leaving it in gear is similar, but is not freewheeling.
It should be noted that shifting a car to neutral on a downgrade is against the law in many places.
:smack:
It just occurred to me after posting that opus, that I did not directly answer your question.
The cruise control had gravity helping it. Therefore the throttle had to close down or the car would have accelerated down the hill. Just as you lift your foot off the gas when heading down a hill, the cruise control also closes the throttle. So the sudden RPM drop was from the throttle closing. The reason the engine did not return to idle was explained in my last post.
