210 gearing

[welland99]

Just got back from summer holidays in France in the 210. Had plenty of time to study the on-board computers displays (43 hours driving in 2 weeks - and not one traffic jam!) and am confused about something.

At a steady speed of 70 MPH (using cruise control) the engine turns about 2600 RPM. When accelerating, the revs before the last gear change is about 3200 rpm.

So, I presume this means that:
70MPH in 5th gear = 2600RPM and
70MPH in 4th gear = 3200RPM.

So far so good.

But when climbing hills and the box changes down, it sometimes seems to change to an in-between gear.

On slight hills, the revs rise to about 2750 RPM (still doing 70 MPH), and on steeper hills they rise to about 3200 RPM.

So the question is this. What is happening at about 2750 RPM? Is this a 4-and-a-half-th gear?

 

[Tinkerer]

I think that you will find that in all but top gear the system uses the torque converter which means that the engine is not 'locked' up to the gear train as in a manual gearbox and so an element of 'slip' is introduced which will give rise to the variation of engine speed to road speed that you are seeing. In top gear under certain conditions (that I could not be sure of) I believe that a type of clutch engages which effectively locks the torque converter so that no slip exists. But then someone is bound to be along in a minute to tell me that I am talking a load of rollocks!

 

[johnsco]

Tinkerer is basically-correct.
There are lock-up clutches on 5th gear and sometimes on the other gears, but the lock-up is progressive, so that it should be imperceptible as it engages.
The lock-up clutch is to save fuel, and the progressive nature of its action gives the transmission a smoother characteristic.
It also reduces the amount of energy lost in the torque converter, this reducing the amount of heat dissipated into the ATF (which is, in any case, energy being wasted)

 

[welland99]

Thanks for your replies guys. I get the picture that the auto transmission is a complicated beast (and it does its job very well).

But, I'm not clear about why there is advantage in introducing slippage (either through the torque converter or clutches) as I have experienced.

When cruising at 70 mph under light throttle and more power is required to ascend a hill, what advantage does the slippage condition bring? Is it to allow smoother down shifting to fourth (if that becomes necessary)?

 

[Aerialmark]

Just got back from summer holidays in France in the 210. Had plenty of time to study the on-board computers displays (43 hours driving in 2 weeks - and not one traffic jam!) and am confused about something.

At a steady speed of 70 MPH (using cruise control) the engine turns about 2600 RPM. When accelerating, the revs before the last gear change is about 3200 rpm.

So, I presume this means that:
70MPH in 5th gear = 2600RPM and
70MPH in 4th gear = 3200RPM.

So far so good.

But when climbing hills and the box changes down, it sometimes seems to change to an in-between gear.

On slight hills, the revs rise to about 2750 RPM (still doing 70 MPH), and on steeper hills they rise to about 3200 RPM.

So the question is this. What is happening at about 2750 RPM? Is this a 4-and-a-half-th gear?

I am also touring around Europe and have to agree about the traffic jams, and not a drop of rain either

 

[johnsco]

Thanks for your replies guys. I get the picture that the auto transmission is a complicated beast (and it does its job very well).

But, I'm not clear about why there is advantage in introducing slippage (either through the torque converter or clutches) as I have experienced.

When cruising at 70 mph under light throttle and more power is required to ascend a hill, what advantage does the slippage condition bring? Is it to allow smoother down shifting to fourth (if that becomes necessary)?

If the transmission shifted from 5th in lock-up mode to 4th in lock-up mode, I suspect there may be quite a bump !!

 

[tor_ake]

Slippage lets the engine increase its revs, thereby going into an area where more torque is available (losses made up for). This is apparently enough to negotiate slight hills.
Kind of like having an extra gear "in-between" as OP described it.