Engine Torque Control: A Guide to Automatics & DCTs

[The Jargon]

Hi guys,

I have a question about engine torque control in automatic transmissions and dual clutch transmissions. I've seen cases where the engine torque is decreased on an up-shift and increased during a downshift. Why is this done?

 

[russ_watters]

I have a question about engine torque control in automatic transmissions and dual clutch transmissions. I've seen cases where the engine torque is decreased on an up-shift and increased during a downshift. Why is this done?

Engine torque or wheel torque? How would you be able tell if engine torque is increasing or decreasing when a shift happens?

Just to make sure we have the basics clear though; wheel torque goes down when you upshift because of the gear ratio.

 

[The Jargon]

Engine torque or wheel torque? How would you be able tell if engine torque is increasing or decreasing when a shift happens?

Just to make sure we have the basics clear though; wheel torque goes down when you upshift because of the gear ratio.

Engine torque. I know by reading the CANBUS.

 

[Bandit127]

Engine torque in this context is likely to be secondary to requested rpm. The engine control will be matching rpm to the gear that has been selected to reduce stress on the gearbox and undesirable jerkiness for passengers.

 

[russ_watters]

Engine torque in this context is likely to be secondary to requested rpm. The engine control will be matching rpm to the gear that has been selected to reduce stress on the gearbox and undesirable jerkiness for passengers.

Yes, I was thinking back to my stick driving days how I lifted my foot off the gas when shifting to avoid revving-up the engine. So my question back to @The Jargon is whether this is a momentary change during shifting that you notice or does it continue after the shift?

If it continues and the throttle position isn't changed, I was thinking it may be due to moving to a better or worse spot on the torque curve due to the rpm change.

 

[jack action]

I've seen cases where the engine torque is decreased on an up-shift and increased during a downshift. Why is this done?

I would think it is to make sure that the engine power before and after shifting is the same, depending on the rpm and throttle position. That is the condition for smooth transition.

 

[The Jargon]

Engine torque in this context is likely to be secondary to requested rpm. The engine control will be matching rpm to the gear that has been selected to reduce stress on the gearbox and undesirable jerkiness for passengers.

Hi Bandit, this seems to be the case.

Yes, I was thinking back to my stick driving days how I lifted my foot off the gas when shifting to avoid revving-up the engine. So my question back to @The Jargon is whether this is a momentary change during shifting that you notice or does it continue after the shift?
If it continues and the throttle position isn't changed, I was thinking it may be due to moving to a better or worse spot on the torque curve due to the rpm change.

Hi Russ, it's only a momentary increase before the gear and clutch is engaged. It then returns to it's original level.

 

[HowlerMonkey]

It's the real reason we have drive by wire.....and complaints of bad shifting.

You should drive a car and pull up the datalist of the throttle plate opening vs what you are commanding while driving it.

You will find they are nowhere similar.

 

[Nidum]

It's a modern day refinement in gear change procedure which has been derived from the manually controlled speed matching required for optimal double declutching as used on old vehicles with crash gear boxes . Purpose as said by others is to give very smooth gear changes .

 

[Rx7man]

Not only is it for smooth gear changes, but it really reduces the wear on the clutch plates if the torque is reduced until the force on the plates is up to spec.. The plates don't rotate against each other under load and wear.. it also does make for smooth shifts, and since shock loads are what break things in most cases, if you can reduce the shock load you can have a smaller, cheaper, lighter transmission that can be put behind a more powerful engine that would otherwise tear it to bits without torque control

 

[Nidum]

My late father was once the foreman in a heavy goods vehicle repair garage .

Many of the lorries that came in then were big slow moving ones of 1940/50's vintage . These all had crash gear boxes .

Point of interest here though was that these big lorries required a heavy tow truck to go to the rescue when they broke down or slid off the road . This particular garage had a tow truck converted from a war time Matador gun tractor . Driving this with a broken down truck in tow over the winding and steeply graded Welsh valley roads in winter was a real art . Crash gear box of course but the simple mention of double declutching does not nearly describe what actually had to be done to change gear .

Changing gear on a down grade in particular was a complicated , slow and hazardous process .

Foot brakes were not very effective so with bad roads and heavy load most braking was done using engine retardation . On a long down grade it was necessary to change down to bottom gear under load while losing as little engine retardation as possible during the change . Devil's choice on how to do that . Take too long and you got a runaway . Crash the gears too quickly and you stripped the box .

 

[Rx7man]

I grew up with crash gearboxes on the farm equipment.. One tractor in particular I got very good at.. sure takes practice... I have an old bulldozer (Allis HD6) and with a hand clutch and a crash box I sometimes get it right