AMT gear shift strategy

[SMUG]

I am working on 5 speed AMT shifting strategy.Do you think that the transmission should shift the gear at a vehicle speed where the driver can feel the maximum forward acceleration in each gear( ie max torque at the wheels). If not then what should be the best shift points both from performance and fuel economy point of view??

 

[GregLocock]

For optimum fuel consumption you'll need the sfc map for the engine and the resistance curve for the vehicle

For optimum WOT performance you'll want to shift at the red line.

constructing the map of other shift points from these two sets of data is not entirely straightforward, and involves a lot of judgement calls.

You also need a downshift map, which is not merely the upshift map.

You'll probably need to consider emissions and drivability as well.




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

If the application is for passenger car use, the control will need to be about as comprehensive and versatile as that needed for fuel injection.

 

[crysta1c1ear]

A car I was in a few years ago shifted up when approaching a red traffic light. The reason was that the electronics selected a more fuel efficient ratio. If the brakes had been applied gently during the deceleration prior to the shift, then the gear shift would have been down, to provide better engine braking.

One of the Japanese manufacturers has a patent describing use of the GPS to help in ratio selection, for example by knowing the distance to a hill and the gradient of the hill.

A leading German carmaker has plans to avoid unnecessary shifts while cornering.

Some Honda AMTs can coast on low accelerator, again for fuel economy.

 

[SMUG]

Thanks all

Greglocock ,When you say resistance curve then you mean vehicle forward acceleration in each gear?

What parameters will define the downshift map other than braking?

When you say drivability do you mean torque management or gear shift quality?

 

[crysta1c1ear]

SMUG
Greg had a website with a fuel economy calculation spreadsheet which had a shifting algorithm coded in to it to determine a gear ratio. The aim was something like determining US driving cycle city mileage. From memory, the gear selection algorithm was replicated down the spreadsheet with copy paste. However spreadsheets like Openoffice will allow you to define a formula (or maybe algorithm is a better word) once, and then replicate application of the algorithm down the columns.

You can then try tweaking the gear selection algorithm and immediately see possible effects on fuel economy. For example I think I had a look at the benefits of changing to 6 or more speeds. It appeared to me that benefits above about six speeds seemed fairly limited, and that gains were more easily made after that by agressive shift logic, ie tweaking the gear selection algorithm rather than adding ratios.

I don't know where my copy of the spreadsheet is, but it is Greg's intellectual property really and my changes were for personal use, whereas Greg happily put his spreadsheet on the internet and advertised it here on eng-tips.

Maybe Greg will want to comment. I guess I should have said thanks Greg a year or so ago, but didn't. :+(

 

[GMIracing]

SMUG-

To oversimplify things, you need a few things as Inputs;

Vehicle Mass
Road Load Curve (Rolling Resistance + Aero)
Driveline ratios
Engine Torque map (Torque at Load vs RPM)
Engine BSFC map (g/(kW•h) at Load vs RPM)

Ideally, you would create a shift algorithm (map) that is a tradeoff between Fuel Economy and Driveability.

1st, For the Fuel Economy, you want to operate the engine as much as possible within the “sweet spot” of the BSFC.

2nd, for the Driveability, you want to have smooth Input vs Output (Pedal Displacement vs Vehicle Acceleration)

These ideal maps that result from these two models will be different, so the idea is to find one that is the best of both.

 

[Fabrico]

Downshifting will be almost as complex as upshifting. And, as Greg pointed out, a mostly separate issue.

 

[GregLocock]

Yeah, I killed that download, it had too many good ideas in it, and /nobody/ ever commented on it, until you did just now! I used it as the basis for something else which I decided was too handy to leave in the public domain.

The overall shift schedule doesn't really control shift quality, that is done in another map.

Drivability is just a bin for all subjective aspects of the implications of the trans calibration. For instance tip-in clunks, tip-out clunks (backout shifts), performance feel, shift feel, busyness, etc etc.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SMUG]

I sometime think what was engineering world like before the arrival of internet!!!

Greg and GMIracing thanks for that.

What is the physical meaning of tip in clunks and tip out cluncks on AMT?

 

[GMIracing]

Tip in and Tip out:

The quick application or disapplication of the "go" pedal.

Clunks/lugging/lurching are what happens when the transmission selects the wrong gear, applies the cutches too fast/slow, or gives the engine too much/not enough fuel.

 

[GregLocock]

Ideally the gearbox would give the impression that there is a seamless flow of power with no hesitation, and no funny noises or vibrations. Drivability is the failure to meet that requirement.

Usually I can drive an auto for about 10 minutes before I start to play with the throttle pedal and find out how to trick the calibration into making some nasty shifts. Ten years ago this was quite difficult in some (upmarket) cars, but these days the fuel consumption is so important, and there are so many gears, that everybody uses shift maps with less hysteresis, so it is easier to get them to change gear.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SMUG]

What typical noise/vibrations problems you would expect in order to achieve seamless powerflow specially in the context of autoshift? I am thinking of actuator speed for shift and force influence?

 

[GregLocock]

It's more of a whole car problem, from engine mounts through to the wheels.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BrianPetersen]

If you are talking about an automated-manual transmission like BMW's SMG, one headache is that there is an unavoidable "null" period in every gearchange. The transmission does what a manual driver does ... clutch in, disengage previous gear, engage new gear, clutch out. The time that this takes is why people complain about SMG.

VW's DSG pre-engages the next gear and simultaneously clutches out the previous gear and clutches in the next one; it's most impressive. (My dad has one, by the way.)

If you can figure out a way to shift the transmission under load with minimal engine cut, you can almost eliminate the "null" period. Superbike roadracing gearboxes aren't "automated" - still rely on rider working the shift lever - but they use a momentary ignition-kill for milliseconds to let the gearbox shift. Those are dog-engagement gearboxes without synchro's.

The other headache to getting it seamless is getting the thrust to the wheels to be the same after engagement of the next gear as it was in the moment before the shift in the previous gear, otherwise you're going to get a lurch. And you have to do something about the flywheel effect; a sudden change in engine revs will make that flywheel inertia go somewhere - there's another potential lurch. Newer cars with drive-by-wire throttles can fiddle with the throttle setting (or ignition timing, etc.) in order to achieve this. VW DSG automatically blips the throttle on downshifts to rev-match.

Controlling all this stuff ain't simple, as others have noted.

 

[patprimmer]

Why not minimise the first flywheel, and have a flywheel after the gearbox to absorb lurches.

I guess I already know. Cost and rotating mass.

Regards

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