Torque converter and gradability 1

[Tads3]

I have been asked to carry out automotive trials on a new vehicle design. One of the tasks is to establish the maximum grade for continuous ascent and stop/restart.

The design team have provided me with the tractive effort curves for the vehicle which clearly show that 1st gear (6speed auto box) is not low enough to climb the required gradient. However the torque converter curve is superimposed on this curve and at a high torque multiplication ratio the curve does cross the gradability line.

How realistic is it to assume that the torque converter will provide the desired performance? Is it the case that the vehicle will climb the slope but there is a danger of excessive gearbox temperatures or is there a point where the torque converter cannot be relied on to provide the necessary extra gear reduction?

Thanks for your help, Tom

 

[BrianPetersen]

In the absence of any real information, one has to assume that the specified gradient is a very steep slope, far steeper than found on normal highways. Usually this sort of thing is something that the vehicle has to be able to do in order to get up someone's steep driveway, or something of that sort. Normally this type of usage is only for short periods (a few seconds, typically) at a time. For that, heat buildup isn't normally going to be a big issue.

If the torque converter does what it's claimed to do then the torque multiplication will be as specified (but normally greatest only when the output side is "stalled", i.e. held stopped). Crawling up a hill as described above is only going to happen at a crawl speed ...

In exceptional situations, you CAN burn up an auto tranny from heat, but we're talking about abusive applications like 4x4 rock crawling, or trying to get a vehicle out of a situation that it's stuck in. Normally there is a transmission oil cooler to handle the heat from *some* degree of abuse. This is why folks often recommend auxiliary oil coolers for trailer towing, etc.

For an OEM application, you can always just put the usual oil cooler, and throw the vehicle into a "safe" or "limp-home" mode if the tranny temperature gets too high ... There's usually a sensor for it these days anyhow ...

 

[patprimmer]

I support what Brian said.

The torque convector will multiply torque up to stall speed. The closer to stall the greater the heat build up.

We have no idea what transmission cooling capacity you have, but mass of fluid, cooling capacity at the speed the vehicle will be operated at during that time, the degree of heat generated, and duration of heat generation in excess of cooling capacity are all important considerations.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

What do they mean by 'continuous' ascent? If it means what I think it means then you need to get the efficiency curve of the TC and do some sums.





Cheers

Greg Locock

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

 

[romke]

i would assume that the extra torque that the converter can deliver when the vehicle is nearly or completely stalled, can only be safely delivered in short intervals and not when continuous torque of that magnitude would be required. in other words, with the gearbox you have it will not be possible to ascent the maximum grade for any length of time. if you still want to use the extra torque from the converter for anything longer then say 1 minute, you will have to invest in extra cooling capacity to make sure the tranny oil temperature stays below the maximum the transmission manufacturer or the oil supplier would regard safe for prolonged use.

in normal vehicle transmission designs the torqueconverter is supposed to supply something extra occasionally, but not usually on a permanent basis. there are exceptions to this: the transmissions of shovels and vehicles equipped with Voith automatics are usually purposefully designed to make a lot more use of the converter - and equipped with cooling to match

i remember when i was working with earthmoving equipment that we had a stall test which required the transmission to be stalled for a minute, and where the rise of transmission oil temperature had to stay within certain limits - the torqueverters included a meltplug that would melt above a certain temperature to prevent overheating too much - by simply emptying the converter - not an experience you would like to make when climbing a slope!

i therefore suggest you talk over your problem with the transmission supplier - it may well be that the overal gear ratio has to be reduced to get the required torque to the wheels.

have they also supplied you with tractive data for the driven wheels on the slope with the anticipated type of terrain? you might end up with a transmission that can supply the necessary torque and then notice that the wheels can not deliver it...

 

[Tads3]

Thanks for the answers guys. I think I have what I need now, time to go do the maths!