A lost Asset in Auto Transmissions

[Renze]

I believe the point you are not understanding is that in the past, many automatic transmissions for 4wd and 2wd vehicles allowed you to start out in second gear. There is an advantage to this because in 1st gear, it is difficult to apply really low amounts of torque to the wheels with your accelerator pedal. It is very unforgiving. 2nd gear allows better low torque control. 4WD or 2WD doesn't matter. I have had 4WD vehicles stuck in snow that would spin all 4 tires but with careful low torque easing of the throttle was able to rock the vehicle out.

What you describe also applies for gas engines with manual transmission. Whilst pulling a trailer on wet grass with my S70 TDI, i simply let the clutch up slowly in 1st gear without moving the throttle. The wheels spin one, two times, while the revs drop to 500 and there you go, rolling with static friction between the tires and the wet grass.. My neighbour is having a V70 2.5 10V gas, and he has to push the accelerator pedal, which makes him spin the tires fast enough to not stall the engine, giving dynamic friction between the tires and the surface, giving him less traction.

For the same reason my Diesel clutch lasts longer than a clutch in a gas vehicle. Its much easier to control spin (either tire spin or clutch spin) when the torque is controlled by the fuel injection, not delayed by the distance the mixture has to travel between throttle valve and combustion chamber, and what else

my family are horse people, and i remember my father hating the 740 2.3 automatic when they had to go with the horse trailer in a wet field that served as parking lot. you had to push the accelerator to generate torque at the rear wheels, but once you spin, the slip in the torque converter would change to slip at the tires, so that you had no traction at all anymore, and had to drop the revs to stop spinning and then increase the wheel torque again. Mostly you lost momentum. With his later 740 turbodiesel the speed was in relation to the accelerator pedal position, irrespective of tire grip. When he got a Camry 2.2 on LP gas after that, he started complaining again.


I have been offroading, climbing 45 degree hills with a front wheel drive car by relying on Diesel torque and a governor that keeps the revs under control if you loose traction for just a second.


Getting back about automatics taking off in 2nd: Why would anyone want to drive off in 2nd gear, with less torque at the wheels, and more slip in the converter ? Old farts that cant drive, can use the ESP to prevent doing donuts...

 

[Farmer495]

Getting back about automatics taking off in 2nd: Why would anyone want to drive off in 2nd gear, with less torque at the wheels, and more slip in the converter ? Old farts that cant drive, can use the ESP to prevent doing donuts...

You must not have snow and ice. The second gear start eliminates most of the drive strain slop in the torque converter BEFORE the wheels want to spin, you are basically lugging the engine and easing away with out tire spin, same thing as you describe with low end torque with diesel/manual trans and being able just to pull away at low RPM. I've used 2nd gear in manual trans gas cars and trucks to pull away on ice where first gear would just leave you spinning. Mu current diesel/manual 3/4 ton I normally start in 2nd anyways (granny low first) so in 4x4 it just idles away. If I am trying to start a loaded stock trailer in mud or wet grass I will often use 4th gear(in low range) as it is slightly higher than 2nd gear high range and can pull away with out clutch slippage and the tires bite instead of spin.

 

[Renze]

You must not have snow and ice. The second gear start eliminates most of the drive strain slop in the torque converter BEFORE the wheels want to spin, you are basically lugging the engine and easing away with out tire spin, same thing as you describe with low end torque with diesel/manual trans and being able just to pull away at low RPM.

Yes the higher gear ratio in combination with the converter stall ratio limits the max torque at the wheels, but so does a controlled right foot.

 

[MikePA]

... but so does a controlled right foot.

It is also a function of how sensitive the accelertor pedal and associated controls are. I've owned vehicles where, at the beginning of it's 'travel', the accelerator pedal acted more like an on/off switch. And, yes, the dealer looked at this and it was working as designed and was only noticeable under low traction starts.

 

[jejeosborne]

Sitting in the drive thru yesterday I learned my f150 would launch from 3rd gear when selected manually. Very smooth gentle acceleration from 3rd.

 

[Renze]

It is also a function of how sensitive the accelertor pedal and associated controls are. I've owned vehicles where, at the beginning of it's 'travel', the accelerator pedal acted more like an on/off switch. And, yes, the dealer looked at this and it was working as designed and was only noticeable under low traction starts.

That description fits my fathers previous Camry 2.2 and his current Kia Cee'd 1.6, and my BIL's Ford Focus... I drive away by tapping the accelerator "on" and then take my foot off the gas, then engage the clutch and push the accelerator "on" again as soon as the revs drop to near 1200rpm. My first car was a Volvo 440 1.7 running on LP gas, which had both torque and accelerator response like a Diesel, you could release the clutch pedal and it would take off. Yet it ran around 18mpg which is quite bad for an 1050kg weighing car... The past 10 years i allways had Diesels, i like the predictable accelerator response.

So, manufacturers better solve the cause of the problem by making better proportioning accelerator pedals, rather than trying to fight the symptoms by taking off in 2nd.

Sitting in the drive thru yesterday I learned my f150 would launch from 3rd gear when selected manually. Very smooth gentle acceleration from 3rd.

Yet very inefficient, because all surplus engine power is reduced to waste heat by converter slip. At my previous employer which manufactured wheeled loaders using a ZF transmission. ZF programmed the ECU to drive away in 2nd, the 1st was only accessible by kickdown with a pushbutton. When we found substantial fuel savings at a customer where we did a pilot project where the ECU was reprogrammed to take off in 1st, all other customers wanted their ECU's to be reprogrammed with this new software.

Thats why modern automatics have up to 8 gears: To eliminate converter slip as much as possible because converter slip equals energy loss. In a wheel loader a converter reaches peak efficiency of about 83% at 70% slip, and that minimum of 17% energy loss is in the torque converter alone. In stall conditions (full engine power is spinning the converter pump wheel yet the wheels are not moving) 100% of the energy is turned into waste heat by the torque converter. Thats why a trucks "towing package" often includes a transmission oil cooler. As soon as you start to turn the wheels, the total driveline efficiency rises, yet it can never be as efficient unless the converter lockup clutch engages, thereby eliminating all converter losses.

Off course torque converters in a truck transmission dont have a torque ratio of 2.2 to 2.5 so they have a peak efficiency higher than the 83% of the wheel loader converter, yet still, the torque converter is a dynosaur in terms of driveline efficiency. Even in wheel loaders the converter lockup clutch is rapidly gaining popularity, to use the transmission as a conventional powershift from gear 2 onwards, thereby making substantial fuel savings.

 

[jejeosborne]

It is also a function of how sensitive the accelertor pedal and associated controls are. I've owned vehicles where, at the beginning of it's 'travel', the accelerator pedal acted more like an on/off switch. And, yes, the dealer looked at this and it was working as designed and was only noticeable under low traction starts.

Exactly. First gear in these new 6 plus speed transmissions are so low that it is difficult to limit torque with precision. 1/8 inch pedal travel equal large torque increase. 2nd or 3rd gear start is much less torque with same pedal travel.

I guess some people have superhuman muscular control and can move the accelerator with such precision to not want the option to start in a higher gear. I guess I could go to the gym and train as well as remove all friction from my throttle linkage so I too am superhuman.

 

[MossRoad]

Plus many new vehicles are drive by wire.... the foot pedal is not physically connected to the fuel injection. It's done with wires and electrical current. Virtually NO physical feedback through the foot pedal.

 

[aczlan]

Plus many new vehicles are drive by wire.... the foot pedal is not physically connected to the fuel injection. It's done with wires and electrical current. Virtually NO physical feedback through the foot pedal.

Which if you think about it means that it would be fairly simple for the ECU to say "when stopped in first gear, make the first half of the pedal travel not do much to facilitate driving on slippery conditions"

Aaron Z

 

[MikePA]

Which if you think about it means that it would be fairly simple for the ECU to say "when stopped in first gear, make the first half of the pedal travel not do much to facilitate driving on slippery conditions"

Typing outloud to solve this;

INPUTS
Stopped? - Easy to determine, mph or kph = 0
Gear? - Ask the transmission (don't assume 0 mph = 1st gear)
Slippery? - Not so easy. As has been mentioned in this thread (what hasn't ) Traction Control senses wheel spin, which heats the snow, which then turns to ice.