Film-Tech Forum ARCHIVE: A strange Q about the way automatic transmissions are designed

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Posted by Lyle Romer (Member # 1266) on 08-06-2009, 09:57 AM:

Here's what I've been wondering. Manual transmission models of cars (even with the same number of gears) seem to be slightly more efficient. I assume there is less parasitic loss in a manual transmission design.

So, my question is, to make an automatic why not just attach actuators to a manual to do the shifting automatically and control it with a computer to tell it when to shift?

As an aside, why don't large trucks have automatic transmissions? Is there something about the design that doesn't allow them to handle the gear ratios needed?

Posted by David Stambaugh (Member # 1102) on 08-06-2009, 10:07 AM:

quote: Lyle Romer
So, my question is, to make an automatic why not just attach actuators to a manual to do the shifting automatically and control it with a computer to tell it when to shift?

VW-Audi DSG Direct Shift Gearbox

My 2009 Jetta TDI turbo-diesel has the DSG transmission. It's pretty awesome, though also a bit quirky. I get 34 MPG around town and 45MPG highway without even trying.

Posted by Robert Minichino (Member # 3495) on 08-06-2009, 10:31 AM:

It's hard to engage a dry clutch smoothly yet fast enough not to wear it excessively, so automated manual type transmissions are mostly limited to sporting applications. The dual-clutch gearboxes get around that by using two wet clutches where the next gear is pre-selected on another shaft and the clutches transfer power from shaft to shaft; since the wet clutches are more tolerant of slipping they can perform smoother shifts.

Regular automatic transmissions use a fluid coupling (torque converter), which is where most of the loss happens. Modern automatics use lockup torque converters where at a certain speed at cruise the converter's input and output are locked together so there isn't as much loss. Modern automatics are about as efficient as manual transmissions, but do cost somewhat more. Often the difference in fuel efficiency between manual and automatic models is due to gearing and interactions with the economy test driving cycle.

Posted by Galen Murphy-Fahlgren (Member # 4535) on 08-06-2009, 02:30 PM:

There are plenty of automated "manual" transmissions, you just can't afford most of them. David mentions VW AGs DSG, which is the most affordable and really quite quick, although it is heavier and potentially less reliable, though this still has to be seen. Ferrari has an automated transmission which is the fastest shifting in the world, although it purportedly requires a well controlled throttle foot to operate smoothly (I haven't driven one). BMW has its SMG (Sequential Manual Gearbox), which has been around since some time during the E46s production run. I believe this is a pretty much standard Getrag 6-speed manual with a computer controlled clutch and shifter, and obviously some sensors to accommodate this functionality. Everyone I know who has newer BMWs has a real manual, so I've never driven this either.

While I disdain these types of transmissions less than a torque converter or old fluid drive automatic, I still don't understand the point. Do you know how to drive a car or don't you? I personally abhor the latest trends in automotive design, particularly Volvo's and Mercedes' automatic braking systems. I will quit driving before I hand over any more control to a car.

Semis traditionally use manual transmissions because the number of gears would make an automatic transmission very expensive to produce and so that the driver can have better control over the truck, like using engine braking on steep downhills. Some semis now use automatic transmissions since the cost of automatic transmissions is decreasing.

Posted by Sam Graham (Member # 2889) on 08-06-2009, 05:00 PM:

Then you have CVT transmissions, which are basically two varying pulleys and a belt.

My current car has one. Took some getting used to. You hit the gas and the rev stays at a constant speed while the car still increases speed. It's like experiencing a slipping clutch with the opposite result.

My clutch foot hates me right now.

Posted by Chad Souder (Member # 343) on 08-06-2009, 07:35 PM:

quote: Lyle Romer
Manual transmission models of cars (even with the same number of gears) seem to be slightly more efficient.

This maybe a stupid question, but wouldn't it depend on the gearing ratio? I don't know if any automakers do this, but if they saw a reason to have a different ratio, the automatic could get better mileage, at least highway mileage, even with fewer gears, couldn't they?

Posted by Galen Murphy-Fahlgren (Member # 4535) on 08-06-2009, 08:33 PM:

Chad, that is debatable. Some people insist that modern torque converters are as efficient as dry clutches (ie equally lossy), but I've never seen evidence (dyno tests, EPA test sheets) to support this. However, most automatics are programmed to shift to an engine speed below optimal fuel efficiency, presumably because consumers are oblivious and prefer marginally lower cabin noise.

Posted by Jon Miller (Member # 163) on 08-06-2009, 10:04 PM:

quote: Lyle Romer
As an aside, why don't large trucks have automatic transmissions? Is there something about the design that doesn't allow them to handle the gear ratios needed?

Well, you can specify an automatic transmission for big-rig truck tractors...not a torque-converter/planetary gear design like the average slush-box car but, essentially, a manual transmission and clutch with computer-controlled servos; the Eaton Ultrashift, for example. No clutch pedal is necessary for this type of transmission; lower-end automated manuals use a traditional foot-operated clutch for starting and stopping.

These transmissions have been on the market for quite a few years now, and, according to Tim Reed's reply to a comment I made several years ago in another thread, they work quite well.

Posted by Monte L Fullmer (Member # 2797) on 08-07-2009, 03:36 PM:

Going back to the question on tranny effiency is that the manual tranny is 100% engaged between the motor and driveline or transaxle due to the clutch assembly actually locking the tranny to the motor making the line of power to the wheels as one line of power. The only small disadvantage with the manual is, when the clutch is disengaged from the motor is the loss of power inerta between shifting being the results of that cause.

The automatic is actually a huge power soaker since it never has the 100% engagement between motor and driveline or tranaxle due to the torque converter having just fluid making the connection between motor and driveline, but the 4X4's love them since there is no power loss between gear shifts-100% engagement between gear shifting.

But manuals win in saving front brakes since when the gas is let off, the motor's compression will aid in breaking down the forward mass of the car and no need of full braking. Automatics, on releasing the gas, the vehicle just rolls due to the lack of total connection of tranny and motor and full braking will have to occur. Why you eat up front disc brake pads and rotors ten times more on an automatic car than with a manual car.

..is also why you're seeing 7 speed auto trannies to get the MPG up there in the range of a good ol 5 speed manual tranny.

-Monte

Posted by Robert Minichino (Member # 3495) on 08-07-2009, 03:50 PM:

Monte, modern automatics have lock-up torque converters that provide that direct engagement during cruise.

Posted by Monte L Fullmer (Member # 2797) on 08-07-2009, 08:14 PM:

quote: Robert Minichino
during cruise.

True, but town and stop and go driving is where the difference comes into play since the lock-up system can't be engaged.

Posted by Charles Greenlee (Member # 3856) on 08-11-2009, 04:24 PM:

Also, on an auto, when you let off of the throttle to brake, at lower speeds, you are now having to brake against the engine's pull. Try having an auto in gear and take your foot off the brake petal. Even on level ground, the car starts rolling. A manual, once the clutch is disengaged, suffers no pull from the engine. You only have to brake against your car's momentum.

I drive a manual. In an instance where my car was in the shop, I drove a similar automatic car. I had to brake either a bit harder, or a bit earlier. The torque converter never fully disengages. I found it most suiting just to bump the shifter into neutral when braking on the rental.

This is also what hurts the automatic's fuel economy. When at a stop, since the torque converter never fully disengages, the engine must fight against it, which would reduce idle speed and potentially stall the engine. However, most automatic's throttle bypass (controls idle speed) are set to be open slightly more to allow a little more throttle at idle, which keeps it from stalling. It's not but a very nominal amount, but it's there.