The Ergosphere
Wednesday, September 29, 2004
 

Kill the hydramatic

Q:  How do you take a car that gets 26 MPG going up a hill and turn it into a car that gets 6 MPG?

A:  Make it send all its power through a nice, lossy torque converter.

Yes, I'm serious.  I've got a car with a bunch of nice features, including a highly efficient turbodiesel engine and a trip computer with a not-too-inaccurate MPG meter... and an automatic transmission.  (Wasn't available any other way this model year.)

What's wrong with that? For efficiency, plenty. A torque converter is a way to amplify torque at the expense of power; the slip between the two sides causes fluid to be slung around inside like an O-ring being rolled down a rod, and the stator turns some of that circulating motion into extra torque. This is good for getting away from a standstill, but a 3:1 torque multiplication at stall is still... 0% mechanical efficiency. At low speeds, it's easy for 75% or more of the input power to be lost as heat in the torque converter.

(For those who are lost here, consider the situation of a car standing still on a hill, being pulled by the engine just enough to stop it from sliding backwards.  You could accomplish the same goal by putting a brick behind a tire and shutting the engine off, but instead the engine is spinning faster than normal and being loaded more heavily than a normal idle just to keep the car in the same place.  All of this power and fuel is being wasted in the slippage of the hydraulic torque converter.)

75% losses means using 4x the fuel that is ideally required for the job.  Modern transmissions have "bypass" clutches which lock their torque converters at higher speeds and eliminate the slip losses, but in local and stop-and-go traffic this isn't possible.

Toyota has a better way, in the Prius at least.  The Prius has an automatic transmission, but no torque converter.  Instead, it has a gear arrangement such that the slip between the engine and final drive moves, not fluid, but a motor/generator.  The energy that would be lost in a conventional automatic is turned into electricity instead, and that electricity either feeds a second motor (to add torque much more efficiently than hydraulic fluid) or is stored in a battery.  A little bit of slip at the engine can maintain a lot of stall torque at the second motor, making the whole affair much more efficient in low speed, stop-and-go driving.  And the electric motors used in cars, unlike torque converters, can create torque at zero speed with a minimum of power being wasted.

Yes, two motor/generators are more expensive than some brazed sheet metal formed into a doughnut-shaped chamber filled with type F.  But ask yourself this:  how much are you willing to spend to cut the number of madrassas training the next generation of Mohammed Attas? 
Comments:
It's a false comparison to think that these are your only two choices. The extra money spent on the torque converter replacement might have been more efficiently spent on any number of other things that might move our goals in the WOT further than increasing fuel efficiency.
 
The best of both worlds:  teach everyone how to drive stick shifts.
 
I am a lifelong automatic transmission engineer. The 6 mpg you experience going uphill would be 4 mpg with a manual. The torque multiplication improves gas mileage, not lowers it. Manual transmissions are today a waste of money, time and maintainence. Further, the coefficient of drag in a synchromesh transmission more than out weighs the slight fluid slippage found in todays converters. Then there is the question of longevity. The clutch looses. Don't tell me how great a Toyota Prius is. Wait until the green tree huggers get to pay $10,000 for a battery pack. If it wasn't for the b.s. environmentalists, we would have $.50 gallon gas and trucks that get 45 mpg. When you have phony greens getting paid $6 per car for a phony smog device that never worked, I say we junk the system and start over. Drill for oil everywhere. Our creature comforts pollute less, not more. When 3000 people in France die from a 95 degree heat wave due to no a/c, and we loose 2 people, it says volumes about the uselessness of the tree huggers.
Oh, by the way, you forgot to talk about the 57 mpg your truck gets on the downside of the hill. Quit listening to socialists. Execute them.
 
Anonymous (without a Blogger account or even a nom de plume) said:

"I am a lifelong automatic transmission engineer."

That's nice.  I've worked with automatic transmission engineers, and I know about k factors and multiplication and slip.  I took the product of torque multiplication and one minus the slip to get the efficiency a loooong time ago.

"The 6 mpg you experience going uphill would be 4 mpg with a manual."

It was 26 MPG up a hill at a speed high enough to shift to 4th, where the software allows the torque converter to lock up.  The very same hill in slow traffic was 6 MPG by the trip computer.

Climbing the hill in 4th with the converter locked took less engine speed than going up in first with the converter clutch open.

"The torque multiplication improves gas mileage, not lowers it."

Converter slip does not create energy, it dissipates it.  The advantage gained by multiplication is outweighed by the 2-3x higher engine speed required to generate the necessary input torque; a low-slip converter loses less in this process, at the expense of higher idling-in-gear losses.

"Further, the coefficient of drag in a synchromesh transmission more than out weighs the slight fluid slippage found in todays converters."

Is that so?  The synchromesh has no hydraulic pump, and no pump losses.  Unless the clutch is actually slipping, it has no clutch losses.  Under the conditions of starting and acceleration (open converter, high slip, high hydraulic pressure with consequent high pump load) the automatic loses badly.

Modern automatics are quite close to manuals on the EPA driving cycles, it's true.  But most people can point to their own proof that EPA driving cycles have little to do with the way most cars are actually driven, and even on EPA cycles the automatics come out worse.

"Then there is the question of longevity. The clutch looses."

Last year I sold a 4-speed with over 80,000 miles on it.  Original clutch.  (And you mean "loses".)

I admit that I'm not your average driver.  I can be gentler... or more calculatingly brutal... on a clutch than most people.  Performing a full-torque launch in a turbocharged car is not going to maximize the life of the disc, and I've done a few.  (I replaced clutches in that car every time I had something done to the transmission, but I never replaced a clutch for its own sake.)

More to the point, mechanical clutches are not necessary to reduce or eliminate these losses.  Eddy-current brakes can produce drag without mechanical wear.  If you need 50 N-M of torque into the gear train and you can get it at 800 engine RPM and 1:1 multiplication (drag) or 1700 engine RPM and 1.5:1 multiplication, your total efficiency is likely to be higher (much higher) without multiplication; also, the coupling torque of the eddy-current brake can be modulated in a way that a torque converter cannot.  In other words, killing the hydramatic will yield gains.

An eddy-current brake is effectively an alternator dumping its output into a resistor.  If you substitute a real alternator for the eddy-current brake and capture the output, you can feed it back to another device which produces additional torque.  This will have losses, but it is not going to be difficult to exceed the efficiency of a hydraulic torque converter.  The power required for a given amount of stall torque goes way, WAY down.  The issue quickly becomes the expense.

If you are going to invest in the hardware to do this, it becomes more and more profitable to add energy storage to the mix.  Voila, you've got a hybrid drivetrain uncannily like the Prius.

But mentioning the Prius seems to send you off the deep end.

"Don't tell me how great a Toyota Prius is. Wait until the green tree huggers get to pay $10,000 for a battery pack."

You can build a Prius-equivalent battery pack from off-the-shelf cells for something like a tenth of that, and the price drops month after month.  Soon lithium-ion cells will cost less than NiMH and the weight will drop steeply even as the capacity triples for the same bulk.

It was over a century ago that internal combustion edged out the battery-electric for dominance in the automotive world, but it looks like batteries may have the last laugh.

"If it wasn't for the b.s. environmentalists, we would have $.50 gallon gas and trucks that get 45 mpg."

We would?  Do you mean that the abolishment of environmentalists would make gasoline cost less than Middle East crude, or that they're responsible for the decline of US oil production since 1970 and the consequent importance of the Middle East?

And how would a dearth of environmentalists make a 12-MPG Durango get 45 MPG?  Do tell me about this, I'm all ears.

"When you have phony greens getting paid $6 per car for a phony smog device that never worked..."

Are you telling me that the auto companies PAID for smog devices which contributed nothing toward meeting their emissions limits?  Paid who, for what devices?  Again, I'm all ears.

"Drill for oil everywhere."

If there was oil everywhere, why would we be importing from the Wahhabist Theocracy of Arabia?  (all ears...)

"When 3000 people in France die from a 95 degree heat wave due to no a/c, and we loose 2 people, it says volumes about the uselessness of the tree huggers."

Air conditioning is run by electricity, and France doesn't get more than trivial amounts of juice from petroleum.  Neither does the USA.  Did you have a point to make?

"Oh, by the way, you forgot to talk about the 57 mpg your truck gets on the downside of the hill."

On downills the trip computer reads 70-90 MPG when it's in gear, off the scale in neutral.  And it's not a truck, it's a 5-passenger sedan with full creature comforts and the capability of spanning the continent on a half-dozen tanks of fuel.

"Quit listening to socialists. Execute them."

I listen to clueless ideologues of all stripes, but only to mock them better.  Reality has a way of beating even the most obtuse over the head, and the more firmly they deny the true cause the more they suffer.
 
I agree with engineer-poet. I have sold my '80 Honda Civic with 120,000mi and the original clutch, sold my 1985 Honda Accord at 150,000 mi and the original clutch and got 130,000mi on my 1990 Acura Integra before the clutch went out.
 
I listen to clueless ideologues of all stripes, but only to mock them better. Reality has a way of beating even the most obtuse over the head, and the more firmly they deny the true cause the more they suffer

Great quote! Look at today's oil price: $100 or even $140 a brl. Housing market crash, and Wall Street melt down!

A tree hugger, a proud 53 MPG green Prius owner
 
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