• Aug 18th 2010 at 7:55PM
  • 116
Not so long ago, it was common for automatic transmissions to be referred to as slushboxes, since that's how they often behaved. Rather than use a mechanical clutch, traditional automatic transmissions use a fluid coupling between the engine and the gear-sets to transmit drive torque. This provides some benefits, but isn't a perfect system.

Unless a mechanical clutch is worn out and slipping, it transfers nearly 100 percent of the torque that goes in. Automatics use a torque converter that consists of three main components: the pump, turbine and stator all within a cavity filled with hydraulic fluid. The pump is connected to the engine and, at lower speeds, it spins within the fluid without driving the turbine. As the engine speeds up, the slippage within the fluid increases and the turbine begins to rotate. This, in turn, drive the gears and the wheels. The stator increases the turbulence between the other two components providing a torque multiplication effect. There is generally no direct connection between the pump and turbine other than the fluid, which is why the efficiency is anywhere between zero and about 80 percent. All this removes a pedal, but it used to mean automatics got much worse mileage than manual transmission vehicles. Read on after the jump to find out what engineers have done to overcome this discrepancy.

In the last 25 years, there have been three major advances to automatic transmissions that have made the biggest difference in fuel economy gains: more gear ratios, lock-up torque converters and electronic controls. Lock-up converters incorporate a mechanical clutch that can hard-couple the pump and turbine when the vehicle is cruising with no transmission shifting. The clutch allows the torque converter to achieve near-100 percent efficiency. In recent years, engineers have also been able to utilize electronic controls to increase the proportion of time that the torque converter is locked, further increasing efficiency.

Those electronics have played a much bigger role than just controlling the torque converter clutch. Since the mid-1990s, engineers have integrated the management of the engine and transmission making the entire system work together. In combination with electronic throttle, spark and fuel control, engineers have been able to optimize how the engine behaves during shifts as well as during acceleration.

Since fuel efficiency is measured on standard driving cycles on a dynamometer, the engineers are able to calibrate how the throttle responds regardless of what the driver actually requests with the accelerator pedal. This way, actual vehicle response can be closer to the demands of the cycle so the transmission typically shifts at lower engine speeds. The increasing number of ratios – automatics have gone from three speeds in the early-1980s to six, seven and eight speeds today – has also allowed engineers to calibrate shift patterns that keep the engine closer to its most efficient speed regardless of vehicle speed.

Despite the mechanical efficiency advantages of manual transmissions, the transmission is controlled by the vagaries of the driver trying to follow the test protocol. The result is that in most cases, the automatic transmission can now match or beat the manual. Going forward, automatics are likely to improve even more as torque converter automatics are gradually supplanted by dual-clutch transmissions (DCT). Volkswagen was the first automaker to introduce DCTs in some of its European models in the mid-2000s under the brand name DSG (and S-Tronic in Audi vehicles). DCTs are now becoming increasingly common from companies like Ford, Porsche, Volvo and others.

A DCT basically works like two manual transmissions in parallel, with one clutch engaging and disengaging for the odd numbered gears and the other for even numbered gears. As one engages the other disengages, the gear-set is shifted to the next one up or down. The use of mechanical clutches – like standard manuals have – makes the DCT more efficient and full electronic control lets the engineers make the same sort of optimizations they now do on torque converter automatics. The computers win again.

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    • 1 Second Ago
      • 5 Years Ago
      Here's my take from an article I wrote a long time ago...


      • 5 Years Ago
      The auto transmission in my 13 yr old Civic is doing great, has 166k miles on it. I've got nothing against manuals though. For the mpg hypermilers, you need one. I try to drive efficient with my auto and it's annoying quite often. However, I am not about to get another car with a manual because of it, too costly.
      • 5 Years Ago
      1: Gear Ratios. In almost every car on the road, The Automatics have a much taller top gear and thus turn much lower RPM at highway speed. This discrepancy seems to have grown larger in recent years. I suspect they want automatics to look better to up-sell.

      2: Automatics can be setup to game the EPA test. GM even tried to game the MT in the Corvette before with a lockout on 1-4 shifting.

      Even given this Consumer reports normally gets better city MPG in their testing, as top gear doesn't matter here and MT will be locked up all the time in city driving.

      Highway MPG goes directly with top gear ratio. The bigger the discrepancy, the bigger advantage AT have. When they are equal MT get better highway numbers as well.

      I have some hopes that we will start seeing more 6spd MTs with proper top gears for fuel economy.
        • 8 Months Ago

        That is an excuse I have heard before.

        That is more red herring than real issue. You still need to gear down to accelerate to pass in all but the most leisurely manner. I still gear down for big hills.

        I don't feel disadvantaged compared to an AT driver, because I do something better than the AT. Anticipate. I gear down to pass before I even start to accelerate.

        Who is seriously going to complain about a few highway shifts when in comparison you shift nearly constantly in the city.

        I also observe that older MT I owned were taller geared than a lot modern buzz boxes and it was no problem. I had much better highway fuel economy and much more relaxed engine RPM.

        Just like the Chev Cobalt XFE. 5speed MT and turned a leisurely ~2000 RPM at 60MPH (at least 1000 rpm less than typical compact MT).

        I don't remember any hue and cry about the XFE safety or anything. It you drive MT, gearing down to accelerate is just normal.

        And that was a 5spd. With a 6 speed you can have 5th almost where it is now, with a nice tall super overdrive 6th. Lazy or paranoid drivers can keep it in 5th. It is after all up to the drive to match gear to conditions.

        • 8 Months Ago
        @NRB " They make that sixth gear pretty darned tall, yet modern autos are still beating them."

        This is not the case. Try to find a car that has a MT with top gear that is close the AT top gear. It doesn't matter if it is 5 or 6 speeds, just the top gear ratio:

        60 MPH RPM and highway MPG reported by Consumer Reports:

        Honda Fit AT: 2160 RPM, 39 MPG
        Honda Fit MT: 2875 RPM, 37 MPG

        On the Fit there is a massive advantage for the automatic in gearing yet it only gets a small MPG bonus.

        Looking around I haven't found a single car that the MT has a top gear as tall as the AT, but the 4cyl accord is close:

        Accord MT:2020 RPM, 34 MPG
        Accord AT: 2280 RPM, 35 MPG

        Here the gearing is much closer, still an advantage to the automatic, yet the MT still gets a tiny MPG advantage despite the disadvantage in gearing.

        If you give equal gear ratios to the MT it will get better MPG. Manuals are hamstrung by short gear ratios.
        • 8 Months Ago
        I remain skeptical.
        • 8 Months Ago
        I remember hearing about the 1-4 shifting thing, that's so that a guzzler tax wasn't added on to the LS1 camaros and Corvettes that came out in the late 90's.

        Drive by wire helps a lot in EPA tests because it means you can set up the computer to lug the motor at low RPM and decrease your throttling losses drastically. It does help MPG some, but in reality most people will just put the foot to the floor to initiate the gear shift..

        Automatics have been able to compare to MT's in fuel economy, but they're still typically 1-2 seconds behind in 0-60 tests. There's not enough lipstick in the world to make those pigs look good ;D

        I agree about the 6th gears. MTs could be getting fantastic HWY mpg.
        • 8 Months Ago
        @NRB "why don't they put tall gears in manuals to achieve better highway mpg?"

        Up-sell and buyer satisfaction. Its a lot easier to sell an Automatic as a $1000 upgrade when it gets better MPG and the buyer will feel more like he is getting his moneys worth. But when AT costs more money, gets lower performance and lower MPG it is a harder sell.

        When GM did XFE on Cobalt the did it only for the MT and they did do nice tall top gear. You might question why they only did it for the MT which sells in very small volumes. Likely because it was easier. A MT with tall gear and you are most of the way there. Surprising only GM has done the right thing so far. I am hoping with increased CAFE requirements they will stop handicapping MT cars.
        • 8 Months Ago
        Snowdog: "This is not the case. Try to find a car that has a MT with top gear that is close the AT top gear."

        Why? What's the motivation for manufacturers to not provide a tall final gear in a stick?

        Btw: Honda motors tend to need to wind up pretty good to get anything done, so they're probably not your best example. Especially true for the torqueless Fit.
        • 8 Months Ago
        Snowdog, you are very correct. As a MT driver who likes low highway cruising RPM's, I an very familiar with this. It is difficult to find a current MT outside of Corvette that doesn't buzz the engine up around 3000-4000 rpm once you are on the highway (and I don't mean 60mph, I mean on the Interstate 75-85+ mph). I am hoping that the new CAFE standards help to change this. The Cobalt XFE is an example. Hopefully more will follow.
        • 8 Months Ago
        If drivers are too lazy to shift a MT on the highway, they are likely buying automatics anyway.

        A lot of my friends also drive 4cyl MT cars and the number one complaint is the annoying short highway gearing. Usually nothing to do with MPG, just the annoyance factor of high RPM at cruising speed. When it comes up, everyone wants taller highway cruising gear.

        I have my hopes on Ford doing the right thing with the 6spd MT in the new Focus.

        • 8 Months Ago

        You are the one claiming that equal gear ratios show AT getting better MPG.

        So I ask to show even one example of this.

        I also provide one example where a MT has inferior gear ratio and still gets better MPG.

        • 8 Months Ago
        I don't know what you mean by it being an "excuse", do you mean that you disagree that it's the reason carmakers make it that way or do you mean that it's probably the reason, but you think it's a bad one?

        If it's the first one, I do think it is, it's the most reasonable possibility from what I've seen. If it's the latter, then I can somewhat agree.

        Then again, while I would prefer a taller gear in manual cars, I understand why they would not do it. I don't think it's a bad reason as such, it's just a reason that works for a "casual" driver and not an enthusiast nor an hypermiler. Sure you shift a lot more in the city, but different environments breed different expectations, and drivers on the highway don't like shifting in general. North American drivers tend to be lazy, so they probably want to cater to that, even if driving a manual shows that the drivers aren't as lazy as most other drivers. North American drivers also like having relatively powerful cars, having to shift frequently to do maneuvers on the highway does not give the impression that the car you drive is powerful, rather the opposite.

        I know that many drivers wouldn't have these misgivings and would love the better fuel economy, but the carmakers are companies seeking profit, and they may have analyzed the situation and decided the business case for taller gears just wasn't there. Ideally they'd offer fuel-efficient variants with different gearing, that shouldn't be too difficult, but that would be splitting up a market that is very small to begin with, the market for manual cars.
        • 8 Months Ago
        Basically this: ATs game the EPA testing regimen.

        EPA testing allows automatics to select their own shift points, and throttle control takes care of the rest. It's not just about the gear ratios. It's the fact that the AT can pick which ratio it'll be in during the EPA test, whereas an MT car cannot. The EPA has very strict guidelines about what rpm you can upshift a manual at. It allows ATs to decide for themselves. This injustice led some manufacturers to include an upshift indicator to indicate best shift points. I'm not sure whether the EPA follows the shift indicators or not. While smart drivers will find this out by themselves, most won't.

        In real life, an AT driven properly can get better economy than a bad driver with an MT, but a smart driver with an MT can get much better economy... especially in heavy traffic, where the torque converter takes a heavy toll.

        I've driven all iterations of "new" automatics. Five speeds, six speeds with locking converters, CVTs, Torque-converter equipped CVTs, Dual clutch automatics... and the only thing that comes close or actually exceeds the fuel economy of a manual in real world driving (keep in mind that I'm a "smart" driver) is Honda's CVT. Because their CVT is actually lighter than a manual... yet their rubber band trannies are notoriously fragile when abused.
        • 8 Months Ago

        You forgot the final drive ratios.

        Fusion MT: .68 * 4.38 = 2.98
        Fusion AT: .74 * 3.06 = 2.26

        So again. The AT has a much taller top gear ratio.
        • 8 Months Ago
        Again, I ask why?

        If all it takes is a taller gear, and a manual is (should be anyway) better at transferring power, why don't they put tall gears in manuals to achieve better highway mpg?

        Why not a 42mpg Fiesta with the manual option?
        • 8 Months Ago
        nrb, I know this is late but here's the answer to your question about why manual transmissions' top gear aren't made as tall as automatic transmissions even if they would get much better fuel economy that way.

        The answer is drivability. The lower the RPM, the less power the engine can generate, so making the top gear taller makes top gear acceleration worse.

        In an automatic, it's not really a problem as the transmission will downshift itself quickly to a lower gear if the driver asks for more power by pressing down on the accelerator. So the only thing to do for an automatic is to make sure that the transmission won't downshift constantly on the highway because of headwinds. That allows pretty tall gears.

        In a manual, the driver has to manually downshift, so when trying a quick maneuver requiring acceleration while on the highway, the manual driver is disadvantaged, he either has to take some time to downshift or try accelerating in top gear, where he has little power. This slows down the maneuver and may even make the driver feel in danger. He also may underestimate his need for power and downshift only to 4th when he should be in 3rd for example.

        So to avoid giving the driver the impression he is trapped in a gear where the engine is gutless and to avoid forcing driver to downshift whenever they want to accelerate on the highway, the carmakers make top gears in manual cars shorter, to be able to give more power when needed.

        That's why they do it, even if giving people taller top gears would help fuel economy much on the highway.
        • 8 Months Ago
        "Highway MPG goes directly with top gear ratio. The bigger the discrepancy, the bigger advantage AT have. When they are equal MT get better highway numbers as well."

        I keep hearing that argument, but I have a hard time with it. If it were that easy, they would have done it during the prior gas crisis of the 70's. Actually, they did. Isn't that about when we started seeing (common) four speed trannies and overdrive? Manuals still kicked autos in highway mpg.

        Six speed manuals were more common and are a lot easier to build than six speed autos. They make that sixth gear pretty darned tall, yet modern autos are still beating them. Believe me if it were that easy for Ford to get 42mpg from a manual Fiesta, they'd have done it.
        • 8 Months Ago
        That's not what I'm saying. I'm saying that it's common for modern AT transmissions to get better highway mpg than their manual counterparts. You're saying that it's solely due to gear ratios. I'm suggesting that I have a hard time accepting that. On an economy car, the manufacturer is incented to provide as high an mpg rating as possible. If the manual can do better than the automatic simply by providing a tall final gear, they'd do it.

        As to your question, I went to Google. The 2011 Ford Fusion has a top ratio of 0.68 on the manual and 0.74 on the automatic, yet the auto gets better highway mpg.
      • 5 Years Ago
      I have a car with an automatic transmission. It's a 1997 Ford Escort with about 210,000 miles. We bought it new. The only trouble with the transmission I've had was a leak in the fluid line. One quick repair later, and it's still running just fine. In fact, while a lot of little things have gone wrong outside of those two things, the engine and transmission themselves have held up quite well (knock on wood). I know a lot of people with automatics in the same situation. If the engine and transmission are designed and made right and taken care of, it doesn't matter if it's automatic or manual, they will, in all likelihood, run for a long time.
        • 5 Years Ago
        I made a mistake in word ordering in my previous post. Here's a fixed version that's more clear.

        I have a car with an automatic transmission. It's a 1997 Ford Escort with about 210,000 miles. We bought it new. The only trouble with the transmission I've had was a leak in the fluid line. One quick repair later, and it was no worse for wear. The engine and transmission still, after over 200,000 miles, work just fine, and I get comments from the mechanics each time I take it in for maintenance that it's in remarkably good shape for a 13-year-old 200,000+ mile car. A lot of little repairs have been needed over the years, but only a few were things outside of stuff that you have to expect to wear out (breaks, tie rods, etc), and the engine and transmission themselves have never experienced a failure. I know a lot of people with automatics in the same situation. If the engine and transmission are designed and made right and taken care of, it doesn't matter if it's automatic or manual, they will, in all likelihood, run for a long time.
      • 5 Years Ago
      And I strongly agree about your point, that ATs *only* get higher EPA ratings because it's programmed EXACTLY to drive the way EPA tests.

      an auto tranny is too damn complicated. It's going against the grain to improve on this contraption. CVT has more potential to beat MT because of its simplicity. they should jsut stick to improving CVTs. heck, CVTs theoretically can be faster than MTs as well as better mileage.
      • 5 Years Ago
      "Slushboxes" have mechanical lock-up these days so they don't lose efficiency while cruising (only while accelerating). Also, the gearing is higher than a MT because the torque converter multiplies torque during acceleration/while unlocked, thus negating the need for lower gearing for a given performance level and allowing lower engine RPM at cruise. This is how a modern AT can meet or beat an MT for fuel economy in the same vehicle even though efficiency is slightly less, not because they gamed the EPA test.

      To see what the torque converter does to torque at low speed, go try to drive a MT vehicle with wimpy engine and 4WD/AWD without low range (where torque but not traction is the limiting factor) up a steep hill and witness a bog or stall, then go drive the same car with AT/torque converter up the hill and it will have a much easier time climbing the hill (the engine will rev a bit and the transmission can heat up if it's a long hill, but you'll climb the hill easier than MT because you have more torque at the wheels). Example: Subaru Forester MT vs AT, many have posted this experience. Unless the MT has a granny gear, it can't produce as much torque at the wheels as a transmission with a torque converter (which cuts power in order to multiply torque while unlocked). On the other side of the power vs torque issue, a MT will usually accelerate better on the flat because it doesn't sacrifice power to torque converter "slush".
        • 8 Months Ago
        The Peugeot 3008 hybrid is programmed so that whilst the automatic is changing, the electric motor blips to cover the transition.
        I don't know if that eliminates the problem of power loss whilst unlocked.
        • 8 Months Ago
        I am not saying one transmission is better than another, I am trying to explain what trade-offs each one has.

        Middle Way proves they don't know how a torque converter works. They multiply torque on the output when slipping/unlocked, just like a granny gear does. The extra torque doesn't come out of thin air, it comes with a sort of "gear reduction" effect from the torque converter (they don't call it a torque converter for nothing). You divide power when you multiply torque through gearing/slipping because you've decreased output RPM while maintaining input force/torque (since power is basically torque over time, decreased RPM with a given torque decreases power). On top of that a torque converter cannot transmit all the force when slipping because some of the energy goes into just moving fluid mass and creating heat from shearing.

        The torque converter is quickly stalled/locked in a dyno test so it is unable to do torque multiplication during the peak engine torque, on top of the AT being geared higher than the MT in the same car, peak torque at the wheels in a dyno test will almost always be less with the AT.

        Real world low speed/off-road a torque converter will almost always produce more starting/climbing torque than a MT, by way of its torque multiplication and slippage allowing higher engine RPM and input power/torque. Real world high speed acceleration a MT will almost always produce more acceleration and power than an AT, by way of not having any torque converter slippage or hydraulics parasitic loads. MPG can swing either way depending mostly on gear ratios. Modern 6/7/8/n speed ATs and dual-clutch transmissions are narrowing the differences. CVTs? Now that's a whole different set of trade-offs.
        • 8 Months Ago
        The Rx-8 does not lose 20hp when connected to an automatic.
        That horsepower is still there, the problem is that the automatic transmission can not handle an input speed greater than 7500rpm.

        It was a stupid move on Mazda's part to change the automatic from the 4 port to the 6 port wankel.
        • 8 Months Ago
        They actually still have quite a bit of loss.

        Coolant has to be pumped through them to keep them cool; they also have 2 to 3 times more fluid running through them. They are usually larger and definitely heavier, to the tune of 20-40lbs ( i am not sure if those extra pounds are in rotating mass.... nonetheless, it's less than ideal ).

        Torque convertors do not make torque appear from thin air. What they do is modulate it a hell of a lot better inbetween shifts.

        Your hill theory has more to do with what happens in between a shift than regular output. This is due to the clutch not being able to handle what it's been tasked with. For example, in every manual car i've driven, if i have more than one passenger and i'm starting up a very steep hill.. i'm gonna have problems! but once i get past 2000rpm i am fine from then on. No torque problems there.

        Go look at some dyno graphs of automatic cars versus manual ones. The manual transmissions are always putting out more torque at the wheels, even with a newer automatic transmission.

        In fact, there are a number of newer cars that have a lower rated horsepower and torque when an automatic or CVT transmission is selected. The RX-8 is a good example... It loses 20hp when the automatic transmission is selected.
      • 5 Years Ago
      All of this depends heavily on the vehicle - and the driver, both for driving habits and maintenance.

      I'd happily drive a manual again, but hubby doesn't want to, so we're now on automatics. We've had three manuals, never put a clutch in any of them, including the one I learned to drive manual. One I don't remember the mileage on (pickup), but the others had 9 years-156K and 12-years-175K. No transmission problems on any of those.

      Our current vehicles (automatics) are a 2000 Accord (204K) and a 1997 Chrysler Town&Country AWD with extended wheelbase (240K). The T&C has problems and is being replaced, but it is NOT the transmission.

      The one that had 175K (1987 Mazda 626) we bought after driving a friend's - but they destroyed theirs in short order with their aggressive driving habits, then messed up the Acura they bought. I don't drive slowly, don't drive aggressively, but do drive a lot, mostly long work commute.
      • 1 Year Ago
      It's amazing to drive a new car with an automatic transmission. There isn't any hard shifting and the shifts seem to be so much quicker than before. I love the fact that they are not only improving quickness and efficiency but also have gearboxes up to 9 speeds! I think that these new transmissions will be lasting for a long time. http://shiftritetransmissions.net/clearwater-fl-transmission-repair-services.htm
      • 3 Years Ago
      Sticks are obsolete , but boyracers won't admit it .They must not have big city commutes.
      • 3 Years Ago
      The reality is a lot simpler. Manufacturers make the gear ratio much taller on AT to boost highway MPG more than MT, so they can sell you a more expensive AT.
      • 4 Years Ago
      This is a new US Patent (7,931,107; can anyone explain how it works and the benifits?
      • 4 Years Ago
      80's are finished. Automatic transmissions are better. Because computers are smarter than us now... :)

      Great article about AI, including modern car electronics


      Dean L @
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