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Transmissions Compared: Which Is Best To Maximize MPG? Page 2

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porsche pdk main630 01

porsche pdk main630 01

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Dual-clutch transmission (DCT)

First developed by Audi, dual-clutch automatics are now used by virtually every carmaker. One clutch operates the odd gearsets, the other the even gears. When the car or driver selects another gear, clutch engagement is switched instantly from one to the other. Drive is maintained, so it's quicker and smoother than a single-clutch unit.

Because drive is never interrupted, they can be almost as economical--sometimes more so--than a traditional manual or automatic. Less energy is wasted in moving from one gear to another, and as cars with DCTs often come with gearshift paddles, the eco-minded driver can select their own gears to improve economy.

DCTs are expensive though--picking the DSG option on a Volkswagen Jetta TDI adds $1,100 to the price, and gas mileage is identical--so you'd have to be choosing DSG for a reason other than saving money on gas.

Continuously-variable transmission (CVT)

As the name suggests, a CVT constantly varies gear ratios. This means the transmission can allow the engine to run at its most efficient speed regardless of the situation--high revs for power, low revs for economy, independent of road speed.

 

There's never any step in the power and torque being delivered, making these transmissions very smooth most of the time. At the same time, many drivers dislike the way a CVT lets engine revs rise and fall irrespective of road speed, and some carmakers even build in transmission modes with artificial steps to make them more appealing.

In terms of economy, they're potentially better than any of the others, at least in low load situations where engine revs never climb too much. A Nissan Versa 1.6-liter with CVT beats a five-speed manual equivalent by 3 mpg in combined EPA testing, at 33 mpg to 30 mpg.

Honda CVT transmission

Honda CVT transmission

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Some hybrids, particularly those from Toyota and Ford, use an "electronic CVT", that uses planetary gearsets instead of belts, to combine power between a gasoline engine and electric motor. These are even more mechanically simple, smoother, and thanks to the electric running, even more economical.

Which is best for economy?

It depends. Ultimately, the car you choose has much more bearing on the amount of gas you use than the transmission you equip it with.

Like for like, you'll usually find a manual to be more efficient than an automatic, similar to a single- or dual-clutch automatic, but not as efficient as a CVT. But some carmakers do offer torque-converter autos that beat their manual equivalent, and DCTs can also use less gas.

The best news is that you have more choice than ever, and that the efficiency of any transmission is improving all the time--but with a manual gearbox, the best way to improve economy is by starting with the "nut behind the wheel"--i.e, work on your own technique!

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Comments (10)
  1. I'll take the single gear reduction box for an electric motor:)
    Cheers
     
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  2. I would separate Toyota and Ford hybrid transmission because it multiplies torque with electricity rather than belt and cones mechanically.

    eCVT is a single speed transmission with torque multiplication done with electricity.
     
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  3. Do you in fact know that the Ford hybrid uses belts?
    They consulted Toyota and have licensed or obtained patents which would indicate their hybrid transmission is similar to the Toyota HSD.
     
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  4. Ford never consulted Toyota on the development of their hybrid system; they did however trade some minor patents with Toyota prior to production to ensure there was no conflict. Ford's Gen I and Gen II systems used a powersplit eCVT transmission from Aisin Warner, the same supplier as Toyota, so the transmissions were very similar. Ford's new Gen III hybrids (C-Max and Fusion Hybrid and Energi models) uses a new and more capable Ford-developed powersplit eCVT. The function is similar -- two internal electric motor generators plus an ICE interconnected through a planetary gearset.
     
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  5. Sorry, I meant Toyota and Ford hybrid transmission should be grouped together into electric CVT.

    All other (Honda, Nissan, etc) are mechanical CVT.
     
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  6. True the gear ratio on the planetary gearset doesn't change, but calling a powerplit a single speed transmission doesn't quite do it justice. For anyone interested, I recommend playing with the following demo to understand the relationship between the two motor/generators and the ICE:

    http://eahart.com/prius/psd/
     
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  7. CVTs are slowly but surely becoming the standard transmission on midsize and smaller vehicles. Obviously with hybrids for efficiency, then Nissan jumped in a few years ago, and now Honda is dropping CVTs in their four cyl accords. This trend will continue as the need to obtain better mpg increases and consumers see how simple they are to run.
     
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  8. I don't quite agree with your assessment. I'm not including eCVT's are quite different than CVT's used for ICE's.

    CVT's do have some benefits, but they also have some issues. They are expensive to manufacture as the pulleys require very precise machining. They are not as efficient as they might seem due to hydraulic pumping losses required to keep tension on the pulleys. Honda claims reduced cost and increased efficiency with their new CVT.

    Not everyone is going to CVT. Ford uses DCT's for Fiesta and Focus autos. Others are adding more gears. I don't think CVT's will be a single solution.
     
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  9. Not including hydraulic or pulley CVTs and thinking more long term. It's almost inviteable...almost all CVTs and hybrids/EVs mid next decade for new small n midsize vehicles.
     
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  10. For HEV's, PHEV's, and EREV's, an eCVT's transmission can represent a good solution. Ford did a complete technology scan before deciding to develop a new eCVT. Honda finally is going to field a full HEV/PHEV in the Accord; their new eCVT has a clutch that allows direct connection of the ICE at higher speeds which increases efficiency (similar to the Volt). I haven't seen any details on what Nissan is doing.

    You also can have a full hybrid with a larger version of an Integrated Starter Generator (Honda calls it IMA) with a clutch that allows the electric motor to drive the vehicle without the ICE turning. That's what Hyundai uses (and this type of system is also used in some larger RWD vehicles).
     
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