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Old 11-02-2013, 07:48 PM   #1 (permalink)
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Transmission Efficiencies. MT, Auto, CVT.

This question has come up a lot: How efficient is a CVT/MT? Why do some CVT cars have better EPA mpg ratings than their MT counterparts?
Since i've looked into this in the past, it's better to link direct sources than to respond from memory. I'm not an expert, these are my general observations...

Transmission type Efficiency
Manual neutral engine off.100% @MPG <----- Fun Fact.
Manual 1:1 gear ratio .......98%
Manual other gear ratios .96-97%
CVT toroidal ......................93%
CVT belt ............................88%
Automatic .........................86%

http://docs.google.com/viewer?url=ht...20To%20CVT.pdf

Quote:
None of the CVTs have efficiencies as high as manual transmissions: about 96% in all but the 1:1 gear which is about 98%...... -Note that manual transmissions in standard automobiles do not require water cooling. If a particular CVT requires water cooling, that is proof that it is less efficient than a manual transmission.
Read more: Physics Help and Math Help - Physics Forums
If a CVT can be up to 8% less efficient than a MT, why doesn't a car with a CVT get 8% worse fuel economy?

From a mathematical viewpoint, a transmission is only one component of many that determines overall efficiency. BSFC Engine thermal efficiency% X Trans efficiency% X Rolling resistance% X Friction etc etc. All of these eat at the fraction of how efficient the engine is. For example MT vs CVT at 35% engine efficiency: (.96 x .35) - (.88 x .35) = 0.028 , Or you lose 2.8% overall efficiency MT vs CVT in this instance.

On page 34 of this link, you can see a CVT can cherry pick the best BSFC curve for efficiency. Sometimes at the expense of torque or power.
https://docs.google.com/viewer?url=h...pdf?1363999370

Many Manual Transmissions are geared for low end acceleration, NOT efficiency. Many don't have a tall enough top gear for decent highway cruising; going a couple hundred rpm over what you would want it to. Not to mention a proper overdrive gear for efficiency.

People are stupid! A CVT will seamlessly lower an engines rpm at any road speed. Giving better mpg and lower emmisions. And there have been vast improvements in CVT efficiency lately which go unreported as proprietary information from automakers.

All this leads to more MPG's from a less efficient transmission. A couple less horsepower lost, a second less to 60mph, and 50 more lbs.

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Last edited by sheepdog 44; 11-16-2013 at 09:00 PM..
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Old 11-03-2013, 09:58 AM   #2 (permalink)
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CVT's will rise. Materials are better and some peoples expectation of high engine power is less so they can handle it. But belt loss will always be high compared to a gear. Honda has an all new CVT for the new FIT. We will see how it goes. The CVT in the gen2 insight seemed to work out to be less than fuel efficient. And they are somewhat limited on the max gear ratio change. Cars with the regular manual trans option always get shorter gears which is the only reason their planetary auto versions get a higher rating. Dual clutch transmissions seem to be the best of both worlds and Honda has figured out how to put the electric motor out on the end of one shaft instead of on the flywheel which makes engine off driving much more elegant than what was in the gen2 Insight. I wonder how long it will take them to make a gen3 which could rival the Volkswagon XL1 at a much lower price.
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Old 11-03-2013, 10:53 AM   #3 (permalink)
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Quote:
Originally Posted by sheepdog 44 View Post

Many Manual Transmissions are geared for low end acceleration, NOT efficiency. Many don't have a tall enough top gear for decent highway cruising; going a couple hundred rpm over what you would want it to. Not to mention a proper overdrive gear for efficiency.
All because they have no economic reason to correct this. They can charge more for a CVT or Automatic.
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Old 11-03-2013, 11:17 AM   #4 (permalink)
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Another efficiency reference, from:

Jatco's job: Boost CVT efficiency, acceptance

Quote:
CVTs have plenty of room for improvement. The transmissions, for example, lose about 15 percent of their efficiency because of mechanical loss, compared with a mechanical loss of only 10 percent with dual-clutch transmissions, Usuba said.

Engineers will get there by reducing friction in the oil pump and bearings and by reducing the amount of oil splash caused by the gear bands as they slosh through the lubricant reservoir.
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Old 11-04-2013, 01:22 PM   #5 (permalink)
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How many CVTs have other than a TC between them and the engine/motor though? I have recollection of one using a magnetic powder to disengage the drive at idle but other than that I think they do all use TCs.

Using a TC does allow taller numerical ratios (in planetary transmissions and CVT) because the slip in the TC is equivalent in effect to lowering gear ratios. About 40 years ago Alex Issigonis had gearless Minis running that used only the TC to provide the gearing.

eg. Issigonis Prototype 1970 Mini 9X &lsquo;Gearless&rsquo; - Atwell-Wilson Motor Museum
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Old 11-07-2013, 05:21 PM   #6 (permalink)
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Quote:
Originally Posted by Occasionally6 View Post
How many CVTs have other than a TC between them and the engine/motor though? I have recollection of one using a magnetic powder to disengage the drive at idle but other than that I think they do all use TCs.
The cheapest ones mostly used in small motorcycles still use a centrifugal clutch pack instead of a TC.
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Old 11-10-2013, 09:06 PM   #7 (permalink)
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Quote:
Originally Posted by nemo View Post
All because they have no economic reason to correct this. They can charge more for a CVT or Automatic.
The cost for them to correct it is almost zero, they pick the ratios that they think will be a ballance of making the buyer happy and getting good EPA tests.

I think the automakers are considering 2 key factors when selecting MT gear rations.

1. Since a MT won't downshift automatically if they have too tall of a top gear some buyers will be upset that they have to downshift for a hill or a pass and feel that the car is underpowered.

2. It's been mentioned previously in another thread that the EPA test for MT has set shift points. If a tall gear will give real world FE savings but cause the engine to perform in a way that requires the EPA tester to shift to a lower gear then the result could be a lower EPA score.
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Old 11-10-2013, 11:57 PM   #8 (permalink)
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One has to also remember that from the factory, to get good emissions scores transmissions are designed to downshift rather than increase load. Higher load increases NOx, which is the hardest to eliminate with the catalyst. Manual transmissions don't have automatic downshifting, so the top gear needs to be less efficient to not only allow more passing power, but also to reduce emissions on the EPA cycle.
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Old 11-11-2013, 01:02 AM   #9 (permalink)
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Quote:
Originally Posted by minispeed View Post
Since a MT won't downshift automatically if they have too tall of a top gear some buyers will be upset that they have to downshift for a hill or a pass and feel that the car is underpowered.
Even before the massification of automatic transmissions in the American market, the average American customer wanted to avoid shifting at all, no wonder huge engines bolted to transmissions with a small # of gears were so popular, with plenty of torque at idling already, so they could eventually keep it in top gear all day long. While an European or a Japanese would be satisfied with a 4-pot and a 4-speed manual transmission, the average Joe would be pissed off with less than 6 cylinders and more than 3-speed

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