"youshiftem" (torinosport302)
09/11/2014 at 12:18 • Filed to: None
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"youshiftem" (torinosport302)
09/11/2014 at 12:18 • Filed to: None | 0
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How does removing gears with CAGS (skip-shift) save gas, and adding gears (like Crysler's new 8-speed) also save gas?
HammerheadFistpunch
> youshiftem
09/11/2014 at 12:20 |
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They both skip gears you know.
Bad Idea Hat
> youshiftem
09/11/2014 at 12:22 |
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Science!
(Good question, I want to know the exact reason as well. I believe it's due to where the car operates in its rev range versus where the engine is most efficient)
jariten1781
> youshiftem
09/11/2014 at 12:27 |
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They both do it by taking out the human element.
Skip shift saves gas by forcing low rpms...it's the sledgehammer approach.
The new wiz-bang electric controlled 8+ cog boxes also force low RPMs and have more available ratios to to put the engine in the sweet spot so it's less noticeable. It's just a computer controlling it rather than a mechanical lockout.
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youshiftem
> HammerheadFistpunch
09/11/2014 at 12:27 |
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I figured the 8 speed must skip, but if your goal is efficiency, wouldn't there be an ideal 4 ratios that would keep weight and power loss to a minimum?
TwinCharged - Is Now UK Opponaut
> youshiftem
09/11/2014 at 12:32 |
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I think it's to do with the gear ratios - to save fuel, they move to higher gears to allow for more efficiency. Both the skip shift and 8 speed allow for gear skipping to choose higher gear ratios, so the engine doesn't have to spin as quickly to move the car. In the C7, if you stick it in 7th and cruise at 40mph, you're barely ticking over since the engine is spinning at around 800rpm.
zeontestpilot
> youshiftem
09/11/2014 at 12:34 |
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Good question. From my limited car knowledge, usually the more gears you have means that as you go higher in the gears, less torque is used. Less torque means that the engine doesn't have to work too hard to keep you moving. Therefore less gas is used.
There is also something about gear ratios, which I'm not to knowledgeable about. But if something is skipping gears, it may have something to do with that (don't quote me).
....so senior oppo members, did I get it right? Am I close? Can the initiation to Junior Oppo member finally begin?!?
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HammerheadFistpunch
> youshiftem
09/11/2014 at 12:35 |
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Weight an parasitic loss on the new 8-9 speeds is as good or better than than the old 4 speeds. The goal is ratio spread, so that you have short enough gearing around town and taller gearing for cruising. Most of these transmissions have 2-3 or 4 overdrives specifically for cruising, around town you can keep your engine in a more productive part of the power band and in peak volumetric efficiency, but thats not as big a deal as the overdrives/ratio spread.
Jayhawk Jake
> youshiftem
09/11/2014 at 12:35 |
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This is my engineering judgement guess knowing little about automotive efficiency but...
Many speeds helps because in acceleration you can stay at a lower RPM. Less gears with longer ratios is great once you're cruising, but when you accelerate you have to get to higher RPM and use more gas.
This is why a CVT is good for efficiency: you decide how much you want the engine to turn then just change the gearing to move the car forward. The more gears you have the closer you are to continuous.
Arch Duke Maxyenko, Shit Talk Extraordinaire
> youshiftem
09/11/2014 at 12:36 |
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You could put a wide ratio gearbox in a car, but what the 8 speed is doing is essentially giving you a wide ratio but with gears between to keep the rmp loss lower.
RamblinRover Luxury-Yacht
> youshiftem
09/11/2014 at 12:38 |
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Nope. In most cases the gears are obtained by combining the planetaries in different combinations, rather than there being X number of actual extra gears. So, weight and lifespan aren't affected as much as you'd think, and power loss is incidental compared with the ability to keep the engine in its wheelhouse more perfectly. You'll not necessarily have that much more spinning between a 6 and an 8 - might not even have any more.
It might be most efficient climbing a hill to go 1-4-6, on flat land to go 2-4-5, or any number of other combinations. The more choices, the better equipped the trans computer is to say "I have this resistance, the driver wants this speed, and the engine wants this rev: everybody meet in the middle".
You can tell a Finn but you can't tell him much
> youshiftem
09/11/2014 at 12:40 |
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Skip shift forces low rpm's which uses less fuel. It is a crude fuel saving method, but it can work.
The new 8&9 speed transmissions are more efficient by keeping the engine operating closer to its most efficient point over a wide range of operating conditions.
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HammerheadFistpunch
> youshiftem
09/11/2014 at 12:40 |
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I should add that, if you aren't familiar with how auto trans work, they don't really care how many gears you have in there, its just planetary sets that are either engaged or not and the newer variable pressure pumps and tight tolerances mean that you are actually experiencing less pumping and drag losses than ever before...so why not add a few more planetary sets?
MontegoMan562 is a Capri RS Owner
> youshiftem
09/11/2014 at 12:41 |
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simplest explanation:
both keep your RPM low.
low RPM = less gas
speed is irrelevant in mpg it's all about the RPMs.
That's why I always laugh when they talk about speed limits to help with gas, cars are geared different so 70 could be better rpms than 55 for me. Depends on the car.
crowmolly
> youshiftem
09/11/2014 at 12:43 |
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CAGS is probably in the top 3 "stupid-like-a-fox" features.
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youshiftem
> RamblinRover Luxury-Yacht
09/11/2014 at 12:43 |
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Very well said.
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crowmolly
> Bad Idea Hat
09/11/2014 at 12:47 |
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You are correct. It is based on calculated fuel use given an RPM and load.
KusabiSensei - Captain of the Toronto Maple Leafs
> youshiftem
09/11/2014 at 12:57 |
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Short answer: Science! Keeping engine speeds low tends to correlate with less fuel usage. Either skipping gears (CAGS) or providing more gears (to keep torque production high enough to continue moving the car, but low enough to not require higher fuel usage) works to save fuel.
Long answer:
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KusabiSensei - Captain of the Toronto Maple Leafs
> Bad Idea Hat
09/11/2014 at 13:08 |
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Partly, yes. CAGS and the ZF 8 and 9 speed autoboxes are more towards keeping the engine RPM as low as possible, and using that fact to curb fuel use.
If one had a perfect CVT (with no starting problems, like needing a clutch or torque convertor of some description), the ideal RPM would be where you get the most work from the engine for a given fuel charge. Work is really Torque (simplified, but sufficient for here), so maximum work for a given fuel charge (take 14:1, the stoichiometric charge) would occur around the torque peak.
Like a generator's engine.
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Bad Idea Hat
> KusabiSensei - Captain of the Toronto Maple Leafs
09/11/2014 at 13:10 |
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I'm going to print this out and stick it to my wall. Thank you.