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Old 06-25-2012, 12:35 AM   #1 (permalink)
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automatics vs manuals (in-gear deceleration differences - fuel cutoff?)

A second ago i became curious about the difference between auto and manuals while coasting in gear. i drive an automatic, and it smoothly decelerates when i let off the gas. i drive a stick, and it slows down much faster, with a slight jerk right after i let off the accelerator. which makes me think i get worse gas mileage in a stick, even though sticks almost always get better mileage(though the difference has become quite small with newer cars)
so i googled it.
"The fluid coupling between engine and transmission is almost a one way street, transferring about 90% of the power from the engine to transmission, but transferring very little the other way. The clutch locks the engine to transmission with no losses as there are in the fluid coupling, so energy is transferred either way equally. The fluid coupling works much like the ratchet mechanism in a bicycle that transfers power to the wheel when you apply pressure to the pedals, but otherwise the back wheel free-wheels and clicks allowing you to freely turn the pedals backwards while coasting along. The person above who talks about gearing is not quite correct. In a 4 or 5 speed manual transmission, the gear ratios are very close to those in a similar automatic with 4 or 5 speeds. Engine braking works in a manual only because the power coming from the wheels gets back to the engine. In an automatic, virtually no power gets to the engine from the wheels because of the slippage in the fluid coupling."
so the inefficiency of the fluid coupling is the reason autos get worse ratings, it looks like.
but with this in mind, does deceleration fuel cutoff work in automatics? it doesn't sound like it should... if the wheels aren't connected to the engine then they can't force it to turn and let the injectors shut down...

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Old 06-25-2012, 03:29 AM   #2 (permalink)
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Depends on the transmission. Basically all automatics these days have a lockup clutch on the torque converter to avoid said losses as often as possible. They can be programmed to provide engine braking by locking up under deceleration, and I'd guess that there would be a fuel cutoff function too. I think all Hondas will engine brake to some extent.

This is a big reason why automatics are starting to score higher on EPA tests. They typically run wider spaced gears with a much taller final gear, are programmed to lock the torque converter a lot, and are programmed to shift up early under light acceleration. This matters for the EPA test since most of the scoring is done on tests where acceleration is extremely leisurely, which is bad for engine efficiency. The automatic transmission can get around this by shifting up sooner and increasing engine load, whereas a manual transmission is not allowed to shift until some speed.

Automatics still need some amount of power to select gears by activating clutches, and provide hydraulic pressure for the torque converter, so a manual transmission in the right hands with around the same gearing will get better fuel economy.

Last edited by serialk11r; 06-25-2012 at 03:35 AM..
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Old 06-25-2012, 08:13 AM   #3 (permalink)
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Quote:
Originally Posted by Banana Jack View Post
A second ago i became curious about the difference between auto and manuals while coasting in gear.
Coasting in gear is a braking technique.

Quote:
which makes me think i get worse gas mileage in a stick,
Not overall though.

If you shove it in neutral rather than keep it in gear, it'll roll (coast) a long, long way, slowing down ever so gently.
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Old 06-25-2012, 10:45 AM   #4 (permalink)
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In my car, when I let of the gas a little deceleration fuel cut off occurs instantly. It feels a lot like engine braking in a manual transmission car. At that point scanguage reading shows zero fuel consumption (0.0L/100km) and the car slows until a low speed 35km/hr (22mph) is achieved, at that point then it shows a very low consumption reading 2.0-2.5L/100km (90-100mpg)

Feathering the gas at lower velocities repeats the above activity over and over and results in quite high fuel economy.

In my manual counterparts of the same car, they have confirmed that coasting in gear yields significantly better fuel economy than does putting it into neutral and coasting (EOC would be the only way to beat it IMO). Another point of interest is that the RPM in top gear at 90km/hr (55mph) is higher in the manual than it is in the automatic, so there is some compensation there for those that have automatic transmissions, I suspect there are a lot of cars like that these days. I do know that the deceleration fuel cut off in the 10th gen Corolla is very good.
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Old 06-25-2012, 12:48 PM   #5 (permalink)
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Also depends on whether or not you have electric throttle.

My focus has electric throttle, if I let off the pedal ... it will keep going. Same with the 01 ford truck.
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Old 06-25-2012, 01:46 PM   #6 (permalink)
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Read your owners manual on how to operate the automatic transmission, most owners manuals tell you how to down shift an automatic transmission for engine braking and it's something that SHOULD be taught in drivers ed too.

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Originally Posted by Banana Jack View Post
"The fluid coupling between engine and transmission is almost a one way street, transferring about 90% of the power from the engine to transmission, but transferring very little the other way. The clutch locks the engine to transmission with no losses as there are in the fluid coupling, so energy is transferred either way equally. The fluid coupling works much like the ratchet mechanism in a bicycle
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Old 06-25-2012, 02:25 PM   #7 (permalink)
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Mine also has electronic throttle. 2009 Corolla.
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Old 06-25-2012, 03:10 PM   #8 (permalink)
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Quote:
Originally Posted by Banana Jack View Post
"The fluid coupling between engine and transmission is almost a one way street..."
This is wrong. The fluid coupling in virtually all automotive torque converters is symmetrical. What makes an automatic transmission coast when the selector is in D is that there is an over-running (one-way) clutch inserted in the drive train. This clutch is not part of the torque converter, it is inside the transmission. In L or 2 or 1 that clutch is disabled. Put the automatic in L and you'll feel the same drag as a manual transmission when you lift off the accelerator.
Most of the rest of the original quote is also incorrect. There is slippage in a torque convertor, and so most have internal lock-up clutches that eliminate that inefficiency.

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Old 06-25-2012, 03:31 PM   #9 (permalink)
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Quote:
Originally Posted by euromodder View Post
Coasting in gear is a braking technique.
well, i always coast in gear in an automatic, 'cause i don't know how i feel about coasting in neutral, since i've heard it can be bad and i don't know very well how an auto tranny works.
in a stick, i've been coasting in gear for a while because of DCFO, then i joined this forum and saw the thread about point 44 of the hypermiling tips, read that thread, and i might change my driving style now. depending on how soon i need to stop.

Quote:
Originally Posted by euromodder View Post
Not overall though.

If you shove it in neutral rather than keep it in gear, it'll roll (coast) a long, long way, slowing down ever so gently.
but you do most of your driving while on the pedal, right? at least, that's how you get places. so my thoughts were, it seems like there's more resistance in a stick while just rolling than an auto while just rolling, so that resistance is still there while i'm accelerating, making for lower fuel economy.

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Originally Posted by Ryland View Post
Read your owners manual on how to operate the automatic transmission, most owners manuals tell you how to down shift an automatic transmission for engine braking and it's something that SHOULD be taught in drivers ed too.
well i'm not really THAT concerned about it...i don't ever plan on owning an automatic, i was just curious.
and the only reason i would ever want to engine brake in an auto would be for dcfo(because if we dismiss fuel consumption, i'd rather use my brakes to slow down than my engine. brakes are cheaper.) and i still don't know...since it seems most automatics don't usually have engine braking, they shouldn't be able to have dcfo...

as for the guys with electronic throttle control, i couldn't figure out exactly how it works, but my guess is that it isn't actually coasting when you let off the throttle, but continuing to feed gas for a little bit, slowly lessening the acceleration so you don't get that dragging feeling.

Quote:
Originally Posted by mort View Post
This is wrong. The fluid coupling in virtually all automotive torque converters is symmetrical. What makes an automatic transmission coast when the selector is in D is that there is an over-running (one-way) clutch inserted in the drive train. This clutch is not part of the torque converter, it is inside the transmission. In L or 2 or 1 that clutch is disabled. Put the automatic in L and you'll feel the same drag as a manual transmission when you lift off the accelerator.
Most of the rest of the original quote is also incorrect. There is slippage in a torque convertor, and so most have internal lock-up clutches that eliminate that inefficiency.

-mort
interesting. and good to know. i'll have to try that.
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Old 06-25-2012, 09:00 PM   #10 (permalink)
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The latest greatest autos have closed the gap on EPA mpg estimates and in some cases, exceeded manuals.

The trouble is, this is done under controlled conditions that do not use hypermiling techniques that can substantially boost manual mpg numbers. One of the biggest is engine off coasting. It can be done with an auto, but, is not advisable. This is due, in part to it potentially damaging an auto. Personally, I would never EOD an auto for a different reason-safety. With a manual tranny, you can instantly bring the engine back to operating speeds, restoring all power features, in a blink of an eye. An auto takes longer. Too long, for me.

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