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Old 07-31-2007, 07:01 PM   #1
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Automatic Transmission + efficient driving.

Automatic Transmissions and Efficiency

Alright, first post. Everything here comes from a day of reading and about an hour on the road today, in a 1999 Toyota Sienna with a 3.0L V6, if you want to know. I didn't do any of the fun stuff like coasting on neutral or PG or whatever other risky stuff you guys do. I am a relatively new driver, and really don't feel I have close to the experience needed for the fancier tricks.

First of all, I guess it would help if I explain the torque converter, albeit in a very stylized (and, therefore, inaccurate) way. Think of a rotating fan in front of a really small windmill, about the same size. When power rotates the fan, it blows air to the windmill. The air then turns the windmill. Essentially, a torque converter works the same way. The flywheel rotates with the engine, pushing fluid across the blades connected to the axle, and turning that. Not the most direct method, but it works with only a 4%-8% drop in efficiency, and it allows the engine to idle without stalling. Because of this "fluid coupling" the engine is never directly connected to the transmission (except at higher speeds, but then you're not using the TC). Engineers design the TC to deliver no power at idle (or very little, i.e. if you put the car in park and let go of the brake on a flat road, the car will move forward very slowly, that's the TC at work) but to deliver as much power as possible once you press the accelerator. In reality, it is a sort of curve. At idle, the T is very inefficient at delivering power to the wheels, and it gradually increases in efficiency as the power increases. That's intentional, they are designed that way. Giving the wheels too much power at idle is bad for the brakes, if they slip. The bup-bup-bup of the car sometimes when you are waiting at a traffic light is the brakes slipping due to the engine delivering too much power to the wheels. When you let the brakes and accelerate, the TC applies more torque to the wheels, so that it can move. Unlike manual cars, the engine RPM is is not indicative of the speed, because there is no hard connection between the engine and wheels.

Regulation of shifts is much more complicated than this, but here's what happens. There are three important parts to know. The pump (connected to the TC) spins with the engine, and the resulting fluid pressure indicates the speed of the engine. The governor does the same thing, connected to the wheels. The throttle valve indicates how much you push the throttle. All three push a piston, called the shift valve, in different directions. The position of the piston determines what gear the tranny should be in.

When feathering the pedal, the car shifts very late, and therefore is not the most efficient. The AT is not going to shift unless it has enough power to do so. At the same time, because of the TC, not all of the power is going to the wheels anyway, making it even more inefficient. If you slam the pedal, the auto engineers assumed you wanted to accelerate hard, and again the tranny delays the shift to give you more power and higher RPMs, and you can take a guess as to how inefficient that is. If you do a little experimenting, you can find the "sweet spot" acceleration which is the most efficient. I would call it brisk, but I don't want to give a quanitiative description, because it is likely different for every car.

At freeway speeds, the TC power drain becomes negligible (or it locks, whatever), so here RPMs do make a difference. The van runs 55-60 at 1900 RPM, which seems to me to be great. I really have no idea what mileage I got, but I was pushing the pedal only a hair and still keeping a good speed. I read that once at speed you only need 10-15 hp to keep moving, and it sure felt like it. At 1900, the van also had enough power to go up and down the hills without any trouble.

I really noticed the effect of big trucks on mileage. There was a tanker in front of me, going at about the same speed, and I could take my foot completely off the pedal and still continue at speed. I was easily three seconds behind, which is safe. I was there for about six minutes or so, when the tanker took an exit. A chevy 4x4 on the side, however, really screws up my speed because of its wake, which makes the van sway, and I lose speed somehow.

That's all I found out yesterday. If anyone wants to disagree or give me tips or something I would really like it.



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Old 07-31-2007, 07:16 PM   #2
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Everything sounds right cept you asking for tips? What are you trying to achieve besides high fuel mileage? You sound very aware of your vans aero, and you know the causes and effects of your actions while using it!
Welcome to GasSavers, and keep up the good work!

"You have to know the truth, and seek the truth, and the truth will set you free."
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Old 07-31-2007, 09:37 PM   #3
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Nice Writeup

I'd recommend a ScanGauge or similar Feedback device for real-time FE...

Did you have any automatic driving style or mods questions?

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Old 08-01-2007, 07:57 AM   #4
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skale7, Here's what I've found works with my a/t in city driving: At my preferred acceleration throttle position (~50&#37, the trannie will stay in 3rd (not down-shift) above 25mph. And above 45, the t/c will remain locked up. I try to stay above these thresholds so I can take those hills in the highest gear possible.

By doing this, I've been getting 22mpg on my daily, one-mile, 25mph, hilly, through-stop-signs-and-red-lights, take-the-daughter-to-school commute. I think I used to get 14mpg when I let my speed bleed off to a crawl at the top of each hill.

I hope that helps. Your thresholds may vary, of course, but the principle should apply to your a/t driving.
Roll on,

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Old 08-01-2007, 01:10 PM   #5
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I really need to find those things out on my car. The speeds/rpms for the right gear and how to accelerate. Current;y I just see the instantaneous display at 10mpg or so and say "sh.t!" and then don't know how to help it without slowing down too much. Do you have any tips to finding the right speeds and where to throttle?
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Old 08-02-2007, 06:34 AM   #6
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Andy, I think these guys have it right. Get to your tallest gear as quickly as you reasonably can, because that's where you get the most distance for each engine revolution. Don't floor it and go open loop, but don't baby it either.

(When I first started trying for efficiency I would keep my RPMs below 2000, 2200 tops while accelerating. I thought I was being smart until I got my ScanGauge and saw how much lower the MPG is in 1st and 2nd (3rd actually isn't too bad). Giving it a little more gas and getting into the higher gears sooner -- as long as I'm not in a 25MPH stop-sign-laden environment -- seems to be a better choice. I switched from a "granny gas" TPS value of 12 or so to 20 at first, last tank was 25, this tank I'm trying 30.)

In next-to-highest gear don't push too hard or you're delay the shift -- remember, the goal is to get to your tallest gear quickly. (Though if you have a lockup TC, you may benefit from a little more gas and a higher RPM shift in exchange for faster lockup... can anyone coment on that?)

As soon as you get into lockup (or your tallest gear without), back way off on the gas (try to anticipate that if you can). Run your RPMs as low as you feel you reasonably can. If you have a lockup TC that's probably going to put you around 45MPH or so but, like you said, you'll have to find the values for your vehicle.

(On the VW -- which doesn't have lockup as far as I can determine -- if I run in the 1600s in 4th (around TPS 10) I see better than 50MPG on the flats at around 40MPH. That's on a vehicle rated 22 combined, with no significant mods, just a little extra air in the tires (45PSI). There's a route into town that runs more-or-less parallel to the interstate with minimal stoplights and a lot of 30/35/40MPH segments. I'm now taking that in preference to the 65MPH+ interstate, getting much better mileage, enjoying the ride a whole lot more, and only spending about five more minutes on the road each way.)

If you don't want to drive s - l - o - w - l - y (and I can't blame you), try paying attention to the gentle inclines/declines of the road -- the ones you don't even perceive as hills. (If you have a GPSr with an elevation or rate-of-climb readout, that can be a great help.) On even subtle downgrades you can often lift your foot slightly -- I mean very slightly, so little you'd hardly notice; maybe one tick of TPS -- and see quite a significant increase in instantaneous MPG. On the same degree of incline, keep your foot locked in whatever your "flat road" position is and let your speed drop off one MPH or so in trade for holding the same MPGs you had on the flats.

Those are my best guesses as an inexperienced hypermiler. Hills, I'm still trying to figure out.


P.S. I would kill for 1900RPMs at 60MPH!
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Old 08-02-2007, 11:16 AM   #7
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Originally Posted by skale7 View Post
...When feathering the pedal, the car shifts very late, and therefore is not the most efficient. The AT is not going to shift unless it has enough power to do so. At the same time, because of the TC, not all of the power is going to the wheels anyway, making it even more inefficient. If you slam the pedal, the auto engineers assumed you wanted to accelerate hard, and again the tranny delays the shift to give you more power and higher RPMs, and you can take a guess as to how inefficient that is. ...
Thank you for sharing what you've been able to find on auto trannies. I've mananged to become partially educated on them and learned some new bits from your description.

I do have a question re. the first line in the paragraph I quoted. My experience is that the tranny will upshift earlier if you feather the pedal - that is, let up on it. Do you also find this is true? If you keep your foot in it, it stays in the lower gear longer. That is, all other things being equal and if the road speed and rpm's will allow an upshift without lugging. The higher gear is more efficient; again - as long as it's not lugging the engine. They manage to program it so that situation is avoided.

My (older) auto tranny is fully mechanical, not computer controlled, and has no t/c lockup either. I've loosened the kickdown cable a bit which allows upshift sooner, or you can say it delays downshift. I'm still wondering and trying to learn if that decreases friction pressure on the clutch plates, allows more t/c hydraulic slippage, or has other negative results.

Do you have any information that helps resolve that?
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 08-02-2007, 06:28 PM   #8
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I really want to actually measure my FE, instead of this guesswork. However, my mother drives the car much more than I do, and that makes the measurement kinda pointless. I could still play with the odometer next time she fills gas.

My dad has a Mercedes E430 auto. It has a mileage meter built in. He gets about 19MPG average, I got around 23 last time I drove it, in mixed highway/neighborhood driving, which is decent, I guess. Lots of room for imporvement.

Bruce, never happened to me, it just sits at 25mph in low gear. However, today I managed to 'induce' a few shifts by flicking my foot off the accelerator and pushing it down again. I still don't have it down though. Try it and see what you can do. Seriously, though, I don't think I'll do it very often, since I think I have the acceleration figured out fairly well.

Actually, it was 1900 down the hill and ~2050 up the hill. It's Atlanta, the roads are never really flat.

I think that gas used is based on throttle as well as RPM. Anyone know?

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Old 08-02-2007, 10:44 PM   #9
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What's so risky about coasting in neutral with an auto? Even Toyota said tht it's fine.
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Old 08-03-2007, 07:47 AM   #10
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Maybe later. Keep in mind I've only been on the freeway about three times so far. I usually stay on the smaller roads.


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