Observations from playing with an automatic

Discussion in 'Fuel Economy' started by roothorick, Jul 15, 2015.

  1. roothorick

    roothorick Active Member

    My car is an ancient automatic four-speed, that for financial reasons I can't get rid of. Oh well. Few things I've noticed trying to get the most out of this car.

    One, with an automatic, there's another major factor affecting the powertrain efficiency: torque converter losses. Basically, torque converters inherently have some slip, meaning the RPM output by the engine is higher than the RPM going into the transmission. This gets progressively worse as RPM goes up. You generally want to keep the car in the bottom of its highest gear ratio to minimize torque converter losses, even if this puts it a good ways below the break-even between air resistance and wheel friction.

    Two: most any automatic has TWO ratios per gear. Meaning, my four speed has effectively EIGHT gears. Why? Overdrive. There a second sort-of-transmission with two gears, starts in a lower gear but will drop into the higher gear within a certain torque range. Obviously you want to keep this one in the higher gear as much as possible. Coming completely off the gas will cause it to jump back to the lower gear; I found I can keep it locked in by holding the throttle just above five degrees. This tends to prevent deceleration from coasting at lower speeds though, and I'm not sure how much I'm saving anyway.

    Speaking of, I've found you can "provoke" the transmission. Automatics are very predictable about the conditions in which they'll upshift/downshift; generally, this is a ratio of speed to throttle position, with a lower bound on the speed it'll shift at. As I approach that lower bound while accelerating, I'll dial back the throttle, which will usually significantly reduce or outright kill acceleration, but provoke the transmission into dropping into the next gear much sooner than it normally would. This, of course, means lower RPM, which has all the effects that normally does. At this point, I'll bring the throttle back up and continue accelerating.

    Of course, the former doesn't apply to "manumatics" with a solenoid-driven clutch instead of a torque converter, and I don't know how CVTs behave.

    Anything else people have observed?

    -E- On a side note, I've noticed that P&G simply doesn't work on autos. These almost never can be flat-towed, so you can't kill the engine coasting. The bigger problem, though, is that the ECM wants to keep the engine RPM close to the transmission input RPM, but the torque converter is one-way and won't do this on its own. This means it's CONSTANTLY feeding the engine, and significantly more fuel than it would at idle. This kills the glide.
     
    Last edited: Jul 15, 2015
  2. RedylC94

    RedylC94 Well-Known Member

    Those are mostly reasonable observations, although ....
    There's only one mechanical ratio per "gear" on conventional automatics. You might be confusing the difference between locked-up and not-locked states of the torque converter with ratios. It's true that you want to keep it in the higher-seeming lock-up state as much as possible. Some transmissions, mostly newer ones, tend to be more willing to stay locked than others.

    With most automatics, you can safely "P&G" in "N" with the engine idling. Although not as efficient as proper engine-off coasting, that's still better than staying in "D" in some circumstances.
     
  3. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    What he said. Engine-on glides in neutral are far better than leaving it in Drive. My own rule for driving automatic ? Do the best you can , and accept the transmission's limitations.
     
  4. roothorick

    roothorick Active Member

    Torque converters can lock? In hindsight, that should've been an obvious improvement... I wonder, then, if I'd get better fuel economy somewhere above 40MPH, try to find that break-even point. (Yeah, 40 seems high for such a big car, but it's surprisingly aerodynamic for a Detroit tank.) I'm still in the process of calibrating Torque so experimentation will have to wait a while.

    I remember the ECM pulling the RPMs up even in neutral but I can try it again. Might be a car-by-car thing.
     
  5. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    I think the original post above refers to the basic laws of planetary gearing that you can have underdrive (reduction), overdrive, or direct drive, depending on which element is held and which is input vs output. For "first gear", you hold the ring gear, power the sun gear and the planet carrier is the output. If you were to swap input and output - hold the ring gear, power the planet carrier and the sun gear is output, you get a very tall overdrive.

    The second post refers to the fact that the geartrains in automatics are laid out to provide underdrive, overdrive or direct in each gearset to create multiple ratios. It's not practical to lay out a four speed automatic to provide eight forward speeds because it would be very complicated, large and heavy, and very unreliable.

    Along the original post's thought process, some six speed automatics are actually four speed automatics with an extra gearset that runs as either reduction or direct drive. One example is Chrysler's front drive six speed automatic. It is a four speed that shifts 1,1,2,3,3,4, with an auxilliary gearset that runs L,D,D,L,D,D (low and direct drive). It has a "four prime" gear that is second gear with the underdrive gearset "on", so it can kick down to 4-prime, and then shift 3,3,4 with the underdrive gearset returning to direct drive for the last two ratios.

    It is interesting to read the various patents for 8,9, and 10 speed automatics and see how the different companies solve the same problem with different approaches.
     
  6. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    There is also energy lost in driving the front pump in the transmission, and in the clutch plates and bands (your transmission has three bands) that are not applied. Your car has a "forward band" that applies to allow the car to move forward, and it also has a 2/4 band that is released in first, third and reverse but still creates drag in those ranges. It also has a coast band that is released but still dragging a bit unless you shift into 3,2, or 1 on the shifter.
    It does, but you need a very wide range in speed variation. And the benefit is marginal. I had a Chevy with a slightly different transaxle and could get some good figures with P&G but you really had to work at it. Go too fast and you lose energy to air drag. Go too slow and your instant mpg is too low to pay off because the big V6 still burns a lot at idle.
    Correct. You have the 4T65E transaxle. Not flat towable. The 4T45E, used in the smaller cars (Malibu, Cavalier, Cobalt) is flat towable up to 65mph. Do not kill the engine in your car when it's moving at more than walking speed.
    There is a friction clutch that locks the impeller and turbine together and energy loss is minimal. But as a hydraulic automatic, it is still losing energy in the pump and in the released clutches and bands.
    The tach should rise to around 800-900rpm - the same speed it runs in neutral when the car isn't moving. Fuel burn is going to be high because it's a V6 but you may be surprised by the instant mpg you see when the car is rolling down the road with your foot off the pedals, whether the lever is in D or N.

    If your '98 Lumina has the 3.1L engine, the unadjusted EPA numbers are 21.79 city and 37.09 highway. Good luck getting 22 around town (without advanced techniques), but the 37 highway is within reach if you keep speeds in the 45-55mph range and you have enough air in the tires and a very steady right foot.
     
  7. roothorick

    roothorick Active Member

    Interesting stuff. But I'd expect these to be relatively constant vs. speed.

    Maybe that's why it seemed to always work out worse -- I just suck at it. I've never had access to a manual (and therefore never learned to drive stick), or, for that matter, a hybrid of any description, so I've never been able to practice on anything even remotely forgiving to know what good P&G looks like.

    My impression has been that MOST automatics aren't flat-towable but most manuals are. And if you have AWD you can just forget the whole thing.

    This car doesn't have a tach. No, really -- this is what the base model instrument cluster looks like. VERY minimal.

    I debated dropping a Monte Carlo instrument cluster into it -- from what I've read, as long as it's from a 96-99 column-shift automatic that's not a police package, it will work 100% and I don't even have to calibrate the speedo because it's digitally controlled by the ECM. But I ultimately concluded it's not worth the effort.

    I can however use a SG or Torque to read the RPM and the ECM's desired idle speed. In park, once warm, the latter sits at exactly 730.5, and the former hovers around 680. So, 800-900 seems a little high for me.

    And yeah, I've seen my instant MPG numbers coasting. Coasting down off of 41 I'd see 200 sometimes. More typically hits 80 and goes downwards as the speed cools off.

    Yep it's the 3.1. I thought all the base models had 3.1 and they'd make you get an LS (or LTZ aka 4-door Monte) if you wanted 3.4 or 3.8.

    Currently I'm actually getting 24-27 in town and I don't think anything I'm doing is all that advanced, mostly just stoplight timing and lots of coasting. It may have to do with the area I live in -- they're absolutely obsessed with roundabouts here. They recently shut down an entire highway offramp just to replace its two spotlights with roundabouts and add a few turn lanes. And they just shutdown another rather notorious offramp for, most likely another roundabout. Stoplights are still common, but they're almost always sensor lights and pretty predictable if you can see cross traffic. Most anywhere to anywhere involves 30-35MPH roads so I'm more than willing to push it to that magic 40MPH number, which also helps.

    Highway numbers have been similar. Instant hangs out around 25 at 80MPH which I found pretty impressive. Rather aerodynamic for such a big car. After my math in the other thread I'm gonna be going a bit slower now, so that should improve. We'll see where this goes.
     
  8. roothorick

    roothorick Active Member

    I was able to test this out recently... it's a car by car thing. My car, going into neutral doesn't change the RPM or GPH at all. But my friend's 99 Accord, when you pop into neutral you immediately see the tach needle drop, and it hangs out below 1k no matter the speed.
     
  9. jcp123

    jcp123 Caliente!

    Oldie but goodie I guess.

    I don't think there's any rule for a car's idle in N. Fords seem to be the worst I have seen, both my Mom's Escape and my SVT Focus would hang out at around 1100rpm rolling in neutral, and would lower only when you came to a full stop. Plus another quirk: in a cold start, the high idle would NOT come down after the vehicle was warmed up unless you came to a full, complete stop for a few seconds.

    My Echo raises the N idle by about 100rpm while rolling and really only starts to drop it when you're under about 10mph.

    I can't really remember most of the other vehicles I have driven and I'll have to pay attention to it in my wife's van (only had it a month).

    As to losses, yeah. I don't really understand transmissions very well, but an auto has a lot more going on in there. It makes sense that the losses would be higher, even if the final drive were the same as a manual.
     

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