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Old 27th September 2006, 18:58   #1
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Question Automatic Transmission = Lower Mileage : Why?

Hi Guys,

I was having this discussion with some friends over Lunch.

We all agreed that Automatic Transmission gives a low mileage but no-one could really explain why this is so?

Please note that I am talking about the normal automatic transmission technology being used in say the Honda Accord.

Conceptually, the gears change at the optimum RPM as tuned by the Manufacturer so they should hence be giving a better mileage. However, this not the case. Why?

Would love to have some technical explanation on this. Alternately, if there is a thread that discusses this issue, please share the URL as I could not locate one.

Regards
Manveet
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Old 27th September 2006, 19:21   #2
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This is more a case of non-automatic-tranny users airing their opinions, than fact. I have done ~50k kms on a Old Honda City AT1.5. All in-city driving - no highway atall. I consistently got an average of 14/15 km per litre with AC on. I don't think the manual 1.5 would yield better mileage.
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Old 27th September 2006, 19:27   #3
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In MT, you can select at which RPM you want to upshift. If at that RPM, torque is not sufficient, you'll feel jerks. In AT, car computes an RPM to shift which is often a bit overestimatation (guranteed torque after upshift) So, more rev = less FE. Also most AT cars so far have only 3 (or sometimes 4) gears so there is less flexibility in chossing best RPM. Since most fundamental AT cars use epicyclic gearbox it was difficult to accommodate more than 3 gears there. See www.howstuffworks.com for details of AT.

Modern AT with 5 or even 7 gears have almost same FE as of MT cars.
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Old 27th September 2006, 20:32   #4
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Its purely mechanical....an automatic transmission uses a torque converter which is basically like two fans facing each other with one end attached to the engine and the other to the gearbox. This setup is completely filled up with oil. When the engine spins, it causes the other fan (turbine) to spin, hence rotating the gears and the wheels.

Now there's no actual physical linkage between them apart from the oil unlike the regular manual transmission where the clutch completes the physical linkage so to say between the transmission and the engine....so naturally these transmissions become less efficient.

Drifter

Last edited by drifter : 27th September 2006 at 20:36.
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Old 27th September 2006, 20:33   #5
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Quote:
Originally Posted by sbasak
Modern AT with 5 or even 7 gears have almost same FE as of MT cars.
naah .. even the quoted FE is lower for Auto than for Manual.

Would appreciate if you could pick a standard contemporary car, I suggest the Accord, and explain this concept.

Thanks
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Old 27th September 2006, 20:40   #6
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Quote:
Originally Posted by drifter
Now there's no actual physical linkage between them apart from the oil unlike the regular manual transmission where the clutch completes the physical linkage so to say between the transmission and the engine....so naturally these transmissions become less efficient.

Drifter
Ok. This sounds logical. But your explanation makes sense wrt effeciency, but I think effeciency in itself is an arbit concept - is this the same as fuel effeciency. Somehow I am not convinced that this would be the sole reason.

Perhaps you could add some more info ?

Thanks
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Old 27th September 2006, 20:44   #7
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Quote:
Originally Posted by Bomi
This is more a case of non-automatic-tranny users airing their opinions, than fact. I have done ~50k kms on a Old Honda City AT1.5. All in-city driving - no highway atall. I consistently got an average of 14/15 km per litre with AC on. I don't think the manual 1.5 would yield better mileage.
I get a consistant mileage of 8.5-9.0 kmpl on my OHC 1.5 Manual. My friend gets 12. You get 15 on Automatic. Someone else gets 11.

Don't discount other people's knowledge and opinion. We are here to learn.
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Old 27th September 2006, 20:48   #8
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Yes im talking with respect to fuel efficiency. The gear ratios in automatics are different from their manual counterparts so you dont really feel the power drop. But power loss and fuel consumption in automatics is always greater than manual transmission. But with modern technological advances, the differences have come down.

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Old 27th September 2006, 21:21   #9
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drifter that was a great explanation of a torque converter. torque converters, different ratios, and higher shift points combined together give you that difference in fe.
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Old 27th September 2006, 22:43   #10
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Quote:
Originally Posted by drifter
Its purely mechanical....an automatic transmission uses a torque converter which is basically like two fans facing each other with one end attached to the engine and the other to the gearbox. This setup is completely filled up with oil. When the engine spins, it causes the other fan (turbine) to spin, hence rotating the gears and the wheels.

Now there's no actual physical linkage between them apart from the oil unlike the regular manual transmission where the clutch completes the physical linkage so to say between the transmission and the engine....so naturally these transmissions become less efficient.

Drifter
True only in old designs. Now all slushboxes a lock up clutch that closes and locks the 2 turbines so to speak. (at cruising speed)

Still, autos have a huge oil pump that sucks up the power, thats the main reason. CVT and DSGs are a little better.
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Old 27th September 2006, 23:08   #11
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Quote:
Originally Posted by manveet
Hi Guys,

I was having this discussion with some friends over Lunch.

We all agreed that Automatic Transmission gives a low mileage but no-one could really explain why this is so?

Please note that I am talking about the normal automatic transmission technology being used in say the Honda Accord.

Conceptually, the gears change at the optimum RPM as tuned by the Manufacturer so they should hence be giving a better mileage. However, this not the case. Why?

Would love to have some technical explanation on this. Alternately, if there is a thread that discusses this issue, please share the URL as I could not locate one.

Regards
Manveet
Auto trannies up until the CVT gave give lower FE primarily due to their fixed and longer shifting times over a manual - the engine would have to run at a higher(upper limit) rpm for the auto gearbox to change. However current CVTs - originally started by Subaru worldwide, made famous in hamara Bharat by the NHC, are more efficient than any manual transmission.
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Old 27th September 2006, 23:27   #12
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I don't want to get CVT's and DSG's into the picture, thats precisely why I was referring to the Accord.

Lemme rephrase the question - Why does an AT Honda Accord give a lower mileage than an MT? (under similar driving conditions)

Now my friends are really smart and take a lot of time getting convincedSo I gave them all the gyaan on shift ratios and all, but they reply by saying that the CPU in the car times the gear changed based on say a formula, how can THAT lead to a lower FE than MT which is based on driver judgement. So I need to be TOTALLY clear if I have to explain the funda to them. Most answers (except 1-2) seem global gyaan to me.

All I've learnt that I didn't know before is that there is no actual contact and the oil pump reduces FE. But that's not enough for me to convince them. Especially since I spend all my time in office on team-bhp

I'd REALLY appreciate something more tangible. Pictures would help
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Old 28th September 2006, 00:10   #13
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guys let me explain this whole lower FE on an automatic in layman's terms.

have you seen the traffic in bombay?? jokes apart, it all depends on the driving style. We tend to change to a higher gear as per our driving style say in a santro ud change from ist to 2nd even as low as 20 kmph but in an auto ud have to go to 30-40 as per the limit set. you cant change it even if u wanted to (altough its possible to shift quicker in an auto, higher up).

2) wnen we slightly step on the gas, the tras gets a signal that we wanna hurry up to overtake or whatever and it steps down one or two gears (kick down) depending on the speed and gear ure in which in a manual can be avoided and the car remains in a higher gear gradually picking up speed, thus saving gas.

3) drive an auto steadily and dont be lightfooted or ull land up with horrendous FE. steadily step on the gas and when overtaking dont get pedal happy, gently tap the gas and ull just shift the right gear down to overtake.

I hope its explained and remember a km or two less per litre is worth it if ure driving through the crazy traffic

Last edited by Mpower : 28th September 2006 at 07:32.
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Old 28th September 2006, 01:07   #14
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Quote:
Originally Posted by sbasak
In MT, you can select at which RPM you want to upshift. If at that RPM, torque is not sufficient, you'll feel jerks. In AT, car computes an RPM to shift which is often a bit overestimatation (guranteed torque after upshift) So, more rev = less FE. Also most AT cars so far have only 3 (or sometimes 4) gears so there is less flexibility in chossing best RPM. Since most fundamental AT cars use epicyclic gearbox it was difficult to accommodate more than 3 gears there. See www.howstuffworks.com for details of AT.

Modern AT with 5 or even 7 gears have almost same FE as of MT cars.
this seems to be closest.

in MT, you can change the gears at ur own will, u can see far down the road and preapre to downshift/upshift.
AT will sense the change in load only after running on it for sometime, then there will be some lag in the shift. in the mean time it will have burnt some extra fuel.

also, on a varialbe load road, u may know that it is going to last for say 5 mins. but AT will keep shifting to get the optimum RPM. after all it's just a combination of sensors and actuators.

keep in mind, once the car has settled on a particular gear (AT or MT), there will absolutley no diference in the way they behave. so the highway cruises will not show much difference between AT and MT.
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Old 28th September 2006, 01:11   #15
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Quote:
Originally Posted by V-16
3) drive an auto steadily and dont be lightfooted or ull land up with horrendous FE. steadily step on the gas and when overtaking dont get pedal happy, gently tap the gas and ull just shift the right gear down to overtake.
exactly, if u push the pedal just like u do in MT (push-wait to upshift-push-wait to upshift....) u can actually dictate the gear upshifts. comes with practice though. you can see ur success in the tacho, the moment u release pedal to wait, the rpm shud drop much below expected, indicating the car has upshifted.
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