Go Back   Team-BHP > Under the Hood > Technical Stuff


Reply
 
Thread Tools Search this Thread
Old 28th September 2006, 01:14   #16
Senior - BHPian
 
vivekiny2k's Avatar
 
Join Date: Feb 2005
Location: cincinnati, jabalpur,chennai
Posts: 1,241
Thanked: 163 Times
Default

Quote:
Originally Posted by drifter
Its purely mechanical....

Drifter
are you suggesting that at a steady highway cruising speed, AT and MT have different slippage between engine and wheel?
vivekiny2k is offline   Reply With Quote
Old 28th September 2006, 02:46   #17
Distinguished - BHPian
 
Mpower's Avatar
 
Join Date: Mar 2005
Location: Bangalore
Posts: 10,432
Thanked: 1,682 Times
Default

Quote:
Originally Posted by V-16
guys let me explainthis whole lower FE on an automatic in layman's terma.

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
Well you are right as far as old gen autos are concerned. Modern autos have enough speeds (Lexus has and 8 speed and Merc 7 speed. Have you seen a manual with that many?) and some have economy and sport settings that will rev the engine higher or lower.
More newer ones have fuzzy logic that try to read the driver's mood for speed and adjust the shift pionts accordingly.

Most importantly, many of them (new Civic) provide paddles so that the driver can shift when ever he wants. How can we explain low FE is this case?

Only the 2 points mentioned by Drifter and me are the biggest contributors. During transient speeds especially, the auto is notoriously inefficient due to the torque converter slip.

Quote:
Originally Posted by theMAG
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.
CVT is better than conventional auto but not as good as manual. Ask any NHC CVT owner. Otherwise Indians would have lapped up the NHC CVT in high numbers. Shift time has nothing to do with FE. Ask any drag racer and he will tell you that autos shift faster than any human.

Last edited by Mpower : 28th September 2006 at 03:03.
Mpower is offline   Reply With Quote
Old 28th September 2006, 08:27   #18
Distinguished - BHPian
 
condor's Avatar
 
Join Date: Jun 2006
Location: Speed-brkr City
Posts: 10,758
Thanked: 4,348 Times
Default

Quote:
Originally Posted by Bomi
This is more a case of non-automatic-tranny users airing their opinions, than fact.
The manufacturers themselves quote better FE figures for M/T than A/T's. M/T's giving better FE is not just opinions of 'non-A/T-users'. I think the guys have explained things quite well.
condor is offline   Reply With Quote
Old 28th September 2006, 11:39   #19
Senior - BHPian
 
Join Date: Jul 2006
Location: Bombay
Posts: 1,007
Thanked: 229 Times
Default

Yes, it is because of torque convertor issues as drifter explained that causes auto trannies to lose some efficiency. Generally at crusing sppeds, the difference is quite less, but in traffic with continuous differing speeds, losses are quite high.

Drive the AT/MT civic and feel the difference. In the AT, you can certainly feel that the linkage is not direct. It always feels that you are driving with a slipping clutch. OTOH, the shifts are very smooth, quick and there are no jerks.
sandeep108 is offline   Reply With Quote
Old 30th September 2006, 18:24   #20
jat
BHPian
 
Join Date: Dec 2005
Location: SINGAPORE
Posts: 265
Thanked: 3 Times
Default

As mentioned, comparing Honda Accord MT vs AT, the AT will have lower fuel economy. Reasons being, as described earlier:

1. The transmission losses - AT will consume some power for auto operation in terms of hydraulics etc and/or if it has a torque convertor then due to slippage which will be wasted as heat.
2. The car performs best accelaration or fuel economy depending where you are operating the engine. At the lower end (for petrol cars), it gives better fuel economy and at higher end better accelaration. Thus with manual transmission, that is with higher number of gears, you have a better choice of keeping it closer to operating point for fuel economy by changing gears earlier (same goes for accelaration from standstill where you change late).
jat is offline   Reply With Quote
Old 2nd October 2006, 02:57   #21
Senior - BHPian
 
V-16's Avatar
 
Join Date: May 2005
Location: mumbai
Posts: 4,575
Thanked: 844 Times
Default

Quote:
Originally Posted by Mpower
Well you are right as far as old gen autos are concerned. Modern autos have enough speeds (Lexus has and 8 speed and Merc 7 speed. Have you seen a manual with that many?) and some have economy and sport settings that will rev the engine higher or lower.
More newer ones have fuzzy logic that try to read the driver's mood for speed and adjust the shift pionts accordingly.

Most importantly, many of them (new Civic) provide paddles so that the driver can shift when ever he wants. How can we explain low FE is this case?

Only the 2 points mentioned by Drifter and me are the biggest contributors. During transient speeds especially, the auto is notoriously inefficient due to the torque converter slip.


CVT is better than conventional auto but not as good as manual. Ask any NHC CVT owner. Otherwise Indians would have lapped up the NHC CVT in high numbers. Shift time has nothing to do with FE. Ask any drag racer and he will tell you that autos shift faster than any human.


Well firstly no one is talking about the fuzzy logic trans nor the new merc/lexus 7 and 8 speed shifts, i think the query was for the regular autos available in india coz anyone who can afford to buy the merc does'nt cre two hoots about the FE.secondly there is no setting in any car except two that is economy (regular) and sport (which changes the shifts in higher rpms) no third option is available thirdly any drag racer will also tell you that the torque and power and rpm settings in a a dragster differs to a normal everyday use car and that when the car and we're not discussing dragsters. Then again, having paddle shifts or triptronics do not change the habits of car drivers andthese are not short shift autos like in dragsters . Yes your opinion is also a factor but not the single largest contributing factor in this issue as the t/c slip is consideranly reduced in modern cars too. all these collectively add up but to say that just one point is singularly responsible...no way!!
V-16 is offline   Reply With Quote
Old 2nd October 2006, 11:35   #22
Senior - BHPian
 
drifter's Avatar
 
Join Date: Sep 2004
Location: Bombay
Posts: 1,469
Thanked: 13 Times
Default

Also have you noticed that sudden surge of torque you get when you start off from standstill in an automatic? Auto boxes are designed to give close to maximum torque on startup...so that they dont have problems in climbing up hills. But in city traffic all that torque is wasted as its not really required.

In a manual, we compensate this by applying varying amount of throttle and clutch. But in the auto, they dont have that option, its all torque or no torque so to say. Hence another reason for lower FE.

Drifter
drifter is offline   Reply With Quote
Old 2nd October 2006, 12:49   #23
rks
BANNED
 
rks's Avatar
 
Join Date: Nov 2005
Location: ??
Posts: 1,237
Thanked: 8 Times
Default

The primary reason for lower FE in an AT has to be that an AT will operate at lower gears for significantly longer periods of time. My guesses as to why this should happen is sketched below.

MT gives more flexibility in choosing the speeds at which you shift, which may enable you to drive in lower gear for longer periods. E.g. you are climbing slightly. As per Santro manual you should shift from third to fourth at 40 kmph and fourth to fifth at 50 kmph. But since you are climbing, you could choose to alter these to 50 and 60 kmph respectively in an MT -- so that you can remain at fifth for the rest of the climb with the added momentum. But an AT may shift to fourth/fifth as soon as you hit 40/50 (or just fourth at 40 if there is no fifth gear) and then your speed will drop, resulting in an upshift etc., leading to a loss in FE. Maybe more sophisticated ATs will avoid this scenario. Similarly while going downhill, you can downshift at lower speeds, say, to fifth at 40 kmph.

You can also use engine braking more effectively in an MT. I don't know if this leads to an improvement in FE. For example, you are approaching an intersection at 60 kmph in fifth gear -- you could choose to upshift at this speed without braking and pass the intersection in third. Whereas in an AT, you might find yourself having to brake and then accelerate, resulting in a waste of fuel.

In an MT you can also upshift to third and execute quick overtakes that may be difficult in an AT. Once you are clear of the slow-moving vehicle, you can shift to fifth -- whereas in an AT, your driving style may have to be different, and you are stuck for longer periods behind slow-moving vehicles -- and hence operating for longer periods in lower gear.

I am just offering guesses here, as I don't have much experience or knowledge of ATs.
rks is offline   Reply With Quote
Old 2nd October 2006, 14:02   #24
Senior - BHPian
 
drifter's Avatar
 
Join Date: Sep 2004
Location: Bombay
Posts: 1,469
Thanked: 13 Times
Default

Quote:
You can also use engine braking more effectively in an MT. I don't know if this leads to an improvement in FE.
Yes it does....modern engines use almost no fuel during engine braking. That is also one point against AT.

Drifter
drifter is offline   Reply With Quote
Old 3rd October 2006, 10:04   #25
Senior - BHPian
 
Join Date: Jul 2006
Location: Bombay
Posts: 1,007
Thanked: 229 Times
Default

Quote:
Originally Posted by drifter
Yes it does....modern engines use almost no fuel during engine braking. That is also one point against AT.

Drifter
That is why paddle shifts of civic AT appeal. Also civic's AT does use some sort of logic control, it does seem to find an appropriate gear even when slowing down without the paddle shifts. This helps in use of engine braking and lessens brake pad wear. Of course it is not as much as MT, but it shows that there have been considerable advances in AT tech.
sandeep108 is offline   Reply With Quote
Old 3rd October 2006, 13:22   #26
BHPian
 
speedofreak's Avatar
 
Join Date: Oct 2005
Location: Dilli
Posts: 104
Thanked: Once
Default

Quote:
Originally Posted by drifter
Also have you noticed that sudden surge of torque you get when you start off from standstill in an automatic? Auto boxes are designed to give close to maximum torque on startup...so that they dont have problems in climbing up hills. But in city traffic all that torque is wasted as its not really required.
Drifter
i corroborate this claim.....i have an OHC AT .....its feels pretty awkward to drive it in bumper to bumper traffic .

as per what i picked up from this thread ....its only the overestimation of torque that the causes AT to drop in mileage .....baakee sab not convincing.
speedofreak is offline   Reply With Quote
Old 3rd October 2006, 17:44   #27
Senior - BHPian
 
vivekiny2k's Avatar
 
Join Date: Feb 2005
Location: cincinnati, jabalpur,chennai
Posts: 1,241
Thanked: 163 Times
Default

Quote:
Originally Posted by speedofreak
i corroborate this claim.....i have an OHC AT .....its feels pretty awkward to drive it in bumper to bumper traffic .
.
i have driven a volvo and a mitsubishi, and i saw that the way they respond is totally diferent. the volvo even had a switch to decide "sport" or "economy" mode, which wffected the way gear shifts happened.

but still i never faced any problems as mentioend in the bumper to bumper traffic. I can drive the cars as smmothly as i want. and i can even dictate the gear upshifts with my driving style. I have not driven an OHC though.
vivekiny2k is offline   Reply With Quote
Old 3rd October 2006, 18:21   #28
Senior - BHPian
 
Join Date: Jul 2006
Location: Bombay
Posts: 1,007
Thanked: 229 Times
Default

Quote:
Originally Posted by vivekiny2k
but still i never faced any problems as mentioend in the bumper to bumper traffic. I can drive the cars as smmothly as i want. and i can even dictate the gear upshifts with my driving style. I have not driven an OHC though.
Very true. The AT upshifts depending on pressure on pedal. By keeping pedal to the metal, I can make either Corolla AT or Civic AT shift only at redline. Be easy on the pedal and it upshifts sooner for better FE. In fact in any AT the gear shifts are usually very smooth and seamless. No problems in any situation yet with AT.
sandeep108 is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Lower mileage in the East Coast winter. Why? arjithin Technical Stuff 19 22nd October 2017 16:27
Mercedes Benz 300D (W123 series): Manual Transmission to Automatic Transmission. wartikars Post-War 5 3rd January 2014 12:39
Why no Automatic Transmission with Base Variant? Simhi The Indian Car Scene 55 28th March 2013 14:59
Mileage labels on cars soon; 10L fine for violation of mileage standards coolclouds The Indian Car Scene 22 14th June 2012 15:13


All times are GMT +5.5. The time now is 04:32.

Copyright 2000 - 2017, Team-BHP.com
Proudly powered by E2E Networks