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Why is this anyways?
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(my real question) If you have one of those new-fangled cars that lets you shift the automatic transmission manually (if you wish), does your gas mileage improve when you do that?
- Because the linkage between engine and wheels is much more direct in a stick. When you have the clutch out in a stick, your engine’s speed translates straight to the wheels.
A stick with the clutch all the way out works much like an old-timey watch or clock’s gears. One RPM in 5th gear in a particular '96 Mustang will work out to a set number of feet.
To understand how automatics work, imagine a pair of box fans submerged underwater. The first fan is mated to the engine. The second fan is mated to the wheels. The engine spins the first fan, much of the power going into the first fan makes it to the second fan, and the gears attached to the second fan turn the wheels [1].
Which of those two methods sounds like there will be more power wasted?
Gross oversimplification, but that last part explains a healthy part of the losses associated with automatics. I also left out the concept of “stall speed”, since I don’t understand it well enough to explain it properly, but suffice it to say that is another substantial difference.
Old automatics were teh suck when it came to energy efficiency. Newer automatics are getting very close to manuals when it comes to gas mileage, but haven’t quite bridged the gap. I have a belief that the less powerful the engine and the lighter the vehicle, the more of a difference the transmission type makes, but I can’t explain the reasons, I just recall getting that impression when comparing EPA mileage ratings in car reviews. - The underlying mechanism in “Manumatic” transmissions are conventional automatic transmissions and as such they will have the same efficiency.
The only other way that a manumatic could beat a conventional auto tranny would be if you had a smarter shift pattern. The only problem is that the computer that runs an auto tranny’s shift pattern has been engineered to give maximum gas mileage [2], and the best most drivers can manage is to match it
[1] Actually, read the below link. Much nicer explanation than I can give.
http://about.edmunds.com/ownership/techcenter/articles/43836/article.html
[2] That is due to CAFE regulations. http://www.mpgplus.org/perspective/cafe.html explains it nicely, although I believe it is a lobbyist site.
In theory I think manuals will do a little better than today’s automatics. However I think that assumes that the manual is used properly. Don’t stay in low gear too long, don’t stay in high gear too long when pulling uphill and stuff like that. I seriously doubt that the average driver with a manual in city driving can do better than the automatics. Out on the road where you spend most of the time in high gear? Probably the manual will do a little better.
Excellent explanation Mr. Slant. One more thing: in some cars, the manual transmission may have a higher top gear ratio than the automatic. For example, in my manual Jeep Wrangler the ratio of my 5th gear is .78:1. So for each revolution of the engine, the driveshaft spins 1.28 times. The automatic is only a three speed, and the third gear ratio is 1:1. Basically, the engine in my Jeep is turning at a lower RPM at the same speed than a similar Jeep with the auto. This can be complicated by the final drive ratio (axle ratio), tire size, etc., but hopefully you get the point.
I’m pretty sure this is “The Master’s” answer as well. I’m too lazy to look it up though.
Just thought I mention that the Toyat Prius dispenses with a conventional transmission entirely. The gas engine and electric motor/generators are connected by a planetary gear system which is directly connected to the wheels.
Brian
Most A/T has a locking torque converter for the final drive/overdrive gear, so there is a direct relationship between the engine rotation and the vechical distance. So in highway driving you will see the A/T mpg approach the manual mpg, sometimes so close that they will be the same on the sticker.
I’ve never seen an automatic car get close to the EPA numbers in the city, but on the open road with the torque converter locked up and in a gear higher than most manuals have in the box they’re pretty much equal. Manual cars usually get the EPA rating both city and highway, with the exception of certain sportscars (CRX, Corvette) that never get into high gear on the EPA test cycle.
An automatic can be, and usually is, programmed to recognize that it’s on the EPA test cycle and shift accordingly. It’s a very dirty trick but it works. If you get a copy of the graph of the cycle and drive exactly like it you’ll notice the transmission shifting very early and very hard (to use the rotational momentum of the engine to turn the wheels) and the torque converter locking up in the lower gears.
A manual gearbox has a clutch which is locked solid 95+% of the time. An automatic has a fluid clutch which is locked maybe 10% of the time and never in the city. The rest of the time, the fluid clutch/torque converter is busy turning energy you paid for into heat. I’ve never seen a street manual transmission with a heat exchanger of its own; I’ve seen plenty of automatics so equipped…
Another factor is that with a manual-transmission car in the city, you can spend most of your time coasting. Whenever you see red lights ahead, take the car out of gear and let it cruise up to the light. It’ll save you money on brakes and gas. The light might even be green when you get there.
Automatics also weigh a lot more than manuals. The penalty is usually between 100 and 200 lbs. A Honda B-series manual gearbox & differential weighs 60 lbs. The equivalent automatic weighs 180. Considering that a good approximate formula for city gas mileage is 60000/(car weight in lbs) this is worth a mile per gallon or two alone.
The two fans underwater thingie that Mr. Slat described is called the torque converter. Torque converters tend to suck because they never make a good connection between one side and the other. You are always wasting energy just sloshing the fluid around.
It used to be that you had a choice between a stick shift, which makes a good solid connection between the engine and the wheels, and an automatic, which had a sloppy connection between the engine and wheels but required less from the driver. Modern automatic transmissions have what is called a lock-up torque converter, which is an attempt to make the best of both worlds. At low speeds, it functions like a regular torque converter, so the user doesn’t have to worry about stalling the engine out at stop lights, or managing the clutch as it shifts between gears. Once you get going, the torque converter goes into lock-up mode, where it locks the two spinning fan blade thingies (how’s that for a technical term) in the torque converter together so that you have a solid connection between the engine and the wheels. When it’s in lock-up mode, you have a good solid connection between the engine and wheels, and you aren’t wasting energy sloshing fluid around.
Someone who is good with a clutch can still outperform even a modern automatic transmission, but the difference isn’t as pronounced as it used to be. Also, no matter how good the transmission gets, it can’t look ahead and see what conditions are in front of the car. Someone with a clutch has more information available, and can shift more intelligently because of this.
Sure they have the potential to shift more intelligently than an automatic does. But have you watched the typical driver? There are lots of distractions. People get to talking and forget to shift out of low gear or charge up to a stop sign and forget they have to stop until the last minute. The automatic doesn’t get tired, or lazy, or distracted.
The advent of small and reliable computers to program shifts using inputs from various sensors has already made quite an improvement over old-style automatics and will make more in the future I suspect.
More information?, How. You may be thinking of more options, but with the manumatic a/t this advantage is not as great, but still there.
You can see what the road ahead is doing. An automatic transmission can’t.
For example, is that change in slope a minor hump or the beginning of a huge mountain climb? You know. Your transmission doesn’t. You have more information to work with.
What about Continuously Variable Transmission? A friend of mine bought a car with CVT a few months ago. The gas mileage, it turns out, is pretty crappy. He chalked it up to buying a large car with all kinds of extras that suck up energy, but could it just be an inefficient transmission?
A manual trans can’t see either, again the manumatic mode would negate this aspect, but even most a/t’s will allow some manual shifting (normally downshifting only)
I think you are over estimating what good this is. The automatic transmission can detect the actual load on the engine with much more accuracy than your calibrated mark one ass sitting in the driver’s seat.*
Until the load starts to increase, there is no advantage that I can think of to downshift early. If the hill is a small hump, the trans shifts down once, if it is a large hill it may shift down more times.
Between 6 speeds, a whole range of sensors, a high speed data network, and a torque converter that can both fully lock up or part way lock up at speeds as low as 35 mph, I don’t think you give up nearly as much as many people would think.
Looking at some EPA numbers here is what I see
Volvo V70R (300 HP AWD)
manual 6 speed 18/24
Auto 6 speed 18/25
Puts a bit of a hole in that slushbox arguement don’t you think?
or if you want to go down market a bit
Volvo S40i (168 HP FWD base car)
Manual 5 speed 22/29
Auto 5 speed 22/30
Volvo S40 T5 AWD (218 HP AWD)
Manual 6 speed 20/29
Auto 5 speed 20/28
Cite
Overall you don’t give up much to drive an auto box anymore*
Of course a stick is much more fun to drive.
*I am assuming that we are talking about a state of the art electronically controlled transmission, not a slush-o-matic out of a '54 shitbox.
Pretty impressive numbers. I didn’t realize the newest transmissions had come along quite that far.
Goes to show that the Volvo manual gearbox isn’t any good… check the difference between the manual and automatic versions of the Civic.
Also remember that the automatic is programmed for optimal EPA mileage at the expense of everything else. Although the plural of anecdote is not data, I have a sufficiently large sample size to make the categorical statement that there is no automatic car that meets its EPA ratings in actual service.
The automatic transmission-equiped Scion tC is rated the same for city driving as its manual counterpart and higher in freeway driving. I think it’s just the gear ratios in this case, though.
And my tC gets just a little better than the rated EPA for city driving. It’s rated at 23 mpg and I’ve always gotten 24-25. Once I think I got 23.5.
My old '99 Mustang always got 17-18 mpg a tank. And it was a manual.
I think it’s the opposite, a gearbox is pretty darn simple and efficent to make, a good A/T however is a lot more complex and much harder to make efficient. It would seem that Honda is lacking in A/T tech.
Uh, we’re not allowed to accuse people of trolling, so I’ll just assume you’re being funny. I’m not going to do the research, but I imagine the difference you see is due to shitty little engines. Now I don’t mean the Honda engine is shitty; I mean that small engines are shitty compared to big engines. I remember when I was waiting for delivery of my brand new 1995 Honda Civic EX 5-speed. The dealer kept my 1990 Si (tradein), and gave me a 1995 Honda Civic EX Automatic. That dang dealer has to be thanking his stars that I didn’t test drive an automatic, or I’d not have bought thea newer Honda Civic. Whining and gear-hunting and making all of those damn hamster noises that cheap cars are known for making. I can see why it wasted so much gasoline – it spent most of its time seriously over-revved. I loved my 5-speed manual, though.