I heard somebody say that a manual transmission in a car is more efficient than an automatic transmission in the same vehicle. Knowing that manual transmission gears use static friction and therefore produce waste heat, is the above statement true or just crap?
My WAG: manual transmissions are more fuel efficient because the driver anticipates need; extra power is applied sooner. With an automatic, the system doesn’t “learn” of the need until after the driver mashes the accelerator, so it requires more power to “catch up” to the level that a standard transmission would have reached by the same point in time.
As always, with posts about cars, YMMV.
Tom and Ray’s take on the question.
I was always under the impression that the reason a slushbox
got better mileage than a standard was because there were an extra gear in a manual (5sp man vs 3 or 4sp auto).
Also an auto requires a pump to actuate the hydraulics responsible for shifting gears and lubrication; a manual has gears simply sitting in a puddle of oil.
It’s because of the way the power is transferred to the driveshaft.
In a manual transmission, the power is transferred in a direct coupling of the clutch plate to the driven plate, which is, apart from friction and heat on take-up, very efficient.
In an automatic transmission, power is transferred via a structure called a torque converter, which consists of two turbines which spin independently of each other within an oil-filled chamber. The driven half of the torque converter spins at the rate determined by the currently selected gear cog. The fins of its turbine create a vortex in the oil, which acts on the other turbine, causing it to spin, thus tranferring the power through to the driving wheels. Because the oil actually transfers the power,rather than direct coupling, the nick-name “slushbox” was coined.
However, as auto transmissions were refined, some (if not most by now) have the facility to “lock”, usually in the highest gear, hence overcoming the inefficiency of the torque converter.
Whackmaster, what makes you believe an automatic transmission has no gears? It has gears plus a torque converter. Check out
http://www.howstuffworks.com/automatic-transmission.htm
http://www.howstuffworks.com/torque-converter.htm
http://www.howstuffworks.com/category.htm?cat=Auto
I asked this question of a mechanical engineer who was heavily into racing - actually designing and building competitive race cars, Formula One and Grand Prix stuff.
His answer was that modern automatic transmissions in good repair are so efficient that in order for a manual transmission to be more efficient the driver would have to be extremely skilled at operating the clutch and gearshift, and never make a mistake.
As for why the auto transmissions are so efficient, some of the answers have already been offered: overdrive, lock-up torque conveters, computer control, etc.
As my dad (a mechanical guy) once told me, you cannot have something without taking from somewhere else. Meaning that when you add extra functions to a car, energy to run it has to come from somewhere. A noticeable example of this is if you are in a low powered car and air conditioning kicks in, you can feel the accelerator go down slightly to compensate for the extra energy needed.
Although as noted by other people, automatics are much more efficient now, but certainly in the past manual was more efficient.
In terms of mechanical efficiency, the manual simply is more efficient. There have been numerous threads here you can search on to find more information.
As to the OP - what do you mean “static friction”? And as to the waste heat issue - have you ever been under the hood of a car with an automatic transmission before? Most automatics, especially on larger cars/trucks, produce so much waste heat that the transmission fluid needs to be cooled by making a radiatior specifically for it, or by having them use a small radiator attached to the main radiator of the car.
Some other factors y’all may not have considered…
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Gear selection - There are times when automatic transmissions tend to be in the wrong gear for the particular situation (staying in 1st or 2nd longer than it should, even when you’re not stomping on the gas). An experienced manual transmission driver can avoid these.
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New overdrive gears on manual trannies - My new Vette came with a 6-speed manual. The 6th gear allows me to cruise at 75 MPH at about 1,300 RPM. I haven’t seen where the RPMs are at 75 MPH on the open highway in the 4-speed automatic version, but I’d bet that it revs higher.
BTW, I can’t believe that Tom and Ray insinuated that manual transmissions represent more maintenance than automatics. After all, would you rather replace your clutch once or twice during the life of the car or rebuild an automatic?
… they allow the driver more control (you can select the exact gear required, instead of having some complex system of hydraulics and electronics trying to guess which speed you want to be in).
… if your car won’t start, you can push-start a manual.
… most car thieves (particularly juvenile car thieves) can’t drive a manual.
… a clutch might cost a couple of hundred dollars to replace. An automatic transmission might cost a couple of thousand dollars to rebuild.
… you are actually driving a manual, where as in an automatic you’re only steering and using the pedals.
… you can increase the life of your brakes as you can use the selection of gears to slow down your vehicle.
… automatic transmissions are heavier and do not generally have a direct path to the engine (the torque converter, as other Dopers have pointed out).
Granted, the line between manual vs. automatic is become blurred - you now have manuals with automatic clutch (Mercedes A-Class, Alfa Romeo 156 Selespeed) and automatics with manual shift (Tiptronic, as used in Porsche, Audi, Mitsubishi etc).
On the other hand, you have continually variable transmissions (CVT) which is an automatic with one infinitely adjustable gear ratio (Honda Civic, Honda HR-V or ‘Joy Machine’ as it is known in some markets, Nissan Micra etc).
Personally, I can’t wait for sequential shift (ala Alfa Romeo, Mercedes Benz etc) trickle down to shitbox Kias and Hyundais (i.e. stuff I can actually afford. 