Aces High Bulletin Board
General Forums => The O' Club => Topic started by: Fulmar on July 31, 2008, 09:06:39 AM
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I had a co-worker ask me the question today just chewing the fat. Why do manuals get better gas mileage than autos. Now before you jump on the horse. I do know that newer cars have really closed the gap to be within 1-2 MPG or equal on some cars, especially with the CV transmissions. But lets step back 10-15 years when it was more than likely true.
Now I've googled the question and really haven't come up with a good answer. Is it a weight thing, with the auto transmission being heavier than a clutch system? Is it a rpm thing where a manual driver can more accurately gauge when a shift is needed? Combination? Or something completely different?
Any input is good...
As an example, in my 2006 Focus (manual) I average 1-4 MPG better than my wife's 2005 Focus (auto). Same engine/options.
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From what I know, all else being equal, most auto transmissions slip a bit while running. You are slushing things around instead of direct gear connections on an engaged manual. I think that has been improved on more recently.
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Well think about when you drive a stick, When you come to an intersection and you see your going to have to stop you push the clutch and coast to the intersection and that entire time the engine is at idle power. Really pay attention to how you drive a manual and you'll notice that the engine spends allot more time at idle power than when you drive an automatic. Same thing when your shifting. Your pushing in the cluth and letting off the gas to shift gears. In an automatic the engine doesn't let off the gas it just shifts. More idle power time = better MPG
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A auto trans holds the car back...and 5 speed pretty much is free wheeling.
Less moving parts in a 5 speed...one clutch in a 5 speed...many clutches in a auto.
less friction in a 5 speed
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Kaw1000 is incorrect - a traditional auto doesn't have a clutch. Google 'torque-converter' for the correct answer. Sloshing a bunch of fluid around isn't as efficient as a direct connection through a conventional clutch.
More recent auto transmissions such as the DSG box from Volkswagen have a pair of electronically controlled clutches rather than a torque converter. These allow the next ration to be pre-selected for near seamless shifts. These boxes can achieve similar or better mileage than equivalent manuals.
By the way, coasting in a manual transmission is generally frowned upon by most advanced driving schools.
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Google 'torque-converter' for your answer. Sloshing a bunch of fluid around isn't as efficient as a direct connection through a conventional clutch.
By the way, coasting in a manual transmission is generally frowned upon by most advanced driving schools.
I believe that the 'torque-converter' answer is proabably one of the most likely answers to this question. I will add one other thing I think is also a big contributor.
When stopped at a stop sign or traffic light in an automatic the transmission is not completely in neutral and the engine is still under considerable load which I think can use more gas. To test this sit still in a manual with it in gear and clutch presses and listen to engine now put auto in gear and hold brake. I am pretty sure that you will notice difference in amount of load that the engine is under. When in city conditions this will have a significant bearing on efficiency.
As far as the coasting. I think when he said about coasting to stop he was referring to the braking period not coasting freely, thus leaving the car in a controlled condition. I learned to drive a manual transmission many years ago and I believe they are teaching the same thing now that they did then. It is not considered a good idea to coast freely but also not a good idea to downshift either (due to unecessary wear and tear on clutch and transmission and decrease in fuel efficiency). At some point you will need to "coast" holding the brake because if you don't the vehicle will begin to "buck" and eventually stall.
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Kaw1000 is incorrect - a traditional auto doesn't have a clutch. Google 'torque-converter' for the correct answer. Sloshing a bunch of fluid around isn't as efficient as a direct connection through a conventional clutch.
More recent auto transmissions such as the DSG box from Volkswagen have a pair of electronically controlled clutches rather than a torque converter. These allow the next ration to be pre-selected for near seamless shifts. These boxes can achieve similar or better mileage than equivalent manuals.
By the way, coasting in a manual transmission is generally frowned upon by most advanced driving schools.
Kaw1000 is Correct....there are clutches in automatic transmissions..I've only been in the car bizz for 30 years.
as a matter of fact if you want to put a small wager on it I will prove to you that there are several clutches in
a automatic transmission....so how much do you want to bet??
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Kaw1000 is Correct....there are clutches in automatic transmissions..I've only been in the car bizz for 30 years.
as a matter of fact if you want to put a small wager on it I will prove to you that there are several clutches in
a automatic transmission....so how much do you want to bet??
not a big transmission man here but I know that automatic transimission do have clutches in them. I am not sure how many, whether it be multiple or a single but in the older transmissions they are controlled by the torque converter (I believe). The torque converter itself is not how the power is transferred from the engine to the wheels.
A torque converter is a big donut shaped container in front of the transmission (in the bell housing) fluid is forced into this torque converter by centrifugal force. The faster the torque converter is turned by the engine the more fluid goes into the converter. This in turn takes fluid out of the clutch control device "not sure of name or if there is one" in turn this engages the transmission.
Again I am not a big transmission guru but this is very simplified and as close as I can imagine the process to be.
I am sure somebody will chime in here and correct me and/or elaborate some.
edit: I spent the early days after getting my license having to teach myself how to work on cars because I never had enough money to buy a reliable car and also did not make enough money to pay someone else to work on them for me. So since this subject came up I am also hoping that someone who knows can come in here and either confirm my very simple explanation or explain what I have wrong. The above explaination was just the only way I could imagine the torque convertor would be used since automatic car and truck transmissions are not hydraulic transmissions which are very different.
:salute
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(http://www.samarins.com/glossary/transmission.jpg)
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Manuals do not always get better mileage. I.E. The FJ Cruiser automatic gets better mileage than the manual version.
http://www.fueleconomy.gov/feg/bymodel/2008_Toyota_FJ_Cruiser.shtml
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MPG is typically calculated in the last gear, at cruise. If you take the two same cars, one w/ manual, one w/ auto, it is very common for the last gear in the manual to have a closer ratio to 1v1 than the auto. Also, the auto never really locks up completely like the manual does, allowing for slippage as someone mentioned. These are your two reasons. Have a nice day. :aok
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One other factor to remember is that when at a stop the auto transmission is still putting a drag on the engnie requiring a higher idle throttle setting. You are burning fuel to overcome the hydraulic resistance of the transmission even when stopped.
More power is required to move more bits including hydraulic fluid that also creates resistance for the same amount of ground covered. While auto trannsmissions do lock up at higher speeds, most of us spend most of our driving time at lower speeds where the transmission is still in slip mode.
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I believe that the 'torque-converter' answer is proabably one of the most likely answers to this question. I will add one other thing I think is also a big contributor.
When stopped at a stop sign or traffic light in an automatic the transmission is not completely in neutral and the engine is still under considerable load which I think can use more gas. To test this sit still in a manual with it in gear and clutch presses and listen to engine now put auto in gear and hold brake. I am pretty sure that you will notice difference in amount of load that the engine is under. When in city conditions this will have a significant bearing on efficiency.
As far as the coasting. I think when he said about coasting to stop he was referring to the braking period not coasting freely, thus leaving the car in a controlled condition. I learned to drive a manual transmission many years ago and I believe they are teaching the same thing now that they did then. It is not considered a good idea to coast freely but also not a good idea to downshift either (due to unecessary wear and tear on clutch and transmission and decrease in fuel efficiency). At some point you will need to "coast" holding the brake because if you don't the vehicle will begin to "buck" and eventually stall.
If you take a drivers test, and do not down shift to second gear and leave it there until you are almost completely stopped (i.e. a few feet), you will be failed. Period. The logic behind this is that anytime the vehicle is not in a gear, or engine is not engaged to the power-train, you do not have control over the vehicle. If you had to use power to get out of a predicament and were in neutral, you couldn't do it.
In addition, using the brakes in any vehicle to slow down is the least efficient way of slowing down (but still the fastest). Energy is wasted as heat.
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I've got one of those hybrid transmission where I can either set it to automatic or manual, usually drive it manual as I typically get slighter better mpg than when I've got it set to automatic. The only time I drive on automatic is if I'm stuck in L.A. traffic.
ack-ack
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....you will be failed.
I knew those "Epic Failure" signs were meant to hurt MY feelings! :cry
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If you take a drivers test, and do not down shift to second gear and leave it there until you are almost completely stopped (i.e. a few feet), you will be failed. Period. The logic behind this is that anytime the vehicle is not in a gear, or engine is not engaged to the power-train, you do not have control over the vehicle. If you had to use power to get out of a predicament and were in neutral, you couldn't do it.
In addition, using the brakes in any vehicle to slow down is the least efficient way of slowing down (but still the fastest). Energy is wasted as heat.
yes transmission in gear with clutch pressed thus still being in a neutral (no power to wheels) state. need to add power to get out of jam just release clutch and hit gas. This was the way I learned to drive in driver's ed class in high school and passed with no trouble. Maybe this has been changed since I learned to drive 19 years ago. The funny thing is though I don't drive this way though, I downshift my truck all the time mostly out of habit.
I agree that energy is wasted in the form of heat when stopping but stopping is wasting energy. Braking can be faster if you hit the brakes hard enough, however downshifting and using the engine to stop the vehicle increases the RPMs of the engine which in turn uses more gas. Not very fuel efficient. Unless you are in a hybrid car that is designed to recover braking energy none of the energy used to stop the vehicle is recovered and by the way slowing down using the engine produces heat too in both the clutch assembly and the heat in the engine itself is increased only it is managed by the radiator and subseguently released to the atmosphere.
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yes transmission in gear with clutch pressed thus still being in a neutral (no power to wheels) state. need to add power to get out of jam just release clutch and hit gas. This was the way I learned to drive in driver's ed class in high school and passed with no trouble. Maybe this has been changed since I learned to drive 19 years ago. The funny thing is though I don't drive this way though, I downshift my truck all the time mostly out of habit.
I agree that energy is wasted in the form of heat when stopping but stopping is wasting energy. Braking can be faster if you hit the brakes hard enough, however downshifting and using the engine to stop the vehicle increases the RPMs of the engine which in turn uses more gas. Not very fuel efficient. Unless you are in a hybrid car that is designed to recover braking energy none of the energy used to stop the vehicle is recovered and by the way slowing down using the engine produces heat too in both the clutch assembly and the heat in the engine itself is increased only it is managed by the radiator and subseguently released to the atmosphere.
Increasing the "R's" of a engine without increasing the amount of fuel delivered to the combustion chamber, via the throttle, does not use more fuel, especially in today's cars where the mix is computer regulated.
If you freewheel downhill on a bicycle and then put it in gear, do you get more tired?
If down-shifting, or slowing down the engine wasn't efficient, 18-wheelers would not have Jacobsen Engine retarders (Jake Brake if you prefer)
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I stand corrected on the clutches in auto transmission thing - apologies to Kaw1000. Makes sense really - an auto box still has to disconnect the final drive to switch ratios.
In the UK, where I learned to drive and where almost everyone learns and drives manuals, you need to work your way down through the box when slowing or you'll fail. You may only coat when required to prevent a stall when coming to a complete halt. Immediately on stopping you are required to apply the handbrake and put the gear selector in neutral. As long as you don't downshift too early, you shouldn't be stressing the clutch too much and you certainly improve the life of your brake pads.
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need to add power to get out of jam just release clutch and hit gas.
If you took your car out of fifth gear and were just using the brakes to slow down, what gear would you use to power out of an emergency? The second it took you to decide on which of the five gears you should be in, could be a second too late.
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Historically, the car with an automatic transmission was geared differently than the standard transmission car. Partly to help reduce the engine RPM under load as the torque converter output shaft never matched the input shaft RPM due to slippage within the hydraulic unit.
Add the transmission oil in the auto transmission which works against rotation as well, and the increase in rotational mass of the clutch packs being sunk in oil, and the engine simply has to work harder to maintain the same RPM as the standard transmission counterpart.
Newer auto transmissions are using a clutch like a standard, but still have the multi-plate wet clutch pack and mass problem. They get closer to a real standard transmission, but not quite there.
Oh, as far as braking goes. Some engine management systems cut the fuel delivery completely when you use the engine as a brake. Some do not. The ones that do not will use more full per tire rotation as they are running a higher RPM at the same tire rotation with the same amount of fuel being allowed in per revolution.
Now, accelerating with an automatic will almost always use much more fuel than a standard due to sever factors. The slippage in the automatic itself. The higher load per revolution due to lower RPM loading (i.e. you press the gas pedal farther down to get similar acceleration as the standard transmission).
At idle the automatic will use significantly more fuel than a standard transmission at idle simply due to the loading the automatic places on the engine. Again, automatic transmissions which use standard transmission clutches will use the same amount of fuel at idle as the standard transmission counterpart.
There is also a matter with the oil pump in an automatic transmission. The engine is always working against that pump. There is no pump in a standard transmission which has to maintain pressure.
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If you freewheel downhill on a bicycle and then put it in gear, do you get more tired?
If down-shifting, or slowing down the engine wasn't efficient, 18-wheelers would not have Jacobsen Engine retarders (Jake Brake if you prefer)
when you freewheel on a bicycle your legs aren't moving. If you had to make your legs keep up with the pedals going down the hill and had to use the power in your legs to slow down then you would become tired. You would need to apply resistance to the pedals to slow down that way. What you describe would be similar to keeping the transmission in gear like I said but keeping the clutch in the entire time.
Truck driver's down shift to gain extra stopping power while applying the brakes due to the excessive weights involved. Not in place of them.
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Increasing the "R's" of a engine without increasing the amount of fuel delivered to the combustion chamber, via the throttle, does not use more fuel, especially in today's cars where the mix is computer regulated.
I did want to note though the newer cars may not as much fuel during the slowing period but I find it very hard to believe that it would use no extra fueldue to the increase in gases exchanged during the increase in rpm's. those gases would need to be exchanged every time the piston moves up and down. the computer does not change the cycle of the valves opening and closing or the piston and crank from moving. I agree that this method would not be as wasteful as 20 years ago but still would use more fuel than brake power only
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If you took your car out of fifth gear and were just using the brakes to slow down, what gear would you use to power out of an emergency? The second it took you to decide on which of the five gears you should be in, could be a second too late.
no need to decide which gear. you move through the gears without releasing the clutch. Most people who drive manual transmissions all the time usually know which gear would be appropriate for certain speeds and would be moving down through them as they stop anyway.
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Not to worry, we'll soon get a reply from CAP1 who will proceed add a reply without taking the time to read the entire thread basically repeating what has already been said. And they lived happily ever after.
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If down-shifting, or slowing down the engine wasn't efficient, 18-wheelers would not have Jacobsen Engine retarders (Jake Brake if you prefer)
Just a point here. The Jake brake is required as a diesel engine does not provide engine braking without an exhaust brake like a gas engine does. The jake isn't to slow down the engine it's to allow the engine to have resistance so it can be used to assist the brakes or save the brakes from use in down hill grades. It's also no where as efficient as the brakes are at higher speeds but does come in handy.
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Just a point here. The Jake brake is required as a diesel engine does not provide engine braking without an exhaust brake like a gas engine does. The jake isn't to slow down the engine it's to allow the engine to have resistance so it can be used to assist the brakes or save the brakes from use in down hill grades. It's also no where as efficient as the brakes are at higher speeds but does come in handy.
not sure here but I was told as a young lad that the way the jake brake works is it actually shuts off the exhaust and that build up of pressure what causes the truck to slow
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Almost all of the automatic transmissions built since the mid to late 1980's have a clutch in the torque converter that is actuated by a solenoid and a hydraulic valve. The clutch completely eliminates the slippage of the torque converter. In fact, if the clutch, solenoid, or the valve fails, the torque converter will get hot enough to turn blue.
An automatic transmission has a hydraulic pump that produces as little as 15 psi, and as much as 400 psi. The pump absorbs horsepower, the more pressure and volume required, the more horsepower absorbed. Even if the converter clutch is locked up, the pump still absorbs horsepower.
Few manual transmissions have a pump in them, those that do have a low pressure pump that supplies lubrication. Most manual transmissions are splash lubricated and have no pump at all.
These days, it is the pump in the automatic that absorbs horsepower and thereby increases fuel consumption. Lighter components, torque converter clutches, more efficient designs, and over drive ratios have closed the gap. Also, despite the clutch in the torque converter, the converter still slips when the clutch is not applied, and the clutch is most often applied at part throttle cruise, at speeds above 40MPH, in direct and over drive gears. In lower gears, and under acceleration, the converter slips in order to multiply torque, and some power is lost to slippage. So, most often, there is a bigger gap between manuals and automatics when they are not on the highway.
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captn.. I know the old turbo hydros ate up about 40 HP.. what about the new electronic ones?
lazs