Why the mpg difference - auto vs manual

[dkff49]

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.

[VonMessa]

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) 

[68Wooley]

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.

 

[ChickenHawk]

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.

[Bones]

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.

[dkff49]

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.

edit:

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

end edit

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.

[DiabloTX]

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.

[Maverick]

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.

[dkff49]

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

[Captain Virgil Hilts]

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.

[lazs2]

captn.. I know the old turbo hydros ate up about 40 HP..  what about the new electronic ones?

lazs