Manual vs automatic What is desirable for MPG?

[Isaac Zachary]

Originally Posted by mort

My 20 year old Volvo with only 4 gears in the automatic has a shifter position that allows the driver to hold any gear and up or down shift whenever they please. I'm sure most modern automatics have that feature, often as paddles.
-mort

This brings up an important point. How much control does an automatic give you? Some do let you limit to a certain gear, but not select it. Some give arbitrary "L" selections that could still be one of various gears that the transmission control unit still decides. And a few, not "most," that do let you hold a particular gear.

Most automatics that I've seen seem to cater to drivers who don't have much of a clue of what the transmission even does and have very few shifting options, or options like "Sport" and "Eco".

[Ecky]

Originally Posted by Isaac Zachary

One advantage of the CVT no one has mentioned yet is it can keep the engine from constantly accelerating and decelerating. When shifting, you're losing small amounts of kinetic energy everytime the engine decelerates.

I'm not sure this makes a difference?

What causes the engine to decelerate when you take it out of (or between) gear, is friction (fluids, rings, accessories), which is always present, even when the engine is in gear. Were the components frictionless, the engine would not decelerate between shifts.

[Isaac Zachary]

Originally Posted by Ecky

I'm not sure this makes a difference?

What causes the engine to decelerate when you take it out of (or between) gear, is friction (fluids, rings, accessories), which is always present, even when the engine is in gear. Were the components frictionless, the engine would not decelerate between shifts.

But it is increased friction, especially in a manual. In a traditional manual you are closing the throttle and engine braking every time you need to shift, which is another reason dog clutches could be better.

Also in an automatic, the greater the speed difference between the two halves of the torque converter the greater the losses. Having several gears helps, but the losses from shifting are still there.

From what I can tell, the ultimate transmission for both engine and transmission efficiency is the series hydraulic hybrid, which is a type of CVT.

[Ecky]

Originally Posted by Isaac Zachary

But it is increased friction, especially in a manual.

This is the point I'm not sold on.

Find a slope where you're neither gaining nor losing speed while rolling in neutral. Now, put it in gear and find the exact throttle position where you're neither accelerating nor decelerating. At this point, as long as you hold the throttle exactly where it is, you can take it in and out of gear and 1) RPM will not drop when it leaves gear, and 2) fuel consumption will not change whether in or out of gear.

Similarly, taking your foot off the gas while leaving it in gear will decelerate the vehicle more quickly than throwing it in neutral. Exactly as quickly, in fact, as the inverse of the amount of fuel used to spin the engine at the RPM it would be turning while in gear.

The engine is the lossy component, whether it's connected to the transmission or not. A CVT is almost always better at keeping RPM down and load up, granted, but I don't see how shifts themselves can make any difference.

[Isaac Zachary]

Originally Posted by Ecky

Find a slope where you're neither gaining nor losing speed while rolling in neutral. Now, put it in gear and find the exact throttle position where you're neither accelerating nor decelerating. At this point, as long as you hold the throttle exactly where it is, you can take it in and out of gear and 1) RPM will not drop when it leaves gear, and 2) fuel consumption will not change whether in or out of gear.

No one drives at zero torque and shifts in and out of neutral with zero torque.

Try reving really high as you go down the road and pulse and glide in a single gear without using any neutral or clutch. Accelerate up a few miles per hour and take your foot off the accelerator and let the car slow down a few miles per hour and then do it again.

Driving a typical manual means snapping the throttle shut and causing higher pumping losses.

Driving an automatic means that the torque converter will be forced from time to time to hit less efficient RPM differences that churn more hydraulic fluid into heat.

[hayden55]

Engine off coasting in neutral with an eCVT prius ,electric air conditioning, no clutch to slip, electric brake pressure accumulator is pretty nice.
But behind that i would typically choose a manual on most cars especially if like pre 2015.
Driving at high load and going back to neutral with a manual will get you similar mileage to an eCVT hybrid especially if you also shut the engine off but its a lot more work and feels like it.

[Isaac Zachary]

One thing that a lot of people complain about is how much work it is to drive a manual, especially to hypermile one. On the other hand, I find my automatics and CVT's downright boring. There is nothing to do but sit back and let the car decide everything for you. Many people like to replace that with torque and speed, but I've never liked carnaval rides and didn't buy my car so I could feel like I'm in one.

[MetroMPG]

Originally Posted by Isaac Zachary

I find my automatics and CVT's downright boring. There is nothing to do but sit back and let the car decide everything for you.

One thing I've learned is *every* automatic car I've driven that has a "manual" mode will let the driver upshift sooner than even the factory "eco" mode programming. Even some CVT's permit it. Some geared automatics will also permit you to start out in 2nd gear in manual mode, but won't when in "D". (My folks' new-to-them Kia Soul does this.)



Thread:



"Short-shifting" a Jeep Patriot & Audi A4 CVT (forcing early "upshift") saves gas


So that can make driving modern automatics slightly less boring, and slightly more efficient.

[Isaac Zachary]

Originally Posted by MetroMPG

One thing I've learned is *every* automatic car I've driven that has a "manual" mode will let the driver upshift sooner than even the factory "eco" mode programming. Even some CVT's permit it. Some geared automatics will also permit you to start out in 2nd gear in manual mode, but won't when in "D". (My folks' new-to-them Kia Soul does this.)



Thread:



"Short-shifting" a Jeep Patriot & Audi A4 CVT (forcing early "upshift") saves gas


So that can make driving modern automatics slightly less boring, and slightly more efficient.

I find it funny that we compare the modern automatic to the not-so-modern manual. Manual transmissions haven't changed much since synchromesh was introduced. Automatics are totaly differnt animals than what they used to be.

If someone would make a modern manual transmission, (maybe an automatic with the option to shift into whatever gear the driver wants is a modern manual transmission) then maybe we could have a fair comparison.

I think sequential shift dog clutch transmissions should be offered as modern manuals.

[hayden55]

Originally Posted by Isaac Zachary

If someone would make a modern manual transmission, (maybe an automatic with the option to shift into whatever gear the driver wants is a modern manual transmission) then maybe we could have a fair comparison.

I think sequential shift dog clutch transmissions should be offered as modern manuals.

I would imagine it is because they are noisy and clunky. I assume you could get the non straight cut gears to get rid of the hellacious supercharger whine they give off? But i wouldn't imagine people would put up with dog clutch engagement either