3 ms·

This is less a hybrid thing and more a new transmission thing. It, of course, isn't free. The efficiency of a CVT is a good 10-20 percent lower than previous

by taeric 1y ago

This is less a hybrid thing and more a new transmission thing. It, of course, isn't free. The efficiency of a CVT is a good 10-20 percent lower than previous transmissions. That said, currently, the win from keeping the engine at either the max power or the max efficiency speed is substantial. There are some really good videos out there going over how newer CVTs work. Looks like some people are working on ones that are teeth driven, to reduce the loss from being free belt driven. Borderline magical stuff, all told. (Obviously, not magic magic. But very very impressive designs.)

klaff 1y ago
I have a Subaru (ten years old w/ >200k miles on it's chain-type CVT), and I thought the justification for it was that it was more efficient than previous systems. From a mechanical engineering standpoint, the Subaru CVT uses a fairly conventional lock-up torque converter at the input, but that gets locked as you pass something like 15-20 mph (once the lowest gear ratio is satisfactory w/o the torque converter function) and beyond that all shifting of the CVT is done w/ the torque converter locked. In addition, the clamping force of the sheaves is adjusted per the torque load of the transmission to minimize the frictional losses. Anyway I'm curious about data comparing efficiency of conventional and CVT automatics.
- taeric 1y ago
  My understanding is that the overall system is more efficient. It is only the transmission that is less so. The videos online that look at various CVT systems is truly an amazing resource that I regret not having when I was younger. :D
  - klaff 1y ago
    If overall efficiency is what you see reflected in the EPA mileage numbers (or similar European tests), in what scenario would a lower efficiency transmission (lower Pout/Pin over some range of operating points?) lead to or even allow better overall efficiency?
    - taeric 1y ago
      I'm not sure I understand the question? You get the engine to hit its sweet spot and to hold it there. That gain outweighs the loss from the new transmission. This would be similar to hitting the optimal torque point. The idea there would be that you can get out of the acceleration phase faster, so that you can transition to a more efficient gear to maintain the speed for longer. The wikipedia looks to cover this well. One of the cites is specific on the efficiency of the CVT. I think I overstated how much higher the loss is, so maybe that is confusing things? I thought it was 10-20, but the cite on the page shows it solidly around 10.
      - klaff 1y ago
        Ah, thank you, I see the argument. I'm still a bit skeptical though when many non-CVT automatics are 6 or 8-speed models that there would be much "sweet spot" benefit left by being able to make relatively small changes in engine speed. Like many things probably depends on exactly what comparison is being made.
        taeric 1y ago
        I mean, we are comparing 6-8 to effectively infinite? Question then comes into just how much is lost by being outside of the sweet spot for an engine. Which, I agree that I would not have thought it would be that big of a deal. Just going off modestly paying attention in my car, I see massive benefits keeping the car around 300 rpms lower. Interestingly, my car gets better gas mileage around the 40ish speeds than I do at full highway speeds. That somewhat surprises me. It is very dependent on not having a heavy foot, of course.
        vel0city 1y ago
        Generally speaking your car will get better fuel economy at 40mph than it will at 60 or 70mph. Your car gets so much more wind resistance the faster you go. The main reason why city mileage is usually lower is because of all the stopping.
roelschroeven 1y ago
The post you commented on was talking about Toyota hybrids though, who don't use a CVT in the sense you're talking about. They use a series-parallel hybrid transmission which is sometimes called eCVT, but works completely different from a classic CVT. There are no pullies, belts, chains, none of that. What they do have is a couple of motor-generators and a differential to link the system up with the engine and the drive shaft. No friction losses like CTVs have. See https://prius.ecrostech.com/original/PriusFrames.htm https://prius.ecrostech.com/original/PriusFrames.htm, or look up "Hybrid Synergy Drive" on Wikipedia or Youtube or your favorite search engine.
- taeric 1y ago
  Ah, totally fair in that this particular transmission is better than the belt driven CVT stuff. I had thought they were still a bit worse than other transmissions, but it looks like if they are, it is on the order of 1% or so. Is fascinating to watch these things work.

by taeric 1y ago

klaff 1y ago

I have a Subaru (ten years old w/ >200k miles on it's chain-type CVT), and I thought the justification for it was that it was more efficient than previous systems. From a mechanical engineering standpoint, the Subaru CVT uses a fairly conventional lock-up torque converter at the input, but that gets locked as you pass something like 15-20 mph (once the lowest gear ratio is satisfactory w/o the torque converter function) and beyond that all shifting of the CVT is done w/ the torque converter locked. In addition, the clamping force of the sheaves is adjusted per the torque load of the transmission to minimize the frictional losses. Anyway I'm curious about data comparing efficiency of conventional and CVT automatics.

taeric 1y ago
My understanding is that the overall system is more efficient. It is only the transmission that is less so. The videos online that look at various CVT systems is truly an amazing resource that I regret not having when I was younger. :D
- klaff 1y ago
  If overall efficiency is what you see reflected in the EPA mileage numbers (or similar European tests), in what scenario would a lower efficiency transmission (lower Pout/Pin over some range of operating points?) lead to or even allow better overall efficiency?
  - taeric 1y ago
    I'm not sure I understand the question? You get the engine to hit its sweet spot and to hold it there. That gain outweighs the loss from the new transmission. This would be similar to hitting the optimal torque point. The idea there would be that you can get out of the acceleration phase faster, so that you can transition to a more efficient gear to maintain the speed for longer. The wikipedia looks to cover this well. One of the cites is specific on the efficiency of the CVT. I think I overstated how much higher the loss is, so maybe that is confusing things? I thought it was 10-20, but the cite on the page shows it solidly around 10.
    - klaff 1y ago
      Ah, thank you, I see the argument. I'm still a bit skeptical though when many non-CVT automatics are 6 or 8-speed models that there would be much "sweet spot" benefit left by being able to make relatively small changes in engine speed. Like many things probably depends on exactly what comparison is being made.
      - taeric 1y ago
        I mean, we are comparing 6-8 to effectively infinite? Question then comes into just how much is lost by being outside of the sweet spot for an engine. Which, I agree that I would not have thought it would be that big of a deal. Just going off modestly paying attention in my car, I see massive benefits keeping the car around 300 rpms lower. Interestingly, my car gets better gas mileage around the 40ish speeds than I do at full highway speeds. That somewhat surprises me. It is very dependent on not having a heavy foot, of course.
        vel0city 1y ago
        Generally speaking your car will get better fuel economy at 40mph than it will at 60 or 70mph. Your car gets so much more wind resistance the faster you go. The main reason why city mileage is usually lower is because of all the stopping.

roelschroeven 1y ago

The post you commented on was talking about Toyota hybrids though, who don't use a CVT in the sense you're talking about. They use a series-parallel hybrid transmission which is sometimes called eCVT, but works completely different from a classic CVT. There are no pullies, belts, chains, none of that. What they do have is a couple of motor-generators and a differential to link the system up with the engine and the drive shaft. No friction losses like CTVs have. See https://prius.ecrostech.com/original/PriusFrames.htm https://prius.ecrostech.com/original/PriusFrames.htm, or look up "Hybrid Synergy Drive" on Wikipedia or Youtube or your favorite search engine.

taeric 1y ago
Ah, totally fair in that this particular transmission is better than the belt driven CVT stuff. I had thought they were still a bit worse than other transmissions, but it looks like if they are, it is on the order of 1% or so. Is fascinating to watch these things work.