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| Confused about PHEV, Hybrids, etc... |
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| bdunham7:
--- Quote from: nctnico on July 26, 2022, 07:31:12 pm ---That depends on the type of hybrid; the only hybrids to consider need to have a true Atkinson cycle engine. Such engines are much more efficient so even on highways, you'll get a much better mileage compared to a conventional ICE based car. Unfortunately there are only a few manufacturers that make such hybrids (typically with a drive train from Toyota). --- End quote --- A 'true Atkinson cycle engine' is an entirely different animal and not something you will see in any modern car. Modern "Atkinson' and 'Miller' cycle engines are simply marketing designations for regular Otto-cycle engines with modified valve timing and higher compression ratios (as calculated from swept volume and combustion chamber size). Yes, these setups can be significantly more efficient at the cost of reduced power (for a given engine size) and limited operating range, but the name is just that and nothing more. Many modern engines, including non-hybrids, use variable valve timing and lift along with mechanical compression ratios higher than would be otherwise workable to achieve similar results under the right conditions. |
| gnuarm:
--- Quote from: bdunham7 on July 28, 2022, 05:29:44 pm --- --- Quote from: nctnico on July 26, 2022, 07:31:12 pm ---That depends on the type of hybrid; the only hybrids to consider need to have a true Atkinson cycle engine. Such engines are much more efficient so even on highways, you'll get a much better mileage compared to a conventional ICE based car. Unfortunately there are only a few manufacturers that make such hybrids (typically with a drive train from Toyota). --- End quote --- A 'true Atkinson cycle engine' is an entirely different animal and not something you will see in any modern car. Modern "Atkinson' and 'Miller' cycle engines are simply marketing designations for regular Otto-cycle engines with modified valve timing and higher compression ratios (as calculated from swept volume and combustion chamber size). Yes, these setups can be significantly more efficient at the cost of reduced power (for a given engine size) and limited operating range, but the name is just that and nothing more. Many modern engines, including non-hybrids, use variable valve timing and lift along with mechanical compression ratios higher than would be otherwise workable to achieve similar results under the right conditions. --- End quote --- If you think modified valve timing does not produce a "true" Atkinson cycle, you aren't grasping the nature of the Atkinson cycle. The mechanics are a trivial detail. What is important is that the power stroke is longer than the compression stroke (or consider power and compression ratios, either way). The compression ratio is limited by the anti-knock properties of the fuel. So with a given compression ratio, the Atkinson cycle provides a longer power stroke, a higher power ratio, and more power from the same amount of fuel. The fact that the piston moves the same path for the compression and power strokes is not relevant. The valve timing makes the compression stroke effectively shorter than the power stroke, creating an Atkinson cycle. |
| bdunham7:
--- Quote from: gnuarm on July 28, 2022, 06:06:05 pm ---If you think modified valve timing does not produce a "true" Atkinson cycle, you aren't grasping the nature of the Atkinson cycle. --- End quote --- I fully grasp the concept of the Atkinson cycle and I don't want to debate the details with you and I don't know what you mean by 'power ratio' anyway. The actual result is simply that the exhaust temperature is lower, meaning more energy has been extracted than would be otherwise. However, none of that is my point--what I was pointing out is that many engines today use this general method of increasing efficiency but only a select few market it as an 'Atkinson cycle' or 'Miller cycle'. Therefore, IMO, making your purchasing decision based on whether the marketeers have attached this particular name to the product is not helpful. |
| gnuarm:
--- Quote from: bdunham7 on July 28, 2022, 06:41:59 pm --- --- Quote from: gnuarm on July 28, 2022, 06:06:05 pm ---If you think modified valve timing does not produce a "true" Atkinson cycle, you aren't grasping the nature of the Atkinson cycle. --- End quote --- I fully grasp the concept of the Atkinson cycle and I don't want to debate the details with you and I don't know what you mean by 'power ratio' anyway. The actual result is simply that the exhaust temperature is lower, meaning more energy has been extracted than would be otherwise. However, none of that is my point--what I was pointing out is that many engines today use this general method of increasing efficiency but only a select few market it as an 'Atkinson cycle' or 'Miller cycle'. Therefore, IMO, making your purchasing decision based on whether the marketeers have attached this particular name to the product is not helpful. --- End quote --- What you described (or actually didn't really describe) in other engines is not the Atkinson cycle. The Atkinson cycle has a very specific detail, the compression ratio is lower than the power stroke (expansion) ratio. The point of it is to allow more energy to be extracted from the expansion stroke than with a regular cycle, specifically by using different ratios on the compression stroke and the expansion stroke. Anything else is not an Atkinson cycle. Other methods of achieving more power extraction may achieve similar results, but it's not an Atkinson cycle if it doesn't have the same features. |
| nctnico:
If you look closely at the animation in this video from Toyota, you'll notice that the length of the strokes is different: https://sherbrooketoyota.ca/en/videos/atkinson-cycle-engine So it looks like Toyota is doing more than just having different valve timing. |
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