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| Confused about PHEV, Hybrids, etc... |
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| bdunham7:
--- Quote from: nctnico on July 28, 2022, 07:29:11 pm ---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. --- End quote --- Length of what strokes compared to what? Compression vs power on the same engine, like the original Atkinson cycle engine? That would require an insanely complex mechanism that I can assure you does not exist on any current common product. Now in general, these types of engines often do use a longer overall piston stroke than similar size/power 'normal' engines, but the movement of the piston is completely set by a rigid connecting rod and crankshaft. |
| edy:
What I saw in that Toyota video is that the cylinder heads move exactly in the same fashion and amplitude in all 4 phases of action (fuel/air intake, compression stroke, power/explosion/expansion, exhaust stroke). However, what I see happening is that in the early part of the compression stroke the intake valves remain open so that the cylinder is not being pressurized from the start... instead as the cylinder head moves up, because the valves are still open, it lets some of the air/fuel mixture push back out into the intake chamber area. Finally when the intake valves do close, the cylinder starts to pressurize, compressing but against a smaller volume of fuel/air. This reduces the effective "size" of the cylinder, as if a smaller cylinder is being compressed. I assume that part of the fuel savings is simply because it burns less volume on each stroke, you are taking say a 2.8 L engine and effectively only using it as a 2.4 or 2.2 L engine (for example). Perhaps the efficiency is also improved as there is more complete combustion of the contents of the cylinder since there is less fuel trying to completely burn up in the time that the power/expansion stroke occurs. Somewhere there must be a trade-off in power/torque and at what RPM, but perhaps a computer can figure out based on the speed of the car and what is required of it this is a worthwhile trade-off to milk a bit more efficiency out of the engine with nuanced valve timing. |
| tom66:
You have to account for the fact that the hybrid system improves the performance allowing for a less than powerful engine tune at the expense of lower performance which is supplemented by the hybrid system. The hybrid motor adds about 40 hp to the engine output under peak acceleration in the Prius; it adds 100 hp to my Golf GTE. Most people don't need the full 100 hp from their vehicle for long periods of time (exceptions apply, yadda yadda, but this is generally true for most driving profiles) and this allows the use of smaller engines and/or less power-optimised tuning for these users. |
| gnuarm:
--- Quote from: nctnico on July 28, 2022, 07:29:11 pm ---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. --- End quote --- Sorry, I don't see that and they don't talk about it. |
| gnuarm:
--- Quote from: edy on July 29, 2022, 05:11:50 pm ---What I saw in that Toyota video is that the cylinder heads move exactly in the same fashion and amplitude in all 4 phases of action (fuel/air intake, compression stroke, power/explosion/expansion, exhaust stroke). However, what I see happening is that in the early part of the compression stroke the intake valves remain open so that the cylinder is not being pressurized from the start... instead as the cylinder head moves up, because the valves are still open, it lets some of the air/fuel mixture push back out into the intake chamber area. Finally when the intake valves do close, the cylinder starts to pressurize, compressing but against a smaller volume of fuel/air. This reduces the effective "size" of the cylinder, as if a smaller cylinder is being compressed. --- End quote --- Exactly, that's why this is not an accurate representation of where the advantage is. In reality, they would design a slightly longer stroke for the Atkinson cycle engine, which would provide a higher compression ratio, if it weren't for the different valve timing. So now the Atkinson cycle engine has the same compression ratio, but a higher expansion ratio, all with the same stroke length, but not the same stroke length as the corresponding Otto cycle engine. --- Quote ---I assume that part of the fuel savings is simply because it burns less volume on each stroke, you are taking say a 2.8 L engine and effectively only using it as a 2.4 or 2.2 L engine (for example). Perhaps the efficiency is also improved as there is more complete combustion of the contents of the cylinder since there is less fuel trying to completely burn up in the time that the power/expansion stroke occurs. Somewhere there must be a trade-off in power/torque and at what RPM, but perhaps a computer can figure out based on the speed of the car and what is required of it this is a worthwhile trade-off to milk a bit more efficiency out of the engine with nuanced valve timing. --- End quote --- You are comparing two engines based on the power stroke. Compare them based on equivalent (effective) compression strokes and compression ratio, and you get a different picture. Now they can have the same compression ratio, but the Atkinson with a longer expansion stroke to make it more efficient. |
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