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Hybrid Vehicles Hybrid Vehicles - Archived Discussions How do Hybrids work? Newbie questions encouraged! 
139 messages, Last post on Aug 02, 2007 at 5:38 PM
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Replying to: midnightcowboy (Sep 20, 2005 4:36 am) That blanket statement doesn't apply to Prius and other HSD vehicles. They are hybrids with *PERSISTENT* electrical systems. Regenerative braking only contributes a small amount of electricity with respect to how much the generator creates... which is far from small. It is 10kW, which is the same size as the current Civic-Hybrid uses for its thrust motor. With the 2006 Civic-Hybrid, that increases to 15kW... which is still considerably smaller than the 50kW Prius currently uses. Civic-Hybrid and the other IMA vehicles are hybrids with *PASSIVE* electrical systems. With them, the regenerative braking is the primary source of electricity. JOHN
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Replying to: midnightcowboy (Sep 20, 2005 4:36 am) That is a misconception. In reality, the electricity is simply directed back to the thrust motor immediately, rather than using it to recharge the battery-pack. This is surprisingly efficient, due to the way the Planetary-CVT is designed. The reason for the misconception is that people don't realize how frequently this happens. 10 times per minute is quite common, which is fast enough to take advantage of an efficiency opportunity without allowing the penalty of charging to take place. In other words, their are inefficiencies with the gas engine that it prevents from occuring. JOHN
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Replying to: john1701a (Sep 20, 2005 4:50 am) This is one of the primary reasons the highway mileage on the Prius and other Toyota HSD deriviative hybrid systems is lower than the city mileage. EPA Prius 60/51 60=city mileage 51=highway mileage. |
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Replying to: john1701a (Sep 20, 2005 4:57 am) This has been discussed over and over. You might want to look at some of the early discussions in the Prius 2004+ thread This is one of the primary reasons the highway mileage on the Prius and other Toyota HSD deriviative hybrid systems is lower than the city mileage. EPA Prius 60/51 60=city mileage 51=highway mileage. |
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Replying to: midnightcowboy (Sep 20, 2005 5:31 am) |
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Replying to: midnightcowboy (Sep 20, 2005 5:31 am) That most certainly is not how HSD works! JOHN
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Replying to: john1701a (Sep 20, 2005 8:06 am) "What are you talking about? That most certainly is not how HSD works! JOHN" The EPA on a Prius is 60/51 The first number means the expected miles per gallon in the City, 60 miles per gallon. The second number is the highway miles per gallon, 51 miles per gallon. The City miles per gallon is 9 miles higher than the highway. All cars are more efficient on the highway, but the Prius and other HSD based vehicles are not. One wonders why the little motor generator MG1 spins and spins needlessly converting ICE to electrical at the penalty of energy conversion loss when the traction battery is fully charged! Have a good day, MidCow
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Replying to: midnightcowboy (Sep 20, 2005 10:14 am) Second, that reasoning is based on common assumptions... not how the system actually works. There are far more efficiency savings opportunities in city type driving than there are when cruising on an open highway. The HSD system takes full advantage of them by preventing the inefficiencies that would normally occur, hence the MPG observation. And where the heck did that "fully charged" claim come from? That isn't even close to correct. The Multi-Display very clearly shows 6 bars (68.57%) almost all of the time. I've personally only seen it up to 8 bars (80.00%) briefly 3 times over the past 40,600 miles. That's not full by any definition. JOHN |
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Here's a newbie question for you: How does regenerative braking actually work? I've heard again and again that it converts kinetic energy, but I don't know by what means. Where on a hybrid vehicle is the actual mechanism that converts braking energy, and how does it do so? |
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Hi midcow, on post # 88 you stated the following " One wonders why the little motor generator MG1 spins and spins needlessly converting ICE to electrical at the penalty of energy conversion loss when the traction battery is fully charged! It is first necessary to to have a clear idea of what the Toyota HSD is. It is a power split device and takes the form of a planetary gearbox except that unlike conventional planetary gears where the the outer ring gear is fastened to the gearbox casing and is therefore stationary, in the HSD this gear is allowed to rotate (upto 6000rpm  100mph) and is connected to the drive wheels via a 5:1 ratio. MG1 is connected to the central sun gear and though it may appear to be spinning needlessly to midcow and others, it is in fact splitting off some power which it must do (about which more later ) The engine is connected to the planetary carrier and of course provides the power which gets split. The three planet gears are distributed and held symmetrically around the carrier and make contact with the internal teeth of the ring gear. Clearly (I'm hoping it's clear!) if the sun gear was removed there would be nothing for these three gears to 'bite' on and the engine would be rotating easily while those three planet gears would be spun around by the internal teeth of the ring gear as they passed by. So almost no torque would be imparted to the ring gear and the car would sit still. If now the sun gear and MG1 assembly is mounted there would still be no torque if the sun gear with MG1 were allowed to start flying off in the opposite direction. At this point the engine cannot put any useful power into the system so there is no power to split and MG1 would - in midcow's words - spin needlessly. But supposing the electronics to MG1 are turned on. And the electronics says "I am going to continue to allow MG1 to be pushed backwards at 6000rpm but I will not allow it to be pushed backwards at 6001rpm." Of course with a little more engine throttle you know this is bound to happen. But the electronics are probably measuring the shaft speed about 100 times/sec with great accuracy at this time and when this tries to happen will change the electrical conditions to make MG1 start to regenerate probably as much 22kw. This will have the effect of applying a restraining force of up to 20lb-ft on the sun gear. The planet gears will pass this force on from the sun gear to the ring gear which due to the 2.6 ratio between sun and ring will exert a torque of 50lb-ft. This example pertains close to full throttle. It shows how quickly torque can be generated and it attempts to explain the power splitting process and how a small torque at the sun gear can control a much larger torque from the HSD output. It should be mentioned that the engine is effectively supplying both these torques . At lower throttle openings the same strategy applies with MG1 always extracting power from the sun gear in order for the HSD output (the ring gear) to be able send power to the wheels. So MG1 never spins needlessly. BTW where does the generated power go?, it goes to the electronic inverter which powers MG2. MG2 is made to devour all the electrical energy brought in by MG1 unless the battery happens to need charging in which case it cuts back to allow the battery to absorb the excess. MG2 happens to be conveniently connected to the ring gear where it drives the wheels by the same 5:1 reduction as previously mentioned. Why is MG2 twice as big as MG1 ? The answer is because during acceleration MG2 not only has to absorb all the power from MG1 but also absorb up to 20kw from the battery as well. But what's the point of all this ? A very good question, I'm glad I asked ! The Prius is capable of a lot more things than has been mentioned here. To get through everything would take half a dozen posts at least. And some involve speculation because not many people play with synchronous motors and inverters. And, I might add, those that do utilise industrial servos do so without needing to understand the why. Getting two servos with the same inverter bus when either can be called on to motor or generate under a wide range of speeds particularly when mechanically coupled with a planetary gear and with the addition of a prime mover in the mix is something that needs a write up from the designers themselves. And this is what we are not getting. The real point of a spinning MG1 is to form a CVT function. The so called Continuously Variable gear ratio Transmission. Toyota chooses to use an electro-mechanical implementation while Honda have chosen to use a wholly mechanical implementation as found in the 2006 HCH. I have to say that now that Honda have dispensed with the manual transmission option on the Civic hybrid it is to be hoped they can concentrate their resources to really nail this device and improve its reliability. Something that industrial CVTs were lacking when they were discarded years ago. The reason for the CVT was originally for it to be an alternative to automatic three speed transmissions which were no more efficient than now particularly on subcompact cars. Today the reason is to allow a much smaller engine perform as if it were a larger engine when needed but to have that fuel consumption of the smaller engine most of the time. When using a manual transmission, the power profile of a constant torque engine is triangular, power increases with rpm. The area under the curve represents the energy delivered over time. The power profile of the output after an engine goes through a CVT, however is closely rectangular - at low revs the torque is much higher than in the previous case dropping slowly until at high revs the torque will become the same. The area under the CVT's rectangular profile will be almost double that area found under the manual transmission therefore the CVT will have delivered twice the energy in the same time. This can be played two ways by doubling the road performance of an existing engine or accepting the same performance from a weaker engine. The Toyota Prius and HCH obviously go for the latter and the fuel economy it brings. I would say that CVT's can provide fuel economy approaching double that of the manual transmission car and then that's it folks ! There will have to be another shift in thinking. T2
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