Snakeoil
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| reply to justin
Re: Hybrids
If I understand your question:
Why do hybrids get lower MPG on wide open highways vs city/rush hour traffic?
I remember hearing/reading that is they way they are designed. They are setup to give their best preformance in traffic vs wide open highway. Meaning all the stopping helps generate juice to charge the batteries, so the gas engine is used very little in city driving. But when on the open highway, you place more of a demand on the batteries to maintain curising speed, which means the gas engine has to run more often, to recharge the batteries. By the gas engine running more often, and for longer periods of time, you burn more fuel.
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3SGTE
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| reply to justin
Every gasoline engine has an operation range (RPM/load) where it generates the most output for the least amount of fuel burned.
In conventional vehicles, the process of accelerating is extremely inefficient, the engine operates far outside its most efficient range.
In a Prius, there are 3-4 factors that work in one's favor, especially in the city.
1. No idling at a stop.
2. CVT operation
CVT operation allows the engine speed to remain in the efficient range during acceleration. If operator inputs are reasonable, battery energy is used as a supplement during acceleration, again to help the engine stay in its efficient range. If the battery needs to be recharged, again, charging can occur with the RPM/load in the most efficient range.
3. Atkinson cycle.
The engine controls intake valve timing to deliberately underfill the cylinders with air fuel mixture, this accomplishes two things. Reduced pumping losses, and more of the expansion energy of the burning air fuel mixture can be captured, as opposed to wasted pressure going out the exhaust valves. (A greater percentage of energy can be extracted from the gasoline.) Electric supplementation and decoupled engine speed allows this, in a conventional vehicle the engine response/output would be inappropriate.
4. Regenerative braking.
This one is a simple one. Energy otherwise wasted is recaptured.
As for conversion efficiencies, they have been considered. Part of the powerflow from the engine to the wheels in a Prius is mechanical, part is electric. This is how the CVT ratio is determined. The balance between peak engine efficiency and power conversion efficiency is always considered as the system decides the optimum balance of mechanical/electric drive.
Likewise/as a result, the system does not use the batteries strictly to prevent engine operation. Overall efficiency is prioritized.
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iLive4Apple
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| reply to iLive4Apple
Actualy the Prius has no CVT or transmission at all.
»www.cleangreencar.co.nz/page/pri···cal-info
»www.cleangreencar.co.nz/page/pri···smission
And this page better explains how the Prius works |
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3SGTE
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1 edit | said by iLive4Apple  :Actualy the Prius has no CVT or transmission at all. The Prius has an Engine.
The output from the engine needs to be transmitted to the wheels.
There is a mechanism that does so, therefore it is a transmission. The transmission in the Prius does not operate at a fixed ratio. The transmission ratio is effectively Continuously Variable. Most certainly the Prius transmission does not use a steel belt, but I certainly did not say that, and that does not "define" a CVT.
To refer to something as a CVT does not need to imply that it has a specific mechanical construction. The term (IMHO) is concept based.
Is a "rotary" any less an engine because it has no pistons? NO. Mechanical construction/configuration does not dictate the definition of engine, any less than it does for transmission or CVT.
What I wrote was not a complete explanation of how a Prius or its transmission operates. I wrote a response to Justin's questions about efficiency and tradeoffs. There are innumerable additions and greater detail that could have been added, but a line needed to be drawn somewhere.
Playing word games is disingenuous. It doesn't help to convey the points that I was trying to make.
Edit: Didn't mean to come across as harsh, the links are good, but kind of overkill for this thread.
Added "effectively", I originally didn't feel it necessary in the context of this discussion. |
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iLive4Apple
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| Sorry I wasn't pointing my comment to you, I worded it badly also. I was trying to put what was on the site about to Prius and hit the post button to early. Sorry if it read rude. I don't understand how my Prius can even leave the driveway because it supposedly does have one gear ratio acording to that site. I am not sure how it would though. That seems kinda strange.
"The Prius Transmission
The Prius transmission produces one of the effects of a CVT but not the other. The spin rate of the engine can be selected to produce the required power but otherwise to spin no faster than is necessary to maintain fuel efficiency. The Prius engine tone therefore sounds as if the car has a CVT because it does not rise as the car picks up speed. Instead, it rises and falls with power demand, in other words, how hard you press on the accelerator pedal. The Prius transmission does not, however, multiply up engine torque at low vehicle speed. This is because it has only one gear ratio. Effectively, the engine is coupled to the wheels as if the car is always in top gear. This would be a crippling limitation, if not for the presence of a powerful electric motor in addition to the petrol engine. With this motor adding its considerable torque, people have said that the car feels as if it's always in 1st gear!"
»www.cleangreencar.co.nz/page/pri···smission
Also what is different about the Prius is that it has an Atkinson engine instead of an Otto Cycle to prevent pumping loss. What is pumping loss? |
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3SGTE
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| said by iLive4Apple :What is pumping loss? Basically, the energy required by the engine to pump air.
The air in the crankcase that creates resistance to downward movement of the pistons, and the energy required to "suck" the air into the intake manifold and compress it in the cylinders.
Unnecessary excess compression of the intake charge is a pumping loss that is avoided in the Atkinson cycle.
(Atkinson effective compression ratio is different from the expansion ratio.) The smaller C/R vs E/R is the main point of the reduction of pumping loss.
On some of its vehicles, I believe that BMW uses its Valvetronic system mostly without throttle control, pumping loss reduction is among the main benefits. »en.wikipedia.org/wiki/Valvetronic
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Jerry33
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| reply to justin
said by justin :I don't buy that explanation. There is no magic energy for free here (regenerative braking aside). If you burn X amount of gas to move the car using petrol only, you are DEFINITELY going to burn more gas to charge batteries to turn electric motors to move the car at the same speed. Yes, but... The Prius' engine is sized for steady state running rather than acceleration like a conventional car. Therefore, it almost always runs in the 36% to 40% efficiency range rather than the 20% efficiency range that a normal car has. At very low power demand the Prius runs on electric only because the hit you take from energy conversion is less than the poor engine efficiency those kind of power demands produce. The motors also provide the extra acceleration required so that you don't notice that you are in a car with a very small engine compared to the car size.
Every bit of energy in the Prius comes from the gas tank, so things like regenerative braking just let you throw away less energy as heat. Compared to a conventional car, it appears like the Prius is getting something for nothing--but that's not the case. It's just using the difference between 20% efficiency and 36% efficiency. As a driver you can help the Prius get more mpg by using techniques that minimize electricity and engine use. |
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iLive4Apple
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Helena, AL | Now you can recharge all your battery with regen braking which is used especialy when in all electric mode. |
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Jerry33
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| said by iLive4Apple :Now you can recharge all your battery with regen braking which is used especialy when in all electric mode. Regen braking is not used more (or less) in EV mode than when in combination mode or gas only mode. It only depends on how hard you press the brake pedal and if any of the safety features (ABS, TC, etc.) are currently engaged. Because the Prius has two motors, one is always acting as a generator (though they switch personas depending on the circumstances). In any case, even if you charge the battery with regen, the power still came from the gas engine. |
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iLive4Apple
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Helena, AL | But when you are coasting or braking the gas engine is completely turned off. |
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justin
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| he means the power came from the gas engine because how do you think you got to sufficient speed in order to regen brake in the first place? the gas engine - or the batteries, which were charged by the gas engine.
Everything still comes back to the internal combustion engine. The prius just gets good mpg because it is
* aerodynamic
* fuel efficient (small motor, only used when necessary)
* re-uses some momentum (braking creates some power)
* tricky transmission (keeps motor near optimum efficiency rpm)
In short, the car is just not wasteful. |
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dandeman
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3 edits | reply to justin
Correct. The Prius in effect throws away less energy that came from petrol as does a conventional vehicle.... There's significant savings that can be had in a drive train that levels out the energy demand.
Is there something fundamentally wrong with that concept in your view?
How do conventional cars throw away energy?
1. every time you touch the brakes.. even through the Prius has conversion losses in regen braking, it is not 100% loss as in a conventional car.
2. automatic tranmsission torque convertor heat (energy) loss. This is significant.. the Prius dual motor generator, single planetary gearset has higher efficiency.
3. everytime you go up a hill, you give the car more gas to maintain speed.. you do the same thing with the accelerator in a Pruis, but part of the additional energy comes from stored energy the down hill section you most likely traversed.. Again not perfect energy conversion, but energy lost than in a conventional vehicle, which has no energy leveling capability on rolling hills.
4. And of course I could go on and on with idling, car not in motion modes, in which there's zero mpg efficiency in a conventional vehicle.. |
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iLive4Apple
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| reply to justin
Yes, the little 1.5 liter engine uses the Atkinson cycle engine which prevent's pumping loss, where as regular gas engines use the Otto cycle which has pumping loss and waste gas. And the transmission is efficient somehow. Of course when I am getting on the interstate it rev's up very high. But I make it on the interstate. It drives just like a normal car except you can't feel it shifting gears. |
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justin
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| reply to dandeman
said by dandeman :Is there something fundamentally wrong with that concept in your view? No. not at all. |
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3SGTE
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| reply to iLive4Apple
said by iLive4Apple :Yes, the little 1.5 liter engine uses the Atkinson cycle engine which prevent's pumping loss, Not "prevents", reduces.
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Dennis
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| reply to iLive4Apple
Looks like i'm late to the Prius thread here  , but I drive a 2001 Honda Insight. Lifetime avg 54.7mpg w/ 47k miles on it.
cute car, good for commuting, but only have 2 seats. Next car will probably be a Prius if I go hybrid again. I'm a technology whore. |
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iLive4Apple
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Re: Hybrids
I was going to get an insight but I found out it only has 2 seats and my kids can't ride in that. Can yours go in just in all electric motor. Mine can only go up to 45 MPH without the gas engine on. |
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MPScan
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Boston, MA
| reply to iLive4Apple
While I am all for Hybrid's and the revolutionary technology they are using today ..
... It's going to be a blast when, in the next year or two, the 1st gen hybrid's need battery replacements which aren't going to be cheap.
How much will that cost compared to the savings gained in the additional MPG efficiency? |
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justin
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| said by MPScan :It's going to be a blast when Why is it going "to be a blast"? You are THAT emotional about someone elses choice of transport that you'd be cheering any troubles they have? |
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