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The one thing I've always wondered is why sometimes the Prius will convert energy from the gas engine to the electric engine and then back to the wheels. How does that help FE? I know you lose energy in the conversion, so why do it?

Hi Alex:

___I will take a stab at it.

___Think of EV as around 80 + % efficient and the ICE at or around 30 + % when in a range where charging usually occurs for an overall efficiency of maybe 25%. Regen conversions are even less efficient so I will leave that out of the discussion given I am speaking beyond some of my knowledge here anyway … Now place yourself into a situation where you are accelerating from a stop light or sign or stuck in a stop and crawl. The ICE/Automobile as a system - real world efficiency is easily less then 10%. Moving a few feet and stopping with the ICE running for the short distance spurt is the worst while EV mode can easily make up for the charging hit during that scenario as one example.

___The big advantage of the electric drivetrain is that the Prius II uses the Echo/Yaris sized motor tuned for even higher fuel efficiency while propelling a much larger and far more massive automobile in a manner most find acceptable.

___Good Luck

___Wayne

Are we talking about actually charging the battery or converting mechanical engergy to electrical and then directly back to mechanical on the other side of the gearbox? Those are two different things but the Prius does both. As Wayne said, the Prius uses the battery as sort of an "energy sink" while energy demand is too high for EV but too low for efficient engine operation. Charging the battery adds load to the engine, making more of the gasoline into usable energy. You can use it later via stealth or battery assist.

Sometimes I notice the electric motor powering the wheels along with the engine without involving the battery. In this case the system is using the motor-generators to multiply torque. They can provide more low-end grunt at a given speed than the ICE can do efficiently all by itself.

You people just skipped right over my question. I'm not asking why the Prius will use the battery as well as the gas engine, I know why it does that.

I'm asking why it'll go from the GAS engine to the ELECTRIC engine, then back to the WHEELS? I'm not talking about the battery at all.

Relax, and read my post again.

On second thought, I'll try to go into more technical detail.

The Prius achieves very high ICE efficiency by running at ~80% load most of the time. This is achieved by cracking open the throttle to a fairly wide opening and changing the gear ratio so that the ICE runs at whatever speed happens to provide the amount of power that a driver wants. Whereas a mechanical CVT would do this by shifting the bands to the right part of the conical sheaves, the Prius eCVT does this by sending power across the planetary gearbox between MG1 and MG2. I forget which way around it is, but the MG on the engine side makes power which is sent to the MG on the opposite side of the planetary gear box, the speed of which controls the gear ratio for a given ground speed. Sometimes the first MG makes more power than the second MG needs (under low sytem load) so the excess power issent to the battery to be used later. This, I understand, is the question you don't want me to answer.

Other times, at high system load, the car need all of that power to go to the wheels so it stops sending it to the battery pack. That's what's going on when you see electricity going from the engine to the motor to the wheels. This works well because, as I said, those motors generate very high torque relative to the ICE running by itself.

Edit Opps... looks like Tim was coding a reply at the same time as me... double post.

Agreed, I think brick covered it. One point slightly off topic but to add to the understanding is knowing how the Prius transmission works (which you may or may not know). There are some good articles about the Prius Planetary gear and the Power Split Device (PSD). These govern how some of the power out of the engine gets split, when, and why.

There is another thread here (http://www.greenhybrid.com/discuss/showthread.php?t=10928) (although the site seems to be moving away from FE discussions) that speaks to it in some detail (100+ posts) you might review.

Sorry but I can't recall the writeup I read on the PSD and Planetary gear, but I came out of it understanding a bit more.

Unfortunately I don't know what is specific to Prius II vs Prius Classic, but I'll leave that up to you.

11011011

Right, maybe that's exactly the point. It all comes down to how the eCVT works. The reality is that it must convert some energy to electrical in order to keep the planetary gear set balanced.

Hi Alexstarfire:

___Tim did describe what Toyota and others say is going on. As for the efficiency gains with it, that one is debatable given this is an area where the Prius I/II can perform the mid 70 mpg cruise and the low 60 mpg cruise with a minimum of change in terms of speed, ICE load or terrain. At or around 55 - 60 mph that is ...

___Good Luck

___Wayne

Right, maybe that's exactly the point. It all comes down to how the eCVT works. The reality is that it must convert some energy to electrical in order to keep the planetary gear set balanced.

That is my take on it. IIRC, MG2 is on the engine side. For certain vehicle speeds and torque requirements, MG2 actually has to run backwards to make the engine speed match to the wheel speed. It is probably this mode that he is taking about, it takes energy to run the motor and that comes from the engine.
Gary

Yea, I seemed to have missed what Brick said the first time. It's like a skipped over a key word or something. Thanks for the clarification. I didn't know that the electric motor provided that much torque. I mean it is electric after all.

Take a look at
_ http://techno-fandom.org/~hobbit/cars/serpar.gif
_ http://techno-fandom.org/~hobbit/cars/Nomograph-plus-PSD.gif
In the former, the paths of power are illustrated in about
the simplest way possible, and the two sides of the blue
and orange "square" always work in parallel to bring power
to [or from] the wheels. Think about it: the big motor is
good for 50 kW, and the gas engine is good for 57 kW. They
are about evenly matched. The smaller electric M/G can
handle 10 to 15 kW, when needed. Note that the battery is
just another wart hung off the side of this and does not
need to supply or receive energy for the driveline to work.
.
The blue "punkin" in the middle represents the planetary
system, which really is the same as a differential but with
an asymmetric ratio. Principle is the same. Put kitty
litter under the ice-bound spinning wheel, and the other
wheel gets more torque. Same with this -- load down MG1
in some way, and the shaft going to and through MG2 gets
more torque from the engine.
.
The electrical flow can be in either direction, depending
on conditions. Sometimes MG1 powers MG2, and sometimes
[in the "overdrive" or "heretical" situation] it's the
inverse. The reason MG1 can be smaller is that as it's
connected to the sun gear, it's whipping around way out at
the left side of the nomograph "stick". Much wider RPM
changes, but it doesn't have to produce or soak up nearly
as much torque because it's got a 3:1 leverage advantage
over the other parts.
.
Regardless that power has to take a partial electrical hop,
the system is still VERY efficient. The torque split from
the engine is 72% / 28% with most of it going straight to
the final drive and wheels, so what's left over that needs
to go the electrical route is less than a third of the
engine's output power anyways. The pair of motors and
inverter handles this easily, losing a miniscule amount of
energy in the process. Possibly on par with what you'd
give up in a step gearbox with more rotating parts, in fact.
.
It's horrendously complex and beautifully simple all at the
same time.
.
_H*

Hi Hobbit:

___Thanks for the explanation! I will have to read your links all over again …

___Good Luck

___Wayne