Prius breaks 60 km/h EV speed barrier

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coulomb
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Prius breaks 60 km/h EV speed barrier

Post by coulomb »

I heard a week or two ago that someone had figured out a way to get a Prius with an extended battery pack to stay safely in EV mode over the standard ~35 MPH (60 km/h) barrier.

Here is video confirmation that it can be done:

http://evworld.com/EVWORLD_TV.CFM?storyid=1719

Apparently they use "stealth mode", where the engine is still turning, but is not fed fuel. I'm a bit uncomfortable with this, as it means that the cylinder walls and piston rings are wearing away needlessly. (Edit: I assume that they have control of the valves so you don't get compression deceleration when driving.) The engine can come on at any time, but it could do that from zero RPM anyway. In the demo, they had to accelerate slowly or the petrol engine would come on. When set up correctly, the engine comes off as soon as it is not needed. (In the videod demo, this part possibly wasn't set up yet).

For those that need a range extended EV, this could be a really interesting option, which can be had right now at a reasonable price.

I'm still trying to find the article I was reading some months ago, where the speed relationships of the two motors and engine were explained with graphs. The "overspeed" problem that usually limits EV mode to 60 km/h must be to do with MG1; it's the one that can start the IC engine (by spinning backwards? Not sure.) MG2 is essentially connected to the diff input. I think it must be "designed" to spin at higher speeds than the other one. So when a Prius is converted to pure electric, I think they just use MG2 and ignore MG1. That seems a terrible waste.

Edit: metrificated. Now if only I could change the topic title...
Edit2: Doofus, you CAN change the topic title Image
Last edited by coulomb on Wed, 08 Jul 2009, 07:24, edited 1 time in total.
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Prius breaks 60 km/h EV speed barrier

Post by Electrocycle »

that sounds very inefficient to be turning the engine the whole time.

MG1 will always have to turn in the same direction - either working as a motor (for starting the engine and stealth mode) or working as an alternator when the engine is running.

I don't think there's any valve control on the engine (as far as decompressing the engine goes) but since the air is working as a spring it shouldn't add greatly to the losses (air is compressed when the piston rises, then helps push the piston back down - minus some air loss past the rings)
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Prius breaks 60 km/h EV speed barrier

Post by woody »

I think they do have valve control - the motor is atkinson cycle (high compression, but late valve closing on the compression stroke).
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Prius breaks 60 km/h EV speed barrier

Post by Electrocycle »

it's a miller cycle without the supercharger :)

Reduces the wasted energy, but also power. They have variable valve timing, but they can't shut off the valves, so the engine will still be pumping air through when it's being spun by the motor.
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Prius breaks 60 km/h EV speed barrier

Post by coulomb »

Electrocycle wrote: that sounds very inefficient to be turning the engine the whole time.

MG1 will always have to turn in the same direction - either working as a motor (for starting the engine and stealth mode) or working as an alternator when the engine is running.)

Err, no. It can motor or generate in forward or reverse, a true 4-quadrant machine. I found the link I was looking for:

http://www.ecrostech.com/prius/original ... Device.htm

Speed equation:
MG1 = 3.6 * ICE - 2.6 * MG2.

From the above article, converted to km/h:

Image

I've added an orange line for MG1=-6000 rpm and ICE=0. It intersects with the MG2 line at 62 km/h.

So when driving forward (MG2 is at a fixed speed compared to the wheels, so MG2 > 0) and the ICE is stopped (ICE in the above equation, MG1's speed will be negative.

At other times, MG1 can be running forwards, e.g. road speed is 110 km/h, accelerating hard, ICE will be 4500 RPM, MG1 will be 5700 RPM, either motoring or generating depending on the state of the battery.

So here's a dilemma: an article linked from the above (http://groups.yahoo.com/group/toyota-pr ... ograph.pdf; registration to Yahoo Toyota Prius group required, free and painless if you have a Yahoo password), states that "stealth mode" implies that the ICE speed is zero. But with ICE=0 and with the maximum speed of MG1 at 6000 RPM, we get the infamous 62 km/h (it seems to vary a little bit depending in who you talk to) EV speed limit.

So either the linked PDF is wrong and stealth mode doesn't imply ICE=0, or they are using something other than stealth mode to achieve EV speeds of > 62 km/h. I think it's the former.
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Prius breaks 60 km/h EV speed barrier

Post by Electrocycle »

ah ok, I was assuming that MG1 is attached to the ICE (working as a starter and / or assist motor or generator) and MG2 is attached to the output, with a power sharing gear setup in between.

So, the way it works is with power sharing between each stage - which is why they need two motors.
By feeding power between them the whole system works as a CVT without the variable mechanical linkage between input and output.
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Prius breaks 60 km/h EV speed barrier

Post by acmotor »

Here is a nice animation.....

prius split
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Prius breaks 60 km/h EV speed barrier

Post by Electrocycle »

it's an interesting way to do it, and solves the usual problem of electric motors not having a wide enough usable rpm range without having to use a very large motor (or gearbox, or motor that can do very high rpm with a large reduction)
Last edited by Electrocycle on Sun, 05 Jul 2009, 13:44, edited 1 time in total.
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