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Old 01-25-2009, 08:51 AM   #1
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Prius Engine Break

If you leave it on engine break you can effect neutral at any speed, but I guess the engine is running when you're in neutral? Anyone know how the engine break works in the Prius? In a normal car, you use engine breaking by having the gear at high rpm to slow the car down. This uses no fuel in many manuals (DFCO) (not sure about automatics). But the engine break in the prius has the engine on. Why? Does it charge the battery when you use the prius' engine break? If anyones about Prius' engine breaking technology, I'd be intererested to know! Thanks.
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Old 01-26-2009, 06:27 AM   #2
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When you let your foot off the gas slightly, you go into a coast mode. There is no engine braking. This is a lot like neutral, the engine is off (unless it thinks it needs to be on for heat or other), and you coast easily.

When you let your foot off the gas totally, you enter into a regenerative mode where the electric motors become generators and send juice to the traction battery. This acts like an engine brake.

We are not advanced hypermilers and have not yet experimented with the use of neutral. Except for rolling through a carwash, I'm not sure it has many official (and desireable) uses. But, I'll look that up someday.
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Old 01-26-2009, 06:39 AM   #3
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IIRC, in B mode, the pruis fully opens the valves so that the engine starts pumping air, which helps to slow the entire system.
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Old 01-26-2009, 09:06 AM   #4
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Yeah, B mode leaves the engine running all the time. Meant for coasting down hills and such.

From what all I've read, in that mode only 30% of the energy that is normally recovered during coasting is recovered.
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Old 01-26-2009, 12:57 PM   #5
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I've driven my mom's and read some descriptions from Wayne Gerdes

A Prius will EOC if the pedal is in the correct position, no accel or decel, provided that it is going less than 41 MPH. Above that speed the engine stays engaged and idles. Pull your foot up a little more and it starts charging and goes into DFCO
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Old 01-26-2009, 07:44 PM   #6
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I think the older prius would keep the engine running at any speed over 39mph but 2007 and on you can go 60mph and have the engine off and coast or run on electric drive. It sure feels weird with you find the neutral spot how the car just rolls silently along without any shifting or turning off of ignition. When in engine BRAKE mode the engine slows the car but providing a load - never tried it but I imagine if the traction battery was full then the engine could still provide some braking action but I am not sure at what speed the electric clutch motor drives the engine rpm.
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Old 01-27-2009, 07:29 AM   #7
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Just because the engine is spinning on a Prius, it doesn't mean that it is burning gas. The motors and engine are connected. So, to prevent damage to the motors, the engine is spun up at higher rpms/speeds. If input from the engine is not needed, the valves are closed and fuel is cut.
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Old 01-27-2009, 04:26 PM   #8
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The clutch/generator/Starter? between the engine and the differential would slip to regulate how fast the engine is driven when engine braking and acts like a variable transmission/torque converter. I can't remember if the electric motor does the regen braking too but I guess it has to or else the clutch would tend to spin the engine when slowing down.
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Old 01-28-2009, 01:34 PM   #9
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No clutch on a Prius.
Here's an article on the B mode.
http://techno-fandom.org/~hobbit/cars/b-mode.html

And an overview of the entire Prius hybrid system.
http://prius.ecrostech.com/original/PriusFrames.htm
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Old 01-28-2009, 07:06 PM   #10
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YEAH there is a clutch . . . it's an electric clutch / generator that couples the engine torque to the differential . . . not a friction clutch but still a clutch unless you want to call it a torque converter? But a torque converter actually varies the torque between input and output.
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