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Question about a Prius (or an electric car)
Old 10-12-2011, 02:47 PM   #1
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Question about a Prius (or an electric car)

OK... today when driving to lunch I was sitting at a red light behind a Prius... now, my car is a manual and usually I just sit there without my foot on the brake.. but I noticed that the driver of the Prius had his foot on the brake... why?

IMO, and from the little I know, the engine does not even run while stopped.. so why would the car move at all if the person is not pushing on the 'go' pedal.... IOW, why would the car move at all if the driver does not want it to move... there should be no electricity going to the electric motor (that would seem to be a waste if it is)....

I have seen this before and have thought the same thing... why are they holding down the brake Does the car want to move if you do not hold down the brake If so, then why


Since I am on the subject of this car.... how is the AC unit Does the engine have to be on for it to work What about an all electric car? How much energy is expended on the AC unit.... and how does that affect your range of driving
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Old 10-12-2011, 03:01 PM   #2
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I had these same questions about the vehicle Fred Flintstone drove ...
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Old 10-12-2011, 03:09 PM   #3
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Perhaps the Prius owner is worried that you could rear-end him and push him into traffic where he gets hit by a Hummer. Hence the foot on the brake.

Myself I wonder about and worry that the refrigerator light is still on whenever the door is closed.
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Old 10-12-2011, 03:45 PM   #4
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I'm with Master Blaster. Even though my mainland car was a manual, I typically took the car out of gear when stopping for a red light, but still kept my foot on the brake to lessen the chance of being pushed into an intersection by an errant driver from behind.

I too have wondered about the auxiliary equipment on a hybrid. How does the heater work if the engine hasn't warmed up? What runs the AC? What keeps the power brakes and steering active. Apparently, these are not significant issues as hybrids have taken over Oahu, but I'm still curious.

Since battery capacity is such a critical factor for hybrids (and all electrics) it would seem every effort would be made to minimize auxiliary electricity usage (e.g., probably use LEDs for every light source, maximum (legal) window tinting to limit AC usage, low-bypass recirculation of conditioned air, limited wattage of any standard sound system, etc. I once heard that standard headlamps (this was circa 1980) could cost 1 mpg.

I suppose I could Google for answers, but have wanted to ask a hybrid owner in person. I'm sure several on the forum own hybrids. My guess is many have become intimately familiar with the workings of their vehicles.
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Old 10-12-2011, 04:06 PM   #5
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I don't have a Prius but I do have a hybrid (I won't say which for fear of being attacked as not sufficiently frugal). I hold my foot on the brake at red lights because I always have held my foot on the brake at red lights. I have no idea what would happen if I took it off -- if I remember to try it next time I drive the car drive I will report back. As for AC, I have sat waiting for DW for long periods and the A/C works. The internal combustion engine will come on periodically as needed.
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Old 10-12-2011, 04:30 PM   #6
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My wife always takes our manual shift car into neutral and rides the brake at stop lights. She says it hurts her foot to keep it extended pressing the clutch down, since her legs are short. And if you're not using your right foot for the clutch, it seems natural to use it for the brake, to keep the car from drifting. But this has nothing to do with electric cars, so it's probably not relevant.
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Old 10-12-2011, 04:41 PM   #7
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Quote:
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My wife always takes our manual shift car into neutral and rides the brake at stop lights. She says it hurts her foot to keep it extended pressing the clutch down, since her legs are short. And if you're not using your right foot for the clutch, it seems natural to use it for the brake, to keep the car from drifting. But this has nothing to do with electric cars, so it's probably not relevant.
Her technique limits throw-out bearing wear. Always did it this way and never replaced a throw-out bearing (or a clutch). But, YMMV.
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Old 10-12-2011, 04:45 PM   #8
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My Escape hybrid will creep if you take your foot off the brake. It was designed that way to make it feel like a regular automatic transmission car.
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Old 10-12-2011, 04:52 PM   #9
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Originally Posted by Koolau View Post
Her technique limits throw-out bearing wear. Always did it this way and never replaced a throw-out bearing (or a clutch). But, YMMV.
Our just put the car in neutral and not touch the clutch or the brake... as long as not on an incline you just sit...
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Old 10-12-2011, 04:54 PM   #10
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My Escape hybrid will creep if you take your foot off the brake. It was designed that way to make it feel like a regular automatic transmission car.

That seems strange as I would think that they have to have electricity going to the motor all the time to be able to creep... maybe it is so little it is a 'who cares' except for an electric only car...
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Old 10-12-2011, 04:55 PM   #11
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The A/C compressor is electric. There is no "neutral" in the sense of a normal transmission. The wheels, electric motor/generator and gas motor I think are all mechanically linked to the planetary gear system, and the computer manipulates the planetary gear system depending on which force is going to drive another. (Regenerative braking, electric motor drives wheels, gas motor charges batteries by driving the electric motor, gas motor powers wheels.)

You stay on the brake or put the little joystick in "Park" to let the computer know you want to stay in place. The computer mimics the behavior of an automatic transmission. I'm not sure, but I think they put artificial gear-shift bumps in there and possibly affect how the car accelerates to make it "feel" like driving a normal car.

There are a bunch of batteries, and if they run low enough the engine will start to charge the batteries. This can happen whether the car is moving, sitting at a stoplight or in park. There is no "normal" starter in the car; the electric motor spins the gas motor on nearly instantly.

Edit: This post specifically refers to a Prius Hybrid Synergy drive system. Other hybrid drive systems may be completely different.
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Old 10-12-2011, 05:09 PM   #12
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For those of you interested in Prius details - technical and otherwise - check out PriusChat. Also has info on hybrids in general.

I have a 2010 Prius IV and absolutely love it - never thought I would say that about a mode of transportation.
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Old 10-12-2011, 05:15 PM   #13
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Here is an interactive model of the planetary gear system. EDIT: I mean the graphic if you scroll down on the page. Hover over the components and it will tell you what the gear is mechanically linked to.



This page has some photos of the system and a whole bunch of info I haven't even tried to skim.
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Old 10-12-2011, 05:28 PM   #14
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Originally Posted by Texas Proud View Post
That seems strange as I would think that they have to have electricity going to the motor all the time to be able to creep... maybe it is so little it is a 'who cares' except for an electric only car...
It is sophisticated enough that it doesn't put electric power to the motor if the brake is on.
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Old 10-12-2011, 05:35 PM   #15
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Quote:
Originally Posted by Texas Proud View Post
..........
Since I am on the subject of this car.... how is the AC unit Does the engine have to be on for it to work What about an all electric car? How much energy is expended on the AC unit.... and how does that affect your range of driving
On the original Escape hybrid, the engine had to be running to power the AC compressor. On the newer ones and the Fusion hybrid, there is an electric AC compressor. All-electric cars have electric AC compressors and resistance heaters. They take a big bite out of driving range.
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Old 10-12-2011, 09:16 PM   #16
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Quote:
Originally Posted by donheff View Post
I don't have a Prius but I do have a hybrid (I won't say which for fear of being attacked as not sufficiently frugal). I hold my foot on the brake at red lights because I always have held my foot on the brake at red lights. I have no idea what would happen if I took it off -- if I remember to try it next time I drive the car drive I will report back. As for AC, I have sat waiting for DW for long periods and the A/C works. The internal combustion engine will come on periodically as needed.
I have a Prius, and I also keep my foot on the brake at red lights because that's what I've always done. You have to step on the brake to bring the moving car to a halt, and I doubt that I'd ever form a new habit of taking my foot off the brake once the car stopped, unless there was some specific reason to do so. Now that I'm thinking about it, I wonder—if you let up on the brake pedal of a hybrid once the car has come to a stop, do the brake lights stay lit or not? If I remember to try it next time I go for a drive, I too will report back.
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Old 10-12-2011, 09:58 PM   #17
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So the answer is psychology rather than physics. Interesting!
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Old 10-12-2011, 10:14 PM   #18
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I don't know anything about hybrids, but I do know that if you're old and absent-minded like me, it's possible for your vehicle to creep foward or backward when it's supposed to be stopped and you not be aware of it. So, IMHO, it's a good idea to have the habit of keeping your foot on the brake when stopped until you shift into park on an automatic or apply the hand brake on a manual.
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Old 10-12-2011, 11:01 PM   #19
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Quote:
Originally Posted by Texas Proud View Post
OK... today when driving to lunch I was sitting at a red light behind a Prius... now, my car is a manual and usually I just sit there without my foot on the brake.. but I noticed that the driver of the Prius had his foot on the brake... why?
IMO, and from the little I know, the engine does not even run while stopped.. so why would the car move at all if the person is not pushing on the 'go' pedal.... IOW, why would the car move at all if the driver does not want it to move... there should be no electricity going to the electric motor (that would seem to be a waste if it is)....
I have seen this before and have thought the same thing... why are they holding down the brake Does the car want to move if you do not hold down the brake If so, then why
The Prius will crawl at 1-2 MPH when your foot's off the gas (and off the brake) just like an automatic-transmission car. I don't know why the Prius does that.

If you want to stop that then you can put the car in neutral or in park, but to go back to drive you have to remember to put your foot on the brake while you're moving the paddle shifter. So most people leave it in drive and leave their foot on the brake.

The car's computer will keep running all the auxiliaries, and it'll start the engine as necessary to recharge the main battery. That's all separate from propulsion.

Quote:
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Since I am on the subject of this car.... how is the AC unit Does the engine have to be on for it to work What about an all electric car? How much energy is expended on the AC unit.... and how does that affect your range of driving
Quote:
Originally Posted by Koolau View Post
I too have wondered about the auxiliary equipment on a hybrid. How does the heater work if the engine hasn't warmed up? What runs the AC? What keeps the power brakes and steering active. Apparently, these are not significant issues as hybrids have taken over Oahu, but I'm still curious.
They're all powered through a DC-AC inverter. The heater has resistive strips (ours are probably thickly coated with dust) and the A/C makes the inverter work hard to keep cold air flowing. As the inverter discharges the main battery, the Prius' computer(s) turn on the internal combustion engine to recharge the battery.

Same deal with the rest of the car's aux equipment. That inverter is almost as critical to the car's operation as the main battery is to its propulsion, but they're both pretty reliable.

I have some old habits that are no longer relevant but are still emotional triggers. For example when the Prius is turned on, after a few seconds the internal-combustion engine fires up for about 45 seconds and charges the battery. It has nothing to do with anything in the battery or the ICE-- it's just to warm up the catalytic converter (with ICE exhaust) so that the car qualifies for its EPA rating as a "practically zero emissions vehicle".

I know this. But when my daughter turns on the car and then starts fiddling with her iPod while the ICE burns gas, it drives me nuts. I can still hear my mother from 1976 screeching "Stop wasting gas!!" Yet no gas is being "wasted" other than as designed.
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Old 10-13-2011, 07:51 AM   #20
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I'd guess that the Prius rolls a bit with the brake off to make for a more gentle acceleration. You take your foot off, the car edges forward, so when you hit the gas pedal (do they call it that?), it's a smoother transition from 2 mph than from 0 mph.

I agree with the safety reasons for keeping your foot on the brake too. It's what I do in my manual at a stop. I put it in neutral, so it would not roll unless I'm on a hill. But I don't want to get bumped into an intersection, and I want the brake lights on so that a car coming up from behind me knows that I'm stopped. Finally, it's not always obvious if I'm on enough of a hill that I'd roll ahead or back, so if I always use the brake I'm not going to misjudge ground that looks flat.
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