Question about a Prius (or an electric car)

[Texas Proud]

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

 

[rescueme]

I had these same questions about the vehicle Fred Flintstone drove ...

 

[MasterBlaster]

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.

 

[Koolau]

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.

 

[donheff]

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.

 

[GregLee]

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.

 

[Koolau]

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.

 

[travelover]

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.

 

[Texas Proud]

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...

 

[Texas Proud]

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...

 

[BigMoneyJim]

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.

 

[MBAustin]

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.

 

[BigMoneyJim]

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.

 

[travelover]

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.

 

[travelover]

..........
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.

 

[kyounge1956]

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.

 

[Meadbh]

So the answer is psychology rather than physics. Interesting!

 

[JakeBrake]

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.

 

[Nords]

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.

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

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.

 

[RunningBum]

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.

 

[ERD50]

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? ....

I have seen this before and have thought the same thing... why are they holding down the brake ...

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.

A big +1 to RunningBum's comments. In fact, when I first read the OP, I asked myself - 'Why don't you hold the brake down when stopped with a manual transmission?' I see benefits, and no downside - Prius, manual or anything.

-ERD50

 

[Texas Proud]

A big +1 to RunningBum's comments. In fact, when I first read the OP, I asked myself - 'Why don't you hold the brake down when stopped with a manual transmission?' I see benefits, and no downside - Prius, manual or anything.

-ERD50

OK, I will answer...

I don't see the benefit. Around here, most of the time when you stop you will have someone stopped behind you pretty quickly... I live in a very flat place... the car does not roll most of the time, and when it does I will hold the brake...

My question back: how many times have you been rear ended while being the first person sitting at an intersection Me, none... I have been rear ended twice, once while doing an emergency stop and my car stopped faster than the guy behind me... the other I was in line at a light and saw the guy in my mirror about 15 seconds before he hit me...

So, the fear of being pushed into an intersection is so low IMO that it is not a good reason to hold down the brake..

Now, for the engineers out there... what would fail quicker.... a brake system that was under pressure at every stop for extra hours on end when added up over time, or one that was used to stop the car and then depressurized (to tell the truth, this is not why.... I just don't care to hold down the brake if I don't have to)....

 

[RunningBum]

My question back: how many times have you been rear ended while being the first person sitting at an intersection

Once. At a busy intersection. I was damn glad I had my brakes on. Avoiding one potentially very serious injury seems like enough, no? If not, how many deadly injuries do I have to avoid for it to be worth the inconvenience?

I don't think you put any wear on the brake system to hold a car in place that is not moving, but I can't be sure that holding the pedal down doesn't do something, so I'll leave that for someone more knowledgeable to answer.

 

[ERD50]

bold mine...

OK, I will answer...

I don't see the benefit. Around here, most of the time when you stop you will have someone stopped behind you pretty quickly... I live in a very flat place... the car does not roll most of the time, and when it does I will hold the brake...

My question back: how many times have you been rear ended while being the first person sitting at an intersection Me, none... I have been rear ended twice, once while doing an emergency stop and my car stopped faster than the guy behind me... the other I was in line at a light and saw the guy in my mirror about 15 seconds before he hit me...

So, the fear of being pushed into an intersection is so low IMO that it is not a good reason to hold down the brake..

Now, for the engineers out there... what would fail quicker.... a brake system that was under pressure at every stop for extra hours on end when added up over time, or one that was used to stop the car and then depressurized (to tell the truth, this is not why.... I just don't care to hold down the brake if I don't have to)....

I don't see your logic. 'How many' times something has or has not happened has little to no bearing on the decision. The decision should be based on 'am I improving my odds, what is the downside, and does that downside outweigh the benefit?'.

When I get on a plane, I sure want them to go through the entire checklist, and not skip some things because 'well, THAT part never fell off mid-flight before!'

And for your second example of being rear-ended - who is to say that your brake light would not have caught the guys attention? Again, not really meaningful for a one-time example, but it is a possibility, with no downside. Yes, the brakes will have some added pressure on them for some added time over the life of the car - but that pressure is much lower than )just enough to keep the car from moving) than that used to stop the car, I doubt that it would be responsible for any early failure.

To answer your Q though - I've been rear-ended three times, and avoided a fourth by seeing the guy in my rear-view mirror, scanning the intersection to be sure it was clear, and flooring it and getting on the horn, and I managed to get out of his way (he stopped mid-intersection, I think my sudden movement and horn alerted him, and maybe the fact that my brake lights then 'flashed'' as I took my foot off to accelerate). If I had been in Neutral, it would have slowed my response. He didn't miss me by much.

Anecdotes aside, it just makes sense to me that seeing the brake lights ahead adds one more level of awareness to what is going on ahead. Why even have brake lights if that's not the case?

-ERD50

 

[W2R]

I guess I must be really lucky. When I was 15 and taking driving lessons, my instructor clearly told me that my foot had to always be on the brake when waiting at a red stoplight.

I never realized until this moment that some people don't do that.