Toyota Prius Forum banner
1 - 20 of 29 Posts

·
Registered
Joined
·
21 Posts
Discussion Starter · #1 ·
I'm a new prius owner - stereotypically happy - and this may be a bit presumptuous. I hope to get answers from the 'old hands' here, who know a lot more than I do.

I've noticed that if I let up on the gas pedal the prius does not coast freely. Instead, the regenerative braking is engaged and the car slows slightly. I read that this was done so that the car would drive similarly to a conventional (automatic shift) car, which slows slightly when the gas pedal is released.

My gut reaction is: I don't like this. I'd rather the car coast freely. When the car coasts freely, it is getting maximum gas mileage. But if the regenerative braking is engaged, there is a loss of efficiency.

I know that I can get the car to coast if I press the accelerator very lightly - just enough to stop the brake, but not enough to engage the engine. But it's difficult - if I press too hard the car goes into Stealth mode, or gets very slight engine acceleration, and if I press not hard enough then the brake is still engaged - either of which is not the best for gas mileage.

So I think: wouldn't gas mileage be better if the car just coasted and did not engage the brake when the gas pedal is let up?

I'm sure this issue has been thought about before. It may be that the loss of gas mileage is trivially small, or there may be other considerations. Does anyone know?

Thanks.

PS. So far, the gas mileage is great! I'd seen complaints from new prius owners about disappointing gas mileage, so I was prepared for the worst. Instead, I watched the Consumption display rack up 50+ mpg on the highway as I drove home from the dealer, and calculated 40 mpg for my 3-mile commutes to work. 40 mpg is bad for a prius, but great for the driving conditions.
 

·
Registered
Joined
·
1,397 Posts
The car is designed to work like any ordinary car with an automatic transmission. All cars will "drag" the engine when you let up on the accelerator while they are in gear. With a manual transmission, you can push in the clutch or shift into neutral to disengage the transmission from the engine and coast freely. But aside from that, cars are designed to provide engine braking when your foot is off the gas so you don't have to use up so much of your brakes.

I hope this helps. I really can't imagine that you truly don't like the feature given how pretty much all cars work that way.
 

·
Registered
Joined
·
420 Posts
The light pressure on the gas pedal, even with the regeneration lines showing, is still decreasing your generator drag. (The screen only shows whether regeneration is occurring, not how much is occurring.)

As Richard Schumacher wrote, you'll develop the touch over time. You'll also learn not to be irritated when the car absolutely refuses to go into gliding mode, which happens a lot in colder weather or in hot weather when the A/C is blasting.
 

·
Premium Member
Joined
·
640 Posts
I've gotten into the almost unthinking habit of just flipping the gear into Neutral (pull left, hold for 2 secs.)...do it on level or slightly downhill stretches whenever I know I can go at least a 1/4 mile "free", where leaving it in D would have caused drag requiring some acceleration...try it!
 

·
Registered
Joined
·
1,397 Posts
richard schumacher said:
Normal and typical are not necessarily desireable.
The original poster didn't sound like he was trying to "pulse and glide" and getting frustruated with not being able to glide. He said he didn't like something about the car that every other non-manual transmission car does. Unless it is his first car like that, I'm just surprised how not liking it was possible. Of course if he was trying to do the pulse and glide thing then I just didn't realize it from the post.

Personally, I think making the car behave the way other cars do is the right thing to do from Toyota's perspective. I'm also referring to the "creeping" that occurs when you take your foot off the brake. I think doing it the way they did it makes most people happy. If the car surprised most people (by perhaps starting to roll backward when they took their foot of the brake on a slight hill), more people would legitimately be unhappy with it.
 

·
Registered
Joined
·
7,161 Posts
Avogrado, I want to commend you on making your evaluation so quickly as a new owner. It won't take you long to become a Prius expert, especially if you are technically/mechanically inclinded and decide to access the tech manual from Techinfo.toyota.com.

As stated, Toyota chose to make the Prius respond like a conventional car with automatic transmission. This provides better one pedal control over the car in wider driving conditions, where a manual transmission would have you control 3 pedals. Many of us have voiced that it would be nice if the car gave you settable options regarding this and other vehicle handling aspects, but there could be safety and liability issues.

Above 42 MPH, the engine is always spinning and consuming some fuel though a very minute amount if gliding. This is probably why you can't seem to get a 'dead-zone' at or above this speed when you try to glide. Remember, the energy screen shows whether energy flows, or not, it doesn't show how much. Even so, energy might flow but not show. So, if you are teetering between energy from ICE and energy only from MG2, that's close enough to achieve your goals.

The alternative is to put the car in Neutral. I personally don't see a problem with that, legally or technically, but others have expressed concerns which may have merit. I did discover though that if ICE is running, putting the car in Neutral will not shut it off, and may actually prevent it from shutting off. If ICE is already off, it won't be able to start in Neutral. I observed this phenomina by letting the car go to 2 bars SOC where ICE started, let it charge up to where it would come close to shutting off, put it in Neutral. ICE continued to run though the sound from it indicated it was not under load. When I put it back to park, ICE turned off.
 

·
Registered
Joined
·
1,028 Posts
redwein said:
The original poster [...] didn't like something about the car that every other non-manual transmission car does.
Yes, that's the "normal and typical" part...

Unless it is his first car like that, I'm just surprised how not liking it was possible.
"Not possible" to dislike it merely because every other car does it? Everyone gets old and dies, but that doesn't mean we all like it. I do appreciate why [email protected] designed it this way: they didn't want to surprise, confuse, frighten and [email protected] the great mass of potential customers merely to surprise, delight, impress and satisfy others.
 

·
Registered
Joined
·
21 Posts
Discussion Starter · #10 ·
Thanks DanMan and others.

It's true that I am used to manual-shift cars, so I am used to coasting. Of course, the prius needs to automatically 'creep' forward from a full stop; otherwise starting on a hill would be REALLY challenging.

I've tried to pulse-and-glide; it's 'natural' in Michigan where I live - we have lots of long, slight downhills. So I do a lot of coasting, just driving normally - in a prius I try to press the accelerator just enough to turn off the regenerative braking.

I wondered whether it was worth the effort, so I just went and did a test. I drove a stretch of deserted scenic river road - 35 mph - one time gliding as much a possible and the other time driving 'normally'. The MFD showed 79 MPG overall with 'gliding'; and 66 MPG driving "normally".

(OK, the best way to improve mileage is to drive deserted scenic river roads. But gliding helps also.)

About the design - DanMan has a point, the current system lets you have a lot of control with just one pedal; you can slow down slightly without having to brake. I'll learn to glide eventually - it is not nearly as hard as learning manual transmission was.
 

·
Registered
Joined
·
1,028 Posts
redwein said:
I think doing it the way they did it makes most people happy. If the car surprised most people (by perhaps starting to roll backward when they took their foot of the brake on a slight hill), more people would legitimately be unhappy with it.
Agreed. [email protected] solved the problem in the least surprising way. They could have provided a no-creep function to address the stopped-on-a-slope problem: when the car is READY, motionless, and no brake or accelerator is applied, use just enough motor power to remain motionless. This would work in all situations. As it is, it still rolls backward on a sufficient slope, and of course rolls forward on no slope at all.

There is no simple strategy for no-brake/no-accelerator which optimizes fuel economy in all situations. If the car glided by itself, I'd bet that most people under most circumstances would end up braking fairly hard close to a stop. This would not allow for very much regeneration, and so result in lower overall fuel economy. Personally I would have preferred that the default coasting behaviour recharge the traction battery at its optimum rate (which may in fact be the case), regardless of how much or little drag it caused.
 

·
Registered
Joined
·
27 Posts
Here's a hypothetical:

If "gliding" results in such superior fuel efficiency, wouldn't the Prius get even better gas mileage if it were able to glide at all speeds and not just between 30-40mph?

And taking that a step further, wouldn't this be a really cheap way of making a car that got great gas mileage? I wonder if anyone has thought of it:
What if there was a car that had no battery pack and no regeneration system - but it automatically went into glide mode when coasting at any speed (transmission goes into neutral with ICE off). Plus, it automatically shuts off the ICE at stops. For those of you that play golf, it would be like a golf cart that could glide.
 

·
Registered
Joined
·
1,397 Posts
wayneswhirld said:
I've gotten into the almost unthinking habit of just flipping the gear into Neutral (pull left, hold for 2 secs.)...do it on level or slightly downhill stretches whenever I know I can go at least a 1/4 mile "free", where leaving it in D would have caused drag requiring some acceleration...try it!
Here is something I found that makes a case against driving in neutral:

http://www.grist.org/advice/ask/2003/10/28/umbra-neutral/

It is pretty consistent with what I have been taught over the years. That is, sometimes you need to unexpectedly accelerate to avoid an accident and being in neutral can hamper that. They also claim that it is illegal in some states, probably due to that reason.
 

·
Premium Member
Joined
·
640 Posts
redwein said:
wayneswhirld said:
I've gotten into the almost unthinking habit of just flipping the gear into Neutral (pull left, hold for 2 secs.)...do it on level or slightly downhill stretches whenever I know I can go at least a 1/4 mile "free", where leaving it in D would have caused drag requiring some acceleration...try it!
Here is something I found that makes a case against driving in neutral:

http://www.grist.org/advice/ask/2003/10/28/umbra-neutral/

It is pretty consistent with what I have been taught over the years. That is, sometimes you need to unexpectedly accelerate to avoid an accident and being in neutral can hamper that. They also claim that it is illegal in some states, probably due to that reason.
Redwein, Thanks for the info. I am aware of the risks and legal implications of coasting. I did confirm in fact that it is illegal to coast (even with a motor on) in VT. I think my risk is minimized since I almost always do this in areas where there's little or no traffic (a good part of my travel) as it seldom would work where you have to maintain the flow of traffic. I could see the most likely risk being if an animal would run across the road in the one case I go over 40 MPH in N and slightly downhill for 3 miles. I would not be able to slow down quite as fast as in D, and perhaps the need to accelerate (even though going fast downhill) might possibly help as I attempted to maneuver. As to saving gas in N, I'll agree it's pretty minimal, but could be as much as 1/2 to 1 MPG over a tankful. So the case has been made against it, but nonetheless, I'll take my chances.

DanMan32 said:
...Neutral. I personally don't see a problem with that, legally or technically, but others have expressed concerns which may have merit. I did discover though that if ICE is running, putting the car in Neutral will not shut it off, and may actually prevent it from shutting off. If ICE is already off, it won't be able to start in Neutral. I observed this phenomina by letting the car go to 2 bars SOC where ICE started, let it charge up to where it would come close to shutting off, put it in Neutral. ICE continued to run though the sound from it indicated it was not under load. When I put it back to park, ICE turned off.
Dan, that's interesting...I didn't realize the ICE is running (or spinning) during coasting as I figured no arrow activity meant no motor activity. So that also means the ICE is running during the glide in P&G. Ultimately a ScanGauge would confirm all this although you seem to have with your test observation.
 

·
Registered
Joined
·
7,161 Posts
Yes, a scanguage (or related) would be the best indicator.

There should be no problem with stopping as fast as you need while in neutral, as you still have the hydralic brakes. Not as efficient as regen, but you have them. Needing to accelerate to avoid a hazard is pretty rare, but I could see a potential issue.

It was discussed that one might over-rev MG1, especially in a classic, but then the ICE will already be spinning if over 42 MPH and not spinning under 42 MPH. Most likely you'll be losing speed, not gaining it, unless decending a hill.
 

·
Registered
Joined
·
126 Posts
Regenerative braking recaptures kinetic energy by converting it to chemical energy and thereby recharging the battery. If you want to NOT use regenerative braking, you are doing exactly the same thing as taking a couple of gallons of gasoline and pouring them out by the side of the road.
 

·
Registered
Joined
·
7,161 Posts
No, not when the engine is not running. The same friction losses occur whether you use regenerative braking or not. With regenerative braking you add kinetic-electrical-chemical energy conversion losses. Regen is great for when you have to stop faster than friction losses would allow. But when friction losses would suffice, better to cut the engine and coast to stop than run the engine further then use regen to stop.

Of course traffic patterns and practicality may prevent you from doing what is ideal, so again regen would be better.

What would be worse is braking when in neutral, as then you are only using friction brakes. You would do this only if you were trying the coast method, but then had to stop quickly and unexpectedly.
 

·
Read Only
Joined
·
51 Posts
wayneswhirld said:
redwein said:
wayneswhirld said:
I've gotten into the almost unthinking habit of just flipping the gear into Neutral (pull left, hold for 2 secs.)...do it on level or slightly downhill stretches whenever I know I can go at least a 1/4 mile "free", where leaving it in D would have caused drag requiring some acceleration...try it!
Here is something I found that makes a case against driving in neutral:

http://www.grist.org/advice/ask/2003/10/28/umbra-neutral/

It is pretty consistent with what I have been taught over the years. That is, sometimes you need to unexpectedly accelerate to avoid an accident and being in neutral can hamper that. They also claim that it is illegal in some states, probably due to that reason.
Redwein, Thanks for the info. I am aware of the risks and legal implications of coasting. I did confirm in fact that it is illegal to coast (even with a motor on) in VT. I think my risk is minimized since I almost always do this in areas where there's little or no traffic (a good part of my travel) as it seldom would work where you have to maintain the flow of traffic. I could see the most likely risk being if an animal would run across the road in the one case I go over 40 MPH in N and slightly downhill for 3 miles. I would not be able to slow down quite as fast as in D, and perhaps the need to accelerate (even though going fast downhill) might possibly help as I attempted to maneuver. As to saving gas in N, I'll agree it's pretty minimal, but could be as much as 1/2 to 1 MPG over a tankful. So the case has been made against it, but nonetheless, I'll take my chances.

DanMan32 said:
...Neutral. I personally don't see a problem with that, legally or technically, but others have expressed concerns which may have merit. I did discover though that if ICE is running, putting the car in Neutral will not shut it off, and may actually prevent it from shutting off. If ICE is already off, it won't be able to start in Neutral. I observed this phenomina by letting the car go to 2 bars SOC where ICE started, let it charge up to where it would come close to shutting off, put it in Neutral. ICE continued to run though the sound from it indicated it was not under load. When I put it back to park, ICE turned off.
Dan, that's interesting...I didn't realize the ICE is running (or spinning) during coasting as I figured no arrow activity meant no motor activity. So that also means the ICE is running during the glide in P&G. Ultimately a ScanGauge would confirm all this although you seem to have with your test observation.
According to the Prius technical description of Neutral is that it disables the transistors to MG2 and MG1 so that neither motor can be powered or used to generate any electricity. Now MG2 rotational speed is always determined by a ratio of the how fast the car is going. But MG1 depends on the rotational speed of MG2 and the ICE. If the ICE was not running when switching to neutral, it is going to remain not running. It required MG1 to spin it up. Also if the ICE is not running you can exceed the maximum speed of MG1 (10000 RPM). If you are driving too fast or more important if you are going down hill and speed up from below 42 MPH to above 42 MPH and the ICE was not running you can over-rev MG1, since there is no way to start the ICE. If the ICE is running the question becomes whether the computers know the rotational rate of MGI (since the electronics too it are shut off, to MG2 also) to be able adjust the speed of the ICE to keep MG1 at a safe RPM. I personally think it is unwise to drive in neutral.
 

·
Registered
Joined
·
7,161 Posts
You're almost right. The 42 MPH limit was with the 6500 RPM limit of the classic. The computers still start ICE spin at 42 MPH, but this is no longer a technical limit. MG1 is indeed now limited to 10K, so the car can go 64 MPH before MG1 limit is reached.
 
1 - 20 of 29 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top