Free-Wheeling/coasting- Bad For The Engine?

[WuWu]

I'v been driving my Prius for 2 months now, and really like it. Getting about 58mpg. Is it legal to go into neutral down hills? And is it bad for the engine? I realise that the Battery isn't being charged up by going into neutral, but if it's already fully charged, does that matter?

[Grumpy Old Man]

Why on earth would you want to go down hills in neutral?

I don't know about it being illegal, but I would of thought it was not recommended.

[Ancient Nerd]

I'v been driving my Prius for 2 months now, and really like it. Getting about 58mpg. Is it legal to go into neutral down hills? And is it bad for the engine? I realise that the battery isn't being charged up by going into neutral, but if it's already fully charged, does that matter?

I think it probably is illegal - if you get caught!

However, there is no point freewheeling with the Prius:

• The engine switches off anyway, so you won't gain any improvement in mpg.
  
• If you are in neutral, you won't get Battery charged by the regenerative braking. So you won't be saving fuel later.
  

The next thought is don't use B, unless you really need engine braking. In the main, it's worse for fuel consumption.

Also, don't forget that there is still capacity left in the Battery when all bars are showing. Leave the Prius to sort out itself, there is some really clever programming in there.

There was once a point in freewheeling, when engines were inefficient. I once drove a Bristol 401 fitted with a freewheel. The easiest way to engage it (IIRC) was to shift into a lower gear, typically first. There was no question of being out of control in neutral. When the car speed slowed to match the gear, it re-engaged. This is totally pointless with modern ECUs, which switch off the fuel on overrun.

[Jan&tone]

I'v been driving my Prius for 2 months now, and really like it. Getting about 58mpg. Is it legal to go into neutral down hills? And is it bad for the engine? I realise that the battery isn't being charged up by going into neutral, but if it's already fully charged, does that matter?

Wuwu, welcome to TOC

Not only is it not recommended to coast IN ANY CAR let alone one that is designed to recover and charge the Battery when it is not accellerating, ie coasting in 'd', it is also extremely stupid! You have only the brakes to control the car and that frankly is not recommended at all....

Never ever 'coast' in neutral in any car please....

In the Prius, when moving forwards you should always be in either d or b (b if you are going down a steep hill to give you extra braking)

Hope this helps

[Opifex]

Everyone seems agreed that Neutral shouldn't be used, but does anyone know why it's there in the first place? When I'm moving it's in D ( or B or R) if I'm stationary it's in P. When do I need N?

[Habu]

Everyone seems agreed that Neutral shouldn't be used, but does anyone know why it's there in the first place? When I'm moving it's in D ( or B or R) if I'm stationary it's in P. When do I need N?

Prius may need to be pushed or pulled when not under its own power.

[DrCez]

I'v been driving my Prius for 2 months now, and really like it. Getting about 58mpg. Is it legal to go into neutral down hills? And is it bad for the engine? I realise that the battery isn't being charged up by going into neutral, but if it's already fully charged, does that matter?

If you're aiming to improve fuel consumption a better (safer) option would be to leave the car in D and feather the accelerator very slightly so that the CHG bar on the HSI display is not lit. The aim is to move the bar slightly into the EV area (but keep it below the half-way mark) at which point there is no artificial engine drag and no/very slight electric motor assist. At speeds below about 40 mph this allows the car to coast with the petrol engine off; at higher speeds it will spin but won't use any fuel. This is equivalent to the glide phase of the pulse and glide technique.

Regarding use of N I have experimented with using it to allow the car to start rolling on a long, *slight* downhill slope in heavy stop-start traffic, before switching to D once the car is travelling fast enough to need braking (and therefore charge the Battery a little). My reasoning is that the car is wasting charge to start accelerating, and with many stop-starts over a distance of, say, a mile or more it could add up to a significant amount of lost charge. The owner's manual cautions against any driving in N but the car seems happy enough (at low speeds, <10 mph).

[Jands]

Not sure if it is illegal but the Highway Code definitely says you should not coast as you are not in full control of the car (or something to that effect).

Having said that, I've seen the odd post on Prius Chat (our fiends across the pond) who do it as a lazy way of gliding (the Pulse & Glide technique to increase mpg) but it is better and recommeneded to glide using the accelerator pedal. I see you have a gen3 so once you reached your desired speed, simply lift off the accelerator pedal for a second (this will switch off the engine) and then re-apply pressure so your HSI indicator is on the left side of the bar (as close as you can get it to the CHG section without entering it).

http://priuschat.com/forums/gen-iii-2010-prius-fuel-economy/66723-2010-prius-pulse-glide-warp-stealth-help.html has good info on it and see the pic from Ken1784.

Hope that helps.

[capnbirdseye]

You have to remember that the Prius doesn't automatically engine brake when going downhill as does a traditional car and altho the regeneration will be in operation to charge the Battery it doesn't really slow you down if at all. As already pointed out the Prius can legally simulate coasting with the engine off and with no transmission drag

The reason for this is the clever technology that enables the computer to open the valves whilst you coast in D so there's no engine drag whatsoever but if you select B then it closes the valves and slows you down in a really good way . I was out for a drive yesterday and used B for as we descended a long steep hill at Broadway Glouc, every car/van etc in front had their brakes on all the way down with engines screaming in low gear whilst I touched the brake just 3 times on the bends and our Prius kept pace exactly with those in front. I've used B about four times since I had the car (6 weeks)and each time it worked flawlessly and it definitely also charges the Battery in B mode as all four times it was green at the bottom of the hill ( Gen2).

I spent hours on the net reading about the Prius whilst waiting for my car , here's a link to a website that gives loads of info which altho covers Gen2 it's mostly relevant to Gen 111, select stuff on the left of the page to expand more

http://prius.ecrostech.com/original/PriusFrames.htm

[kevin h]

One reason to freewheel on a full Battery is it will save fuel.

When the Battery is fully charged and the engine is up to operating temperatur, I have always found that the engine is still running at the bottom of the hill.

Thats on both my Gen 2 & 3.

But, I'd never reccomend it.

[Opifex]

One reason to freewheel on a full battery is it will save fuel.

When the battery is fully charged and the engine is up to operating temperatur, I have always found that the engine is still running at the bottom of the hill.

Thats on both my Gen 2 & 3.

But, I'd never reccomend it.

I found a paragraph about this in this article

http://prius3.wordpress.com/2010/07/24/10-months-driving-in-the-future-a-prius-2010-review-2/

11. Down-shifting/engine braking: If the Battery becomes fully charged (all bars light up) while the gas pedal is released (e.g. going down a long hill), the ICE will freewheel (no gas injected) acting as an air compressor (exhaust valves are kept closed), by releasing excess energy as heat. In this way the ICE offers resistance and therefore braking power, without involving the hydraulic brakes.

The same happens when the B mode gear is selected or when downshifting on a conventional drivetrain. Using B mode gear in normal driving situations is not useful, since energy that could be used to re-charge the batteries is instead wasted in heat generated by the engine which is compressing air…so though one might be led to believe that when breaking the engine is also involved, no extra energy is actually recuperated, rather wasted. Prius drivers, drive in “D” except when “falling” down a hill/mountain.

In other words the engine is still turning over but it is not using any petrol. I believe that using N will disconnect, and stop,the engine but you will not save any fuel and the brakes will have to work harder, and hence wear more, to slow you down.

[Grumpy Cabbie]

It can also cause the electric motors to over spin and potentially cause them serious damage.

Just leave the car in Drive when you are driving it - the Prius is designed to save you petrol - let it do its job. Simple

[timberwolf]

I think Kevin H may be referring to the situation where the car can be stopped with the petrol engine running and the HV Battery fully charged, the petrol engine is running, possibly at higher revs, to force the system to dump electricity by running MG2 to act against the RPM of the petrol engine (i.e. balance out so that there is no forward motion through the transmission) and therefore bring the HV Battery SOC down.

[Opifex]

I think Kevin H may be referring to the situation where the car can be stopped with the petrol engine running and the HV battery fully charged, the petrol engine is running, possibly at higher revs, to force the system to dump electricity by running MG2 to act against the RPM of the petrol engine (i.e. balance out so that there is no forward motion through the transmission) and therefore bring the HV battery SOC down.

This is interesting.

I hadn't come across the abbreviation SOC before but found that Google dictionary suggests it means 'State of Charge'. You learn something new everyday.

I assume that when you say the petrol engine is running, it is being turned by the motor but is not being fed with fuel. It would simply be compressing the air in the engine causing it to warm up so that the electrical energy in the Battery is converted to waste heat. This would indeed reduce the Battery state of charge but I'm puzzled why it should do this when the car is stopped. With the car stopped and the Battery fully charged why doesn't it simply sit there until you start to move again?

I've just thought of one possible answer. If needed, the engine will run to heat the inside of the car. Could this be what you have noticed?

[Grumpy Cabbie]

To cut a long story short, the HV Battery does not like to be over charged. Toyota found that keeping the Battery in a 'sweet' spot SOC allows the Battery to last for years. If you 'over cook' it, over charge or under charge it this will have long term longevity issues.

As such, if the battery is getting too full - such as when going down a really long hill, the engine is run without fuel by the HV battery to try and waste some charge. This is why the engine will still be running (without fuel) even when stationary if the SOC is full. The car will also do other weird stuff if the HV is completely full (or as full as it will be allowed to go) such as running on electric only upto 45 mph but allowing much quicker acceleration than if you use EV mode.

You may also notice that the SOC doesn't go below 2 bars on the HV indicator and that's to stop the battery running down too far for the reasons given above. The HV battery is very over engineered and there are graphs on the US forum which show how keeping it in this sweet spot allows for a potentially long battery life. If the battery was used it in the same way as a mobile phone uses its battery (full charge and discharge cycles) you could probably run the car for a good 5 miles on EV but the battery would only last 6-12 months.

This was the short version! Hope it helps.

[kevin h]

I think Kevin H may be referring to the situation where the car can be stopped with the petrol engine running and the HV battery fully charged, the petrol engine is running, possibly at higher revs, to force the system to dump electricity by running MG2 to act against the RPM of the petrol engine (i.e. balance out so that there is no forward motion through the transmission) and therefore bring the HV battery SOC down.

That's it, I knew I'd seen something about this some time ago.

As everybody is sayin, it a bad idea and gets you nowhere in terms of fuel savings.

[Opifex]

To cut a long story short, the HV battery does not like to be over charged. Toyota found that keeping the battery in a 'sweet' spot SOC allows the battery to last for years. If you 'over cook' it, over charge or under charge it this will have long term longevity issues.

As such, if the battery is getting too full - such as when going down a really long hill, the engine is run without fuel by the HV battery to try and waste some charge. This is why the engine will still be running (without fuel) even when stationary if the SOC is full. The car will also do other weird stuff if the HV is completely full (or as full as it will be allowed to go) such as running on electric only upto 45 mph but allowing much quicker acceleration than if you use EV mode.

You may also notice that the SOC doesn't go below 2 bars on the HV indicator and that's to stop the battery running down too far for the reasons given above. The HV battery is very over engineered and there are graphs on the US forum which show how keeping it in this sweet spot allows for a potentially long battery life. If the battery was used it in the same way as a mobile phone uses its battery (full charge and discharge cycles) you could probably run the car for a good 5 miles on EV but the battery would only last 6-12 months.

This was the short version! Hope it helps.

Thanks for the explanation.

I can see I'm going to have to read a lot more before I understand what's going on.

[r04drunner1]

A fascinating and useful discussion.

To go back to the OP's point about running the car downhill... I have a theory.

Contrary to normal practice in an automatic, when going down hill in a Prius, I reckon it's actually better to lightly sit on the brake pedal rather than try to use the gearbox to slow the car down. In other words, don't use B.

The reason is that, if you have the Prius in B mode, while you will regenerate energy you will also be throwing away energy (heating up the air in the engine). But if you use the brake pedal to keep the car at a safe speed, then you are not incurring that energy loss in the engine. Also, as you are simply regenerating energy and storing it in the hybrid Battery, you will not actually be using the physical brakes. So there is no strain or heating up on the brake pads. (On a normal car, that heating up and resultant brake fade is the very reason why you would not ride the brakes down a hill.)

Of course, if the Battery is full then the above does not apply. You might as well use B.

Also if you are bothered about your brake lights being on all the time, you might prefer to use B. Just "B" advised ( ) that you are wasting energy by doing so...

R04drunner1

[cbcdesign]

You may also notice that the SOC doesn't go below 2 bars on the HV indicator and that's to stop the battery running down too far for the reasons given above. The HV battery is very over engineered and there are graphs on the US forum which show how keeping it in this sweet spot allows for a potentially long battery life. If the battery was used it in the same way as a mobile phone uses its battery (full charge and discharge cycles) you could probably run the car for a good 5 miles on EV but the battery would only last 6-12 months.

This was the short version! Hope it helps.

I have worked in industry designing and building Battery management systems for over 25 years and I have never heard any cell manufacturer claim that fully discharging a Battery reduces its life, quite the oppositie in fact. A full discharge (which should not be confused with zero volts but down to a specific voltage per cell) is good for a Battery and extends its life in fact.

I suspect that the reason the Prius battery is not fully discharged is therefore not due to concerns about reducing battery life but because the vehicle requires a minimum voltage from the battery and fully discharging it to its end of discharge potential could easily result in voltage collapse under even a light load, a condition the vehicle could not withstand for all sorts of reasons including proper function.

Overcharge and undercharging are of course bad for a battery as the first causes overheating and cell degredation whilst the other is really self explanatory.

[Grumpy Cabbie]

I wasn't getting into the technicalities of how or why the Battery is kept between 40% & 80% charge and not allowed to go below or exceed these limits. I have read that if it goes outside this 'window' it causes the Battery life to reduce. Whether this is because it gets too hot or offers a reduced voltage I care not a jot. But the Prius has very advanced Battery management to allow for a long battery life.

I am searching for official literature to put this in a more technical way for those who want the long, complicated story.

[cbcdesign]

I wasn't getting into the technicalities of how or why the battery is kept between 40% & 80% charge and not allowed to go below or exceed these limits. I have read that if it goes outside this 'window' it causes the battery life to reduce. Whether this is because it gets too hot or offers a reduced voltage I care not a jot. But the Prius has very advanced battery management to allow for a long battery life.

I am searching for official literature to put this in a more technical way for those who want the long, complicated story.

Fair enough but I wasn't getting into the technicalities of the window in which Toyota choose to run the Battery either which they do for their own reasons no doubt but merely pointing out that proper cycling of any Battery extends Battery life not reduces it.

I would certainly be interested to read the official literature to see why they choose to charge and discharge their cells over a smaller operating window than normal, I am sure it is quite interesting.

[cbcdesign]

It would appear that Grumpy Cabbie is correct and in the case of the batteries used in the Prius, partial discharge extends the life of the Battery by a significant margin. My apologies Grumpy for talking through my backside!

It would seem that depth of discharge effects the number of cycles that the Battery can tolerate so Toyota have limited the voltage band within which the Battery operates in order to extend the life from hundreds to many thousands of cycles.

We live and learn as they say!

[Grumpy Cabbie]

There was an interesting article on the US forum which showed a graph noting a sliding scale of charge cycles on the Prius Battery. If it was charged and discharged 0%-100% it would last 1,000 cycles (allowing something like 5 miles EV) before being mush (not literally), but when using the magic window between 40%&80% (normal Prius operation) the Battery could handle something like 5 million cycles.

Been trying to find the graph as it was rather interesting read - esp for someone with knowledge of batteries, but obviously when I want it I can't find it!

UPDATE 10 MINS LATER;

I knew if I said I couldn't find it I would! Does the following assist?

http://priuschat.com/forums/knowledge-base-articles-discussion/64978-understanding-nimh-hypercycles-prolong-prius-hv-battery-life.html

[cbcdesign]

There was an interesting article on the US forum which showed a graph noting a sliding scale of charge cycles on the Prius battery. If it was charged and discharged 0%-100% it would last 1,000 cycles (allowing something like 5 miles EV) before being mush (not literally), but when using the magic window between 40%&80% (normal Prius operation) the battery could handle something like 5 million cycles.

Been trying to find the graph as it was rather interesting read - esp for someone with knowledge of batteries, but obviously when I want it I can't find it!

UPDATE 10 MINS LATER;

I knew if I said I couldn't find it I would! Does the following assist?

http://priuschat.com/forums/knowledge-base-articles-discussion/64978-understanding-nimh-hypercycles-prolong-prius-hv-battery-life.html

Very interesting that and very much the polar opposite of the belief within the industry that full cycling is best.

Time to change the way in which we NiMh powered devices too in that case as the same rules would apply to any cell including the small ones we use in our hand held equipment. :)

[Grumpy Cabbie]

I guess it doesn't really matter in a mobile phone where the replacement cost is £20 and the 2000 cycles would more than last the life of the product.

In an EV or hybrid then you're talking £2k - £5k for a replacement Battery and a lifetime of 10 years. You're gonna want to make sure it lasts the distance. Then you have the fact that its got to operate in -40c in Canada to +40c in Australia and still be reliable and last the warranty etc.

Apparantly Toyota really over engineered their HV Battery because any failures would be jumped on by the 'anti' brigade. Also, in certain American States the warranty has to be 8 years/150k miles as a minimum, so Toyota don't want any failures there either.

If you're electronically minded you might want to investigate the enginer (spelt correctly) Electric Vehicle modification to the Prius. There are a handful of UK Prius owners who have converted their Prius and can get 5 or 10 miles pure EV out OR 80-100 mpg in blended mode.

Check out the following; http://www.enginer.us/

Be aware that it is a DIY modification and WILL more than likely INVALIDATE your Toyota warranty.

The following is a link to a US forum where a UK Prius owner is updating details of fitment of the enginer system. Interesting if you're a techie. You can see it wasn't quite straightforward.

http://priuschat.com/forums/prius-phev-plug-in-modifications/82816-4kwh-enginer-geniii-plug-in-hybrid-electric-vehicle-in-uk.html

Or UK installer;

http://www.pluginplanet.co.uk/