UKC

Does anyone know what's the most efficient way to drive an EV (in terms of weighing total distance covered against type of road). I know wild acceleration etc. is going to eat up charge but I more mean about is it better to be stop starting doing <50mph average speed on a-roads, or is it better to cruise at 70mph ish on a motorway but for a longer distance?

From my house Google suggests a 160mile motorway cruise to Snowdonia, but the straightest line is only 132 miles along windy a-roads. Time wise shorter route is only about 15 minutes longer than the 3 hours the motor way takes.

160*2=320 > the suggested range of my car 😭

132*2=264 < the suggested range of my car 😎

Therefore I think I could potentially get all the way to Snowdonia and back on one charge and save the hassle of public charging - but I just wondered if cruising on motorways might be some % more efficient?

I'd expect it to follow the same rules as petrol and diesel cars. it'll take the same wattage to accelerate a 1.5 ton mass regardless of the fuel source, EV's are just have less losses when you press the go pedal.

so by that logic if it'd be more efficient to take the motorway vs windy a roads in a petrol car then it'll also be more efficient in a electric?

I think the US set the maximum speed to 56mph during the 70s fuel crisis because this was deemed to be the most efficient speed.  Air resistance is your enemy, so sitting at 55mph on an A road would probably be better.  However, in practical terms, sitting in the hole behind an HGV or coach on the motorway is the most fuel efficient way to travel, as long as you don't mind stone-chips.  Not the safest way to travel though, as you can see four fifths of f all....

I guess the only difference might be the braking, in that in a petrol car braking just turns energy into heat whereas with an EV at least some would be recouped for future use? If so, electric car "mpg" might be less impacted by twisty roads than a petrol model?

Everyone seems to say that motorways munch through the batteries, but I don't find that to be the case. Remember, 70mph is not compulsory on motorways!! 

I tend to stick behind a lorry on the motorway - it keeps your speed steady and it does give a wee bit of a pull along. Obviously, maintaining your safe braking distance!

I have never really found public chargers to be a hassle, maybe I am just lucky? I believe they should get even better next month when a new company takes over from Charge Place Scotland.

I'd go A-roads, but you can charge at 7kW at the Siabod cafe for as long as you like for the price of a cup of tea, so long as the single charger isn't occupied. 

The flattest route with the wind behind you is good (although you are obviously not going to time your journey with the winds!)

And keep the speed down and your foot light.

Non-answer:  Depends on how you drive on the back roads and on the motorway.  Personally, I'd find it easier to engage "Lorry mode" on the motorway than "granny driving mode" on the twisty, windy back roads with all that lovely torque under my right foot.

Same basic rules as an ICE - moderate your top speed, look/drive ahead etc (*).   This might bias me towards the motorway, because I can tuck in behind a lorry and annoy nobody, where-as driving back roads gently, coasting down gently for bends and so on can wind people up, and pulling over to let them through can eat in to the range.

You don't have the losses associated with changing gears and being at non-ideal revs that an ICE has on back roads but can dodge when motorway cruising.

If you're taking it close to the wire and the outbound journey finishes with a big altitude gain, prepare yourself to have less than the required range when setting off for home - put your faith in gravity and it will return.

(*) regenerative breaking saves some energy from braking when not driving ahead, but with some losses; better to coast down as with an ICE.  Ironically, harder in many EVs.

Also - top tip - fill your boot with rocks or water near the top of each hill and empty them at the bottom.  Good GCSE physics problem to work out, makes surprisingly little difference.

I've experimented and proven to myself that a slower journey is much more efficient.

To be expected given the physics where the drag increases in accordance with the square of the velocity.

Unless you're in a rush then take the scenic route

Coasting in an ICE doesn't save fuel*. If you're coasting then fuel is needed to keep the engine turning, if you're in gear then the momentum of the car will keep will keep the engine turning, essentially backdriving the engine, and the ECU will cut fuel to the engine.

*If you have a start/stop motor and a long downhill and you turn the engine off completely then you might save fuel. You also lose power assist for the brakes so it's not a great idea

As in coasting down to a bend in gear vs late breaking.  I don’t literally mean outing it in neutral and free wheeling.

As in “there’s a bend ahead, I’ll take my foot off the go pedal now so that the car slows to the appropriate speed for the bend by the time I hit it” rather than “gas gas gas brake brake brake bend”.

As well as saving on fuel from going faster on the late-braking approach and saving from fuel’s energy burnt in the kinetic breaks, it saves energy with a modern 12V electrical system because they charge more when zero-throttle coasting down.

Ok, but that it is what coasting means when talking about driving, or at least having the clutch fully depressed, and it is a relatively commonly held belief that this saves fuel

Okay; I take "coasting" to mean moving without applying power.  I should have defined that in my post, but where would internet threads be without ambiguity in terminology?

It must depend on your car, but in my Leaf the A roads are way more economical than the motorway. That's driving at normal speeds on A roads, not pootling along, compared to cruising at 65 on the motorway. In a Leaf, going above 65 is for short blasts only.

In Britain 50mph was enforced on motorways during the fuel crisis.  There were pictures on TV and in the papers of the then fairly new Range Rover being driven by the police three abreast at 50mph.

Driving not to exceed 50mph except for downhill coasting changed my mileage from about 420 to 600 miles from a tank of fuel when I had a job that required me to cut travel costs - the car was a Vauxhall Cavalier SRi, about 5 years old and that had done about 20000 miles per year so not new by any means nor a light car.

I am certainly finding, as expected, that speed is the biggest factor by far.  In an ICE, much of your energy is wasted as exhaust heat, and hitting the brakes turns your hard-won KE into more heat.  Your single biggest loss is work done against drag, and drag goes with v^2.  70^2 is almost twice as big as 50^2.  So go the longer, slower route.  Braking is less of a loss than it is in an ICE as some of that KE is regenerated.

Having said that - ten minutes is a reasonable time to have a break in the middle of a 3 hour drive, and ten minutes on a 50kW charger will give you a decent cushion against disaster.  I start to get twitchy when the difference between my remaining range and distance to destination gets small..!

If only a ten minute charge could actually be accomplished in ten minutes... I have had this dilemma a few times and decided that sticking below 55 for the rest of the journey was quicker and easier than stopping to charge.

One of those that you will have to test for your self. Welcome to EV driving! What I would say, is that just as no ICE car hits the advertised MPG (and therefore range) it will be the same as an EV. The lab tests are the same for both and use a rolling road in mixed speed sample journey.

This is much more likely to represent the twisty stop start nature of A road driving. As others have said doing 70 will not allow you hit advertised range, and if the distance is further on the moterway you may need more juice to get to your destination than the short cut.

In the Leaf it's easy to see how much power you are using at any one time and it's easy to drive as efficiently as possible, take the hills slowly and pick up speed on the downs sitting on the regen moter breaking when up to limit. 

One factor however is that if you decide you do need a stop you may have more options for rapid charging on moterways than A roads, but this is improving all the time and we recently did a tour of Wales in our leaf charging in many of the main towns. Don't be scared of stopping, most chargers now are really simple to use and a quick 25 minute loo/coffee/shopping/emails break gets you plenty of miles back.

However you do it remember it will be much cheaper than petrol per mile, and much better for the locals lungs. 

Err the Cavalier came out in 1975, the 50mph limit was introduced on 8th Dec 1973. 

Read wercat’s post again.

I am certainly developing a preference for the 'just tap your credit card' type over the smartphone app controlled ones..!  BP and InstaVolt at the top of the pile.

It’s almost as if the whole app nonsense was totally unnecessary and pointless.  Whodathunkit?  Integrated charger/vehicle comms for automatic billing is in the pipeline of standards for new vehicles.  That’ll be nice.

Indeed! I linked the two paragraphs together somehow.

Top tip: don’t fill the boot of your ev with water.. 😬

What could possibly go wrong?

I thought BP pulse you need an app? It’s the only one I’ve used and only used it once but took 25 minutes just to get that cable out because the charger crashed or something. 
Which type can you just tap and pay?

I’m sure industry experts will declare the system fundamentally secure.  Just don’t go search Stack Exchange for questions posted by the sub-sub-sub contractors that are actually writing the code…

I've only used BP / Pulse a couple of times, but the ones I've been to could either be started by using the app or by tapping a card.

Perhaps it's only just rolling out across their network? I know Ecotricity are slowly changing all of their Electric Highway chargers to tap-and-go.  Ironically they were the only network where there is a point to the app - you get half price charging if you buy your home electricity from them. 

InstaVolt is certainly tap-and-go, and I've never had a problem with them. It seems to be slowly becoming standard everywhere.

I would refer our learned friend to Highway Code Rule 122:

Coasting. This term describes a vehicle travelling in neutral or with the clutch pressed down. It can reduce driver control because

• engine braking is eliminated
• vehicle speed downhill will increase quickly
• increased use of the footbrake can reduce its effectiveness
• steering response will be affected, particularly on bends and corners
• it may be more difficult to select the appropriate gear when needed.

https://www.gov.uk/guidance/the-highway-code/general-rules-techniques-and-a...

I would suggest that simply reducing pressure on the accelerator pedal in order to lose speed gently, rather than going straight from accelerator to brake, is simply called "slowing down".   Lots of drivers don't seem to be able to manage this comparatively simple skill: if they're not pressing the right-hand pedal then they're pressing the middle one.  I have always understood that if the engine revs actually rise without corresponding input from the accelerator pedal e.g. because you are going down a steep hill then that is what is called "overrun" (although Wiki would appear to disagree).

References to gears and the clutches become obsolete with EVs, the highway code becomes an anachronism and needs to evolve.  Even on ICEs, the comments on steering response are increasingly obsolete as various vehicles move to electronic power steering.  

I could refer you to multiple dictionary definitions of "coasting" that equate to moving without applying power, a much broader term and translatable between different experiences, surely the sign of a robust meaning.

Totally agree.  It grinds my gears when I'm following someone in traffic and they're arseholeing the car in front and all over their breaks.  My default plan is to double the gap I give them, but that often leads to the car behind me doing the same A/B behaviour.

Again, hard to interpret in the era of EVs.  In an ICE I'd call it engine breaking, and it roughly corresponds (in terms of feel, not energy efficiency) to regenerative breaking in an EV.  To me, overrun has connotations of something unrecoverable.

youtube.com/watch?v=zx4KoZV9fBw&

Also because I’d assume the A road in question is the A5 which is not even that twisty.

I disagree. The advertised mpg of my ICE car is 4.1 l/100km, which it regularly hits unless I'm ragging it, predominantly going uphill, against the wind, or in pouring rain. All of which are unsurprisingly not part of the lab tests anyway. 

As above. There is no need for A-roads to be start-stop, especially the A5. B and C roads maybe, but not 95% of A roads. Learn how to drive round a corner (for example, despite the belief of many road users, you do not need to slow to 30 mph to get round bends on the A5), and how to roll into it rather than braking severely. I'd assume the rules for driving an electric vehicle efficiently are the same as an ICE vehicle.

Lots of modern ICE cars have this too - dead useful. 

On a 130 mile trip I'd consider such a stop anyway, and as you say there are loads of chargers. This is often a thing I think electric vehicle users forget - just because you don't need to stop to refuel and ICE car, doesn't mean you don't stop - so long as you do this where there is a charger, the two can bet combined.

Every rapid charger I've used has had a contactless reader on it. I have never found one where it worked. So all it does is add a little bit more faff to a charging session, while I waste time trying it.

They mostly seem to be BP Pulse round here. Are you saying you've found one where the card reader works? And does it also let you release your cable without pressing emergency stop or phoning them? 

I've used InstaVolt quite a few times, and they have always started instantly once I tapped and stopped (and released their cable) as soon as I asked. Indeed, I think tapping a card is the only way you can start these chargers?

I've only used BP/Pulse twice. First one behaved exactly like InstaVolt. The second one wouldn't, but in retrospect my credit card was paying silly buggers that day (kept refusing to tap in shops, and I had to key in a PIN and even sign) so I used the app instead. It took about 60 seconds to tell the app where I was - I'd left a fiver preloaded on it when I first set it up.  I guess that this does represent 10% of a ten minute stop but it didn't feel silly. It stopped as soon as I told the app to stop it.

Ecotricity is quite a bit more hit and miss - I have sometimes wasted five minutes trying to get their app to start a charger up. In their defence they are older chargers, and when they really go wrong they usually default to free vending.

It's not perfect, and this proliferation of networks is ridiculous, but it's working and I've never yet been stranded <touches wood>.

sorry, I wasn't clear.   The Cavalier experience was in the 1990s and enforced by necessity of personal economy - nothing to do with the 1970s.  It was just to show the impact of changing driving style even with a bigger (then!) heavy car.

I wondered but should have read again. When the 50 limit came in I drove a 3.8 Jag with triple 45 DCOE Webers, now that used fuel!

Previously I had the model before the Cavalier, the Victor (a VX 4/90), the last of the GM clones before it all changed to the Opel stuff. Interesting days.

'the then fairly new Range Rover being driven by the police three abreast at 50mph.'

Oh the irony!

Energy consumption/unit distance basically increases with the square of speed, over a long drive this is where your gains are to be had. Stop-start is inefficient despite re-gen braking, as are hills but coasting down and accelerating smoothly and infrequently isn't a big deal. A-roads differ wildly, plenty offer dozens of miles of smooth cruising between features, others pass through village after village with roundabouts, junctions, lights, crawling traffic...

Slowly and smoothly by the shortest, flattest course will use least.

The difference in route distance is quite significant, I'd go for the A-roads, see what's left in the tank when you get there before deciding to head home or charge. The alternative that might work, cruising at 55 on the motorway is low hassle but will probably be slower.

the difference between 55 and 70mph is 1.6x the energy consumption/range.

jk

At the risk of derailing this thread with a bit of pedantry...

I thought energy consumption/unit time increased with square of speed, not unit distance? Drag (and therefore power) is proportional to speed squared. I did a rough test in my Leaf using the power gauge and it confirmed this. On a steady flat motorway at 50 it was using about 15kW, and at 70 it was using 30kW. So the difference between driving at 70 instead of 50 is 2x consumption per minute, which equals 1.45 x the consumption per mile. Which would make driving at 70 compared to 55 about 1.3 x the energy per mile.

In a Leaf, of course. Your mileage may vary.

Sitting behind a truck - do you have your ventilation on recycling? Or doesn’t it make a difference to air quality in your car?

I think I have it right neglecting rolling resistance, an irrelevant simplification for a typical car/tyre/speed combo.

Aerodynamic drag force increases with the square of speed: drag = (Rho.Cd.A.V^2) / 2

Work done, energy, what we're interested in when discussing battery capacity = force x distance. For a given distance the energy required (measured in kWH) is proportional to the drag force which is a function of V squared.

jk

I tend not to run any ventilation, it eats battery! 

Agree with that; in a simple approximation where the only energy used is work done against drag, then F = kv^2 (where k=1/2 rho C A) and work = Fd so work = k v^2 d.  Over a given distance, energy consumed will be proportional to v^2.

But what Hooo has measured is power.  If work = Fd, then dividing both sides by t gives

P = Fv = k v^3

So doubling v should increase the instantaneous power needed by eightfold.  The data Hooo has given us are that 15kW = k x (50^3) (leads to k = 0.12) and 30kW = k x (70^3) (leads to k = 0.09).  Looking at it another way, if we took the value of k from 50mph, it suggests that at 70mph you'd need 0.12 x 70^3 = 41kW, so the car is performing better at high speeds than we would predict.

The difference in these figures suggests that our very simplified model is not good enough, but it's not a million miles out.  We're obviously missing out energy spent on other functions within the car, against rolling friction, and god knows what else.  

I would guess that Hooo's result (that power looks to be proportional to v^2) is a bit of a fluke?

I just had a go at introducing a constant power drain (ie solving for P - Pnought = k v^3 at both 15kW for 50mph and 30kW for 70mph).

You'd get the predicted P proportional to v^3 if you introduced a constant power loss of about 6.4kW at all speeds, but that seems like a lot of power to be spent on heating, power steering, running the car's internal computer and whatever else.

The rolling resistance is the major effect up to around 50-60mph so not suprising it doesn't add up. Sidewinds are also a major problem which the Cd value doesn't cover, on trucks both the roll resistance and the side-wind effects are greater than the simple aerodynamic drag.

Even tyre temperature makes a measurable difference, after about 20 miles the rolling resistance drops about 5%. Tyre profile is a killer, suprisingly all those tiny slits (sipes) are a real problem which is why truck tyres don't have them. On our dirt bike dyno we use a 15% power correction for offroad tyres.

Hmm, the Leaf's numbers probably aren't far out. Rolling resistance isn't negligable as I stated, it's actually ballpark equal to aero drag at fast cruise speed for a modern car! I'm used to working this out for lighter vehicles where the aero drag massively dominates so didn't bother sense checking that bit.

Also cars are heavy so even subtle hills risk introducing error unless you're very careful with your experimental method. A 1:100 slope at 70mph in an 1700kg car requires (or releases) roughly 5kW.

jk

Just had a quick look - it seems cyclists are very interested in rolling resistance!  The suggestion is that the force of rolling resistance is independent of v, so power spent against it should be proportional to v.

I am bored and waiting for someone to repair my dishwasher, so have just tried to solve this for a general equation power = k v^3 + Rv.  Hooo's data give k = 0.054 and R = 166.  At 50mph, their Leaf is using 6750W against drag and 8300W against rolling resistance; at 70mph it's 18500W against drag and 11600W against rolling resistance.

Hooo: science demands that you verify this.  I predict that at 30mph, you will consume 6.4kW; at 40mph it'll be 10.1kW; and at 60mph, 21.6kW.

I am slightly surprised it's so significant.  Clearly we need to run on solid wheels (and get used to our fillings being shaken out).

In reply to Hooo:

Drag force increases as speed^2

Work done over a distance drag = force x distance 

Power is energy / time energy is converted

Power is force x (distance done) / (time to do distance) = force x speed

Hence total energy used is proportional to speed^2, and power spent is to speed^3

Ignoring wind (including side wind which increases drag force).

I was just about to edit my post and say that after thinking about it I was wrong. Power will increase in proportion to v^3.

So the data from my Leaf was a fluke, the perils of relying on two few data points (like 2)

Indeed.  A nice way to think about is that energy (per distance) goes with v^2, but if I go faster, then not only do I need more energy per distance, I'm also doing more distance in a given time, so my energy requirement per time (ie power) goes up by more than just v^2.

Yep.  It would be really interesting to see if power does follow a rule of P = kv^3 + Rv.  If I can figure out how to get my Niro to give me instantaneous power readouts, I might have a try at getting the data to model this.

Is there an ODBII app for the Niro? The Leaf has an app called Leaf Spy, that will give you all sorts of info including detailed instantaneous power.

It's also why the power available for extraction by a wind turbine goes as the cube of wind speed.  When you look at the actual power output vs speed plots for the big turbines, it becomes clear engineering and financial limits kick in at a surprisingly slow wind speed...

All of that planning goes out the window when it takes 2 hours stop/start from Betws-y-Coed to Llangollen.

Simon...