This is purely to help me understand:
I'm new to EVs having only had my Nissan Leaf for a couple of months. One thing I've noticed is that the car's range massively drops off at 70mph vs 60mph.

Why does a similar range/MPG drop off not occur in ICE cars? The amount of fuel used in an ICE car is not much different between 60 vs 70mph.

If it's purely about aerodynamics then surely both EVs and ICE would have the same issue (assuming they have similar drag coefficient)?

I don't get it.

The friction of air massively increases with speed, and the LEAF is not the most aerodynamic shape ever designed.

 

No but then a lot of ICE cars aren't aerodynamic either. According to this Vehicle Coefficient of Drag List - EcoModder: Leaf has a drag coefficient of 0.28 and a Peugeot 206 (a previous car I owned) has 0.33. The MPG at 60mph wasn't that much better than at 70mph so I don't believe it is just about aerodynamics.

It does in an ICE too, but you don't notice due to the size of the diesel/petrol tank.
When I moved to 60 MPH from 70 MPH in my old XC90, I was amazed how much the MPG increased.

As mentioned above, I didn't see any big changes in my 206 60 vs 70mph.

I kinda thought gears might come into it but at 60mph, I would be driving in the 5th (top gear) of the 206 so the gear ratio was the same between 60 and 70mph.

 

It does in an ICE too, but you don't notice due to the size of the diesel/petrol tank.
When I moved to 60 MPH from 70 MPH in my old XC90, I was amazed how much the MPG increased.

 

When I first picked up my Zoe, I had assumed that getting it home would be well within range. It is mostly fast A-road and motorway, and the range dropped far more quickly than I had anticipated. I did make it home, but with not much energy to spare. The lesson learnt is that for more of a 'town' car the anticipated range is what you get at town speeds. Driving at 70mph drains the battery significantly faster than this.

 

No conventional car is very aerodynamic. If they were they'd look more like slugs.
There is a big drop-off in mpg with speed in ICE cars, you can see this if you have an mpg display and measure mpg against speed in otherwise ideal conditions. But people don't tend to do this! (You can also measure a drop in winter as thermal losses are higher, the air is more dense etc.)

Current EVs with small batteries just make these issues more acute, and there are issues peculiar to batteries- it's like driving an ICE with a 4 litre tank...and the tank shrinks as the temperature drops...and shrinks yet again if you drain it more quickly.

 

It's always been known that higher speed is bad for mileage even on ICE's. It's pure physics after all, @donald will be happy to explain the details
The first thing they did in Sweden for the oil crisis in the 70's was to lower the speed limits.

The traffic nowadays is terrible for fuel consumption with lots of stop and go and (to me at least) extremely high speeds on the motorways. But since fuel is historically cheap, fuel tanks large and petrol stations frequent, no-one seems to be really bothered about range.

Unless you have a current short range EV

 

It's always been known that higher speed is bad for mileage even on ICE's. It's pure physics after all, @donald will be happy to explain the details

[Hiho, OK, I'll wade in for fear of seeing too much pub-talk misinformation bandied around. I was trying to avoid the dubious honour of busting through 10,000 posts! ]

Yes, it is all maths and physics. Mostly boring maths....

An ICE runs more efficiently under load. If, just for 'example', it is running at 25% thermal efficiency at 10% max load (quite typical at 50mph) and 35% efficiency at 80mph (say 33% max load) then the 'brake specific fuel consumption' (that is, the total traction kWh per litre being produced) decreases. However, you need more fuel powered kWh to keep going, so the two things trade off against each other.

In an EV, efficiency only drops as you increase motor speed. You might go from 95% system efficient at 50mph to 85~90% at 80mph (think of battery/inverter losses too, with extra current). Not much in it, but the 'wrong way' all the same.

Now say that to complete a journey at 50mph you need 10kWh of 'real' traction energy and at 80mph you need 20kWh. (Yes, it really could be as high as double.) In an EV you therefore need;

Energy (at 80) = [Energy (at 50) x 2] * [95%/90%] = more than double.

In an ICE

Energy (at 80) = [Energy (at 50) x 2] * [25%/35%] = ~40% extra.

So if you get 5mi/kWh at 50mph in an EV it may be up to 2.5mi/kWh at 80mph.

But a journey at 50mpg at 50mph in an ICE will 'only' drop to 35mpg at 80.

This also stands true for cold and wet conditions. An ICE will run a better volumetric efficiency in colder weather as the air is more dense. So what you lose in increased aerodynamic losses with denser air you gain in better engine efficiency. In wet conditions, what you lose in rain 'stickiness' you gain in extra water vapour in the intake charge, which again improves volumetric efficiency. These are almost 'cost neutral' for an ICE, but again draw down an EV by 10% at 0C (air density) and another 5 to 10% by road surface humidity.

Hope that helps....

[FWIW, this is the theory of the hybrid - by running an engine at '80mph power' yet the car at '50mph speed', you get the high efficiency with the low energy demand. The engine then turns on and off, with the excess energy buffered in a battery.]

 

Petrol is much more energy dense than a battery and so the effect of higher speed/drag is much less noticeable in an ICE.

 

The wind resistance goes up with the square of speed so going faster has a dramatic effect on energy consumption. This is the same in an ICE car. Just think about how much further you have to push the accelerator pedal, your car is consuming more.

 

Isn't energy wastage also a factor? An ICE wastes quite a bit of the energy in the fuel on combustion heat and friction. The proportion of fuel being used for motion is therefore less than in an EV where much of the stored electricity is used for motion. Therefore the change in MPG between 60 and 70 isn't that large (although it must exist, OP, because of rising air resistance) , whereas the much more efficient EV shows up the miles per kWh difference proportionally more.

I haven't made that up, have I?

 

An ICE wastes quite a bit of the energy in the fuel on combustion heat and friction. The proportion of fuel being used for motion is therefore less than in an EV where much of the stored electricity is used for motion. Therefore the change in MPG between 60 and 70 isn't that large (although it must exist, OP, because of rising air resistance) , whereas the much more efficient EV shows up the miles per kWh difference proportionally more.

This.

 

I like the fact that I am travelling at around 1000mph even when I'm stood still, if I could jump high enough I could get to Canada in about 3 1/2 hours without taking a step

I'm still trying to find out if my headlamps will work when I get to the speed of light, or will I need to upgrade

 

I'm still trying to find out if my headlamps will work when I get to the speed of light, or will I need to upgrade

They'll still work, but if it's anything like the cars around here there'll be one where it's only the sidelight showing and the other one will have the bulb upside down and shining in everyone's eyes.

 

Depends on the ev. A small town designed car will usually be less aerodynamic and set up for slower speeds. Going higher starts to not only use more power because of drag but also because it gets to a point where it's drawing more than it's happy with from the battery so some of it is going up in heat for the motor and some of it is going in the battery being less efficient while it's in a higher C range. Tesla's seem to drop off far less than most at reasonable speeds but start to really guzzle when really moving - as you would expect.
A good example of this is the 60Ah i3 and the 94Ah i3 - the 94 seems to drop off a noticeable amount less between 60mph and 80mph than the 60Ah. Same motor, ratio, car, etc so it's only because the battery is having an easier time of it.

 

Do you remember the fuel crisis in the early 2000s where lorry drivers blocked the petrol depots? Everyone was queueing at petrol stations and a lot of them ran out. I had to drive back home from university while that was happening and the M4 was full of traffic doing between 50 and 55mph. It definitely has an impact, but it seems that people would rather get to their destination faster having done 40mpg than get there later having done 45mpg. I wonder if they would make the same trade-off if you were able to show them that the faster journey cost an extra fiver?
This website has more of the ICE info: Mpg For Speed - Fuel Efficiency Vs. Speed
Cheers
Duncan

 

Do you remember the fuel crisis in the early 2000s where lorry drivers blocked the petrol depots? Everyone was queueing at petrol stations and a lot of them ran out. I had to drive back home from university while that was happening and the M4 was full of traffic doing between 50 and 55mph. It definitely has an impact, but it seems that people would rather get to their destination faster having done 40mpg than get there later having done 45mpg. I wonder if they would make the same trade-off if you were able to show them that the faster journey cost an extra fiver?
This website has more of the ICE info: Mpg For Speed - Fuel Efficiency Vs. Speed
Cheers
Duncan

Depends on how long the journey and how much of a rush. If you're doing an 8 hour driving day for work that's a 40 mile difference at the end of it so not insignificant and will certainly have an impact on costs, wages, etc.
If you're popping to the shops it's such a short trip that the difference in fuel use becomes tiny anyway.
I'm sure there is a journey set somewhere inbetween where it becomes worth it but if your time is worth anything to you it's usually not.

 

Gearing. A good reason why ICE 'optimal' speeds are pegged above 50; you need to be in top gear, which makes sense. iirc some ICE have a high 'motorway' top gear to minimise revs/maximise efficiency at 70-80 but should still peak lower due to drag.

Beyond that it's all air & engine efficiency (see above from Donald); a cyclist can *easily* do 90mph behind a motorbike with a big enough gear (not sure about the energy per unit distance there!); as early as the 1800s people were clocking 50-60 behind trains which is very impressive considering the kit.

Slower we (EV) go the better the range, on the whole and I certainly no feeling that 50 is better than 40 (on the contrary, circumstantially) - i3s have no focus on drag coefficient (city car by design). I've not tried to get 100miles at 20mph though...

 

I used to have to drive about 30 miles to work each day and I found that if I drove at a slightly lower speed there was a huge difference in fuel consumption and it only added about 5 minutes to the journey (given that most of the journey time depends on traffic conditions anyway). I think you can almost half your fuel consumption if you drive at 50mph instead of 70mph, for example. People rarely realise this with a petrol car because the tank is so big and so it's hard to see accurately how much fuel you've consumed on a given journey. With electric cars, we can see the exact figures.

In general, going from 50mph to 70mph doubles fuel consumption because the aerodynamic drag doubles. I think there might also be an issue of the airflow becoming turbulent rather than laminar beyond a certain speed, depending on the shape of the car. You know that you have exceeded this speed when the car starts to shake and the airflow noise increases. This could explain the sudden decrease in range between 60 and 70. They probably optimised these cars for 56mph.

 

This is purely to help me understand:
I'm new to EVs having only had my Nissan Leaf for a couple of months. One thing I've noticed is that the car's range massively drops off at 70mph vs 60mph.

Why does a similar range/MPG drop off not occur in ICE cars? The amount of fuel used in an ICE car is not much different between 60 vs 70mph.

If it's purely about aerodynamics then surely both EVs and ICE would have the same issue (assuming they have similar drag coefficient)?

I don't get it.

Really really easy air resistance don't know exactly the figures but hear goes....
30 mph 10% 40 mph twice risistance 50 mph three times resistant 60 mph 4 times resistant 70 mph 10x resistance.. Wow so 70 is a big
When I was a lot younger i still didn't know these figures but to me hadent had much money so putting £1 to £3 worth of fuel had to make drastic eco driving to make every penny last soon realised it got gobbled up at 70 or above... Have been driving frugally for 30+ years . will never stop eather . Ben having EV on and off for 30 years aswell keeping it frugal

 

Finally, my issue of having panic attacks on the freeway is paying off!! Since I try to avoid the freeway as much as possible, I'm happy to be getting a great range with my Nissan Leaf.

 

Often the increase in consumption is masked in small diesel cars that often have a large diesel engine in a small body running at quite low rpm. But as Donald says Electric motors tend to back off in performance terms as rpm increases, This is disguised by cleverly tailoring the power delivery of the controller to give the driver no experience of that loss by upping the power being delivered.
On the small Citreoen C1s speed above 45mph really makes a lot of difference, but what makes a bigger difference is the style of driving.
I try and take out a potential owner of a vehicle and give them some guidance on how to drive, anticipate. If you have to stop at the junction with two cars already there, back off, and cruise up dont just drive and brake at the last minute . The cars will still be stopping you from proceeding when you get there.
I used to drive 20miles across country to Ipswich , and the best I did one day was touching the brakes 5 times.Think ahead and Regen is king.
If you want to see the drop off of consumption at higher speed, drive an auto that doesnt have a trans lock up at higher speed, then you can really see how the losses build up with speed.

 

I tend to retract my leaf wing mirrors at 70 it does seem to make a difference only do it when safe to do so . It's about time cameras were allowed imagine fuel saved on all the ice cars travelling at 90 plus if tiny camera used instead of wing mirror come on change the law help the planet every little bit helps

 

But the headlights are designed to push the wind out of the way, so shouldn't make much if any difference.

 

In the 80s there used to be a rule of thumb, that turning air con on (hence why you should buy air con and have it fitted) consumed no more power than having the windows open at 70mph.
Many evs now have very smooth undersides, that are not so easy to do with ICE vehicles, but smooth airflow and reduce drag. All of the issues of higher power consumption are exacerbated in the cold. Tyre pressures can be way down in the cold, but if you up them now remember to recheck when it gets warmer or every few weeks if you are vigilant on this front. Roof rack and roof bars, dont forget to remove the cross bars when you have finished with them.

 

Yes, it is all maths and physics. Mostly boring maths....

I dispute that maths is boring! Its actually simple in this case Drag equation - Wikipedia

 

Internal combustion engines are most efficient at their peak torque, so travelling in top gear at peak torque revs equates to ~40-60mph. Electric motors are most efficient at low revs so low speed is more efficient. This is compounded by wind resistance growing disproportionately with speed.

 

...

An ICE runs more efficiently under load.
....

I guess nothing more need to be said.

E motors don't change much in efficiency based on load ... actually at high load they can be less efficient

Combustion engine, have best efficiency at max load ... so more power is used and more efficient is the conversion between fuel into energy

Clearly faster the car and higher is the air resistance ..

So ... in traditional car very low speed was not the most economical travel speed ... and once reached the ideal speed, the consumption did not increase as dramatic as the square of the speed (air resistance), but this was mitigated by increased load in ICE

On EV car ... square of the speed has direct impact on efficiency ... slower is always more economical/efficient

PS: ICE have a fix waste of energy also when they not provide any power ... this is also another factor that penalize efficiency/economy at very low speed on traditional car ..

 

I guess nothing more need to be said.

E motors don't change much in efficiency based on load ... actually at high load they can be less efficient

Combustion engine, have best efficiency at max load ... so more power is used and more efficient is the conversion between fuel into energy

Clearly faster the car and higher is the air resistance ..

So ... in traditional car very low speed was not the most economical travel speed ... and once reached the ideal speed, the consumption did not increase as dramatic as the square of the speed (air resistance), but this was mitigated by increased load in ICE

On EV car ... square of the speed has direct impact on efficiency ... slower is always more economical/efficient

PS: ICE have a fix waste of energy also when they not provide any power ... this is also another factor that penalize efficiency/economy at very low speed on traditional car ..

Yes, this is exactly as I now understand it thanks to posts here. All the initial posts saying it's exactly the same between EV and ICE, just that it's less noticeable in an ICE car due to the huge range didn't sit well because it did not match up with my anecdotal evidence of fuel consumption I experienced in an ICE car.

Anyway, I can now sleep easy at night knowing I'm better informed :-D