Eletric cars are they worth it - do you have one

Martyn1981

Hi Herzlos. Not scientific, but in nice weather, cruising at 70mph I can get about 4m/kWh (250Wh/mile) in a Tesla 3 or Y long range. But a couple of years back, I did a long drive, around 600 miles in two days in the TM3, and most of it 'might' have been at around 85mph, which reduced efficiency to about 3m/kWh (333Wh/mile).

Recently did a trip to a friend about 60miles away, pretty much all motorway at 65-70mph. Was 230Wh/mile there, and 270Wh/mile home. The outbound trip ended with an uphill section, the return started (naturally) with a downhill. So can't explain the energy difference, as weather seemed lovely, so can only assume wind assistance on the way, and against me on the return.

Regarding your 50 mile range for high speed ICE, it rang a bell - For super/hypercars at 200mph+, they will only be getting a few miles per gallon:

According to Bugatti's chief vehicle engineer, Jens Schulenburg, at top speed (approximately 255 mph), the Veyron Grand Sport depletes its full 26.4-gallon gas tank in 12 minutes. This means that you could cover approximately 51 miles in a rather short time period.

born_again

ElefantEd said:

The reduction in efficiency as speed increases is particularly noticeable in EVs as you get constant feedback, which you tend not to in ICEs. What I've found is that so long as you drive at a constant speed, 50-60mph is actually quite efficient (4 miles per kWH), about the same as changing speed a lot but at a lower average (in different situations of course). Going at 70mph reduces this quite a bit, so unless I'm in a hurry I tend to go at 60 on dual carriageways to save money.

When driving an ICE this reduction in fuel economy is much less apparent!

Which when converted to MPG is 160 mpg 😶‍🌫️

Anyone seen a petrol or diesel do that @ 70mph?

90 miles from Lanc to Yorks via M 62 saw 4.5kWh when picking up E-Niro. Suffice to say, not hanging around as did not realise that concept of speed is harder to judge in a EV 🤦‍♀️

finbaar

I've enjoyed reading this thread and I'm definitely interested in watching the development of electric cars over the coming years. They are no good to me at the moment as I can't charge at home - I live on a main road, and the "local" charging stations are the low power variety and these are at least 3 miles away and there are only a handful of them anyway. I think the nearest fast/high power charger is 25 miles away. Hopefully the tech will change and we will swappable batteries or pumpable charged liquids in the future. I think having a built in battery you have to plug and wait for it to charge is a dead end. I'm keeping my fingers crossed that it is anyway!

MattMattMattUK

Herzlos said:

ElefantEd said:
When driving an ICE this reduction in fuel economy is much less apparent!

I always thought that was because the engine was only 30% efficient anyway. So any increase in mechanical inefficiency was masked by the fuel wasted to heat/noise, but I could never figure out how to calculate it.

So if you doubled the wind resistance (by going from ~60 to ~80) you'd only see the fuel economy drop by 1/6th with ICE but half with an EV.

I'm struggling to find any figures on EV economy at higher speeds, best I can find is for a model S (top speed 163mph) is up to 80mph. That's showing a range of 200 miles @ 80 vs about 250 miles at 70, or a 20% reduction.

Crudely extrapolating the graph here https://www.tesla.com/en_gb/blog/model-s-efficiency-and-range up to 160mph gives it a range of about 50 miles.

Admittedly I can't find any figures on what the economy an ICE car like a BMW 530d is at 155mph either, but I'm sure it'll manage more than 50 miles per tank, which would be barely 4mpg.

Of course I'm taking it to a logical extreme; virtually no-one is spending any real time at 3 digit speeds.

The best example I have seen of it with crazy cars is a Bugatti Veyron 7,993cc engine at 1,001 PS. Driving at an average of 60MPH it will cruise along at 14-16 MPG, driving at top speed of 258MPH it will manage 3.1MPG, depleting it's tank in a little less than 12 minutes/51 miles, which is fortunate as at that speed a new set of tires will be stripped to the fabric in around 14 minutes, or, cruising at optimal speed it would do 400-450 miles. So whilst a rather extreme example it is an important factor.

One factor in Tesla cars beating every other EV on the road in terms of miles per kWh is that they have very low drag coefficients. The above mentioned Veyron, when lowered to track height and with the car in straight line speed trim has a drag coefficient of 0.36, a Tesla Model 3 has a coefficient of 0.23 which is the lowest of any road legal production car and with the refresh is expected to drop to 0.21. The top of the range sport trim BMW 5 series EfficientDynamic claims a drag coefficient of 0.22, however from what I have read it was 0.27 in independent testing and the previous model was 0.31. Most SUVs are >0.40, some as high as 0.55.

Little things make a difference, the Tesla aero covers for the wheels save around 3-5%, getting rid of wing mirrors (replaced with cameras) saves 3-6% on fuel efficiency, things like making sure the wipers are out of airflow when not in use, smaller gaps between doors and body, bonnet and lights, minimal grill etc. With EVs they have pushed hard, I suspect the improvements will go even further, particularly as more manufacturers start making EVs from scratch, rather than reusing existing ICE platforms with some batteries and motors bolted in. 

ElefantEd

finbaar said:

I've enjoyed reading this thread and I'm definitely interested in watching the development of electric cars over the coming years. They are no good to me at the moment as I can't charge at home - I live on a main road, and the "local" charging stations are the low power variety and these are at least 3 miles away and there are only a handful of them anyway. I think the nearest fast/high power charger is 25 miles away. Hopefully the tech will change and we will swappable batteries or pumpable charged liquids in the future. I think having a built in battery you have to plug and wait for it to charge is a dead end. I'm keeping my fingers crossed that it is anyway!

Not being able to charge at home (or, second best, reliably very close to home) is the number one factor why EVs aren't for everyone at the moment. The range issue is very much secondary in my view, and a red herring for all but a few edge cases.