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Discussion Starter #1
following a discussion the other day with a friend, I became interested in the differences in how energy is used, comparing ICE and EV, we've all heard how ICE cars waste almost all of the energy in fuel to heat, and that how EV's motors are almost perfectly efficient at converting electricity into motion, however, I also became interested in understanding this when I read how important low rolling resistance tyres are to EV's and the same with Aero Efficiency. In my attempt to work out how to explain this to my friend, this was my first thoughts :

the numbers are a little bit off the top of my head, but perhaps someone who understands better can correct me.

the point of my discussion was the prove the following :

1, EV's just use less energy, full stop (not to do with cost, just energy usage)
2, Aero is more important to EV's (it's a bigger percentage contributor to overall energy usage)
3, Rolling resistance is more important to EV's (same reason as '2')

obviously we have two very different fuels here, but I was trying to imagine them as 'units of energy'

((feel free to edit an re-post if you have knowledge in this area and can improve the accuracy of what's below))

ICE at a constant motorway speed uses : (in units of energy)
propulsion 2 **
wasted to heat 9
aero drag 4
rolling resistance 1

EV at a constant motorway speed uses : (in units of energy)
propulsion 2 **
wasted .5
aero drag 4
rolling resistance 1

so, since the 'wasted to heat' aspect basically doesn't exist in an EV, doesn't the aero and rolling resistance (which to some degree can be improved) become more important, than what they are in an ICE car which had a large chunk of it's energy going to waste ?

** actually, I've totally confused myself now, if you could take the aero resistance out of the equation completely, imagine a space shuttle in space, then I'm not aware it requires any energy to keep moving when there's no aero drag ? so should propulsion be nearly nothing in the example re the cars ? and if so the Aero drag and rolling resistance are doubly more important to an EV than to an ICE ?

Jesus I'm boring !

how long have I been stuck in this house for ? :(
 

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Start again. You can get essentially the same car, Hyundai Ioniq, in plain petrol + mild-hybrid for tiny efficiency boost; petrol + plugin Phev with about 30 miles electric range, and pure electric with 120 to 200 miles, depending on MK1 or 2.
Aerodynamically the same (trivial diffs at front grille maybe), maybe the Bev has low rolling resistance tyres. Aerodynamics is important to them all for energy saving. Difference is, the petrol will have about 450 to 600 mile range at a guess, and petrol stations are plentiful and easy to use with debit card. But the Bev is clearly very range limited, even the Mk2 isn't as much as we'd all like, so that's why we keep an eagle eye on every erg of energy we use. Petrol drivers can afford to be wasteful, that's how we were all brought up! 30-40% of the petrol energy goes into motion, tough. The rest is wasted as heat, some of which is useful heat for the cabin. End of story really.

Energy consumption making the EV is a totally different discussion.
 

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how long have I been stuck in this house for ?
Too long.

But for the sums you are trying to do you have to include the supply chain that gets the energy to the car and then wheels. Ignoring renewables the burning to power of an EV is done at a power station.

Drag is only 'more important' for an EV because they tend to have shorter range 'tanks' than ICE equivalents and are a drag to refill mid-journey.

[Edit: Much what HandyAndy says actually. Cross-posted.]
 

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Discussion Starter #4
thanks for your reply HandyAndy, however it wasn't a question about the running costs, range, or practicality of a particular power train, rather energy usage and the importance of rolling resistance and aerodynamics applicable to EV's

(I totally understand it's irrelevant to anyone wanting, or bothered to calculate the costs of their motoring, but cost wasn't part of the question :)
 

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Discussion Starter #5
I don't understand the sentence "Ignoring renewables the burning to power of an EV is done at a power station." if you're talking about where the energy that powers our EV's come from, then that also not part of the question (perhaps the energy currently also has an environmental cost to it..... but at least it has the chance to be produced cleanly in the future.... actual total energy usage of the EV is lower than the ICE vehicle, even if it's not produced in an environmental manner, because there's less wastage at the point of use)

The question was purely about where the energy goes in relation to propulsion, aero drag, rolling resistance and other relevant losses
 

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Discussion Starter #6
did you see postings, or the discussion of why rain makes a difference to EV range ? if you did, you'll understand what I'm getting at with my question ! ...... if you didn't see, or don't understand why rain makes a difference to EV's then probably you won't have the answers to my original question, although thank you very much for reading and for your comments so far :)
 

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The question was purely about where the energy goes in relation to propulsion, aero drag, rolling resistance and other relevant losses
Ignoring friction in the ICE engine/transmission they are the same. The propulsion system doesn't change the laws of physics.
EVs tend to be heavier than ICE equivalents, so rolling resistance will be a bit higher.
 

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You are looking at this the wrong way. Just accept that EVs are vastly more efficient in the way the energy is used but unfortunately batteries don’t store much energy per kg weight compared with liquid fuel (or gas come to that). About 10 times less I think but others can correct me. So, in order to get a usable range for a given size of battery the manufacturers can reduce energy losses by reducing wind resistance and tyre resistance until it gets to the point where the car doesn’t work as a practical vehicle in terms of handling or load carrying ability - plus aesthetics of course.

You can do the same with an ICE to improve fuel efficiency but the design compromises may not be worth the energy savings and will hardly impact the range which is not an issue with petrol or diesel cars anyway.
 

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Discussion Starter #9
and ignoring the small matter that the Ice wastes about 80% of it's energy as heat ?

I wouldn't say there's a massive difference with the model 3 against similarly sizes cars, but I'll give you 10% or more
 

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I think that comparing the energy level of gas (petrol) to electricity helps to make this accessible to ICE drivers. One (US) gal of gas (petrol) is 35 kWh, so my Niro's 64 kWh battery equals a little less 2 gals. That can take me about 250 miles. So, ICE cars would need about 10 gals for that distance, so they are roughly 5 times less efficient.
 

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Discussion Starter #11
thank you ColA for your reply, unfortunately everyone so far is missing the point. I'm not asking about cost per mile, or range, just energy consumption.

I was just interested in how these factors play out in the two different types of vehicles, and how I figures that rolling resistance and aero drag were more important for EV's than ICE due to ICE already having massive losses through wasting 80% of fuel energy on heat.

have you seen the posts explaining why wet roads make such a difference to range of EV's, if you've see this, you'll understand what I'm getting at ?
 

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The reason we are missing the point is that there doesn’t seem to be one, anyway, the weight of the vehicle is a bigger factor than drag or rolling resistance.
 

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Discussion Starter #13
thank you for your reply davidtm, but the question is about where the energy goes, ICE is certainly much less efficient in terms of usage of energy because so much is wasted to heat, some say 70-88% is wasted

a quick calculation would indicate that an ICE car might use 120,000kJ (@45mpg) where as an EV might use 44,000kJ (@3.7kWh/mile)

question still is though, is aerodynamics and rolling resistance more important to an EV due to the fact that it accounts for a higher percentage of wasted energy than what it does in an ICE which wastes more energy through heat ?
 

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Discussion Starter #15
there is a point, and the weight of the vehicle has almost nothing to do with the energy cost at a constant motorway speed
 

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Discussion Starter #16
I'll all on a physics forum, but my god, even I'm bored of this now !
 

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question still is though, is aerodynamics and rolling resistance more important to an EV due to the fact that it accounts for a higher percentage of wasted energy than what it does in an ICE which wastes more energy through heat ?
I understand your question - and answer is yes. Whilst we could no doubt spend a long time doing the sums (I've not been in the house long enough yet...), the general direction of the analysis in your original post is absolutely right. ICE and EV vehicles all share aerodynamic losses etc, however ICE spends probably 60-70% of energy as wasted heat whereas EV is maybe 5-10%. Therefore making (e.g.) aerodynamic improvements gives proportionally lower return for ICE vehicles.
 

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As the ice is only 20 % efficient then the difference of rolling resistance aero etc makes a small impact on the overall picture. Putting low rolling resistance tyres on an ice might raise 20 to 21% that isn’t worth much proportional to the waste.
For the more efficient EV it makes more of a difference since the waste is not the dominant energy use.
Is that where you were headed?
 

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Discussion Starter #19
Thanks for the replies, and yes, those were my thoughts. I wonder if tyre manufacturers and indeed companies making aftermarket wheels will target aero and rolling resistance gains to cater for EV buyers, together with tyre manufacturers who are hopefully working on lower rolling resistance thresthat don’t give up grip as a side effect ?

Cheers guys :)
 

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This is for a very powerful electric skateboard, but the same idea as an electric car:

The chart represents full thottle acceleration and can top out at about 32mph on a 31.5% slope (steepest hill in San Francisco, CA). This can be seen from where the purple vehicle thust line dips below the yellow wind drag + slope force line in the bottom left chart (tire rolling resistance is ignored). Notice the green line, top left chart at 32mph shows between 90% and 95% electrical to mechanical conversion efficiency. An internal combustion engine tops out at about 20-35% efficiency by comparison.

129826


A gallon of gas has roughly 33kWh of energy but you can only utilize 20-35% of that as useful energy. By comparison an electric battery charged to 33kWh allows you to use 90-95% of that energy.
 
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