75, 85, 95 mph motorway miles per kwh ?

[190]

Not sure that's right? Aerodynamic drag is proportional to the airspeed squared,

That is true, aerodynamic drag is proportional to the airspeed squared but the power needed to overcome aerodynamic drag increases by the cube of the velocity.

Drag is proportional to velocity squared.

But the power required to overcome that drag is drag times velocity, which is velocity cubed.

For proof, first the maths - Scroll down to the section on Drag and Power

Aerodynamic Drag – The Physics Hypertextbook.

Much easier, a simple calculator

Top Speed Calculator

I acknowledge that both the maths and the calculator are not quite accurate. In practice you need a bit less power because the calculator considers drag and that is not the only thing that a vehicle has to overcome, there are also frictional and thermodynamic losses from the tyres which don't increase at the same rate. At 95 mph these losses are relatively small compared to drag so the relationship is still substantially much closer to cubed than it is squared.


In an IC engined car we don't notice this near cubed relationship so much because the efficiency of the engine changes dramatically but in an EV it becomes much more of an issue

 

[markjay]

In an IC engined car we don't notice this near cubed relationship so much because the efficiency of the engine changes dramatically but in an EV it becomes much more of an issue

I am not sure that this statement is correct? In spite of gearing, which EVs don't have, the fuel consumption of ICE cars does go up significantly with speed (above 50-60mph).

I would say that we don't talk about it much because with ICE cars there are hardly any consequences (apart for the obvious financial ones), i.e. you just stop and refuel at one of the very many petrol stations that 'litter the landscape', so to speak. But with EVs, range anxiety and the need to find a supercharger along the route makes drivers far more aware of energy efficiency.

 

[Steveml63]

According to Tesla the power will be the same between 90% and 10% charge. The exception being lots of flat out acceleration one after the other like a day at the strip......the power gets backed off very slightly to protect the car. One owner said it works out a drop of only about 0.3 mph of his terminal speed each run.......tiny and un-noticeable on the road. (From a US Tesla owner Forum.....so who knows if that's correct or not. I'm just the messenger!!)

I was going by some videos where owners put their Tesla Model 3’s on dynos and tested them at different states of charge.
From memory - very little difference in power or torque from 100% to 75% SOC.
But it drops off below 75% - and is around 100hp less by 10% SOC.
You can definitely feel the difference on my Tesla between high and low charge states.
Example video here with lots of figures.

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[markjay]

I was going by some videos where owners put their Tesla Model 3’s on dynos and tested them at different states of charge.
From memory - very little difference in power or torque from 100% to 75% SOC.
But it drops off below 75% - and is around 100hp less by 10% SOC.
You can definitely feel the difference on my Tesla between high and low charge states.
Example video here with lots of figures.

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Accept third party cookies

It would be interesting to know to what extent the performance deterioration is a direct result of the battery State of Power, and to what extent it's due to the BMS limiting performance when the battery charge goes down, to preserve remaining mileage range?

 

[Steveml63]

It would be interesting to know to what extent the performance deterioration is a direct result of the battery State of Power, and to what extent it's due to the BMS limiting performance when the battery charge goes down, to preserve remaining mileage range?

Agreed - it’s a fairly complicated system!
I have fettled and tuned most of my previous ICE cars and built a couple of kit cars.
You certainly feel a bit helpless with EVs - as there is so little you can do to improve performance, barring correct tyre pressures, good state of charge and reducing weight in the vehicle!

 

[BTB 500]

That is true, aerodynamic drag is proportional to the airspeed squared but the power needed to overcome aerodynamic drag increases by the cube of the velocity.

Drag is proportional to velocity squared.

But the power required to overcome that drag is drag times velocity, which is velocity cubed.

For proof, first the maths - Scroll down to the section on Drag and Power

Aerodynamic Drag – The Physics Hypertextbook.

Much easier, a simple calculator

Top Speed Calculator

I acknowledge that both the maths and the calculator are not quite accurate. In practice you need a bit less power because the calculator considers drag and that is not the only thing that a vehicle has to overcome, there are also frictional and thermodynamic losses from the tyres which don't increase at the same rate. At 95 mph these losses are relatively small compared to drag so the relationship is still substantially much closer to cubed than it is squared.


In an IC engined car we don't notice this near cubed relationship so much because the efficiency of the engine changes dramatically but in an EV it becomes much more of an issue

Interesting stuff - thanks!

Is the increase in IC efficiency just down to engine rpm and position on the power curve? Maintaining 90 mph would require twice as much power (based on aerodynamic drag) than maintaining 70, but the increase in fuel consumption is nowhere near that!

 

[Tonye]

Steady 65 for me on cruise when possible, enough to keep up and enough to pass wagons. What you lose in time is negligible and saves a small fortune on a long trip, don't get me wrong, if there's some lane hog i just tickle past and cut right across them, that usually wakes them up.

 

[Alfie]

You have an AMG and a motorbike and seriously never break the speed limit? I'm not knocking it, I'm just very impressed.

I have an AMG line C220 estate. Whilst its got loads of torque its no where near a proper AMG sadly. Motorbike - gone. I need to update my signature. I ceerainly dont do anywhere near 95mph. Last time I hit 85 was on a french autoroute (136kph). I do not ever knowingly speed. If I see my display showing a few above the limit, I ease off. No point in rushing, It saves so little time and causes very much heightened stress and concentration levels.

 

[190]

Is the increase in IC efficiency just down to engine rpm and position on the power curve? Maintaining 90 mph would require twice as much power (based on aerodynamic drag) than maintaining 70, but the increase in fuel consumption is nowhere near that!

It is in part due to the position on the torque curve. Efficiency is expressed in specific fuel consumption defined as how much fuel does it take to produce a unit of power. In approximate terms specific fuel consumption is the mirror image of the torque curve. it is substantially affected by other issues such as pumping losses leading to efficiency being very much lower in low load conditions. That goes some way to explaining why an engine might be more efficient at 95 mph that it would be at 75 mph because the best specific fuel consumption is achieved somewhere close to the peak torque revs and under high load conditions. Forced induction complicates it even more but that's the jist of it.

 

[ALFAitalia]

Interesting stuff - thanks!

Is the increase in IC efficiency just down to engine rpm and position on the power curve? Maintaining 90 mph would require twice as much power (based on aerodynamic drag) than maintaining 70, but the increase in fuel consumption is nowhere near that!

I understand the theory......but my car is only running less than 1000 rpm more at 90 than at 70 in sixth. It can't possibly produce double the power at 90 (in fact torque is falling away by then) and mpg is only 5 or 6 mpg worse....say around 50 mpg at 70 against 42 or 43 at 90. I'm not doubting your facts.....but something does not add up somewhere.

 

[philnewmerc]

They are calibrated electronically... can this be trusted?

But joking aside, calibration isn't the issue - the point where the nozzle stops delivering fuel when it senses that the tank is full can vary between pumps, as it's not included in the calibration process.

The ambient temperature does matter, because the fuel in the pipeline and in the pump will be at a different temperature to the fuel in the tank. If you are filling-up 70 litres than the volume change of 2-3 litres might not make a huge difference, but if you are topping-up (say) only 10 litres than it will be more noticeable, especially if there were extreme weather differences between the two filling dates.

My point is that 'accurate' is a relative term, and all that can be said in this context is that a brim-to-brim exercise is potentially less-inaccurate than relying on the car's onboard trip computer.

(I say 'potentially' because I am not actually sure what is the basis for the premise that the onboard trip computer is the less accurate of the two...)

It's about 0.1% expansion per degree. So if there was 10° difference it would be 1%, ie 10 litres might become 10.1 litres. That's not a big difference to be fair.

I think the point about kilowatt hours is relevant, not in a paranoid anti EV sense, but because the brim to brim MPG check is very reliable, but what is the equivalent for EVs? I would assume it is looking at your electricity meter. But does anyone check? I've read that charging losses on EVs are 10 to 15% because shoving electricity into the battery creates heat... Not a big deal but people don't include it usually.

 

[philnewmerc]

It is in part due to the position on the torque curve. Efficiency is expressed in specific fuel consumption defined as how much fuel does it take to produce a unit of power. In approximate terms specific fuel consumption is the mirror image of the torque curve. it is substantially affected by other issues such as pumping losses leading to efficiency being very much lower in low load conditions. That goes some way to explaining why an engine might be more efficient at 95 mph that it would be at 75 mph because the best specific fuel consumption is achieved somewhere close to the peak torque revs and under high load conditions. Forced induction complicates it even more but that's the jist of it.

View attachment 146296

This is a very good explanation. It's in part why Toyota thought that running their engine at best efficiency to charge a battery to then drive the car might be good for MPG.
It's also the case that for spark ignition engines they have a throttle, which literally throttles the intake, which causes lots of losses, and this reduces if the throttle is opened more, ie when going faster... It's all a big balancing act and all cars will behave differently, but I swear blind for my car the difference between 70 and 90 is very small...
It is for this reason that diesels, because they are not throttled, should in theory be much more efficient. Incidentally they are extremely efficient when ticking over, which is why cabbies were always quite happy to leave them running, and why stop start for petrol cars saves far more fuel than in diesels (because petrols have maximum throttling when they're ticking over).

 

[markjay]

It's about 0.1% expansion per degree. So if there was 10° difference it would be 1%, ie 10 litres might become 10.1 litres. That's not a big difference to be fair.

Accuracy is a relative term...

...I think the point about kilowatt hours is relevant, not in a paranoid anti EV sense, but because the brim to brim MPG check is very reliable, but what is the equivalent for EVs? I would assume it is looking at your electricity meter. But does anyone check? I've read that charging losses on EVs are 10 to 15% because shoving electricity into the battery creates heat... Not a big deal but people don't include it usually.

Yes, it generates heat, but this is part-and-parcel.

If you want to calculate the cost per mile, then you need to look at the charging bill or log, check how much kWh you purchased, what was the cost, and how many miles you travelled for your money. If some of it is losses due to heat, that goes into the cost as well.

BTW, fast charging requires a warm battery, if left to its own devices that car will start charging at a low pace until the heat generated allows faster charging to take place. For this reason, EVs are equipped with a battery heater to warm-up the battery as you approach a fast charger so that the fast charging can start as soon as you plug it in. Also, if you charge at home on a slow charger the battery won't heat up (not as much) and the losses will be minimal. And, my EV has a heat pump that captures heat from the battery and uses it to warm the cabin in winter.

In short, heat management is just a tiny part of the sophistication that goes into EVs. The problem is that too many people wrongly believe that a modern EV is simply a scaled-up version of the go karting they used to do when younger, and make assumption accordingly. Modern EVs are hi-tech in every aspect, a lot of money went into their R&D.

 

[Bobby Dazzler]

The fuel consumption in my car barely changes from 70 to 90 mph. I assume this is because the engine is working in a super efficient zone on the RPM scale. But though in most cars the consumption will obviously increase, it certainly does not double. However for EVs this does seem to be the case, not that I can find much hard evidence...

There is more to it than aerodynamic drag, aerodynamic drag just becomes more significant as a proportion as speed increases.

 

[philnewmerc]

There is more to it than aerodynamic drag, aerodynamic drag just becomes more significant as a proportion as speed increases.

Maybe it's because EVs have a true reflection of power required to beat drag, whereas ice cars have an odd power/efficiency relationship whereby they are least efficient on a light throttle (big pumping losses) and max efficient at open throttle (or more open throttle).

 

[philnewmerc]

Accuracy is a relative term...



Yes, it generates heat, but this is part-and-parcel.

If you want to calculate the cost per mile, then you need to look at the charging bill or log, check how much kWh you purchased, what was the cost, and how many miles you travelled for your money. If some of it is losses due to heat, that goes into the cost as well.

BTW, fast charging requires a warm battery, if left to its own devices that car will start charging at a low pace until the heat generated allows faster charging to take place. For this reason, EVs are equipped with a battery heater to warm-up the battery as you approach a fast charger so that the fast charging can start as soon as you plug it in. Also, if you charge at home on a slow charger the battery won't heat up (not as much) and the losses will be minimal. And, my EV has a heat pump that captures heat from the battery and uses it to warm the cabin in winter.

In short, heat management is just a tiny part of the sophistication that goes into EVs. The problem is that too many people wrongly believe that a modern EV is simply a scaled-up version of the go karting they used to do when younger, and make assumption accordingly. Modern EVs are hi-tech in every aspect, a lot of money went into their R&D.

Very interesting. It has been worth starting an argument on here for all the useful info that isn't well known!

 

[Bellow]

I understand the theory......but my car is only running less than 1000 rpm more at 90 than at 70 in sixth. It can't possibly produce double the power at 90 (in fact torque is falling away by then) and mpg is only 5 or 6 mpg worse....say around 50 mpg at 70 against 42 or 43 at 90. I'm not doubting your facts.....but something does not add up somewhere.

Firstly, yours is CI - which behaves differently from SI. Secondly, you row back on the throttle pedal at 70mph (or it would keep on accelerating). At that reduced pedal setting, the power output could very easily be half of what is required for 90mph.

 

[Tonye]

When i used to engage the flux capacitor on the old SL mpg actually went up and so did the fan belt, in flames

 

[Bobby Dazzler]

I don't have any real world data but the laws of aerodynamics will tell you that 95 mph requires double the power compared with 75mph.

I can only imagine that EV range is truly dreadful at high speeds. It doesn't sound like the right type of motive power for you.

For the CLS 55 AMG the aero drag is
490 N @ 75 mph
787 N @ 95 mph

For the CLS 55 AMG the running resistance is
943 N @ 75 mph
1239 N @ 95 mph

For the CLS 55 AMG the running resistance power is
31.61 kW (or 43 PS) @ 75 mph
52.62 kW (or 72 PS) @ 95 mph

So aero drag is 61% more at 95 mph than at 75 mph, and it requires 68% more power to maintain the higher speed.

 

[philnewmerc]

For the CLS 55 AMG the aero drag is
490 N @ 75 mph
787 N @ 95 mph

For the CLS 55 AMG the running resistance is
943 N @ 75 mph
1239 N @ 95 mph

For the CLS 55 AMG the running resistance power is
31.61 kW (or 43 PS) @ 75 mph
52.62 kW (or 72 PS) @ 95 mph

So aero drag is 61% more at 95 mph than at 75 mph, and it requires 68% more power to maintain the higher speed.

Very interesting but where do your numbers come from? What is running resistance a total of? I'm not doubting it, just very interested.

I did the brim to brim MPG check a few days after I got back from the southwest to the north east, 430 miles, 78 L, super unleaded Momentum, average speed only 63 mph but was a combination of a couple of hours of busy A30 and M5 and a constant fast lane occupation () of perhaps 70 to 95 nph cruising speed depending on conditions (i e. if there's anything in front of me, and if the speed differential between me and the lane on my left is not dangerously high), then about 45 minutes of very slow town driving at the end. True MPG was 25.1* and the trip computer read 24.2. This is the first car I've ever had which regularly underestimates its MPG by about 1, most others have been optimistic, sone slightly, some wildly, with the diesels being the most optimistic (for comparison my C270 CDI used to tell me high-40s when in actual fact it was low to mid-30s mpg).

*I'm also wondering if the 99 ron helped with one or two MPG increase... Going to try and monitor this on fuely.

Where the CLS 55 is truly appalling is trundling around town, where 10 to 15 MPG is typical (I usually cycle in that case anyway unless I'm giving someone I left or picking someone up from the station). Obviously this is where EVs are king, and ice is quite frankly quite stupid, like a steam engine vs diesel electric, dissipating enormous amounts of heat in order to produce a tiny amount of motive power. However, every time I kickdown and hear the growl, I crack a little rye smile