In an emergency what speed is the most energy efficient?

[joninmelbourne]

Hi there,

With petrol cars I was always told if you are running out of petrol 56mph or 90kmph is the most efficient speed to drive to try and get to the next petrol station. If we are running out of charge, what speed is the most energy efficient to get to the next charger?

 

[garyjac]

How long is a piece of string? Depends on how far that charger is abd what the weather is like I would guess? No one can really say without a specific set of circumstances. However, electric motor energy usage is pretty linear and slowing down to as low as 40 kph can save more and more and (a little) more energy. Much below that has little or no effect, maybe because the car is overcoming the inertia of running up and down the inevitable dips and rises in the road. Anyhow, I would say RichardMcN is in the best position to talk long distance and energy saving, as is MDK.

 

[RichardMcN]

Thanks @garyjac

Theoretically 22mph (35km/h) but there are a lot of factors at play.

One major factor is traffic volume and road conditions. On a straight open road with very light traffic, you can go as slow as you want and people can go round you no worries. Otherwise you will be having to let people past and the stop-starts will chew up more charge than you can afford. If a huge truck is coming fast from behind at high speed and can't overtake, you are pretty well forced to go straight up to 80-90ish until he gets round.

Aircon must be off if you are after range. Otherwise the slower you go, the greater the aircon consumption on a per km basis.

Wind speed and direction can really upset you at low speeds, as can hills.

Also what can you stand? Are you mentally capable of driving at 35km/h in a 110 zone? 50 is my absolute psychological limit - beyond that I would rather get towed.

If you are really going for max range go slow from the start, then you can speed up a little if you are going well !

@MDK is big on slipstreaming road trains - hopefully he will describe his technique which includes actually stopping with the road train at the roadhouse and pouncing again once he has refuelled !

Model S Efficiency and Range

 

[RichardMcN]

And some more from @Techno-phile ...

Just been doing a trial of energy use on a flat stretch of road with no traffic (5:00 am) and minimal wind (maybe slight tailwind).
Speed/energy kph/wh per kms
50/125,60/130,70/141,80/153,90/165
However energy use is incredibly sensitive to any slopes or wind.

 

[garyjac]

I can add my small confirmation that slip streaming works well. I've been under 100 Wh/km behind big rigs...but, that supposes that the road is good (no loose rocks!) and that you can find the aforesaid "big rig"

 

[Chuq]

I know someone who just drove their Nissan Leaf from Hobart to Adelaide. He used truck slipstreaming to massive benefit particularly on the Horsham to Adelaide leg!

 

[ICUDoc]

I can add my small confirmation that slip streaming works well. I've been under 100 Wh/km behind big rigs...but, that supposes that the road is good (no loose rocks!) and that you can find the aforesaid "big rig"

How far behind said rig must you be, garyjac?

 

[TesAus]

Drafting Behind Trucks: Does it Work?

 

[garyjac]

I've drafted (very occasionally) 5 to 7 lengths behind and it is quite amazing the difference it makes. Not really game enough to get closer at 90 - 100 km/hr. As the reviewer said (above), "there are better ways to save fuel", but I trust the car to keep a weather eye on things, especially now with the "look ahead" radar setup. It may be more significant for our cars at greater distances than in the review quoted because of the exceptionally low Cd.

 

[FlatSix911]

Interesting reading Model S Efficiency and Range
The Tesla Graphic assumes the following conditions:

• Constant speed (such as using cruise control)
• Flat ground, no wind
• Climate control OFF or using vent only (no heat or air conditioning)
• 300 lbs of vehicle load (driver plus passenger or cargo)
• Windows up, sunroof closed
• Tires inflated to recommended pressures
• New battery pack (<1 year, <25,000 miles)

Thanks @garyjac

Theoretically 22mph (35km/h) but there are a lot of factors at play.

One major factor is traffic volume and road conditions. On a straight open road with very light traffic, you can go as slow as you want and people can go round you no worries. Otherwise you will be having to let people past and the stop-starts will chew up more charge than you can afford. If a huge truck is coming fast from behind at high speed and can't overtake, you are pretty well forced to go straight up to 80-90ish until he gets round...

 

[joninmelbourne]

so 35kw/h and would autopilot use more or less energy? Think 35km/h would too low on normal roads even in emergency as you would get a hoon crash into you. 50km/h I would be ok with, windows up to reduce drag, no a/c and how about stereo off and dim the screen? Or only a tiny difference with the screen dimmed probably. And range mode on.

 

[Miggy]

Please keep all measurements in metric as we are in 2016 and not in the USA.

 

[Saghost]

so 35kw/h and would autopilot use more or less energy? Think 35km/h would too low on normal roads even in emergency as you would get a hoon crash into you. 50km/h I would be ok with, windows up to reduce drag, no a/c and how about stereo off and dim the screen? Or only a tiny difference with the screen dimmed probably. And range mode on.

Autopilot uses more energy than a careful driver, because it does more accelerating and braking. In a steady drive at the speeds being discussed, the difference is probably trivial.

Neither the stereo nor the screen consume enough power for the difference to be measurable. A/C does matter, though, and so will the windows.

As the chart showed, you'll use more energy at 50 than 35, though the difference is fairly small.

 

[Blue heaven]

Although there is a noticable improvement in range by drafting a truck it's not a recommended practice, in fact following to close behind any vehicle increases the risk of the following vehicle getting hit by objects that appear faster than can be avoided, stones, shredded tyres, road kill and all manner of junk that fall off trailers and Utes because it hasn't been secured properly. Its also handy to remember that some truck drivers are very inquisitive, if you're following close they'll likely drop the left rears onto the gravel edge to see how the Tesla paintwork stands up to stone damage.

 

[moemoke]

In answer to @joninmelbourne question, the slower the better. It's just not safe to go very slow on our roads.
I've never heard of the 96kmh is the most efficient speed, so many variables and depends a lot on the car.
I read in an ICE engine design book that the most efficient speed is shown on a dyno graph where the power
and torque curves cross which is usually in the 2-3000 rpm range, I'm not sure how that equates to a Telsa though.

Another general rule is 'to go twice as fast you need 4 times the power'

Simon Hackett did 501k in his Roadster and an average of 40kmh

 

[Saghost]

In answer to @joninmelbourne question, the slower the better. It's just not safe to go very slow on our roads.
I've never heard of the 96kmh is the most efficient speed, so many variables and depends a lot on the car.
I read in an ICE engine design book that the most efficient speed is shown on a dyno graph where the power
and torque curves cross which is usually in the 2-3000 rpm range, I'm not sure how that equates to a Telsa though.

Another general rule is 'to go twice as fast you need 4 times the power'

Simon Hackett did 501k in his Roadster and an average of 40kmh

Where the power and torque curves cross?

That's a single fixed point defined by the units of power and torque used, and generally won't be the most efficient point.

You really need a BSFC map to know for sure, but in engines without multiple cam profiles, nitrous, or enrichment the most efficient point is generally at full throttle and the engine speed where torque peaks - the highest volumetric efficiency, where the engine "breathes" best.

Of course, that's just the engine, and because our collective preferences for acceleration, no modern car is operating anywhere close to that point at any reasonable speed - the engines are all too big for that, even on hybrids.

 

[Shock-On-T]

The important thing to remember is that a 10kph difference at high speeds matters much more than a 10kph difference at slower speeds.

This is because (as has been mentioned here) the drag is at a squared relationship to the velocity.

For example, the squares of 3,4 and 5 are 9, 16 and 25.

Going from 3 to 4 (up 1) takes the drag from 9 to 16 (up 7).
Going from 4 to 5 (up 1) takes the drag from 16 to 25 (up 9).

Going from 10 to 11 would increase drag by 21.

In other words, there's diminishing returns the slower you go. You'll save a ton by dropping 110 to 80kph, but far less dropping 80 to 50kph.

 

[Shock-On-T]

Safest way to slipstream trucks is to use TACC or autopilot because it's always watching (lidless eye wreathed in flames?) plus the Tesla will always be able to out-brake a truck.
Main risk is someone merging into you.
Also confirm your windscreen excess on your insurance.
Does save a lot of electrons though.

 

[Techno-phile]

I converted the range v constant speed into metric, and it seems too ambitious based on my experience over my 5000 kms round trip to Broome (and even more so for the trip with MDK across the Nullarbor since his car uses more energy than most).
I certainly could not get 380kms of range at 110Kph, or 550kms at 80kph, or 700kms at 50kph.
RichardMcN quoted my figures in his post above which give less range based on 76KwH of usable battery in a S85.

I copy my report to our WA group of owners, which includes some efficiency measures we used. Would be interested to hear your comments on these.

The 3 phase electric highway to Broome is now open.

Rob & Rob researched this the hard way on single phase.

Thanks to AEVA for sponsoring 3p+n outlets at Billabong, Nanutarra, Whim Creek and The Ranges Resort Karratha;
And to Tesla for their destination chargers at Pinnacles Edge Cervantes, Ocean West Geraldton (now being installed), Sea Change Carnarvon and Cable Beach Club Broome.
The other Roadhouses like 80 Mile Beach (20amp), Pardoo (20amp), Fortescue, Overlander and Wooramel already had 3p+n sockets.

All are happy to supply the electricity if accommodation or meals are purchased but Nanutarra asks for a donation to the Royal Flying Doctor in addition.

Since we only have a single 11kw charger on board we could only draw 16 amp 3 phase, so we could only drive for a maximum of 7 hours @90 kph and charge for 5 hours each day (plus the overnight charge). A dual charger 22kw car could drive for 7.5hours @110kph and charge for 4.5 hours each day.
We actually took four days to drive the 2400 kms in daylight hours, with three overnight stops for night charging so were able to explore the towns and bush on foot during the day charging and actually see the scenery passing at the leisurely 90 kph.
A dual charger car might be able to make Broome in three days and two overnights, driving only in daylight, although both Pardoo and 80 Mile Beach only have 20amp 3p sockets.

We had to use energy and time efficiently with our single on board charger, so developed the following techniques.

Since regen is not very efficient (although better than nothing) we tried to use kinetic energy instead. Ie very slow stopping with slight power on to avoid regen, and constant power rather than constant speed over hills so that the kinetic energy rather than battery energy took the car over. Speed decreases on the way up and builds up on the way down with no regen.

Secondly having calculated our most efficient speed in still air of 90kph (allowing for the time our single charger car can replenish the electrons at 11kw), we deducted 3/4 of any headwind vector and added 3/4 of any tailwind vector from this target speed. This meant going as slow as 60 kph at times.

A dual charger car can top up its electrons at 22kw, so the most efficient speed is 110kph plus or minus the wind vector (hence the 7.5 hours driving at higher speed and energy consumption plus 4.5 hours day charging).

Efficient in this context is not maximum range for a given speed, but maximum range in a given time allowing for driving and charging.

Finally, on the run into the overnight destination, the aim was not to waste any electrons (no stale ones in the tank). To do this we established a 20 kms buffer between the kms in the tank and the kms to destination (by driving slowly with Ac off if necessary, so that the dotted line on the energy consumption graph < solid line). Then we increased the speed (with two guys) or added Ac (with my wife) to maintain that buffer ( dotted line = solid line). Once the kms to go were less than 10 times the buffer, and as long as no adverse windshifts or elevation increases were predicted, we gradually ate into the buffer by increasing the speed ( dotted line > solid line).

One section was challenging since we had not tightened the adaptors sufficiently so that the car had only drawn 10 amp 3p (ex 16/32 amp available) making us quite late. We had both a time and an energy deficit so were having to play these off continuously, arriving with 2 minutes and 7 kms to spare.


Finally, the Tesla destination charger at Cervantes had been vandalised while we were up north, so we were reduced to charging on 10amps through the motel door, surrounded by all those ICE addicts. Tesla are sending two new units to Cervantes immediately – brilliant service.

 

[Techno-phile]

Safest way to slipstream trucks is to use TACC or autopilot because it's always watching (lidless eye wreathed in flames?) plus the Tesla will always be able to out-brake a truck.
Main risk is someone merging into you.
Also confirm your windscreen excess on your insurance.
Does save a lot of electrons though.

Yes we used autospeed behind a truck on the way to Broome with a large saving of electrons but a couple of chips on our bonnet.