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Discussion Starter #1
Hey folks,

I was hoping maybe somebody had ideas for increasing EV efficiency.

I have a 1986 BMW 325SE, manual transmission, w/Warp9. I calculate my efficiency at a modest 300-350 Wh/mi (not the best, not the worst).

I also own a Prius. When I "coast" the Prius... it goes for miles. Rolling rolling rolling. While my BMW seems to vapidly lose every precious electron of momentum and maybe get 1/4 the distance for the same flat glide. I check the tire air weekly and fill to maximum sidewall pressure (44psi for both). This is a mostly back-roads commuter, I do about 80% 30-40mph, 20% 40-60mph, 40mi/day. I have 14kwh capacity, that's not the problem, I just want to use less.

I realize the Prius is MUCH lighter, more aerodynamic and has a dozen or so small (and important) reasons why this is so... but is there anything?

- New (better) bearing
- thinner tires
- covering the bottom
- modifying the body (I kinda like the 80's Brat-Pack-Mobile look, but I'd consider it)

-Bruce
 

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You might also look at trying to reduce weight if you can as it affects rolling resistance and power required when accelerating.
 

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I'd be weary about thinner tires. Although that decreases your rolling resistance and plays a tiny part in your drag coefficient, thin tires are not always great for heavier cars, especially when it comes to cornering.

You can check all the connections to your batteries/controller/motor and make sure they're all using the most efficient material at the right size/gauge. Remember, you lose efficiency through heat. I'm actually kind of interested what most people use for their cell-to-cell connections and what seems to work the best. What has the least resistance and so on.

Also, make sure your motor is getting cooled well. They work most efficiently when they're kept cool.

Since you're using a manual transmission, make sure you're keeping the RPM's in the motor's most efficient RPM range.

And finally, check to make sure you're not 'leaking' power through any accessories you're not using or lights that are on that you're not using.
 

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pretty much what kelmark said.

it's better to have too much pressure in a tire than not enough. tires are way more like to blow from the heat generated from the friction of increased flexing caused by lower pressures than by the pressure itself. you'll make your ride a little harsher, but roll easier.

my MTB tires are rated something like 40lbs per square inch, but i've run them at twice that with no problems. of course the load is miniscule compared to a car, but i bet car tires can take twice pressure too. that would be a good topic to google.

if you really want to get insane on your tires, you can use an inert gas like helium or as some race cars do, run extra light hydrogen. (i use the H word every chance i can! LOL)

more practical than that would be swapping your rims to a lighter race style design. at speed, rotational mass has an effect on efficiency. i'd bet too that a softer suspension wastes some potential energy as heat too.

you could also try lowering your weight if possible by replacing body parts with whatever fiberglass you can. as you're driving a beemer, i won't suggest stripping your interior and replacing your dash with lightweight alminum, but any weight reductions you can make should help. if cost isn't an issue, buy a few thousand dollars worth of titanium bolts and cut 100 pounds or so.

you're always going to do better with your prius though. the last car i had was a daihatsu charade (that i beat mercilessly jumping railroad tracks etc. but was NEVER in the shop for over 100k miles!) and was surprised at just how easy it was to push by myself on a level surface. try that with any full size car with 2 people even.

i bet there are also some suspension upgrades like aftermarket tubular A arms you could buy to lighten your ride some too. considering the marque you're working with, expect to pay twice what any such upgrade is worth though.

hope something here helps.
 

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Discussion Starter #11 (Edited)
I'd be weary about thinner tires...
Ok, just BMW sets up all their cars like they're going to the track, very wide, really great handling, but rolling resistance suffers.

You can check all the connections to your batteries/controller/motor and make sure they're all using the most efficient material at the right size/gauge. Remember, you lose efficiency through heat. I'm actually kind of interested what most people use for their cell-to-cell connections and what seems to work the best. What has the least resistance and so on.
I'm running Thundersky 100ah LiFePO4's. My controller is quite weak and is setup to pull 200amps absolute maximum. When I get done from hard driving, everything (the batteries, the controller, the motor, the wires, everything) is stone cold. Zero meaningfull loss there. Accessories pull 0.5-2amps (out of 100ah total capacity, so for a 30min trip... that's ~0.5% total capacity), I'm really not losing much electrically... I'm loosing a TON mechanically.

-Bruce
 

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Discussion Starter #12
...you could also try lowering your weight if possible by replacing body parts with whatever fiberglass you can.
So lighter is a real problem... It's just a heavy "normal" car. Shy of taking a sawsall to big heavy structural parts, there's no way to get meaningfull weight reduction... This is why Tesla chose the Esprit chassis, it's basically a flighter cockpit on wishbone suspension. replacing a few 10lb panels with 5lb fiberglass one's isn't going to save enough weight to make it worth the money, time and hassle. But I'll keep thinking, maybe there's something I can do, that I haven't thought of and I do appreciate the ideas.

-Bruce
 

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Discussion Starter #13
btw, I got the bosses permission to take her to the shop and give her a "check-up"... I'll have them change all the trany fluids with synthetics, grease everything, check the toe-in and alignment and remove any scrubbing.

-Bruce
 

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Hi Bruce

Sounds like you are doing the right things,

Suggestions
Can you compare with the Prius at low speed and at high speed?

What I am thinking is that rolling resistance - bearings, tires and the like are related to weight and do not increase with speed
Aerodynamic drag increases with speed

If the Prius is a lot better at speed and only a little better at lower speeds then you probably want to have a look at an undertray
Have you blanked off the radiator hole - that may help
 

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Most of your gains will come from aerodynamics.

Two significant mods that will not impact your looks are a bellypan and a grill block; these combined would probably cut your highway energy consumption about 25 Wh/mile. A custom rear spoiler could be designed, custom front air dam added, side skirts could be installed, and pizza-cutter style hubcaps could be placed and they wouldn't impact your car's looks a whole lot(it would have that 'tuner' look), but might give you another 10-15 Wh/mile cumulatively.

If you want huge gains(minus 80 Wh/mile more from aerodynamics) without having wind tunnel access, you're going to impact your car's looks significantly.

If you have access to a wind tunnel(highly unlikely), you could figure out how to get aerodynamic gains while maintaining a similar, stock-like style, but you're going to spend a lot of time testing and perfecting that design, and will need to modify the car's body, possibly make custom glass pieces for the windows and windshields. You're also going to spend lots of money going this route; it's bottomless. Very subtle changes in the proportions of a car can be almost unnoticed to the naked eye, but have a huge impact on drag. Note the difference between the Dodge Intrepid ESX2, and the Dodge Intrepid of the early 2000s; strikingly similar in body proportions and looks, but one of them has about 1/2 less drag...

Getting the bearings repacked, the transmission fluid replaced with a more efficient fluid, the alignment adjusted, getting rid of brake drag, is all a very good start. You might see a 10-15 Wh/mile drop in consumption from that alone, maybe more.

LRR tires are a must, especially at steady state low speed driving. There goes another 10-15 Wh/mile at steady highway speeds.

If your EV is less than 3500 lbs, there is absolutely no reason it couldn't get 150 Wh/mile or less at 30 mph. Unaerodynamic, 5,300 lb truck conversions like "Red Beastie" had very basic modifications, LRR tires, and largely ignored aerodynamics; they only need about 250 Wh/mile at a steady 60 mph on level ground with no outside wind! If you have a car that is consuming the same under the same conditions, there is certainly work to do...

Efficiency is the secret to long range. By choosing to improve it, you may eventually double your range, depending on how far you are willing to go with improving it... improving efficiency on a lead acid conversion has the two-fold effect of not only reducing energy consumption, but also increasing the capacity that your battery pack will deliver. The high school team that designed the Fiat X1/9 "Solar Bolt" learned this lesson very well when their car achieved more than 130 miles per charge on a 120V pack of Trojan T145 lead acid batteries! A similar conversion without those eficiency modifications would struggle to get 60 miles under the same driving conditions. Another good example is Dave cloud's "Dolphin"; 200 miles range at 65 mph on 1900 lbs of flooded golf cart batteries, in a 3300 lb car, with extensive streamlining.
 

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What kind of drive drag are you seeing?

Meaning without the vehicle physically moving down the road ... just the drive wheels up off the ground, what kind of load are you seeing to drive at a given rate / speed?
 

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Discussion Starter #17
What kind of drive drag are you seeing?

Meaning without the vehicle physically moving down the road ... just the drive wheels up off the ground, what kind of load are you seeing to drive at a given rate / speed?
These are rough numbers (140v is typical under these loads):

30mph is ~50Amps
40mph is ~75Amps
60mph is ~150Amps (basically floored)

These can be converted to wh/mi and effeciancy as:
30mph is 235 wh/mi (if you didn't ever have to stop)
40mph is 258 wh/mi
60mph is 330 wh/mi

Not horrible, but I want better
 

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Discussion Starter #18
Most of your gains will come from aerodynamics.
It really depend! If you assume I always drive at high speeds and my car's drive system is well tuned, absolutely, you would be 100% accurate. BUT! in reality, I don't drive that much at high speeds and my drivetrain is horribly out of service. I want to start with the easy things, then do the big and hard ones.

I read the whole thing, and I will very likely do the belly pan, I've built an aluminum airplane, so I'm no stranger to AL sheet and pop rivets. I will likely give the belly pan and grill block a go after this. I may glass up a new blunt nose as well, we'll see.

This isn't a research projects, it's a commuter, so the wind tunnel and significant body mods are out. BUT... I'd love to do a hyper efficent commuter EV from an aircraft fuselage some day...

Imagine this fuselage without the rear wings on a 2-seater EV! I don't need a wind tunnel to know this is low drag! Impatient [non-Californian] man's Aptera!

 

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another insanely expensive unobtanium technology would be ceramic bearings. they might not even be made in sizes necessary for a car though or have enough load bearing capacity, but they do wonders for skateboards & bikes.

as to hydrogen filled tires, i guess i could see how tiny hydrogen atoms are more likely to leak than atmospheric oxygen, nitrogen & carbon dioxide, but refill every day? really? they never talk about long term issues like that in indy racing for some reason.
 
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