Kazinky
Posts: 1
Joined: Thu May 12, 2022 1:22 am

Rex replacement

Hello, I wanted to know if it would be possible to replace the built in range extender with something else, perhaps something more efficient.
eNate
Posts: 928
Joined: Mon Sep 09, 2019 5:33 pm

Re: Rex replacement

Yes, Wokeby makes an i3 trunk Extension, it's about 300 pounds lighter and therefore more efficient. 😉

But since you're talking about a replacement REX, I haven't heard of it being done – definitely no mention of aftermarket replacements. I've only seen a couple of owners reporting they removed the REX (due to a mechanical issue they didn't want to pay to repair).
2 0 2 1   B M W   i 3   1 2 0 A h   B E V  O a k l a n d
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lunacyworks
Posts: 1
Joined: Fri Jul 02, 2021 2:19 pm

Re: Rex replacement

I have been considering the same thing. I would be interested in anything you find. BTW what is the efficiency your current REX runs at?
JohnWasser
Posts: 95
Joined: Sat Mar 07, 2020 6:15 pm

Re: Rex replacement

Ouch. I just did a rough estimate of the efficiency of the REX. It gets about 2 kWh per liter. The energy density of the gas is 8.76 kWh/l so that would make the REX about 23% efficient.

One question is what measure of 'efficiency' did you want to optimize? Volumetric efficiency (kWh per liter)? Mass efficiency (kWh per kg)? Value efficiency (kWh per unit of currency)?
MagicCarpet
Posts: 62
Joined: Fri Mar 08, 2019 1:17 pm

Re: Rex replacement

My i3 Rex specs are 200km from battery net output of 27.5 kW/hr = 7.27 km per kW
and an additional 130km from the generator output of 23.5kW net = 5.53 km per kW from 9 Litres of fuel = 14.4 Litres per 100km

for the small size and weight of the Rex, that is a reasonably efficient use of fuel for a 1400 kg vehicle
Mazda promised to add a rotary Rex to their M30
We will see if they are capable of doing that, and if its better than the Kymco motor, which BMW de-rated for noise reasons.
JohnWasser
Posts: 95
Joined: Sat Mar 07, 2020 6:15 pm

Re: Rex replacement

The most fuel-efficient electric generator type is a combustion turbine. This video shows one small enough to replace a REX:

https://www.youtube.com/watch?v=Al8elCF816g

Note that it only produces 700 Watts (the REX does 25kW) and they hope to get the price down to 'several thousand dollars'.
3pete
Posts: 207
Joined: Tue Mar 19, 2019 9:03 pm

Re: Rex replacement

JohnWasser wrote: Sun Jul 17, 2022 7:51 am The most fuel-efficient electric generator type is a combustion turbine. This video shows one small enough to replace a REX:

https://www.youtube.com/watch?v=Al8elCF816g

Note that it only produces 700 Watts (the REX does 25kW) and they hope to get the price down to 'several thousand dollars'.
I'm all for more rotary engines in vehicles and it would be super cool to have a turbine powered car if you need the REX option.

On the complete opposite side of things, I remember reading about Toyota developing a Free Piston Engine Generator, which removes rotating parts entirely and can hit 42% thermal efficiency. This article portrays it as something perfect for a REX job, but the article is from 2014 though, so I guess it's not coming anytime soon.
https://www.roadandtrack.com/car-cultur ... ta-engine/
MagicCarpet
Posts: 62
Joined: Fri Mar 08, 2019 1:17 pm

Re: Rex replacement

Its not efficient.
the version here (and I'd love to have one) puts out 2.6 KW for 4.5L fuel per hour
the i3 Rex puts out 23 KW so how does 45 Litres/hr of Jet-A (Kerosine or Diesel) sound?

https://www.youtube.com/watch?v=wa8kS8E ... turbine101

actually I can show you how it sounds on my previous TEV here which used a off the shelf commercial 25KW output gas turbine:

https://www.flickr.com/photos/skyjeepav ... 377655083/


used around 20 L to travel 5 miles to the Diner.......but it was worth it

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