xrayman
Posts: 11
Joined: Sat Nov 09, 2019 1:26 pm

Poor REX Mpg on motorway

Sun Dec 29, 2019 2:31 am

Hi,



So last night I drove the i3 purely on REX for 110 miles non stop on the M42/M40 (no traffic). Was doing a constant 70-75 mph (displayed). Had to stop to refuel. Couldn't be bothered to charge as it was getting late. Heater was on at 22c.



Basically over the motorway distance covered, it consumed a whole tank plus 3/4 of a tank. (Full tank is 9l I think). So according to my calculations, this works out at just 30 mpg.



Surely this cannot be correct ? Or was it because the speed was too high for efficiency?



Thanks

robthebold
Posts: 110
Joined: Sat May 11, 2019 5:09 pm

Re: Poor REX Mpg on motorway

Sun Dec 29, 2019 9:48 pm

REX fuel efficiency is nowhere near the electric efficiency. I think it's supposed to be in the 30s, but at 75 MPH, it's fair to expect less than that. Consumer Reports magazine in the US computed the REX efficiency to be in the low 30s (MPG) so your experience is on par with that.

jadnashuanh
Posts: 5196
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Poor REX Mpg on motorway

Sun Dec 29, 2019 10:59 pm

If you're using a British gallon (5 qts) versus a US gallon (4 qts), then 30mpg is pretty poor. Otherwise, it's on par with what I've heard from other people. If you can't recharge, and are running a long trip almost entirely on petrol, there are LOTS of other vehicles that are more efficient. The car only excels when it's not needing the REx to run. The REx was introduced as an aid to help people overcome range anxiety. That people use it to extend trips longer than it was designed for is sort of a tribute to the design flexibility. Used as an emergency, get home without needing to call for help, it can provide peace of mind. Don't expect it to be that efficient.
Jim DeBruycker
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)

robthebold
Posts: 110
Joined: Sat May 11, 2019 5:09 pm

Re: Poor REX Mpg on motorway

Mon Dec 30, 2019 8:00 am

jadnashuanh wrote:
Sun Dec 29, 2019 10:59 pm
If you're using a British gallon (5 qts) versus a US gallon (4 qts), then 30mpg is pretty poor. Otherwise, it's on par with what I've heard from other people. If you can't recharge, and are running a long trip almost entirely on petrol, there are LOTS of other vehicles that are more efficient. The car only excels when it's not needing the REx to run. The REx was introduced as an aid to help people overcome range anxiety. That people use it to extend trips longer than it was designed for is sort of a tribute to the design flexibility. Used as an emergency, get home without needing to call for help, it can provide peace of mind. Don't expect it to be that efficient.
An Imperial gallon is still 4 qts (hence the name), but the UK quarts are a little bigger than US quarts. 1 Imp gal is approximately 1.2 US gal. The discrepancy comes from the fact that many different gallons were in use at the time when the US and UK independently standardized units of measure.

(BTW, when one hears that the US is the last country still using the Imperial system, this is incorrect for at least two reasons. The US never used the Imperial system, and there are still other places where the Imperial gallon is used for trade.)

joni3s
Posts: 56
Joined: Wed Oct 02, 2019 11:16 am
Location: VA

Re: Poor REX Mpg on motorway

Mon Dec 30, 2019 9:48 am

I'm sure you've read that 70mph+ is really poor on EV efficiency ( on i3). At that speed, my i3S efficiency is 30-40 miles for 8-9kWs "uphill". Basically a quarter of the 94Ah batt. I do that drive almost everyday. If I take the "downhill" slower highways with lights, I have much much better efficiency.

If the REX is going all out to barely keep up with the SOC, it's not gonna be much better than pure EV.

The EV and REX max efficiency only comes at speeds around 25pmh. Probably why the REX has multiple speeds and stops on occassion at lights and traffic jams.
2018 I3s rex

jadnashuanh
Posts: 5196
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Poor REX Mpg on motorway

Tue Dec 31, 2019 9:59 pm

Imagine a moped engine trying to push a 5x heavier load than originally designed at greater than its probable maximum speed...the little engine that could just won't be all that efficient! It's a tribute to that engine that it can at all, but it's not, to me at least, surprising that it isn't all that efficient about it.

Now, running around town, letting you get home if you miscalculated the needed watts, it works great.
Jim DeBruycker
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)

spartacus68
Posts: 88
Joined: Sun Jul 09, 2017 11:07 am

Re: Poor REX Mpg on motorway

Wed Jan 01, 2020 1:20 am

You're expecting way too much from your i3 REx. It's not a hybrid in the tradional sense. It's electric with a back up petrol engine, rather than petrol with kick in electric. Ideally you want to find a charging point, or preset the car so you maintain a reserve of electric charge.
2015 i3 REx, 60ah, solar orange. Reversing camera, leather interior, sunroof, park assist, professional navigation.

joni3s
Posts: 56
Joined: Wed Oct 02, 2019 11:16 am
Location: VA

Re: Poor REX Mpg on motorway

Thu Jan 02, 2020 3:28 pm

read this on another forum.

think of it min terms of kWh not mpg as mpg efficiency all depends on the driving style (speed, environment).

gas has energy density of 9.6kWh per liter
tank has 9 liters

Energy content of the tank: 9l x 9.6kWh / l = 86.4 kWh

Efficiency of the REX approx. 30%

kWh generated: 26
2018 I3s rex

jadnashuanh
Posts: 5196
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Poor REX Mpg on motorway

Thu Jan 02, 2020 6:31 pm

EPA seems to rate the REx at 31mpg. A stationary engine can be more efficient than a more normal ICE used in a vehicle, as running at ideal speeds is better than moving around, connected to a transmission with speed variations. You have two issues, thermal efficiency, which isn't great on most any ICE, and then, the conversion of that rotary power into electrons that can propel the vehicle. That efficiency is much higher than an ICE to moving the vehicle directly. But, to keep propelling the i3 at high velocity or load, is pitting a puny engine up to a very large task. It works running around town, often without any limitations, but try to run at high speeds or loads for longer periods of time, and it's not at its best use (moving a moped, which, tends to not weigh much, or go as fast!).
Jim DeBruycker
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)

joni3s
Posts: 56
Joined: Wed Oct 02, 2019 11:16 am
Location: VA

Re: Poor REX Mpg on motorway

Fri Jan 03, 2020 10:03 am

jadnashuanh wrote:
Thu Jan 02, 2020 6:31 pm
EPA seems to rate the REx at 31mpg. A stationary engine can be more efficient than a more normal ICE used in a vehicle, as running at ideal speeds is better than moving around, connected to a transmission with speed variations. You have two issues, thermal efficiency, which isn't great on most any ICE, and then, the conversion of that rotary power into electrons that can propel the vehicle. That efficiency is much higher than an ICE to moving the vehicle directly. But, to keep propelling the i3 at high velocity or load, is pitting a puny engine up to a very large task. It works running around town, often without any limitations, but try to run at high speeds or loads for longer periods of time, and it's not at its best use (moving a moped, which, tends to not weigh much, or go as fast!).
For my 2018 i3S REX found the following info

Electricity
Combined MPG on Electricity: 109 MPGe combined city/highway
31 kWh/100mi

Prem Gas
Combined MPG on Gas Only: 35 MPG combined city/highway
2.9 gal/100mi

EPA MPG
MPGe: Miles per Gallon Equivalent
1 gallon of gasoline=33.7 kWh

https://www.fueleconomy.gov/feg/Find.do ... s&id=39835
2018 I3s rex

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