Rex Gas Economy / MPG Numbers - City vs Highway

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EVMan

Well-known member
Joined
Feb 25, 2017
Messages
340
Location
USA, DC
My US 2016 Rex came with the sticker of 39 MPH Miles combined city & Highway.

Now i am little surprised , why BMW did not publish the individual City Drive & Highway Drive numbers , like most other manufacturers do
I suspect the City Drive to be significantly better then Highway , due to the EV drivetrain and regeneration.
Any one from other countries have the individual city & Highway or more detailed fuel economy numbers ???


I think the 2017 33KWH is only 35 MPH Miles combined city & Highway.
The weight difference of larger battery is not so big , to justify this large difference. ( specially in Highway )
WIth improved software / technology , economy should only improve like with other cars ...so the downward revision suggest , something was wrong with the older numbers.

Any EU numbers , to understand whats going on ?
 
It's my understanding that the latest version of the REx can run faster to produce more power, which makes it less efficient. This is to help ensure that it can better hold the SOC so you don't run into a power deficit when pushing hard when the SOC is low. People complained...sometimes, you don't like the end results...but, it attempts to answer their complaints. Things don't come for free.

Separate city/highway mpge values would be mostly meaningless on the i3's serial hybrid power system. It makes sense when the engine powers the vehicle separately, but not when it is charging the batteries - the electric drive motor is the same efficiency all of the time, it doesn't care whether the power comes from the batteries or from the REx.
 
I agree the latest version of the REx 'MAY' run faster to produce more power , IF Needed.
If NOT needed ( city driving) , and it still runs faster , then the 2017 is not optimized, and is bad optimization.

While i understand your logic, but i cannot agree to your inference. As the EPS test environment is constant , the lower efficiency from 39 to 35 should only be attributed to efficiency and weight and driving conditions. We are not talking about the case to increasing the SOC and compare how quickly the battery is charged in 2016 vs 2017.


The REX engine is only suppose to Hold the state of charge and not increase the SOC to charge the battery . It is designed in 2017 model to cut off , if the SOC increase more than the starting level.

during the EPA testing cycle ( which is fixed described conditions) , If the 2017 is running at higher RPM , then it means the 2016 model was under supplying the energy actually and the battery was losing the SOC , which means the 2016 results were not correct.( stealing milage from the buffer )

if the 2016 was not stealing from the SOC buffer , then it means the 2017 is suppling more energy than needed and it should cut the engine at some point. This actually means , bad optimization. - as the engines is running at lower efficiency and then shutting off , rather than running at higher efficiency for a longer time. Its not optimized.

Your argument will hold water , if the 2016 rex was stealing from the buffer in the EPA test conditions , and is a serious charge , but quite possible.

Again city/highway mpge is not meaningless , as the SOC is not supposed to go up in city driving. That is not my objective . My objective is to understand the EPA economy of serial hybrid vs other hybrid cars in city/highway conditions.
 
I would recommend getting the data files from: https://www.anl.gov/energy-systems/group/downloadable-dynamometer-database

For our BMW i3-REx: https://www.anl.gov/energy-systems/group/downloadable-dynamometer-database/plug-hybrid-electric-vehicles/2014-bmw-i3-rex

Bob Wilson
 
As opposed to an ICE that is directly connected to the wheels via a transmission, and varies it's RPM and power outputs based on stop and go, a serial hybrid will run the ICE at a constant speed to average out the load...IOW, it doesn't rise and fall each time you accelerate or stop like a conventional ICE driven car does...so, city/highway doesn't make much sense.

IMHO, the reason that BMW bumped the max power output of the REx was to overcome those that insisted on driving faster and under a larger load than the REx could provide - typically, high speeds going uphill with all of the HVAC/lights, etc. in use. Those conditions could overpower the REx's output. COnsider that if your battery is nearly dead, just how fast can you go uphill at speed with a 34-39Hp engine? Think maybe an original VW bug...the real solution is to watch the SOC and go slower when needed.

Given that the i3 was intended for short trip, commuting rather than highway mile-eating distances...it made sense to make it more efficient. People insist on making the i3 operate more like a conventional ICE, and BMW was able to up the REx's output a bit to help with that with minimal changes. There is a moderate increase in weight to the larger battery pack that made both the BEV and REx slightly less efficient for the MY 2017 version. Throw in that under max load, the REx runs at a higher RPM, and it takes a hit on efficiency. Sometimes, people get what they ask for; in this case more power, but it comes at a price.
 
The pdf does not look easy to read to me , but looks like the range is from 26 miles to 51 miles.

From the engineering perspective , i am little surprised how fuel efficiencies of the different technologie approaches compare

1)serial hybrid efficiency of Rex ( low)
2) Honda plugin in Accord - 2 mode ( single gear for highway ) ( Medium)
3) volt' 2016 5 mode hybrid ( Medium )
4) prius prime efficiency (high)

The difference in efficiency is surprising large.
Only 1 ( rex) has a smaller engine and larger battery , so direct medical connection may not make sense, but efficiency in highway is significantly low , compared to others
2-3-4 have some level of mechanical connection , and larger engine to be true hybrids .

I wonder , as more and more hybrids start to show up , will they be more like 2 or 4.
Volt 5 mode looks too complicated and probably expensive as well. But looks like the best for performance and economy.
 
EVMan said:
. . .
1)serial hybrid efficiency of Rex ( low)
. . .
The 647 cc engine lacks: (1) Atkinson intake valving, and; (2) cooled exhaust recirculation found on the other cars. This means the REx thermal dynamic efficiency is not as high as it could be. I would point out most diesel-electric trains are also serial drive.

I would also add that the body of the BMW i3 is not as aerodynamic as the lower slung, sedans and hatchbacks. We have visibility and easy access but pay a price at highway speeds. But if you can handle 65-70 mph using dynamic cruise control to follow high-balling trucks, you'll see an honest 40-38 MPG.

Bob Wilson
 
Bob,
Thanks for this info. this make sense
1) So in theory , if the rex had a efficient ICE engine , would it be closer (10-15% range ) in efficiency to say the prius ? I guess the answer should be yes. A ICE engine designed & optimized to be run at constant rpm should be more efficient than a engine in prius designed to be efficient in variable rpm. I understand the efficiency of the generator is around 94% which is quite high.

There are also some hi efficiency non cylinder ICE engines available now ( patent pending by Israeli startup)

2) The volt has way to many modes - 5 ( there are some youtube videos available explaining the modes ) . Do you think the added complexity to maintain all those are worth it ? ( in light of , as you said 'diesel-electric trains are also serial drive')

I am just a little surprised , of how all the different hybrids cars are trying to solve the same problem, with quite different combinations and quite different results. At some point , they should start to converge in design and performance.

The consumer has to decide , which makes the most sense.
 
The other cars you list are designed as ICE replacements, with a decently sized fuel tank, and generally, small batteries...the whole design path for the i3 is different. IT is NOT designed for long distances, and the engine was added as an emergency backup that was not intended for continuous use, but some people use it that way. So, you have a purpose built, short-range, city car verses those that are designed as ICE replacements with some battery augmentation, but not the prime means of propulsion.

BMW's i8 uses a quite different arrangement, as do all of their other hybrids. It is still unknown exactly what their next I-series car will be designed for. SO, IMHO, you're trying to compare apples to oranges. The goal was to keep things as light as possible, since propulsion is entirely via the electric motor. You have to ask, how much is it worth to ensure you get home if you misjudge the battery SOC? WIth that in mind, you don't really care all that much about how efficient it is.
 
EVMan said:
. . .
1) So in theory , if the rex had a efficient ICE engine , would it be closer (10-15% range ) in efficiency to say the prius ? I guess the answer should be yes. A ICE engine designed & optimized to be run at constant rpm should be more efficient than a engine in prius designed to be efficient in variable rpm. I understand the efficiency of the generator is around 94% which is quite high.
. . .
Based on my studies of the car drag and what I've learned about engine efficiency with the Gen-1, Gen-3, and Prius Prime, I suspect the car could achieve 42-43 MPG @65 mph. With an after-body taper, it might achieve this at 70 mph but that is as far as I'd go. As others have pointed out, it was designed to be an urban car with intra-City capability.

The BMW i3-REx advantages come from light weight and low rolling resistance tires. I regularly see 4.0-4.5 miles/kWh which matches what we see with the Prius Prime Plus, a heavier car with 15" low-rolling resistance tires. We own both a 2014 BMW i3-REx and 2017 Prius Prime that bookend plug-in hybrid performance.
i3_pip_020.jpg

(NOTE: these are all 2014 models that snapshot the market.)

Had the Prius Prime been available in May 2016, we'd probably would have passed on the BMW i3-REx. We actually bought the Prius Prime when an infantile problem, a broken motor mount bolt, led to the BMW i3-REx being down for two weeks and I had to use our Gen-3, 2010 Prius that did not have dynamic cruise control.

Our hard requirement is dynamic cruise control and collision avoidance which both cars have. With two, SAFE, high efficiency cars, we can leave one parked in the event to major repairs and still get by.

Bob Wilson
 
Bob,
Thanks for this chart. But for the Rex it is not showing correctly the final mpg. it showing above 75 miles , where as it should be 35-39. Other cars are sowing correct final mpg .
 
jadnashuanh said:
The other cars you list are designed as ICE replacements, with a decently sized fuel tank, and generally, small batteries...the whole design path for the i3 is different. IT is NOT designed for long distances, and the engine was added as an emergency backup that was not intended for continuous use, but some people use it that way. So, you have a purpose built, short-range, city car verses those that are designed as ICE replacements with some battery augmentation, but not the prime means of propulsion.

BMW's i8 uses a quite different arrangement, as do all of their other hybrids. It is still unknown exactly what their next I-series car will be designed for. SO, IMHO, you're trying to compare apples to oranges. The goal was to keep things as light as possible, since propulsion is entirely via the electric motor. You have to ask, how much is it worth to ensure you get home if you misjudge the battery SOC? WIth that in mind, you don't really care all that much about how efficient it is.

Jim ,
Ok now i understand in the i3 rex , the additional 650 engine was added for emergency purpose only. Looks like, this is a self designed limitation and thats the reason , they did not care about the efficiency of that engine.

I was not getting it , since from the technology purpose , there is absolutely no reason , why the 650 cc engine has to be only for emergency only. A well designed durable 650cc engine with proper ventilation and the 'hold mode' paired with a large battery , can deliver a lot of punch for longer distances. If you engage the hold mode at 75% , there is enough power buffer to climb high mountains , even with smaller continuous power supply.

I hope the next versions , do not have this limitation.

https://phys.org/news/2016-10-israel-firm-super-efficient-power-car.html
 
So I've updated the chart to show 2017 models:
i3_prime_volt_010.jpg


EVMan said:
. . . the Rex it is not showing correctly the final mpg. it showing above 75 miles , where as it should be 35-39. Other cars are sowing correct final mpg .
The other cars have larger fuel tanks so the combination of nearly equal EV and REx mode approaches but can not reach the lower MPG. Now if we took out the EV segment, the MPG chart would be a flat line at the '35-39' suggested.

Bob Wilson
 
FWIW, there are some arcane rules BMW tried to jump through when they tweaked the i3 for use in the USA. As implemented (most) everywhere else, you can turn the REx on when the SOC drops to 75% or below. In the USA, you can't turn it on (without recoding) manually...it turns itself on to prevent damage to the battery. Once you've reached that situation, if you're stressing the car (high speed, climbing a grade), you could be using more energy than the REx is able to provide, so you could reach a point where it starts to shut things down to protect itself. BMW chose to get the car through the CARB rules with the best benefit to allow the car to be used in things like the HOV lanes (and gather them the most credits).

But, since they did really design the car for shorter ranges, while it infuriates some US users, it still met their design goal. They really didn't want to include the REx, but felt it was needed to overcome the range anxiety for those new to EV's. Nissan has sold hundreds of thousands of their Leaf without one, but BMW speaks to a different crowd. When introduced, it was the most efficient car you could buy and the first mass-produced car with a CFRP frame.
 
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