REX fuel consumption per kWh

[harb]

I was trying to find this information from somewhere but in vain..

I would assume that fuel consumption of REX is somewhat static per kWh produced. There seems to be couple of different rotational speeds related to load that then likely have different fuel consumption but anyways quite close to each other.

The reason behind this is that it would be then easy to compare the cost of using gasoline vs. electricity on longer roadtrips.

Of course you can derive this information from MPG (or l/100km over here) it's unnecessary step if REX fuel consumption is known.

The other factors will impact the overall efficiency like charger efficiency and energy conversion efficiency.

 

[viking79]

This might not be possible to figure easily as if you are driving I would guess the electricity would power the traction motor directly to bypass the round trip losses to the battery.

I would guess the Rex generates a certain amount of power, the traction motor uses that. Any surplus ends up in the battery and any deficit comes from the battery. Ideally you minimize generating surplus power as you lose 10-20% round trip in the battery.

For this reason it is hard to know how much you use the battery vs not so the efficiency will vary as well. I don't think you would gain anything vs mpg.

I think you need to know the thermal efficiency of the engine, does the OBDII data give the necessary info? I would guess it has that, then you could maybe guess at efficiency of generator and figure out gas burned per kWh.

Alternately, if you get 34 mpg at 75 mph and drive for an hour according to ecomodder calculator that would be about 21 kWh on 2.2 gallons of gas (that mpg estimate might be too high at that speed, maybe closer to 30 mpg). This would be about 9.6 kWh/gallon. Ideal is like 33.7 kWh, so thermal efficiency is around 28%. Sounds too high for i3. If 30 mpg would be 25%.

If you assume 25% thermal efficiency you will end up with 8.4 kWh/gallon.

 

[EVBob]

You may find this document about the REx generator useful - namely pages 54-56:

https://attachments.priuschat.com/attachment-files/2016/07/108943_12_W20-Engine.pdf

On a roadtrip last year, we were able to hold SOC (down to the .5%) - with a fully loaded i3 (2 adults, 2 kids + trunk and frunk full of stuff), going approx 60-65mph on flat ground on eco pro and AC set to 70F (outside temps 80-87F). - we got approx 80-85 miles per 2.34 gallons of gas. So in our case approx 34-36 mpg....

 

[bwilson4web]

You might find my recent benchmarks useful:
http://www.mybmwi3.com/forum/viewtopic.php?f=13&t=5423

Also:
http://www.mybmwi3.com/forum/viewtopic.php?f=2&t=4287&p=37324&hilit=mph+MPG#p37324

My testing revealed the primary control law is to maintain the State Of Charge (SOC). However, the engine has four distinct power bands. If the charge is getting too high, the engine goes to a lower power band. When SOC is getting too low, the REx rises to a higher power band. Normally I don't even notice it. However, the engine-generator could be more efficient.

Adding cooled exhaust recirculation would reduce throttle plate losses making the engine more efficient. Converting to an Atkinson cycle by delaying the intake valve close would also help but needs a larger intake manifold and injector timing.

 

[viking79]

I find it interesting it can use the power electronics coolant loop to cool the engine coolant. Isn't that the AC?

Still surprised they don't have a cabin heater tied into the engine loop.

That is a great engine guide. Love design notes like that.

 

[jadnashuanh]

Some have said that the mpg on the REx is in the 34mpg range. That's also about the Hp, depending on the model year and actual rotation rate. A HP is about 746W, so 34Hp*746W ~25Kw. Given the average power usage is about 4-miles/1k, that's 250W/mile. That would imply that if things were perfect, you'd get about 100-miles per hour of running the REx. Obviously, a gasoline engine is nowhere near 100% efficient...you'd be more likely to see about maybe 30-40%...so, say you were traveling at 60mph, you'd be burning all of your fuel in a bit more than an hour, which seems to be fairly consistent with the REx range on the 2.4g tank.

By the time you take into account the losses in generating electricity from the REx, and engine efficiency, it's not great. Lots of cars can get more than 34mpg these days. The i3 works best when you never use the REx if efficiency is your goal. It was designed as an emergency backup, but some people do use it on a regular basis. It works that way, but is by no means the most efficient use of energy.

 

[viking79]

The Rex is designed for continuous operation. It has adequate cooling and lubrication.

 

[alohart]

The Rex is designed for continuous operation. It has adequate cooling and lubrication.

I question that assumption because the REx air filter is sealed in the air intake silencer making it impossible to change when dirty. Replacing the entire silencer just to replace the air filter would be expensive. I don't think that BMW would have made the air filter irreplaceable had it designed the REx for continuous operation.

 

[viking79]

For maintenance, it was clearly designed to have most of the miles on electric, but it was designed to run continuously on gas (i.e., the engine can handle the load). I have put about 3,000 miles on the Rex, it doesn't overheat or anything as something designed only as an emergency range extender would.

 

[alohart]

For maintenance, it was clearly designed to have most of the miles on electric, but it was designed to run continuously on gas (i.e., the engine can handle the load).

I don't think that we can assume that BMW would have done the additional design work necessary to allow a REx engine to run continuously without problems since BMW has clearly written that the i3 is a mega-city vehicle whose REx function is designed to extend the i3's range enough to reach the next charging station. Many REx drivers have demonstrated that the REx engine can run for hours at a time without mechanical problems at least for a few years, but do we really know whether doing so would not result in its premature mechanical failure? I know of one case where BMW USA refused the 150k mile California warranty repair of a REx engine that had suffered a significant mechanical failure at just over 100k miles because the owner had coded HSOC and had run the REx engine routinely rather than to just reach the next charging station. This does seem like a gray area, though.

If I owned a REx and planned to keep it past its warranty period, I would not risk an expensive out-of-warranty REx engine repair by running my REx engine routinely for long-distance travel.

 

[jadnashuanh]

Some of the early design parameters had the designers preferring not to include the REx, but the marketing people said it was needed for early acceptance of people to EV use. If BMW really wanted people to use the car as a hybrid, they'd have included a larger engine and fuel tank, IMHO. A 34-HP engine won't keep the car going at a decent pace in all circumstances...sort of like the early VW Beetle that could barely get up a hill, and certainly not in high gear. It's been awhile since any manufacturer tried to sell a new car with 34-HP in the USA for a reason.

IMHO, if you need the REx frequently, you bought the wrong car. As a what-if, it can make sense, but the car is definitely not optimized for touring.

 

[EVMan]

Some of the early design parameters had the designers preferring not to include the REx, but the marketing people said it was needed for early acceptance of people to EV use. If BMW really wanted people to use the car as a hybrid, they'd have included a larger engine and fuel tank, IMHO. A 34-HP engine won't keep the car going at a decent pace in all circumstances...sort of like the early VW Beetle that could barely get up a hill, and certainly not in high gear. It's been awhile since any manufacturer tried to sell a new car with 34-HP in the USA for a reason.

IMHO, if you need the REx frequently, you bought the wrong car. As a what-if, it can make sense, but the car is definitely not optimized for touring.

I quite don't understand the need and for a larger engine. Bigger engine has more weight and with such a large battery buffer , i just don't see the need of larger engine.
You just need software to turn on the engine early and need software to build the buffer.
Mazda is believed to be working a a shoe box size rotary engine for EVs

Even the Volt and other plugs-in like Honda insight , with much larger engine , has a mountain mode, to build the buffer ( charge the battery , for mountains) . The i3 has much larger buffer, so it can be built slowly.

I use battery intra city needs, and Rex extra long range , once in a while , when i go out city.
Small gas tank apart, don't see the need of bigger ICE
I can use the smog testing mode to build the buffer , if i need it.
All works out very well. Its my 2 in 1 car. No need top keep a ice with its weekly usage and insurance. I am very happy.

 

[EVMan]

The Rex is designed for continuous operation. It has adequate cooling and lubrication.

I question that assumption because the REx air filter is sealed in the air intake silencer making it impossible to change when dirty. Replacing the entire silencer just to replace the air filter would be expensive. I don't think that BMW would have made the air filter irreplaceable had it designed the REx for continuous operation.

You bring up technical point for the low continuous operation. But it is very stage , BMW went out of the way to limit the life of the ICE engine.
While ICE engines have many issues, such small life is certainly not one of them. i have not heard ICE engines having limited life issue. This is a first . Over time , they have become quite durable.

If the i3 is to be part of developing world with 10x high air pollution or desert air quality, the life will be reduced 10X. Strange indeed
May be its a Revenue tool.

 

[jadnashuanh]

There are driving conditions that can easily exhaust the internal battery while still running the REx at full capacity regardless of when you turn it on...thus, if they wanted the i3 to be an all-purpose, long-distance vehicle, they would have made the ICE larger to handle that circumstance...i.e., never having to compromise your progress to allow the battery to recover whether because you wish to run with the prevailing traffic or steam up a grade. MOst all other hybrids have a large enough ICE to not have to compromise on their forward progress. The REx is a compromise to optimize commuting and potential range anxiety...not as a long-distance tourer. Use it that way and accept the compromises. IF the traffic is all running at 80mph up a grade, the i3 will quickly start to exhibit problems if you wish to stay in the pack. Your speed doesn't have to be that high, either, depending on the conditions. Drive around when it's near 0F, and use the heater while driving with traffic can also wreck your SOC and thus your progress. SAAB did a lot of research a long time ago and found that being cold while driving was a safety hazard...one reason why they include heated seats as standard in (every?) market. A cold car will need some heat to keep the windshield clear if nothing else.

So, to keep the weight down and optimize the i3 for its intended purpose, it isn't very big and doesn't have much of a gas tank...more than enough in a commuting environment to get you home, should your plans change a bit.

 

[viking79]

I am not arguing that the i3 is a great long distance car, the sizing of the REx and tank are not suitable as you point out.

What I am saying is for a range extender operating as designed with hold mode enabled (not US handicapping of the Rex so BMW can get additional ZEV credits) the car does just fine.

I can drive with traffic at 75-80 mph no issue. This is enough to get me 220-250 mile each way to Chicago or Minneapolis flowing with traffic. I couldn't make these drives with any non-Tesla EV yet (or not easily). No CCS chargers.

My wife's Clarity PHEV is certainly much better on the highway, no argument from me there. Doesn't mean the Rex is for emergency use only, it is for regular duty, but not really meant for more than maybe 30,000-50,000 miles during life of vehicle. The intake looks like a $400 part, so not end of world if it needed changing after 10 or 15 years to replace the filter.

 

[EVMan]

I have raced and crossed the sierra Navada mountain range over 8000 Feet.
I think , i put most average ICE cars to shame, specifically after 8000 feet, only the empty pickup trucks raced cross me.
I would say, it passed my testing. Its no high fuel consuming sports car , and i would not buy one of those.

The small engine was not the bottleneck. It may not do good extra long range more than 6-8 hours driving per day. It has limitations.

Downhill was not as fast , due to skinny tires and ICE cars did better , but no biggy for me, as i was consuming no energy

I understand your concerns on durability and reliability of the engine. Its not the small engine, its more of a bad implementation.
Why BMW designs ICE engines with limited life is a open question. Most other brands have the opposite objective of building very reliable and durable engines. Its very disappointing they put these issues into the implementation, other wise it just works

 

[i3Houston]

I can use the smog testing mode to build the buffer , if i need it.
All works out very well. Its my 2 in 1 car. No need top keep a ice with its weekly usage and insurance. I am very happy.

How do you use smog test mode for building buffer?

 

[EVMan]

I can use the smog testing mode to build the buffer , if i need it.
All works out very well. Its my 2 in 1 car. No need top keep a ice with its weekly usage and insurance. I am very happy.

How do you use smog test mode for building buffer?

Just YouTube.
There are steps to put car in smog testing mode.
It runs the ICE. It charges the battery too.
You need to keep the back open....
Closing the door cancels it , so dont close the door.

 

[chuckhawley]

I drove 550 miles to Bend, OR in a single day, and I drove back two days later. I did partially charge the car in Bend, but otherwise I used the REX and judicious battery management to keep as high a SOC as possible. I also carried a two-gallon plastic jerry jug so that I could extend my range between gas stations.

My observations:

1. I could maintain about 72-75MPH with no change in SOC on flat terrain. Therefore, I infer that it takes about 34HP to push an i3 at 72MPH on flat land.
2. Since I would lose the "setting" of the SOC when I turned off the car, I constantly found that I was losing the target SOC when I stopped for gas or Starbucks. Presuming it has previously been set at 50% SOC, I might turn off the car as the REX was trying to get back to 50%, but it would be actually at 45%, for example. Then I'd start up the car, and I'd start at 45%, but only if I remembered.
3. I used every downhill to try to crank up the SOC setting to reverse the tendency in paragraph 2. I would turn off "Hold SOC" as I coasted down the hills, and re-engage it at the bottom of the hill.
4. I apparently "burned out" the fuel pump relay in Bend, which really crippled my ability to be flexible. I have never wished for the REX as much as I did trying to a charge station in freezing Bend. The local dealer replaced the relay at no charge.

I don't know how long my REX is going to last, but I use my i3 more as a hybrid than most. That's how it works for me since I am in a smaller market where I need to travel farther than a more urban owner for many of my trips. If BWM and CARB had allowed the full use of the gas tank and had allowed the engine to kick on at 20% SOC instead of 6%, I wouldn't have needed to have the car coded. But boy oh boy, I think my coded car is a necessity for most use.

Chuck

 

[EVMan]

viking79,
One question about clarity.
Can you advise , when you press the accelerating pedal and the cold gas engine comes on, how come its able to provide instant power without any degradation. ? That kind of defies everything I know about, what not to do to ICE. Normally they need some kind of preconditioning and warming for lubrication .
Are lubricants always circulating in the engine in EV mode ?
Even the Prius hybrid will turn it on for some time initially, to warm up the engine.