REx now with more power - 28 kW

fdl1409 · May 22, 2014

Just came up in the German i3 forum:

http://www.goingelectric.de/forum/bmw-i3-range-extender/staerkerer-rex-t4980.html

In the i3 price list of march 2014, the REx is described to have 25 kW at 4.300 rpm.
Later list has changed, now it is 28 kW at 5.000 rpm.

Looked at my own documents, my REx delivery May 15th has 28 kW.
And just someone posted that his REx delivery March 28th also has 28 kW.

So they made up their mind to give it some more power by going up with the revs.

Same with your RExes in the US?
That would ease your worries about going uphill with only 6.5% in the battery.

Frank
Germany

mindmachine · May 22, 2014

Fantastic news, that is great especially for the US since we can't manually engage the Rex early.

Surge · May 22, 2014

Indeed!
Perhaps someone can confirm when these cars will be shipping? Seems like they already have begun, at least in Europe.

fdl1409 · May 22, 2014

So far I know of 3 RExes in Germany including mine built in march or later, all have 28 kW. So all those shipped to the US could have the higher power rating too. As this is the same engine, only with higher rpm, an update should be technically possible for those delivered earlier.

The first information I had about the REx, that was last year, was that it had 23.5 kW. Now 28 for me doesn´t make any difference, because if ever I want to cross the Alps I can switch on the REx early enough. For you, who can´t start it before those 6.5% SOC, this may make a difference if you want across the Rockies. You can certainly do so with significantly higher speed. If you get the 28 kW also.

Will follow up any more news about this.

Frank
Germany

Lecram · May 22, 2014

My i3 has 28kW too, delivered 15 May

bxb40 · May 22, 2014

Not sure I follow - what does 28kW refers to, the power of the motor (its output) or its consumption at a certain RPM? Because if the second, I can see how consumption would increase with RPM, just like the stickers on different cars seem to indicate.

ultraturtle · May 22, 2014

bxb40 said:
I can see how consumption would increase with RPM

That's an often repeated misconception. If consumption increased with only RPM, a gasoline engine would consume more with the vehicle in neutral gunned to 6000 RPM than it would struggling up a steep hill under maximum load at 2000 RPM. Consumption increases with load, not RPM. A skillfully engineered vehicle would operate at near maximum load (typically at a relatively high RPM) whenever possible, because that is where efficiency (power generated per unit of fuel burned) is at its greatest. Gasoline powered generator sets (gen-sets) like the REx engine typically operate at a fixed RPM closest to maximum efficiency throughout their range of loading. My guess is that the BMW engineers tweaked the engine a bit to now provide maximum efficiency near 5000 RPM, and at a greater power output - upping its efficiency to the point that the gas tank capacity now needs to be reduced in order to meet BEVx criteria.

This is a generic efficiency curve for a gasoline engine:

Most highly efficient automobiles operate at less than 20% efficiency because they need multiples of cruise power in reserve to accelerate and climb. Not so with the REx - that is what the 5% to 7% battery reserve is for. BMW can feel free to close in on that elusive 30% efficiency ideal, and it looks like they have taken the first step in doing so. Hopefully future REx tweaks will allow the REx to operate only at near maximum power output, then shut down completely in cycles rather than the current implementation that apparently has it run at reduced capacity while the i3 is in motion.

These are the early days of the first serious attempt at a series hybrid. The i3 REx is an incredible breakthrough, but there is much learning to be done. I have a feeling that the brilliant engineers at BMW are on the right track to evolve the most efficient way to power a practical vehicle using gasoline.

theadlee · May 22, 2014

I can't believe this thread isn't getting more attention. Someone needs to bribe Tom to check his battery capacity and post it. While he is at it, I would like to know the true capacity of the US gas tank

bxb40 · May 22, 2014

OK, so the original post was about the gasoline engine - as there is no gear box, at highway speeds or on an incline, the REX will work harder to maintain SOC by changing its RPMs (UK owners confirmed they could hear different level of noise under different driving conditions). I don't know the power band and characteristic of that engine but I think that the RPM will be high enough trying to catch-up with consumption that the efficiency will go down with RPM under the same load, just like in your plot... But I'm speculating, maybe soon we'll know more from Tom.
Just curious: what is the redline for that engine?

fdl1409 · May 22, 2014

Within Limits, more rpm make possible more power. That it why race cars have such high rpm.

The REx motor is a standard BMW motorcycle motor with 647 ccm, which is used for example in the BMW C 600 Sport. There it is rated with 44kw at 7.500 rpm. For use in the REx, not all the capabilities are used. Noise must be a factor here, and of course efficiency. BMW has now released some more power, which also needs higher rpm.

Frank
Germany

fdl1409 · May 22, 2014

PS: BMW offers the same motorcycle with reduced power of 35kw at 6.000 rpm.

So we have:
- 25 kW at 4.300 rpm old REx spec.
- 28 kW at 5.000 rpm new REx spec.
- 35 kW at 6.000 rpm in BMW motorcycle with reduced power
- 44 kW at 7.500 rpm in regular BMW motorcycle

Frank
Germany

ultraturtle · May 23, 2014

bxb40 said:
... - as there is no gear box, at highway speeds or on an incline, the REX will work harder to maintain SOC by changing its RPMs (UK owners confirmed they could hear different level of noise under different driving conditions). I don't know the power band and characteristic of that engine but I think that the RPM will be high enough trying to catch-up with consumption that the efficiency will go down with RPM under the same load, just like in your plot...

Different levels of sound are likely attributable to different levels of load. Most gen-sets are designed to operate at a fixed RPM, and for good reason. Take a careful look at the plot of the generic gasoline engine above, draw a vertical line at a approximately 3300 RPM, and you will see that is the RPM range where the engine operates most efficiently at 12.5%, 25%, 50%, 75%, and 100%. Will the engine be louder operating at 3300 RPM under 100% load than at 3300 RPM under 12.5% load? Of course it will. The REx engine does not need a gearbox, because it is designed to operate within a rather narrow range of final shaft rotational speed.

Stevei3 · May 23, 2014

Hi guys,
I don't know if it will add something to your discussion, but under various driving conditions, the variation in generator RPM to accomodate for the desired electrical power output can be clearly heard.
Regards, Steven

fdl1409 · May 23, 2014

Hi Steven,

your REx is from mid february. 25 or 28 kW?
I know of one REx in Germany production date 4th march, this already has 28 kW.

True, the REx is NOT operated at fixed rpm and power as other gensets. It tries to follow the momentary power demand, the battery will even out rapid variations. That means the REx must vary rpm during operation.

For 28 kW, it must work at 100 or near 100% power at 5.000 rpm.

Frank

buickanddeere · May 23, 2014

Stevei3 said:
Hi guys,
I don't know if it will add something to your discussion, but under various driving conditions, the variation in generator RPM to accomodate for the desired electrical power output can be clearly heard.
Regards, Steven

Change in rpm or change in throttle opening/ power setting while at a fixed rpm ?

It would be interesting to find more Rx specs

The Rx power of course rated in power applied to the battery pack. Not gross KW/HP measured at the flywheel.

ultraturtle · May 23, 2014

fdl1409 said:
Within Limits, more rpm make possible more power. That it why race cars have such high rpm.

I'm much more interested in efficiency than power for the REx implementation in the i3. I'd rather have it doing its best to pour the most kWh of electrical power per gallon consumed into the battery bucket, because (assuming it is operating above minimum battery SOC) the power to feed the 170 hp electric motor is coming out of the battery bucket, not the gasoline engine. That said, there is the tiny percentage case where both the battery is at minimum state of charge, and the acceleration/climbing demand requires maximum power output of the REx, and I'm happy to see BMW upped that maximum power available.

Stevei3 said:
... under various driving conditions, the variation in generator RPM to accommodate for the desired electrical power output can be clearly heard.

Particularly now that the maximum power level has been increased, this may be the case. Hopefully BMW has programmed the REx to operate at its most efficient RPM all the way up to maximum power at that RPM (a vertical line at about 3300 RPM on the generic efficiency curve plot). With efficiency now optimized, it could then only increase power by moving right on the graph (increasing RPM), sacrificing some efficiency in order to do so.

Luckily the REx output is likely to be a true DC generator, outputing only DC power, as opposed to portable gen-sets used for powering AC loads which are actually alternators. Alternator output is carefully managed to maintain as constant an RPM as possible, since RPM is the same thing as frequency, and it is imperative for AC loads to receive clean, stable frequency power despite widely varying loading. A DC generator has no such need for constant RPM, so it can vary somewhat, with little to no effect on DC output.

I'd love for someone to get a data dump from the OBD2 port at varying REx loading. Sounds can sometimes fool us. I can swear that my portable home AC gen-set sounds like it is operating at a higher RPM near maximum load than it does while lightly loaded, but I know for a fact that is not the case. It is much louder, but not faster.

Stevei3 · May 23, 2014

fdl1409 said:
your REx is from mid february. 25 or 28 kW?

Hi Frank,
according to the registration, the engine of our car is rated 25kw. Our car was produced in january.
Regards, Steven

Stevei3 · May 23, 2014

buickanddeere said:
Change in rpm or change in throttle opening/ power setting while at a fixed rpm ?

Definitly not running at fixed RPM. More info from the onboard system would have been nice, one would have to agree...

Steven

buickanddeere · May 24, 2014

ultraturtle said:
fdl1409 said:

Within Limits, more rpm make possible more power. That it why race cars have such high rpm.

Click to expand...

I'm much more interested in efficiency than power for the REx implementation in the i3. I'd rather have it doing its best to pour the most kWh of electrical power per gallon consumed into the battery bucket, because (assuming it is operating above minimum battery SOC) the power to feed the 170 hp electric motor is coming out of the battery bucket, not the gasoline engine. That said, there is the tiny percentage case where both the battery is at minimum state of charge, and the acceleration/climbing demand requires maximum power output of the REx, and I'm happy to see BMW upped that maximum power available.

Stevei3 said:

... under various driving conditions, the variation in generator RPM to accommodate for the desired electrical power output can be clearly heard.

Click to expand...

Particularly now that the maximum power level has been increased, this may be the case. Hopefully BMW has programmed the REx to operate at its most efficient RPM all the way up to maximum power at that RPM (a vertical line at about 3300 RPM on the generic efficiency curve plot). With efficiency now optimized, it could then only increase power by moving right on the graph (increasing RPM), sacrificing some efficiency in order to do so.

Luckily the REx output is likely to be a true DC generator, outputing only DC power, as opposed to portable gen-sets used for powering AC loads which are actually alternators. Alternator output is carefully managed to maintain as constant an RPM as possible, since RPM is the same thing as frequency, and it is imperative for AC loads to receive clean, stable frequency power despite widely varying loading. A DC generator has no such need for constant RPM, so it can vary somewhat, with little to no effect on DC output.

I'd love for someone to get a data dump from the OBD2 port at varying REx loading. Sounds can sometimes fool us. I can swear that my portable home AC gen-set sounds like it is operating at a higher RPM near maximum load than it does while lightly loaded, but I know for a fact that is not the case. It is much louder, but not faster.

Small Inverter generators for camping etc do produce DC from a Flywheel generator . The electronics package inverts the DC to an AC sine wave. Engine rpms do vary to match the load wattage.

surfingslovak · Jun 23, 2014

Stevei3 said:
buickanddeere said:

Change in rpm or change in throttle opening/ power setting while at a fixed rpm ?

Click to expand...

Definitly not running at fixed RPM.

Yes, I have noticed this also. I believe that the REx had three and possibly four distinct power output and rpm levels. The lowest one provides around 10 to 12 kW of power.

ultraturtle said:
That said, there is the tiny percentage case where both the battery is at minimum state of charge, and the acceleration/climbing demand requires maximum power output of the REx, and I'm happy to see BMW upped that maximum power available.

I would argue that this is not a tiny percentage; it would be good not to marginalize this use case. My best guess is that the change was introduced in preparation for the US market launch. As we all know, the i3 lacks the hold or mountain mode stateside. This can become problematic if a driver is attempting to climb a larger hill, and did not have an opportunity to charge the traction battery. There already have been reports from freshly-minted owners in LA that they did not find the performance of the REx to be up to their expectations. As we all know, perception is reality, and it would behoove the manufacturer to cater to their customer's wants and needs.

REx now with more power - 28 kW

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