Best range while REx is running

[RJSATLBA]

My understanding, confirmed during a brief second test drive (battery was above 75% on first) is that REx runs at constant rpm when in use but can only maintain charge not add to it. A constant REx rpm would seem to mean that for any given amount of fuel the REx should run for a fixed length of time.

Does this mean that at any motorway speed below that at which REx can only just match the power consumption, some of the energy from the REx is being wasted. If so, this would imply that the maximum range on REx would be achieved by cruising at the speed when the REx is just able to match energy use and so get the maximum distance in the fixed time the fuel will allow.

This sounds to good to be true!

 

[ultraturtle]

Does this mean that at any motorway speed below that at which REx can only just match the power consumption, some of the energy from the REx is being wasted.

Probably not. Constant RPM implies constant frequency, and pretty much every fossil fuel powered generator out there does its best to maintain a constant frequency while the load on it can vary from zero all the way up to its maximum rated output. Fuel burn varies with the load, not the frequency. And yes, I do know that it is generating DC power, so frequency may not be fundamentally important.

 

[Stevei3]

Hi Guys,

the (my) REx does not run at a fixed speed. It adjusts its power output to your needs, This seems to have quite an influence on your (my) gas milage.

Steven

 

[ultraturtle]

Gasoline engines are most efficient at an RPM just below max power, and that RPM is fairly consistent from zero to maximum load:

BMW engineers are smarter than the average schlub, and have more than likely designed the engine to operate as efficiently as possible, and therefore restricted to a very narrow range of RPM. It may seem counterintuitive, but RPM may be slightly higher at lower loads. Anyone with a ScanGuage, or similar device can probably decipher its output to get a reading of actual engine RPM and amperage output to confirm.

This also highlights the genius of BMW's implementation of a gasoline engine as a means of extending range. By operating a reasonably low powered engine as a pure generator, it can operate at its most efficient RPM full time and under full load, which approaches 30% efficiency. Contrast this with using a massively oversized engine to power the drivetrain which can only operate efficiently when heavily loaded, accelerating up steep hills. The rest of the time it operates at lower load and RPM, typically delivering less than 20% efficiency overall.

Best implementation from an efficiency standpoint would be to operate the REx engine at full load until the battery reaches it's charge end setpoint, then shut off until it reaches its charge start setpoint, but unfortunately, it sounds like that may not be the case. Another issue probably takes priority, perhaps thermal management of the ICE itself.

 

[Stevei3]

Another issue probably takes priority

Hi,

my guess is they A) made a design choice with an eye on comfort and B) they do not want to charge the battery above the 'hold' range, so they modulate the enigne to deliver less power when less is needed. I guess you are right that it is running less efficient at that moment, but it does consume less fuel.

Steven

 

[RJSATLBA]

Hi

Thanks for all the interesting replies, I thought it would be too good to be true!


Roger

 

[fdl1409]

I expect my i3 REx in June.

I was first disappointed when I learned that the REx does not charge the battery with power not simultaneously consumed unless SOC is very low. On second thought, it is probably better this way. Charging and discharging causes a lot of losses, so running the REx with a lower load and consequently lower efficiency may overall be the better idea.

Won`t need the REx very often, probably much less than 5% of my driving will be by REx.
Still, maximum efficiency is something I always try to achieve.
Normally, the driver will set a certain point, below which the REx will try to maintain SOC. This in my view is not as efficient as could be. So I will try to use the REx in a different way. As far as I know it is possible to use the 8 buttons on the steering wheel for any function, which can freely be programmed. I will try to use one or two of those buttons to start / stop the REx manually. I will start the REx only if power demand is high, if possible close to or above 23.5 kW, which is the maximum power produced by the REx. This will be the case when accelerating after a stop or when driving uphill or with high speed. Sometimes for few seconds only, This way the REx ought to work at its highest efficiency all the time and produce much more range compared to its normal mode of working with varying and frequently low power demand.

Frank
Germany

 

[RJSATLBA]

Hi

I recall seeing a link to a BMW leaflet on efficient use of the REx somewhere else on the forum which specifically referred to programming one of the 8 buttons to allow the engine to be controlled in the way you wish to.

However, surely use of the REx for very short periods to help manage battery longevity may be at the expense of the REx, especially if the short periods mean the REx is often being used cold and without getting the oil up to the temperature required to burn off impurities and moisture. This was one of the issues I raised with the salesman, although I wasn't thinking of the extreme short bursts you have in mind, merely the likelihood that the engine might only be used once every two or three weeks. I have expensive memories of an aircraft engine needing major work after 3 years due to internal pitting and spalling, which Lycoming attributed to insufficient use and we were advised it was a battle we were most unlikely to win. That was with use of around 150 hours per year and I wouldn't expect many REx engines to do as much as that.

 

[ultraturtle]

A couple points:

Charging and discharging causes a lot of losses...

The system is designed to handle an immense number of charging and discharging cycles. Remember that regenerative braking is a charging event that takes place quite frequently and generates up to 50kW of energy. The REx output is less than half of that.

So I will try to use the REx in a different way. ... I will start the REx only if power demand is high, if possible close to or above 23.5 kW, which is the maximum power produced by the REx. This will be the case when accelerating after a stop or when driving uphill or with high speed. Sometimes for few seconds only, This way the REx ought to work at its highest efficiency all the time and produce much more range compared to its normal mode of working with varying and frequently low power demand.

That is an interesting idea. The only glitch is that starting a cold engine and immediately loading it to near maximum output not only creates excessive engine wear, but excessive emissions. I'm pretty sure the system would be designed to prevent that.

I personally can think of only two scenarios where I will manually activate the REx prior to battery depletion (USA drivers will almost certainly figure out a hack to do that within months of the first delivery):

1. When the last 3 or more miles of the journey beyond battery range requires a significant increase in altitude. This will enable full power for the climb.

2. When making a road trip in excess of the the combined range of the battery and gas tank. Selecting REx as early as possible gives an initial 50 mile reserve allowing full emptying of the gas tank - extending the distance between fueling stops.

 

[Stevei3]

I personally can think of only two scenarios where I will manually activate the REx prior to battery depletion (USA drivers will almost certainly figure out a hack to do that within months of the first delivery):

1. When the last 3 or more miles of the journey beyond battery range requires a significant increase in altitude. This will enable full power for the climb.

2. When making a road trip in excess of the the combined range of the battery and gas tank. Selecting REx as early as possible gives an initial 50 mile reserve allowing full emptying of the gas tank - extending the distance between fueling stops.

Hi Turtle, Frank,

as described above would probably a good way to use the REx, if you don't wan to run the battery down to 5-6%.
I am not a RExpert yet, but I observerved a few things so far: when it starts from cold, it does run idle for a bit before ramping up. Also, when started, it has to run for a while before it switches off. When warm, run-in and run-out times are much shorter.
And the threshold for autostart level cannot be set, that remains 5-6%, what you can do is manually set a 'hold' level. The REx will try to keep that level during your journey, until your gas runs out or you switch off the car. You can use one of the 8 buttons on the dash to switch on the engine, not the buttons on the wheel.

Steven

 

[fdl1409]

For me it is mainly uphill driving, when I would use the REx, and of course only when the battery doesn`t give me enough range.
My home is at 390m sea level, and I frequently have to climb hills 600m high, so for some 3-5 minutes power demand would be in excess of 23.5 kW.

True, very short bursts would probably create higher emissions. That is one point I did not consider before.

Will make some experiments once I have my i3, and report about results.

Frank