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Review of W20 training PDF

Tue Mar 13, 2018 8:46 am


I was re-reading the W20 training PDF and found two nipples on the exhaust manifold in the sketch, pp. 37. The description pp. 38 calls them "Connections, secondary air system (only US)". This looks like an exhaust gas tap but there is no description of its operation or what it feeds.

I looked at the engine sketch, pp. 36, and it shows two tubes identified as "Turbulence lines" going from the exhaust manifold to the intake manifold. They are described as:
    . . . The turbulence lines generate a swirling effect as a result of the air flow upstream from the intake valve during the mixture preparation. The combustion becomes more efficient as a result. The function primarily has an effect in the partial load range as with a small throttle opening in the vacuum in the intake pipe is greatest and thus a higher volumetric flow is achieved in the turbulence lines.
Poor English, the bolded text reads exactly like how recirculated exhaust gas works. It adds an inert gas that keeps the combustion temperatures down and lets the throttle open wider to reduce pumping losses.

The only problem is the system overview, pp. 34, shows the "turbulence lines" in blue, cold, coming from the engine block, not the exhaust nipples. What it doesn't explain is where the positive air pressure comes for these "turbulence lines." It shows the positive crankcase valve line feeding the same intake assembly. So the "turbulence lines" remain a puzzle.

On the gas tank capacity:
  • 9 liters / 2.38 gallons, pp. 45
  • 2 liters / .5 gallons reserve, pp 45
  • > 0.8 liter ( 0.21 gal) pp. 61 - minimum fuel level for Rex operation
This explains why the EPA reports 2.88 gallons, 2.38 + 0.5 = 2.88. But the question is whether or not the engine can dip 0.8 liter into the reserve. That would bring the usable fuel up to 9.8 liters, 2.59 gallons. After BIMMERCODE, I'm only seeing a maximum fill of 2.3 gallons.

So I'm wondering if we could code the tank for 10 liters and get a full fill?

Bob Wilson
20k/27k mi 2014 BMW i3-REx
10k/10k mi 2017 Prius Prime

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Joined: Tue Apr 18, 2017 6:06 am

Re: Review of W20 training PDF

Wed Mar 14, 2018 12:10 pm

Thanks for looking into the documents into more detail! I am sure when the i3s start coming off warranty there will be alot of folks who will want to DIY their repairs, or aid the independent shops in figuring things out... threads like this will help folks down the line :-)

I read somewhere that bmw limited the max fuel usage to 2.3x gallons of fuel for everywhere to keep the pump lubricated. In the event someone ran the tank dry, the pump's life would be shortened greatly. It would be nice to be able to get the full 2.8x gallons, but thinking back to my long road trip - kinda glad I wasn't able to - I usually ran the i3 until I got down to 1-3 est miles to the next gas stations. Often times however exhausting all the available fuel and automatically reverting back to straight BEV for the last 1-3 miles to the next gas station to fuel up again.

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Location: San Francisco Bay Area

Re: Review of W20 training PDF

Thu Mar 22, 2018 9:10 am

Interesting stuff Bob. Thanks!

When the engineers at BMW sat down and designed the Rex into the i3, I'm sure they assumed that running the gas tank down to "empty" was not an uncommon thing - unlike an ICE, where no gas means no-go ;) We also know that the tank "size" can be electronically limited.

Since everyone submerges the fuel pump in the gas tank for cooling purposes (and packaging), it would make perfect sense to never allow the gas tank to actually run empty, but simply report it as empty when the gas level is minimally sufficient for fuel pump cooling. ie, leave enough gas in the tank to at least partially cover the fuel pump.

This might explain the discrepancy between gas tank capacity and "useable" gas tank capacity.

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