The BMW i3 is an electric vehicle that can be optioned with a nifty range-extender engine which runs on gasoline — a series hybrid set up exactly like the new Ram Ramcharger. It’s a great way to avoid the problem of slow EV chargers on long trips, save for one problem—it has a tiny fuel tank that lasts maybe an hour or two at best on the highway. That’s led several owners to develop their own frunk-mounted auxiliary tanks to give their cars longer legs out on the open road. Yes, a gas tank in the front trunk. Let’s dive into how it all works. [Ed Note: We do not condone this mod; we think it’s a bad idea. -DT]
For some owners, the i3’s gas tank is too small, at just 1.9 gallons in 2014 to 2016 models and 2.4 gallons thereafter (note that the small tank could be “coded” to go from 1.9 to 2.4 gallons, as it’s artificially reduced via software for regulatory reasons). At best, an i3 with the 120 Ah battery and the range extender will get around 126 miles of EV range and a further 80 miles or so from gasoline. Thus, those taking the i3 on longer trips often desire a much larger tank that would allow them to drive for hours without refueling or recharging the car.
It’s A Tiny Gas Tank But A Reasonably Sizeable Frunk
If you’re wondering just how small it is, well… it’s about as big as a shoebox. A tiny one:
(That filler is on the front passenger’s side fender, and the tank is in the passenger’s front wheel-well area).
The i3 is a subcompact without a lot of space to spare. By virtue of not having a traditional engine under the hood, though, the frunk presents a nifty storage space that’s outside the cabin:
It’s just roomy enough to stash a small additional fuel tank without the hassle of gas fumes permeating the passenger cabin. Some enterprising owners have taken advantage of this by plumbing in an additional tank to the i3’s fuel system to net hundreds of miles of extra range. It might sound difficult and maybe a bit absurd to plumb in an additional fuel tank, but i3 owners have gone for it, and even found a few shortcuts that make the job easier.
How People Have Turned That Frunk Into A Gas Tank
Jason Sharp is an i3 owner from Idaho, and his car has an 8 gallon tank from Summit Racing stashed in the frunk. He found that the overflow line running alongside the i3’s fuel filler pipe was the perfect place to tap in. He cut the i3’s overflow line, plumbing the tank end into the output from a transfer pump hooked up to the auxiliary tank. The auxiliary tank’s vent is then hooked up to the other end of the cut overflow line.
As the i3’s main tank empties, it naturally draws fuel from the auxiliary tank, even with the pump off. “It will pull half a tank into the [i3’s] small tank, but only when both tanks are full,” Sharp told The Autopian, noting that he uses a switch to activate the electric pump to transfer fuel when the vacuum from the main tank isn’t enough to do the job. He states that this setup keeps the fuel system sealed, which is key to avoiding problems. “Otherwise it will trigger a check engine light,” says Sharp.
Yes, if you’ve ever had a leaky gas cap, you’ve experienced the annoyance of it triggering a check engine light. Thankfully, that’s avoidable by keeping the fuel system properly sealed. Sharp’s setup achieves this neatly by the way it’s plumbed into the fuel system. The external tank’s fuel feed is fed directly to the main tank, and its vent runs up to the gas cap. The system remains sealed, and no check engine lights are thrown.
Paul Housley has a similar setup on his own i3, which he uses for long trips a few times a year. Housley flicks a switch mounted at the base of the dash to turn on a pump which fills the i3’s tank from the auxiliary one in the frunk. Like Sharp, his pump runs into the same line next to the fuel filler. “You just turn on the pump as needed while driving and then it slowly fills the tank while you are driving,” says Sharp.“Usually it is best to turn the pump off when the fuel gauge says three-quarters full because it does not respond very rapidly and you don’t want to overfill it,” he adds. He finds the system most useful on country drives where there are no gas stations or EV chargers for hundreds of miles. In those situations, the i3’s short range can be a real liability.
Sharp notes that the additional tank nets him a range of 350 miles or more on gasoline, a huge jump over what’s possible with the stock car’s standard tank. Similarly, Housley claims a total range of over 400 miles with both tanks topped off and a full charge in the i3’s battery. That’s a huge boost over the stock i3, which is capable of 200 miles in its longest-range trim, according to EPA figures.
Plus, it enables the car to go hundreds of miles further with a simple refill of the auxiliary and main tanks, without having to wait for the main battery to charge. Try that with the standard tank only, and a 5-minute stop for gas will only net you another 80 miles or so.
Not everyone goes the pre-made tank route. These photos from Jorge Montes de Oca show his custom tank under construction, built especially to fit the dimensions of the i3’s frunk.
Some may find the modification unduly invasive, but other owners have found solutions that don’t involve cutting into the i3’s fuel system. A guide shared on BMW forum Bimmerfest explains how to plumb an external fuel line into a hole drilled into the i3’s fuel cap, sealing it with RTV and epoxy. It functionally achieves the same thing, but it’s easier to replace a fuel cap if you want to put your car back to stock.
The cheapest version of the hack uses a simple plastic gas can as a tank, with a pickup hose from a small transfer pump running through the gas can’s plastic cap. The cap also has a tiny hole drilled into it for venting purposes. Without this, the gas can would deform and get sucked in by the vacuum created as the fuel was drawn out of the tank. This works, in that it allows the gas can to effectively serve as an auxiliary fuel tank. However, this cheap vent system creates a risk of fuel vapors building up in the frunk.
[Editor’s Note: Here’s a look at the pdf Lewin mentioned; it shows the entire installation process for a jerry can into the i3’s frunk. Here are some screengrabs. Note how the installer drilled a hole into the fuel cap, and just slathered some RTV to seal a tube that a fuel line from the jerry can plugs into:
Notice how the installer actually trimmed their fuel filler door to fit the fuel hose nipple on the fuel cap.
Here’s the on-off switch for the silver electric fuel pump, shown above (the relay is shown just above the fuel pump image). And you can see the final product in the frunk:
It’s all a bit wild. You can read the full installation instructions here. -DT]
It May Not Seem Safe, But It Does Seem Useful
Indeed, all of these setups do pose a certain safety risk. When BMW designed the i3, it didn’t account for a fuel tank in the frunk. Indeed, the front end must deform to dissipate energy in a crash, as with any modern car. With a fuel tank in the frunk, there is the distinct possibility of it bursting like a balloon in an impact,with any fuel inside spraying everywhere. A partially-empty tank is perhaps even more dangerous, by virtue of the easy ignition of the gasoline vapor inside. In the 20th century, automakers learned not to put fuel tanks in positions where they could be easily damaged or punctured. Putting a fuel tank in the frunk is very much contrary to safe design practices, even if it’s only a small 8-gallon tank.
Sharp isn’t unduly concerned about the potential fire risk, though. “Around town I keep it empty,” he says, noting that it’s “not the safest, but I only travel long distance once every few months now.”
Housley also points out that he used a proper fuel cell for the tank, which is at least nominally designed to survive a crash without a major failure. He also he notes that removing his tank only takes a few minutes. “It is only in the car when I know I am going to need it,” he says.
Anyone attempting such a build should consider the safety aspects involved in modifying a fuel system. Beyond the crash risk, it’s also worth noting the dangers in routing your own fuel lines. Run one too close to an exhaust pipe or other hot part of the car, and you can easily burn your ride to the ground. Tying a fuel line to any moving components of the car can also see them torn or cut open, creating a dangerous leak. Even simply zip-tying a rubber fuel hose to a metal line can cause problems if you’re unlucky, with vibration between the two slowly working a hole into the softer rubber line.
If you’re attempting such a retrofit to your own i3, consider the path of any fuel hoses carefully, and make sure you’re using properly rated lines. If you’re using a low-pressure pump to transfer fuel into the fuel filler, for example, there’s no need to spring for the expensive high-pressure hoses needed for EFI installs. However, you’ll still need to get the stuff that’s properly rated to handle fuel without degrading. Wiring for any pumps, relays, and switches should also be carefully considered. You don’t want to accidentally strike a spark while you’re working on your auxiliary fuel tank, after all. It pays to remember that fuel vapor is far more flammable than the actual fuel itself.
The mod is definitely a game changer for road trips. Regardless, even with the additional fuel, the i3 can still struggle on longer routes. “The biggest issue is just climbing hills for an extended time,” says Sharp, adding “I’ve ran the battery down from 75 percent to zero often.” He explains that when the battery has run out, the i3 can struggle to maintain 80 mph uphill using only the range extender. That’s perhaps unsurprising, given the 0.65-liter, two-cylinder range extender is only capable of putting out 33 horsepower. It’s a small fraction of the 167 horsepower available from the electric powertrain when the battery has some charge.
A cynic would say that a BMW i3 is simply not well suited for long-range travel, and that owners should simply buy another car. As these owners demonstrate, though, it’s quite easy to extend the range of an i3 by 100% or more with just a few hundred dollars in parts. If you’ve already got the car, and you want to take the occasional long trip, it’s hard to argue with the value there. It’s theoretically a lot less fuss than trading in your car for another vehicle, and cheaper than renting one for the occasional weekend.
Still, while I applaud the ingenuity, I’m not sure I’d want a gas tank under my hood, especially one that wasn’t originally designed to be there.
Why? Because as shipped it is the worst PHEV ever sold in the US because they decided the ZEV credits were more important that actually offering a Range Extended EV.
It was to comply with CARB’s BEVx regulations and earn ZEV credits without going all in on EVs. https://www.greencarcongress.com/2012/01/bevx-20120129.html
Chevy Volt 😉
The well executed builds with proper fuel cells don’t strike me as being crazy unsafe. Important to know what your doing but I wouldn’t lose sleep about. The ones were people are just throwing a gas can in there are pretty terrifying though…
Didn’t cop spec Crown Vic’s have a fuel tank right in the path of a rear-ender?
They were in a safe place in front of the rear axle behind the rear seat. The cars that ruptured tanks were in crashes where the car was parked and hit by a car at highway speeds 55+ mph, the patrol car also had to have a full trunk of police equipment. In independent crash testing, by I believe Texas highway patrol, the only way they could recreate the ruptures was to weld a pry bar to another piece of equipment in a way that it would puncture the fuel tank in the 55+ mph impact to the rear. Every crash test they did that had normal police loads they couldn’t get the tanks to rupture. The ultimate outcome was the case being dismissed against ford. Ford for their part offered Kevlar trunk liners at no cost to departments who requested them.
The tank is between the rear axle and the trunk in a Panther. In many of the cases it was stuff in the trunk that was oreinted fore/aft that would punch through the front of the trunk and the and the gas tank strapped on the other side. That is why they started shipping new cars and giving out the trunk liners that are designed to fore people to place long items in the side to side orientation by having it split the area side to side.
Congrats, these i3 s will now be as safe as a Trabant, with its above the engine fuel tank gravity feeding it through a motorcycle fuel petcock.
A fuel tank in the crumple zone – quite illuminating.
Wait until you find out where plenty of other gas tanks are …….XJ and ZJ Cherokees are unibody cars that definitely crumple and have the fuel tank just a few inches from the rear bumper. It’s also a complete non issue.
Those sound like models from 20 to 40 years ago 🙂 And they relied on size more than anything else (weren’t their crash test results quite bad even for their time ?).
This thing is small, and this nose is definitely engineered to crumple and fold, and none of the equations included a tank full of gas right in that zone supposed to fold. A youtube video of any i3 crash test is a 5 seconds search away, and it shows exactly what folds and where.
ZJs also all got free trailer hitches so the rear frame wouldn’t crumple, right?
Putting the tank in front of the vehicle is worse than in the rear. You’re more likely to run into something than get hit, and you are a LOT more likely to have a driver than a rear seat passenger.
ZJs all got free trailer hitches so the rear frame didn’t fatigue and break in normal use. Imagine how flimsy it must be in a crash.
And yet exploding gas tanks is a total non issue. I suspect the front of a carbon i3 is at least as strong as the flimsy trailer-hitch-reinforced back of a ZJ.
No – NHTSA forced Chrysler to give away hitches to protect against fires due to their tank location. This is also why you never see tanks mounted there anymore.
https://cars.usnews.com/cars-trucks/advice/best-cars-blog/2013/06/up-to-156-million-jeep-owners-will-get-free-trailer-hitches
My understanding was that every ZJ starting with 94 model year came with a hitch from the factory, and that any 93 models without hitches were part of a voluntary Chrysler recall to get hitches dealer installed. I understood that it was due to unibody cracking warranty claims.
> petcock
I learned a word today
I learned that word freshman year in autoshop. We had a lot of fun with that, as you can imagine.
Riding a motorcycle, you pay attention to the word petcock. Forgetting to switch your petcock back when you hit your reserve light and refill? You gonna eventually have a bad day!
Spends all that money on a fuel cell for safety, then buttons all the hose together with standard hose clamps instead of the rated, shielded units. Hmmm.
Rated shielded hose clamps? There are other kinds of hose clamps? Every car I’ve ever worked on used standard hose clamps for the fuel system from the factory.
Except for the bit of diamond plate the top one looks stock. BMW would never. I’d like to show that under hood view (without the metal) to a professional mechanic and see if they A) realize that’s not what the front compartment of an i3 looks like and B) can guess what fluid goes in that tank. Oh, and I’d like to see their faces when they’re told what it is.
David “let me just hang this open gas bag from the wing mirror” Tracy – does not approve of this dangerous mod. Got it.
I love this: it reminds me when Acerbis made a 28.5 gallon tank for a Honda monkey which set the world record covering 2997 miles on a single tank without refueling…
When I first heard of the i3 I observed that they kneecapped it for artificial (government incentives) purposes. I just want cars to be all they can be so I would absolutely do this mod. Then again I have been known to drive my very safe Corvair on high speed rallies with lap belts and 10 gallons of extra fuel strapped in the frunk.
Fuel Cell is still safer than any 1960s Ford Compact or Intermediate where the Gas Tank was the bottom of the Trunk, or as others pointed out, air cooled VWs
60s Ford compact? More like most cars 1960-1980s with the gas tank in the bottom of the trunk.
This is a good example of fitting a square peg into a round hole. The i3 is a fantastic city car. I’ve been driving my 2014 BEV for about 8 months, and don’t recall the last time I felt range anxiety. Now that said, I recognize the car’s limitations, and don’t try to put it through the paces with a road trip.
We have an ICE vehicle for longer drives, which frankly has enabled us to make the i3 work. 99% of the time, we opt for the i3. The 1% balance is where ICE comes in.
As ingenious as this mod is, I think an occasional rental would be provide more peace of mind than this frankenmod.
For my money, I’d probably just get a different car if I needed to go far on the regular. It’s a cool hack but not one I’d feel safe relying on.
Occasional rental? For $200 a day? No thanks………
Are you renting super cars? I’m spending $300 on a week-long sedan rental this Thanksgiving. About $42 a day.
So the best addition to an electric car…. is a gas tank ?
This is so like that scene from 1941 https://youtu.be/iMCPORA5eDE?si=PPcbPyNexImX1DnY&t=317 On how not to fire an anti-ship weapon.
Somebody please get video of what happens when this thing rear-ends a Pinto. Nevermind… I could probably just see it from here.
Or T bones a Silverado with a side-saddle fuel tank. Fun times.
Take a collapsible race tank that shrinks while emptying, or use te frunk tank to recharge the battery while the gas tank is being used. 99% of the time you aren’t getting in an accident.
The article mentions that when the battery runs out it can be hard to maintain 80 mph speeds uphill on the REX only and further states that this is understandable as the REX is only capable of 33 hp vs. the 167 of electric powertrain. BUT, BUT, the REX is NOT part of the powertrain; as I understand it, there is no mechanical connection between the wheels and the REX. Its sole function is to power a generator that feeds current to the battery (or maybe the electric motor, not sure which). If that’s the case, and the battery is able to send current to the motor, the deciding factor regarding speed would be the amount of current fed to the motors and not the horsepower of the REX, though presumably a more powerful REX (as in the Ramcharger) would be able to drive a bigger generator and thus provide more current. Am I mistaken?
You’re correct. Very roughly (and ignoring charging losses, which are non-zero), if your average draw from the battery is less than 33 hp then you can continue indefinitely on the REX. I suspect the problem comes when the battery is mostly empty, the REX recently started running, and there isn’t enough juice in the battery to get 167hp from the electric motor.
That said, I don’t know the i3 well enough to know how the REX integrates. For the sake of argument, if you are using 20hp to roll down the highway and the REX is generating 33hp, that’s somewhere below 13hp (again, charging losses) that is charging the battery. If the battery got up to 25% charged, would the REX shut down until the battery is drained again?
Thanks
The car is coded to really dampen its own performance when it goes into REX mode (7% battery IIRC). The way around this is to re-code the car using an app so the REX can be activated at any State of Charge. I’ve gotten into the habit of turning on the REX when SoC hits 33%; there’s no performance hit and the car behaves exactly as you describe. SoC drops slowly if I’m drawing more than the REX can generate and creeps up again on the downhill, as it were. It won’t actually *charge up* though; it maintains the SoC you set.
A 33hp motor can turn a 24.6kw generator which cannot supply more than 24.6 kw to the motor without the aid of a battery. If the battery is discharged, the electric motor max output is probably in the high 20s HP after counting conversation and transmission losses. The vehicle definitely needs more than 30 hp to maintain 80 up a hill.
Makes sense. Thank you.
It probably a lot less than that. Assuming a 33hp motor directly turns an alternator (ie no losses in between), you would probably get around 20kW (80% eff) out of the alternator as power to the battery, which would translate to around 18kW actual charge (90% eff) into the battery. Then it has to be turned into power in the motor (80% again) which gives about 14.5kW (20hp) at the wheels.
Most automotive alternators are actually only 60% efficient but I’ll assume BMW used a particularly efficient one.
I’m guessing the generator power goes straight to the motor when the battery is in a charging system. So we could eliminate one of the conversation losses.
Also I don’t know if you are using alternator and generator interchangeably, but I think the charging system is more like the backup power you might have at your house vs a standard automobile alternator. And while it looks like I overestimated the output, I think they are much better than 60% efficient.
But it’s a purpose built generator, nothing like an automotive alternator.
I’d guess in between 90 and 95% efficiency. And then the charging and discharging losses are again between 90-95%.
Yup. Checks out. It puts it in the realm of a mid-20th century microcar at that operating point.
It’s a common problem for range-extender type hybrids. The Volt has a bigger buffer at “empty” battery when it turns the generator on, and 80HP instead of 33, so it mostly avoids the problem, but it also has a “mountain mode” that basically targets 20% SoC instead of 0%.
Pashaw, Derek Bieri does this all the time, and sets the gas can on the seat next to him!
With a proper fuel cell this is still probably 100x safer than my ’65 beetle. Hell, even with the plastic fuel can it might be 100x safer.
Interesting point! Something something Ford Pinto something something boom
I have seen way sketcher things in the off-road community. Front gas tanks are so Air-cooled VW. Glad some people are using a fuel cell at least and I would hope, accept the risks involving DIY gas tanks.
I hear Canadians do this mod except they put the gas in a clear plastic bag.
I like how an article about a dangerous modification includes detailed instructions on how to build said dangerous modification. I also like that it did not include a disclaimer that a DIY gas tank in the front crumple zone is inadvisable.
Are The Autopian’s lawyers out sick today or something?
The entire story referred to the mod as dangerous. We do not advice that anyone do this.
*advise, even.
I wasn’t trying to be critical of this post, by the way. I am used to seeing legal disclaimers for innocuous things and thought it was funny this post didn’t include them. I feel like most outlets wouldn’t publish this or at least would not include so many details. Thanks for respecting the common sense of your readers and posting interesting content others probably wouldn’t.
Do not do this super cool thing we just showed you
The same lawyers that greenlit a story about a chainsaw and leaky batteries? No, those guys are on call 24/7.
I wonder if this is even legal to do.
Diesel truck owners will often add aux fuel tanks to the bed for greater range; when researching if these were available for gas trucks, I thought I remembered reading that it wasn’t possible under current law. Maybe that’s wrong or misremembered, but add that possibility to all the other reasons this is not a good idea.
***Edit: I guess not, there are auxiliary truck bed gas tanks for sale. It’s still a bad idea***
Diesel, at least, being far less volatile, would make it a less sketchy proposition.
It’s surprisingly difficult to get a puddle of diesel to ignite. Or aerosolize/vaporize, which is what it needs to burn. Kerosene is about the same. It’s definitely not good to have either of those spilling everywhere but it’s a lot better than gasoline.
So…… I’ve never seen a bed tank rigged up as an aux tank, but they can in fact be used to fill up the pickup and extend range.
They are meant to be used as transfer tanks, to transport fuel home for your tractor or similar without goofing around with 6 gas cans.
Is it a bad idea? I don’t think they’re unsafe in the slightest if bolted down and vented correctly……
I would think long and hard before doing this. I suspect that it would invalidate your insurance, all of your insurance. If you were to be in a crash where people died I suspect that you might be found criminally liable. Just a (rather grim) thought.
I was thinking the exact same thing.
It seems sad that anyone would have to emphasize that turning the front end of your car into a Molotov cocktail could get you in trouble if anything goes wrong.
You do realize that the back of your car is likely already as much of a Molotov as this is, right?
Are you under the impression that engineers design a car without a gas tank, and then the end user has to figure out where they think it should go?
The frunk of an i3 was not intended to hold any type of fuel container, unlike the built in gas tank in the back.
I was shocked DT didn’t write this, I smell a feature!
Are you sure those aren’t frunk fumes? /s
Though seriously, this was very well written and I was shocked to discover the editor’s notes were from David and he wasn’t behind this piece. Welcome to the Auto Utopia Lewin!
Whoa. I didn’t even look at the byline. i3 = Hollywood Dave like Jeep = Rusty Dave. I just made an assumption. One day I envision DT fighting himself like Christopher Reeve in Superman 3; will Hollywood Dave or Rusty Dave be victorious?
Nm
I would have no problem living with an actual fuel cell up there, but the plastic gas can is probably where I’d draw the line
Even with an actual fuel cell I think I’d want it a bit smaller to leave some space in front of it.
I’d leave space for a few remote-triggered fire extinguishers. If anything happens, I’d want to be able to smother the fumes instantly.
Maybe fill the rest of the frunk with oil-dry to soak up spills. It’ll still burn but there’d be less to drip into the passenger compartment.
I regularly make extended road trips in something infinitely more dangerous than any i3 could ever be, so I tend to take a more relaxed view to this stuff
Can we get The Bishop to draw up a Mad Max-ified David Tracy i3 with external fuel drums?
Jettisonable barrels on the quarter panels like an old Russian tank?
Maybe some surplus aircraft drop tanks for better aero.
Spinning halberds sticking out of the hubcaps
I expect an article describing how David has re-purposed some Jeep parts to make his conversion work