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BMW i3 REx vs. Chevrolet Volt: two different approaches to plug-in hybrids

Aside from general misunderstanding of how plug-in hybrids work, an even less understood nuance is how the range-extended version of the BMW i3 is different from the most well known plug-in hybrid, the Chevrolet Volt. Torque News breaks it down.

The Chevrolet Volt and the BMW i3 REx are two electric vehicles that rely on gasoline backup to alleviate range anxiety. How the two vehicles go about it, though, is what distinguishes one from the other.

Chevrolet Volt: how it works

When it was first unveiled, GM tried to tell everyone that the Volt was a pure series hybrid; that is, the battery supplied electricity to the motor until it ran out, at which point the gasoline engine would rumble to life and generate electricity to feed to the motor. We eventually found out that GM wasn’t being entirely truthful and the real system is actually far more complex.

In reality the Volt is technically a series-parallel plug-in hybrid, meaning the engine is also capable of powering the wheels when necessary. The 16 kWh battery pack allows for approximately 38 miles of all electric range powered by a 149-hp electric motor. When the battery depletes to a certain level, the 1.4 liter 84-hp gasoline engine kicks on in a smooth transition. It functions as a generator to produce electricity for the drive motor.

Interestingly, though, the Volt doesn’t have just a single-speed direct drive transmission as you might expect for what is effectively an EV. It actually uses a planetary gearset with a second electric machine; the 149-hp traction motor is connected to the central sun gear, while the planetary carrier provides the output to the differential. The 72-hp generator motor can be fixed to the outer ring gear, the gasoline engine, or both at once using clutches.

Volt drive modes

This configuration allows for four different “drive modes” with the Volt, as described by Edmunds:

1 – Low speed EV mode. Up to 70 mph, the larger traction motor provides the necessary torque while the engine and smaller motor-generator are disconnected from the transmission and the outer ring gear is locked in place.

2 – High speed EV mode. Up to 100 mph, the Volt is still capable of running without using the gasoline engine. However, the configuration changes to improve efficiency above 70 mph. In this mode, the ring gear is fixed to the smaller motor while the larger motor turns the sun gear. Both motors turning in the same direction allows the planetary carrier to rotate faster and reach high speeds while keeping the speed of both electric machines closer to the optimal efficiency band. If the larger traction motor were to provide sole motive power at high speeds, electric range would be compromised as motors lose efficiency at excessive RPM.

3 – Low speed series hybrid mode. Up to 70 mph after the battery has depleted to a certain level, the engine fires up and is connected to the smaller motor that acts as a generator to provide electricity for the larger traction motor.

4 – High speed series-parallel hybrid mode. Above 70 mph with the battery depleted the engine will be connected to the outer ring gear to assist in providing power the wheels. It also is fixed to the smaller motor-generator to simultaneously recharge the battery, which forces the engine to work harder. In this mode the larger traction motor is still driving the sun gear; the vehicle cannot move unless the traction motor is operating.

Note: A previous version of this story contained inaccuracies in the drive mode descriptions, pointed out by alert readers. We regret the confusion.

So it is clear that the Volt is more complex than you might think. After the electric-only mode is exhausted, the vehicle gets somewhere between 35 and 40 miles per gallon, not great for a hybrid. But it is capable of driving about 350 miles on a tank, which makes it an EV that is practical for long trips.

BMW i3 REx: a different approach

BMW went a different route with the all-new BMW i3. The vehicle is offered as either a battery electric or a plug-in hybrid REx model, and the buyer can choose which he prefers. The optional range extender gasoline engine costs about $4,000 extra, and lengthens the vehicle’s effective range from 72 miles to 150 miles. The unladen battery electric version without the range extender achieves 81 miles of range per charge.

The gasoline engine in the i3 REx is very small and serves only to increase driver confidence, and the 1.9 gallon fuel tank only provides enough juice for an additional 80 miles after the battery is depleted. The two-cylinder engine is pitifully small at 0.68 liters; once it switches on the vehicle’s performance can be negatively affected in some situations. The 170-hp motor cannot call the same power from the battery that it can in EV mode if it exhausts a certain reserve, as it is relying on the tiny engine to spin a generator that provides electricity for the motor.

The most interesting aspect of the i3 REx is simply that it is optional. BMW obviously reasoned that allowing customers a choice would help the vehicle sell to more drivers concerned about the limitations of an 80-mile electric range. Although it makes the i3 viable for trips up to 150 miles, the vehicle is without a doubt compromised by the addition of the range extender and should be driven with only the battery as much as possible.

Which should you choose?

Both vehicles serve their intended purpose, but that purpose is not the same for each. The Volt is the car that is ideal for shorter commutes in EV mode and the occasional long road trip when necessary, and eliminates any shadow of range anxiety. The i3 REx is primarily a pure electric car that just happens to have an emergency backup engine that reduces performance when it is needed. The REx can commute twice as far as the Volt in EV mode but is not viable for long trips.

Both vehicles are not cheap, although the Volt has come down in price and will drop further with the next generation in 2015. The Volt starts at $34,185 while the BMW i3 REx will set you back a cool $45,200. If you’ve got the cash, both are excellent vehicles that go about their different tasks in different ways.


John Goreham    June 16, 2014 - 9:44AM

Well said. A great overview of the Volt. Worth mentioning too that the Volt was an original idea. By that I meant the designer simply wanted the best experience for the owner/driver. The BMW by comparison was designed with the CARB regulations in mind, and the electonauts talk a lot about how the REX was really designed for a certain sticker color for HOV lane driving in CA. A post today by a BMW i3 REX owner describes the car's behavior as "hysterical" when the range extender comes on. The car sends repeated messages about getting juice and suggests local charge points. As you state the REX is really for emergencies, the Volt can be driven on gas as part of a long trip quite happily. This post is not meant as a jab a the BMW.

John Goreham    June 16, 2014 - 9:45AM

Not to go nuts with comments here, but I like to say that the Honda Accord Hybrid, both version, is really included in this group of cars. It has a LOT in common with the Volt. Talk amongst yourselves...

danwat1234 (not verified)    June 16, 2014 - 2:44PM

This article has a few problems.
1.9 gallons not 2.3 gallons. 2.4 gallons for other countries i believe.

i3 has multilink rear suspension version cheaper torsion beam rear suspension for the Volt.

The speed is not limited when using the Rex. You do have a bit of a buffer because the engine kicks on at about 6% SOC. If you are on it for too long of course the car will limit performance but there shouldn't any limitation until it drops down.

George B (not verified)    June 16, 2014 - 4:43PM

In reply to by danwat1234 (not verified)

Correct, it's 1.9 gallons in the US instead of 2.3 gallons elsewhere. It's also worth noting that the review Chris Knapman wrote for the Telegraph, which prompted you to describe the performance of the range extender as "pitiful", is not consistent with my own experience as an owner of an i3 REx. Never once I felt that the performance was limited or inadequate. The REx appears to have several power modes, and as the energy needs increase, so does the rpm of the REx engine. While it could be described as clearly audible at higher rpm, it's not intrusive and does not sound cheap.

It gets the job done, and the driver can be assured of being able to get to the final destination no matter what. About the only scenario where an insufficient power situation could arise are hills with significant gradients, and with a fully depleted battery. In such a scenario, the vehicle might slow down to about 40 mph, depending on the grade and other power requirements.

While not ideal, the i3 has been conceived as a city car, and it can climb smaller hills and inclines of up to 400 feet easily on the battery reserve in REx mode. To sum it up, while the i3 range extender solution is not as powerful and versatile as what's found in the Volt, it's adequate for most situation. It is not correct to describe it as "pitiful".

Luke Ottaway    June 16, 2014 - 5:36PM

Thanks for clarifying the fuel tank size. I may have been a little harsh with the word 'pitiful' - it is good to hear from an owner that can refute the alleged drop in performance. The Telegraph was not the only review to indicate that the gasoline engine is a last resort only, but it was fairly prominent and so that is why I referred to it in the article. The point I was trying to make is that the Volt engineers intended their gasoline backup to be heavily relied upon for long trips, and incorporated the requisite complexity. The i3 engineers took a different approach for a vehicle intended to suit a different purpose.

VoltOwner (not verified)    June 17, 2014 - 3:28AM

Your description of the Volt's 4 modes is incorrect. There is no need to "change to accommodate speeds above 70 mph". Your "Low Speed EV" mode (Ring Gear locked to case) is capable of reaching top speed and is in fact used whenever high power is needed. The second mode is used to increase efficiency, by using the smaller generator/motor to spin the ring gear in order to reduce the main motor RPM down into it's most efficient range, the "sweet spot"if you will. This can occur at speeds down as low as 38 MPH, whenever average power requirements are below the maximum that the smaller generator/motor can produce. The 2 gas modes are just these 2 modes with the engine running and locked to the generator. It is actually incorrect to say that the gas engine drives the wheels directly, since the car will not even move unless the main electric motor is supplying torque, it is correct to say that the gas engine _contributes_ to the torque when in the two motor mode, which adds to the efficiency gains realized by bringing the electric motors into the sweet spot, by utilizing it's torque to counter the main motor directly, whilst simultaneously spinning the generator...

Tod Dennis (not verified)    June 17, 2014 - 10:38AM

My commuter car is a 2014 Chevy Volt while my friend just picked up a new BMW i3 for his commuter. Back story: I have a 46 mile round trip commute to work every day. Throw in errands and I routinely drive over 50 miles per day. Charging with 120 at home and 240 at work I always finish my day on 100% battery power. Needless to say I love my Volt and not just because I'm averaging over 150 mpg but because I also love the quite smooth ride, the comfortable seats, the utility of its design and the way the car looks and drives.

I recently picked up a second home in the mountains. From my business to the mountains I have a 99 mile oneway trip with approx 27 miles of twisting mountain roads and 5700 feet in elevation gain. My Chevy Volt handles this trip with aplomb burning on average 2.5 gallons going up and 1 gallon coming down or 3.5 gallons round trip. Enter the BMW i3. My buddy followed me up last weekend in part to see how my Volt compares to his ultimate driving machine. For this drive I like to hold a portion of my batteries in reserve for mountain driving. With an efficient driving style I average 48 miles per charge during my weekly commute. For drives that exceed the Volt's 48 mile range I like to hold electric driving mode in reserve for times when the gas engine is least efficient and the electric drivetrain is most efficient, usually when traffic is flowing at 55 mph or lower or when traffic is stop and go. For the commute going up to the mountains I use no more than 20 miles on the battery before hitting the mountain roads. By design I start the assent with 28 miles left on the batteries, so I can have a little fun in the twisties and not be down on power when passing.

My friend had no such options with his i3 as the BMW lacks the option for strategic electricity usage while driving and instead forces the i3 driver to deplete their batteries before switching over to its ruckus range extender. When we both got to the mountains my Volt had 28 miles on its batteries while my friend's i3 had switched over to batteries a few miles back while we were still on the freeway. Not wanting to run out of gas on the way up we topped off his tiny little gas tank and proceeded to attack the curves. At first it was clear the BM was the faster car in the twisties. Now the difference between our two cars was not as great as when I drive this same road in my CTS V Coupe but he was quicker for a few miles and then it happened. The i3 went into crawl mode and I reeled him in fast. By the time I reached the small mountain town of Idyllwilld I had nearly 14 minutes to spare while he had a line of cars backed up behind his i3 and even had to pull over on several occasions to let Prius and RV drivers past!

As a side note driving back with a full charge I deplete approximately 3/5 of the battery before hitting the steep decline but when I reach the bottom of the hill I have t a full charge again thanks to regenerative breaking. For the return trip I only burn 1 gallon of gas! As for the i3 it made its way back home without switching over to limp mode but again, I beat him back to Orange County because he was worried that if he drove in the fast lane he would revert back to slow mode before getting off the freeways, not a safe option while driving fast moving Southern California Freeways.

Paul (not verified)    June 17, 2014 - 3:15PM

In reply to by Tod Dennis (not verified)

No doubt, the flexibility that the Volt gives the driver to decide when and how much to conserve battery power is a great feature. The European i3's have it too, but California CARB regulations and BMW's decision to qualify the i3 REx as a BEVx in California combined to deprive U.S. i3 owners of that flexibility. Too bad that i3 buyers in non-CARB states are equally deprived.

Paul (not verified)    June 17, 2014 - 11:30AM

I would like to join the others who have taken issue with this article's description of the i3's performance with the REx engine on. I'm an i3 owner (though only as of a couple weeks ago) and have driven a total of about 150 miles with the REx engine in operation.

First, power (at least for the short bursts when I've needed to use it) is excellent. Just yesterday, faced with a situation where an uphill freeway onramp went from two lanes to one lane and driving with the gas engine on, I punched the accelerator to get past a large truck before losing my lane. The car took off like a rocket. (Impossible without an A/B comparison to say whether the power was identical to that of all-electric mode, but it was PLENTY sufficient.) Later on that same trip, with the gas engine still running, I encountered an amber light that I noticed a little bit too late to stop. I punched it again. Again, the car shot forward quickly enough to give me a thrill. If the car's power is compromised in range-extended mode, I haven't yet experienced it. Perhaps sustained high-power demands, like a long and steep uphill grade at highway speeds, will tax the system. I haven't encountered that yet. I have also driven the i3, loaded with some heavy boxes equivalent in weight to that of 3 or 4 occupants, in range-extended mode cruising at 60 to 70 with flat stretches, mild grades and rolling hills. There was no apparent power or performance limitation, other than the obvious feeling that there was more weight in the car, in those situations either.

Second, the relayed blog post purportedly from a BMW i3 REx owner, describing "the car's behavior as "hysterical" when the range extender comes on" does not reflect my experience in the least. For me, there has been no change. At speeds below about 20, I'm just barely aware of a small vibration and engine noise coming from the back of the car, and I can detect when the gas engine shuts off when I stop at a light. At all other speeds, I don't notice it at all. I haven't seen any change at all in the dashboard instruments--no repeated messages about getting juice, no suggestion of local charging points. Period. None. Sure, you can use the car's electronics to find nearby charging stations, but that's no different than it is any other time.

Bottom line: based on my experience so far, the i3's only real limitation in range-extended mode is the size of the gas tank. The tank is too small (I would even accept the word "pitiful" to describe the tank size) and if the theories being bounced around the web are correct, we have inflexible CARB regulations to thank for that!

Yuli Chew (not verified)    July 15, 2014 - 2:09PM

In reply to by Paul (not verified)

Is the writer resides in Europe? If he/she is in the US and those follows California regulations, the gas engine cannot just simply kicks on at will. The battery has to drop down to about 6% before the gasoline engine can power on. This is to comply with CARB's BEVx definition.

Jim Reardon (not verified)    June 11, 2015 - 4:18AM

In reply to by Paul (not verified)

Once you hit limp mode in the BMW i3 you will know what other have talked about.
I have driven the volt in the mountains, as well as an Escape Hybrid and an I3.
They can all hit limp mode. In the escape, after a short hill climb the limiter comes on and you can't get past 55. On the volt, IF you enter mountain mode a few miles before you hit the mountains, you will probably never hit limp mode buy if you don't, after a few miles of heavy accelerating up the grade, the dreaded warning message comes on, and the speed gets limited. The car starts to slow down, and levels off between 50 and 55MPH. So it is important to put the volt in mountain mode 5 to 10 minutes before getting to the hills. That way the generator can spin up faster, and build a 10 mile buffer up in the battery. It takes 10 minutes to put a 25% reserve charge in the battery, but it will never run out of power when you do this.

johnny mars (not verified)    June 17, 2014 - 3:08PM

These are both very good cars. The Volt has stellar reliability and safety records and a half-billion miles under its collective belt, but the BMW has not yet proven itself. The i3 has a high step up, like an SUV. The steering is responsive but skittish while the Volt's is more relaxed. The i3 would win a drag race but neither is in sports car territory. BMW's interior is airy and the view is better. The sound system is superior, offering more volume and HD radio stations. Info screens on the Bimmer are about the size of the Volt's, and the materials inside are greener, including hemp door panels. On the interior and exterior, Volt is more stylish. For some odd reason, the i3 has goofy looking 22" wheels, shod with skinny tires for efficiency. The rear seat backs fold forward in both vehicles, but you have to open two doors and find the release near the seat bottom on the i3, which is inconvenient. The Volt is front wheel drive while the BMW is rear wheel drive. The BMW is meant to be a city/suburban car, but the Volt has the additional benefit of being a long-range vehicle with no range anxiety. The $12k price difference gets you a few upgrades and the BMW panache, but I'd rather have the Volt and the cash.

Tod Dennis (not verified)    June 17, 2014 - 6:07PM

In reply to by johnny mars (not verified)

"The $12k price difference gets you a few upgrades and the BMW panache" Real world price difference between the Volt and the i3 is closer to twice the 12k you listed or $25,000 more for the Beemer. I negotiated a fully loaded Volt with a sticker price of $41.5k down to $36k. With the State and Federal tax incentives my Volt landed at $27k plus tax and license. Factor in GM 0% financing on the Volt and you could pay more than twice as much for the privilege of driving a BMW with a 1.9 gallon gas tank.

Toppcat (not verified)    June 23, 2014 - 11:57AM

So the i3 is a leaf with better handling with optional range extender that look like a golf cart with wheels.

Tod Dennis (not verified)    June 23, 2014 - 11:03PM

In reply to by Toppcat (not verified)

"So the i3 is a leaf with better handling with optional range extender that look like a golf cart with wheels" A golf cart does not have wheels? And yes, that pretty much sums it up. Only the automotive press could call the i3 the "the world's most desirable affordable electric car" when a Chevy Volt is a better car on the affordable end while the Tesla Model S is far more desirable on the high end.

Rob (not verified)    August 11, 2014 - 10:58PM

The BMW I3 range extending engine does not drive the car - it recharges the electric battery - article is wrong. Performance is not impacted at all.

Luke Ottaway    August 12, 2014 - 10:11AM

In reply to by Rob (not verified)

Correct, it recharges the battery. And the battery keeps some energy in reserve for bursts of power that exceed the limited power capabilities of the engine...but a prolonged climb up a hill, for example, will exhaust the battery's reserves and restrict the car to only what the engine can put out, which isn't much.