I have never understood why the Volt Series Hybrid idea never took off. It is more efficient to turn gasoline into electricity and then drive the car with that than to directly connect the engine to the wheels. Is it perhaps that the cost involved is just too much more than a plugin hybrid to make the small extra fuel savings worth it?
This was how the BMW i3 worked. It was a rather novel design that included an optional small electric scooter motor in the rear that had a 2.5 gallon gas tank. When the battery was low, it would be charged by running the small generator.
This was clearly a wonderful idea but it was hamstrung by a silly California rule requiring the gas range to be less than the electric range to qualify for rebates. With a 6 gallon tank, the car would have been able to do ~300 miles instead of 170 and would have been parked in everyone’s driveway.
An added benefit was that the car could use existing gas station infrastructure when you needed to travel long distances.
> it was hamstrung by a silly California rule requiring the gas range to be less than the electric range to qualify for rebates.
The problem is that BMW wanted to get the same amount of credits as a pure-BEV. California anticipated that a car with 300 miles of gas range might spend much of it's life being driven on gas if there was no penalty for doing so... and in fact that is exactly what happened with many European plug-in hybrids sold as company cars.
California never gave BMW the credits, but BMW decided to keep the dinky gas tank anyway, so that's on them.
> and in fact that is exactly what happened with many European plug-in hybrids sold as company cars.
The counter force to that is that lease car drivers have to meet certain fuel efficiency goals, so, adopt an efficient driving style and plug in the plug-in hybrid instead of ragging it around; I'm not sure what the consequences were though, probably more taxes. Either way, enough of a push for my colleagues with plug-in / hybrids to do the thing.
Some friends of mine in the Seattle area had one and absolutely loved it. It was a very practical car for commuting within its range. The extender was barely audible when running.
I think the goofy styling was probably as much of an issue as the tank size.
> It is more efficient to turn gasoline into electricity and then drive the car with that than to directly connect the engine to the wheels
I thought so too, but my research suggested that the efficiency is pretty much the same if not worse and power delivery is worse. Do you have some links? I‘d like to be wrong on this one.
You also have to keep in mind that the average consumer is…quite stupid, or more charitably quite sensitive. There are lots of cars on the market today with CVTs (continuously variable transmissions) that enable the motor to always be driven in the most optimal RPM. However in software, the engines are intentionally driven at less efficient rpm’s so that the driver can feel steps and lurches as the car accelerates which is somehow more “correct” to the average car buyer.
No need for that. While it's possible to point out particular "performance" car CVTs that do indeed compromise efficiency for aesthetics, that's not done in most cases, at least not to the degree that efficiency is significantly compromised. The actual reason for not doing as suggested and placing the entire burden of acceleration on the CVT (thus keeping the engine at a constant RPM) is that this would be a foolish design.
Between the precision electronic fuel delivery and ignition, variable valve timing, variable plenums and other features of your ICE, all of which are invariably present today when an ICE is the prime mover, the ICE power curve has wide RPM bands in which its efficiency is high. By taking advantage of these wide bands of efficiency, the CVT can have less size, mass, mechanical loss, cost, etc., than would be the case if the CVT were required to precisely enforce a constant engine RPM.
Minimizing the burden on the CVT is crucial because CVTs are a mechanical compromise: they are inherently not as strong as a conventional transmission and generally have higher mechanical losses.
The manufacturers responsible for these designs aren't actually fools, slavishly beholden to some old fashioned transmission aesthetic. They're responding to a large number of pressures and doing the math. The math says that constraining the ICE to an efficient, yet non-zero, RPM band is better than trying to manufacture miraculously agile CVTs.
Priuses do a power split where the engine drives the wheels and the motor-generators act sort of like an electrical CVT. You would have to be out of the power band a lot to make a series hybrid more efficient than a parallel hybrid. The reason locomotives are series hybrids is just that mechanically switching that amount of power isn't practical. To my surprise, even off-highway mining trucks mostly use mechanical transmissions.
Surely the "generator -> charger -> battery -> inverter -> motor" chain is less efficient than a driveshaft. Perhaps the only benefit is the engine can run at an optimal speed, but an appropriate gear ratio should handle that.
Supposedly has rooms for improvements but there is actually that system and real world data: Nissan NOTE e-POWER with LEAF motor and no mechanical engine->wheels connection has actual user reported mileage of 19.72 km/L(46.4 mpg) on regular fuel[0]. This is worse than reported 21.25 km/L(50 mpg)[1] for Prius[2].
> but an appropriate gear ratio should handle that.
See Prius's "Powersplit Device".
I'd describe the Powersplit device to be a combination of generator/alternator, starter/electric motor, reversed-differential (2x power inputs -> 1x driveshaft), and effective gear-ratio. All in one planetary gearset.
EV motor2 determines the speed of the car.
The ICE motor can spin at any speed that the computer determines to be useful. If EV Motor2 is 0-rpm, then the ICE is 100% in generator mode (2000rpm but the car isn't moving: all the energy goes to charging the battery). If the EV motor is at 10mph but ICE is off (0-rpm), then its 100% electric drive mode. And any combination in-between is possible.
EV Motor1 (a smaller, weaker motor) controls the 3rd set of gears (I think the planet gears?? I forget), which determines how ICE relates to EV (changes the effective gear-ratio)
So yeah, the PSD allows the ICE to always function at the appropriate speed (which is either 0rpm or ~2500rpm for efficiency). While the combination of EV-motor1 (changes effective gear ratio of ICE) and EV-motor2 (hard-wired to the final speed) handle the different speeds the user wants in practice.
I do think that the Prius (and Prius Prime) have surpassed the Volt's design, and the proof is in the pudding. Prius Prime has 52mpg, a figure far more efficient than the Volt ever had.
Prius Prime also has 220 horses today for a 0-to-60 time of 6.6 seconds. So today's Prius Prime is a lot faster than the Volt too.
Volt was good when it came out, but technology has gotten better since then. Toyota has seemingly perfected this "power split device", and its beginning to lead into exceptional acceleration and good driving feel (as opposed to being 100% economy focused like before). Volt had better feel than the 2010-era Prius, but 2024-Prius is a totally different car.
I think all Volt fans are in "but what if GM didn't kill the Volt and kept investing in the technology?". And... yeah... that's a fun what-if. But... GM killed the Volt. It sucks, because it seemed like great tech. Apparently GM has kept the drivetrain technology ("Ultium") and has continued to provide R&D, but Toyota's recent advancements are jawdroppingly good.
> I do think that the Prius (and Prius Prime) have surpassed the Volt's design, and the proof is in the pudding. Prius Prime has 52mpg, a figure far more efficient than the Volt ever had.
This is partly because the Prius is optimized as a hybrid-first design, while the Volt is optimized as a BEV-first design. The difference is in the gear ratios of the planetary CVTs: Prius is optimized to minimize the amount of energy transmitted electrically, which maximizes overall efficiency.
However, that design is sub-optimal for a car that you want to act like an EV. As your link shows, the Prius must have the ICE spinning at speeds greater than 42 mph. Not a problem for a hybrid, but that doesn't work for a "range-extender EV" like the Volt. Additionally, (before the current model year) Toyota's plug-in hybrids can't provide full acceleration in EV mode--they always kick in the ICE when you floor it. That's again a consequence of how the transmission is optimized for hybrid operation.
By making compromises to the gasoline efficiency, GM was able to create a car which acts like a real EV most of the time: A 1st gen Volt will go at least 80 mph in EV mode, and won't turn on the ICE (come hell or high water) until the battery is below 5%. And, it will do that for an honest 35 miles of freeway driving.
The second generation Volt uses GM's "2 mode" hybrid transmission, which closes some of the MPG gap by adding one fixed ratio (100% mechanical transmission) as well as high and low speed eCVT ratios (to minimize electrical power in those two speed ranges).
> However, that design is sub-optimal for a car that you want to act like an EV. As your link shows, the Prius must have the ICE spinning at speeds greater than 42 mph. Not a problem for a hybrid, but that doesn't work for a "range-extender EV" like the Volt. Additionally, (before the current model year) Toyota's plug-in hybrids can't provide full acceleration in EV mode--they always kick in the ICE when you floor it. That's again a consequence of how the transmission is optimized for hybrid operation.
True in 2007.
But 17 years later, the Prius Prime 2024 has 100% EV mode even at highway speeds. Toyota has improved the design since that webpage was made in 2007.
Despite this change, the Prius still achieves 57 mpg, even better than ever before.
Prius Prime is a proper EV-only mode, albeit a touch underpowered but its EV mode now covers all possible driving conditions.
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I believe the modern Prius 2024 has 17,000 RPM limits now, as well as tweaked gear ratios and far larger EV and ICE engines. Despite the larger 220hp aggregate engine, the Prius remains absurdly efficient. Both in EV mode and in ICE mode.
This is why I was saying that the Volt has wasted it's opportunity. GM was ahead in many respects 10 years ago, but Toyota has caught up. The GM advantage has been squandered.
> By making compromises to the gasoline efficiency, GM was able to create a car which acts like a real EV most of the time: A 1st gen Volt will go at least 80 mph in EV mode, and won't turn on the ICE (come hell or high water) until the battery is below 5%. And, it will do that for an honest 35 miles of freeway driving.
Prius Prime added a button to enter EV-only mode in this 5th generation design starting in 2023.
It took Toyota too long to add this feature, but now that it's here the 5th generation Prius is a far better choice.
Especially because Prius still has it's trademarked 50+ MPG and 130+MPGe ratings. Top of the line efficiency.
It's somewhat frustrating to see Toyota catch up when GM was so far ahead for so long. Also because GM killed the Volt.
In the first version. In later versions they added a second separate gear set. Then even later they merged the two gear sets back into one but with two separate planetary arrangements.
I'm not a particular fan of EVs in general, but I do very much like the Prius and the careful engineering that went into this drive train. This video[0] does a great job of explaining the overall system in the context of an operating vehicle.
The Volt was a great idea, but I couldn't make it work for me. I need a four door car which regularly holds four people, and the Volt (especially the newer one, which was when I was shopping) has a low roofline. Only time I've hit my head harder on a car roof was when I tried to sit in a new Supra. So I ended up finding something where the physics of getting in and out were more agreeable. Still a sedan, just with fewer headaches.
I wish they had offered that same powertrain design in something other than a Cruze.
The Volt worked best as an EV with an ICE range extender. As an EV, the 40-52 mile range was sufficient for 90+% of daily driving. Adding more range would have little true benefit. It was a series hybrid, but it was mediocre as a hybrid due to the added battery weight.
Nissan has the E-power hybrid that is the pure series hybrid that you describe. AFAIK it is not as efficient as a regular, parallel hybrid. The advantage is in cost as running the gas engine as a generator uses fewer components than running it in a parallel hybrid system.
I got my honda partially because it has the closest thing to nissan's e-power (in many ways the same, but with a couple twists).
At low speeds (< 45 mph) it's either off the battery the whole time, or the motor runs and directs power to the traction motor or the battery.
One wrinkle is that the motor assembly has a clutch that can engage the engine direct to the wheels, but like in city driving, the engine isn't going to be connected to the wheels. But I def would say that at least Honda's hybrid doesn't feel as simple as e-power.
Now this probably isn't really accurate, but architecturally I think I like it because the only thing keeping it from being a BEV is the battery size (only a couple kWh); the electric motor is strong enough to be usable on its own.
I think Toyota has increased the power on the normal prius, and definitely on the prime, but it used to be as part of the power split system, the electric drive motor wasn't sized to be enough on its own.
Yes, that was one of the limitations of the Honda Clarity. The EV motor wasn't big enough and it was all too eager to switch to the gas engine. Looked like it was a comfortable car, though.
GM setup the Volt to alway have full power in EV mode and the vehicle would never automatically switch to gas unless the battery was discharged.
That seems unlikely. A mechanical coupling should be close to near perfect efficiency where as using the engine to drive electric motors requires several conversion steps.
The trick is having a large buffer, so that typically the engine can operate at the point with the best specific fuel consumption or shut down entirely. Getting energy from the engine to the wheels is less efficient than a mechanical transmission, but the increased average efficiency of the engine can more than offset that.
Still seems like a traditional hybrid would be more efficient. I'm not sure what's gained by removing the mechanical coupling altogether (other than cost/reliability).
Honda has an interesting solution in their latest gen hybrids, where the electric motors power the wheels at lower speeds, the combustion engine runs at its optimum RPMs as a generator while the electric motors are working, and at higher speeds a clutch activates that changes the coupling so that the combustion engine directly drives the wheels.
Aside from your suggestion violating the laws of physics, that's actually not how the Volt worked.
The original concept for the volt was that the engine would only generated electricity, but in production models, the engine was connected to the drivetrain.
Almost - In production models the engine does act like a series hybrid generator under most circumstances. At high speeds, the engine was directly connected to the drive train as it is more efficient to do that. the transaxle of the Voltec system is a marvel of engineering. It supports a dynamic switching between series and parallel hybrid mode as well as using the two separate electric motors for either power delivery or can switch one to regen and can rapidly switch between those modes, too.
It wasn’t my suggestion, but what I read way back in the day, and it may not be accurate but it does not violate the laws of physics. It’s entirely possible that (just as one hypothetical example) being able to keep the engine at optimal RPMs at all times in a series hybrid creates more efficiency even after the extra conversion losses.
Exactly. The Volt uses the same planetary gear set style transmission that the Prius does.
Everyone bemoaning the death of the Volt can now just buy a Prius Prime (the PHEV variant). It's the same thing just newer/better. It even looks sporty-ish now.
That remains true for more recent "series hybrids" as well, such as the Honda mentioned in the article. The efficiency gain from engaging the ICE when cruising on the highway is just too good to pass up.
“It is more efficient to turn gas into electricity”
I don’t think that’s correct. It may be true for highly variable/low loads where the pumping losses in the pistons dominates. However the majority of fuel consumption in a car happens at traveling speed (highway miles). That is the area that needs to be optimized for.
My 2015 Honda Accord Hybrid takes this approach. At below-freeway speeds the gas motor runs in series to drive an electric motor. At highway speeds, it engages a clutch and directly connects the engine to a low-loss 1-speed transmission.
> However the majority of fuel consumption in a car happens at traveling speed (highway miles).
Unless of course you drive primarily in stop-and-go traffic, e.g. delivery drivers, taxis, commuter cars, etc. Quite often you won't exceed 50 kph. For whatever reason, I've never seen (or have and forgotten) a car marketed towards this market—probably for exactly the reason that plug-in hybrids perform better in this scenario.
> > “It is more efficient to turn gas into electricity”
> I don’t think that’s correct.
You're right, it's not.
In fact, GM wrote an SAE paper about their "2-mode hybrid" transmission (which was used in their 2008-2013 light-duty trucks and SUVs, and then in later modified form in the 2nd gen Volt), where this is plainly explained.
In the paper they describe exactly the tradeoffs made to optimize fuel efficiency in an eCVT... it turns out that you want to set up the planetary gears to minimize the energy transmitted from input to output via electricity and maximize the amount transmitted mechanically because that is most efficient. You especially want to avoid a round trip through the battery in most cases (except when that allows installing a smaller, more efficient ICE).
That has implications for the CVT's mechanical ratio: GM's "2 mode" which has what basically amounts to an auxiliary overdrive integrated in the eCVT so that it can use smaller motor-generators over a wider range of speeds. Smaller motor-generators means more energy is transmitted mechanically, which means higher efficiency.
This is also basically the same reason the 1st gen Volt gets significantly worse highway MPG in range-extender mode than the (contemporary) Prius Hybrid (~35MPG vs ~50MPG): The 1st gen Volt eCVT was envisioned as an EV with a range extender (where energy usually comes from a battery), while the Prius's eCVT was optimized for driving primarily on the ICE, with the battery only supplementing acceleration.
Stellantis (Dodge) has an upcoming version of the RAM 1500 that does this. It's an electric truck with a 145 mile range on electric battery; once that is expended the gas engine kicks on to run a 130 kilowatt generator.
