Coach's test drive of the Volt

First both the prius phv and volt are parallel serial phevs. It is therefore non-sensical to talk about the difference in efficiency because one is serial. The benefit of a serial versus a parallel serial design is simplicity and lower cost, not efficiency. The volt is less efficient because it uses a less efficient engine, has higher drag, higher rolling resistance, and a less efficient eCVT (because of both software and ratio) at lower speeds.

The volt has a battery with enough power to run in fully ev mode at all speeds, unless it is very cold. It's eCVT and motor sizing are also proper to do this. The prius phv must run in a blended mode mainly because of both its battery power. Even if this was higher, its eCVT and motor sizing would require blending above 100km/h (62mph).

The key is power density not energy. Since parts don't seem to change, it looks like toyota picked the 42mph glide speed based on ability to accelerate when called upon. The regular battery pack doesn't seem up to the task. The prius phv must have enough oomph to accelerate until the ice can be started and adds power.

MG2 is 80hp(60kw), which seems plenty big for non blended ev mode. The torque may not be adequate at higher speeds because of its gearing, and the psd would cause problems with mg1 without a redesign or clutch. The battery power is the reason it maxes out at 38kw instead of 60kw which is a more likely explanation. The goal was simply to make the least expensive modification to the prius to make it a good usable phev. You shouldn't read into it that the smaller batter was chosen for lower pollution, it was chosen because of smaller volume, cost, and weight.

 

When running on gas, Volt is series hybrid most of the time. Even when the gas engine is clutched in, it operates on series-parallel mode. The percentage of parallel ratio should be a tiny fraction.

 

What is the basis for your statement?

CS is a small fraction to start with.. so maybe I agree. It < 30% parallel its its < 30% CS.

But in CS mode it is only in serial when its very slow, or strongly accelerating. I'd expect the serial mode to be the one with the small fraction of CS.

I'm willing to bet that at least 80% of my 1500 gas miles were in parallel. Its really the only way to get 40mpg which I did for most of my miles. Might even be higher but I've played many Mountain-mode games (which reduces my apparent CS_MPG but increased my real MPG).

 

Based on the percentage Jeff N provided on US06 cycle. He did not provide it for other cycles. I assume UDDS will be mostly in series because of low speed.

 

Does it really matter? usb I thought you were mad before that you found out that it was parellel/serial instead of only serial. Was that not you. Drive trains have very similar parts. Volt has clutches to run without spinning mga in its normal EV mode. Gearing is different, as well as software. How much is series and how much parellel in both cars depends on how you drive.

The key take away in CS mode should be mpg (35/40/37 vs 51/49/50) does it really matter how much is parallel & serial? YMMV

It does matter how much is CS versus CD, this is 94mpge for 35 miles versus 95 mpge for 11 miles. If the combination works as well as the rest of the car all good right

Pretending that the CS mode is all serial is not right for GM or Danial GM appears to have chosen more serial at lower speeds so that CS feel is similar to EV. It would be nice if they let the driver choose most efficient, but hey, neither car is great at user options.

 

The US06 numbers I posted earlier came from a 2011 SAE paper on the Volt transmission design. They didn't have transmission mode data for the other EPA test cycles.

The bottom line seems to be that when it is in charge sustaining mode , the Volt will normally be in serial/parallel at steady speeds above 35-40 mph on mostly flat roads (in other words, under light torque demand). During significant acceleration it will temporarily switch to serial mode. I don't have a DashDaq so I haven't seen this on a real car so this is all based on the technical material published by GM.

I think the US06 test cycle contains a bunch of slowdowns and accelerations and this likely accounts for why the Volt is sometimes in serial mode during the test. The city driving modeled by UDDS will usually be driven in EV charge depleting mode.

 

Yup, it also depends on the car also. Prius PHV will always have more parallel (mechanical connection) than the Volt.

It is interesting how you perceived me as being "mad" from previous discussions. The claim was Volt is an EV with range extender (not gas engine). By pointing out that Volt indeed clutch in the gas engine and mechanically turn the wheel makes it a hybrid, not just a series hybrid but also a parallel hybrid. I was not intrigued by the complexity of 3 clutches. But then, 2-mode hybrid is even more complex so Volt's implementation is simpler, not quite elegant as HSD.

 

Puzzling. You would want the mechanical connection during high power acceleration to route most of the gas engine power efficiently.

 

The Pip's greatest strength is that it uses a blended approach to get the most out of all on board energy systems, that means using gas more often when it is more efficient to do so.

Thats wonderful, its a design choice to maximize efficiency. Its pretty much the idea behind all hybrids, to combine different means to achieve a more efficient result than any one alone. The Pip is a super hybrid.

The Volt is not maximized for efficiency, it is designed to remain in electric even when burning gas would be more efficient, until it is no longer able to do so.

Thats wonderful, its a design choice to minimize gas consumption. Once its reached CS mode it uses a combination of ICE and motors to be reasonably efficient (more so than 90% of cars on the road) as a hybrid. But the Volt is not a super hybrid, its an EV that carries around a hybrid powertrain all the time for the 30% of the time it is needed.

Because they are focused on different goals I don't even really see them competing against one another, YMMV.

 

gwmort,

That's a very good fair and objective statement.

I would add that Volt will emit more greenhouse gas due to the design choice. However, it may cost less to operate because electricity is cheaper (ignoring the vehicle purchase price).

Total cost of ownership should be lower for Prius PHV, especially when you consider that it is a midsize.

 

Most of the time it is. I keep reading that the parallel mode is only used rarely. And in any case, whatever it is, its FE in CS mode is pathetic.

 

That's only because so many of the cars on the road are gas-guzzling SUV monstrosities. I appreciate that the Volt goes farther on EV than the PiP does. The Volt goes almost as far on electricity as my Zap Xebra did. But its mpg in CS mode is inexcusably low. Once you've got electric motors and a battery on board, there's no reason to have such low mpg. Once the Volt is operating as a hybrid, it's far and away the most inefficient hybrid on the market.

 

Volt is rated 35 MPG (with premium gas) in the city whereas Prius c is rated 53 MPG (with regular gas).

 

The higher output and earlier availability of the motor's torque is far better for acceleration than waiting for the small ICE to spool up.

This simply isn't true. It's numbers aren't Prius good, but the only other hybrid that is that good is the Prius c. The Prius v gets nearly 20% worse fuel economy than the liftback.

hybrid != Prius

Of the other hybrids that have better ratings than the Volt, most are close enough that the average driver may not see a substantial difference in mpg between them. It's mostly going to come down to their route on which does better. EV range is the Volt's priority. The CS mpg could have been better with a better engine, but it is good for the various constraints in the design.

Fuel Economy
The list includes the eAssist, but I didn't include SUVs or trucks

 

What is a silverado hybrid rated at?
Maybe you should quit making ridiculous statements.

 

As I understand it, this is a consequence of the Volt doing its power-splitting using an output-split rather than the input-split design used in the Prius. The Volt is apparently the first car to commercialize an output-split transmission mode.

In theory, output-split reduces the dependency on the less-efficient "serial" electrical path (engine generates electricity which then drives the electric motor) at highway speeds than an input-split design. It's possible that using output-split also meshes well with the other design goals of having a full power EV-centric powertrain that can easily unclutch the engine from the planetary gears.

 

The Volt has a large enough battery range so that almost all city driving for typical owners is done on battery.

The Volt's highway extended range hybrid mpg is only exceeded by the Prius Liftback, PiP, and C and several Honda hybrids.

Everything else including the Prius V, Camry, Highlander, all Lexus hybrids, Ford hybrids, Kia and Hyundai hybrids, Nissan Altima, etc. either match or have worse highway mpg. Quite a few of those also have matching or lower hybrid combined city/highway mpg when compared with the Volt or are just slightly higher.

I should add that the bit about the Volt's serial/parallel mode being used only rarely is completely wrong. Yes, I know some media articles say this but then we all know the inaccurate garbage written in the past (and sometimes even today) about the Prius. The serial/parallel mode is the default mode when traveling above 35-40 mph during steady speed driving or mild acceleration (low torque demand).

The media reports that say serial/parallel is rare are confusing the Prius input-split design with the Volt output-split transmission design. Those reporters assume the engine is clutched in when there is high torque demand but reality is that it is clutched in under low torque conditions.

 

I consider the most comparable car to the Volt to be the Lexus CT 200h (premium, compact, hybrid), and the hwy rating is exactly the same between the two.

 

Yes, this is a consequence of both gearing and software. At low speeds both are mainly serial, which is likely more efficient as mechanical transmission would be low. Until you get to high speeds the Prius will be more parallel and have greater transmission efficiency. In EV mode the volt will have higher transmission efficiency.

Part of gearing choice seems to be ev-mode. If a gen II volt gets a motor that can run it throughout the speed range, the this gearing could be changed to be more parallel like the prius.

I would say the pure serial EREV is the most elegent, it is also the least efficient in CS mode. The serial fisker karma is still a phev. I consider the diesel electric trains hybrids even though they are serial hybrids Which is a better design depends on your torque loads and the percentage of time in ev mode Most of the inefficiencies of the volt versus prius in CS mode have nothing to do with the transmission. Toyota has had a long time to get the software/ICE/electrical hardware right. They also were able to reduce the weight of the non EV part of the car as well. I expect toyota to learn some things and change the psd for phev in the next generation. This may be as simple as gear ratio and motor sizing.

I do think gm said that the engine is clutched in at lower speeds at higher loads. Timing of the switch may change though. Do you have more information on what the software does?

According to gm engineers, the two motor no ice mode was implemented because mgb did not supply the desired torque at high speeds.

 

Although the CT 200h has interior plus luggage space of 100 cubic feet vs. 108 for Volt. If the 200h had any less interior room it would be rated as a subcompact by EPA (subcompact is 85-99). Oh, and it weighs 600 pounds less but has slower acceleration. I guess the 200h's 1.8L 4 cylinder engine (from Prius Liftback) and poor HSD design must be to blame.

Actually, I assume the 200h is a fine vehicle although I've never driven one. I'm just going by the raw specs.