Plug-In Hybrid MPGs: Is Parallel or Range-Extended Better?

2011 Fisker Karma

2011 Fisker Karma

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These are exciting times to be a car shopper, as the choices in every possible category just keep getting better and more fuel-efficient.

The options for ecologically sensitive hybrid buyers have taken a major step forward with the introduction of the 2011 Chevrolet Volt and the imminent arrival of the 2011 Fisker Karma.

Both of these cars are range-extended electric cars, whose wheels are turned solely by one or more electric motors. (There's a minor exception for the Volt, but it's not worth covering here.)

Then, by June 2012, the Toyota Prius Plug-In Hybrid will also arrive on the open market. It's an evolution of Toyota's Hybrid Synergy Drive design, technically known as a parallel hybrid, with a larger battery pack that can be recharged by plugging it into the grid. 

Which is better?

The big problem with a pure electric car is the size, weight, and cost of the battery pack required to sustain the vehicle for realistic extended driving of 200 to 300 miles in a day. That's where range-extended electric cars (also known as "series hybrid" designs) come in.

2011 Chevrolet Volt drive test, March 2011

2011 Chevrolet Volt drive test, March 2011

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The series hybrid format still uses only electric power to drive the wheels, but it also has a range-extending gasoline (or diesel) engine that kicks in to drive a generator that provides electricity to power the electric motor after the battery charge is depleted.

Series hybrids like the 2011 Volt and the 2011 Fisker Karma are basically electric cars, and within their battery range (25 to 40 miles for the Chevrolet Volt and a projected 50 miles for the Fisker Karma), the engine never switches on, making them truly zero-emission vehicles despite the presence of a gasoline engine.

In a (non-plug-in) parallel hybrid, like all present-day Toyota and Lexus models, the new 2011 Infiniti M35h, and all Ford and Lincoln hybrid models, the electric motor is relatively small and can drive the wheels under light loads only. 

Today's parallel hybrids can use electric-only mode at speeds at high as 62 miles per hour for distances up to 1 mile or more, but under greater load from more demanding acceleration, and at higher speeds, the gasoline engine starts up to provide extra power. 

Toyota Prius Plug-In Hybrid prototype, tested in November 2010

Toyota Prius Plug-In Hybrid prototype, tested in November 2010

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Plug-in versions of parallel hybrids can run longer, accelerate quicker, and stay all-electric longer, but their engines still switch on and off at will when needed.

That makes driving them a very different experience from driving an all-electric Volt or Karma before each one has depleted its battery pack.

It's also worth noting that the prototype Prius Plug-In Hybrid would not recharge its battery pack once depleted.

Real daily driving numbers

The basic (non-plug-in) 2011 Toyota Prius is rated at 50 mpg combined gas mileage, so the 2012 plug-in Prius will easily better that number. Some early prototype reports suggest 70 to 80 mpg with regular fuel should be commonplace.

But even careful driving is not likely to produce a much better number than 80 mpg since in a parallel hybrid, the gasoline engine will always find a way to do some work, especially if the car travels more than the projected 12-to-14-mile range of the battery pack.

prototype 2012 Toyota Prius Plug-In Hybrid, April 2010

prototype 2012 Toyota Prius Plug-In Hybrid, April 2010

Enlarge Photo

In highway-intensive driving at Northeast winter temperatures, one road test produced only 53.9 mpg in a prototype Prius Plug-In Hybrid.

On the other hand, data collected by General Motors from the first few hundred Volts sold indicate that they travel up to 1,000 miles before refueling. Volt owners report on the Chevy Volt Forum that they get gas-mileage averages close to 150 mpg in their real-world vehicle use.

The 2012 Fisker Karma should also produce mileage similar to the 2011 Volt in actual use, as long as its drivers do not need to take regular daily drives of more than 75 to 90 miles too often.

For either car (at prices of $41,000 for the Volt and $95,900 for the Fisker Karma), it's more about radical cuts in gasoline usage than it is about payback. 

Fisker has promised, however, to introduce a similar drivetrain at a more attainable price by around 2014. That model, not yet seen publicly, is currently called the "Nina."


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Comments (15)
  1. "Series hybrids like the 2011 Volt and the 2011 Fisker Karma are basically electric cars"....mind you: only for the first few dozen miles these are electric cars. After that they are gasoline powered cars just like any other gasoline powered car on the road except using an electric motor for transmission and the Volt of course sometimes adds direct ICE drive into the mix, making it basically a series-parallel hybrid like the (plug-in)Prius, except with a radical different power mix.

  2. Chris, the article did mention the ICE addition to the transmission, however by all reports the amount this actually occurs is minimal. So to say the Volt is like the Prius really is not accurate.
    I think the article explains the differences very well.
    However one sentence that struck me was "Today's parallel hybrids can use electric-only mode at speeds at high as 62 miles per hour for distances up to 1 mile or more," I was under the impression that the Pius could only go to around 25mph in all electric mode.

  3. Rich: I'm well aware that the article mentions the Volt's direct ICE drive but I also noticed that this fact was swept under the rug so the Volt could be passed off as a series hybrid while in fact it technically falls in the same category as a series-parallel hybrid like the Prius though the two vehicles use vastly different power mixes of course. This direct ICE drive thing was what "Voltgate"was all about and it caused such a stir because people felt it made the Volt less of an electric car. This is correct if one chooses to define an EV as a vehicle that is powered by an electric motor. But who care what drives the wheels, cars like the Volt are all about new energy sources for transport, so it makes sense to define an EV as a vehicle that uses electrons from an outside source for energy. Now it no longer matters that the Volt has direct ICE drive in extended range mode since it's only an EV for the grid powered miles anyway.

  4. @Chris: I think I'd comment that 'Voltgate' was a classic tempest in a teapot. It mattered a great deal to a very, very small number of auto journalists, and almost not at all to the rest of the world. That includes most Volt buyers, who seem thus far to be quite happy with their cars regardless of whether under a certain limited set of circumstances a minority of the drive torque is contributed by the engine through the planetary gearset to supplement the electric torque. Note: Unlike the Prius, the Volt cannot run solely on engine torque. For a bit more perspective:

  5. I think this is an excellent area of inquiry, but feel a few of points should be highlighted. 1) The MPG numbers are meaningless because they neglect the electricity used. 2) The KWH/MILE numbers for the Prius are likely to be better than for the Volt due to the Prius being a lighter, more efficiency car. 3) Let's say the Volt is "better" than the plug-in Prius. Seems like a significant cost premium for the Volt. 4) If the Volt is better; is 40 miles electric the right number, or should it be 30 and save the cost and weight of the battery needed to drive 40 miles?

  6. One more thing, the Prius occasionally has troubling lags in the acceleration curve. This is not so much a problem from 0-60, but reaccelerating from 20 to 30 mph. Feels a little like turbo lag, but worse. If the series hybrid fixes that problem, that would be great. On the other hand, some reports show that the Volt occasionally has similar problems. Perhaps the Volt's problems can be fixed with software changes. I am less optimistic about the Prius problems being fixed with software changes given that it is a mature product.

  7. Gentlemen,
    In my analysis, as John V. puts it "Voltgate was a tempest in a teapot." Yes, I too felt a bit mislead by GM when the final patent details emerged and we found out that the Volt could get some power directly from the gas engine. However, on reflection, I decided that I would always want the most efficient powertrain, and not the most esthetic (?) one. Hence, I decided I was MORE PLEASED that GM engineered the Volt to have some boost power directly from the gas engine under very demanding conditions (why waste power?). Further, in as much as 99.9% of the time, the Volt IS a series hybrid, so for most of us as day-to-day users, THAT is what the car IS. I very infrequently have days that I am "down," but 99+% of the time I am cheerful and optimistic, so I think of myself as a "positive outlook guy." Let's not be too microscopic on the classification of the Volt, please.
    John Briggs-I have not experienced any kind of acceleration lag with our Volt. The power curve has been smooth with strong output from 0-40 mph, for sure.
    I would prefer another 10 miles of EV range rather than ANYTHING less. We have owned Toyota products off and on for the last 20 years (Previa, 2004 and 2006 Prius, 2007 Camry) with absolutely unblemished satisfaction, BUT I would not consider the upcoming Prius plug-in since it has less than 15 mile EV range. Heck, I might also mention that I am still holding a deposit on an Aptera (optimist that I am).

  8. George: Like I said earlier it's not a big deal when using the right definition for EV's, but since I'm all for using the right terminology when it comes to new energy vehicles I can't quite agree with calling the Volt a series hybrid while it's drivetrain architecture is clearly that of a series-parallel hybrid, just because the direct ICE input is supposedly rarely used. Mind you: after 20 minutes of highway driving the Volt's battery will be depleted and the car will function as a series-parallel hybrid for the remainder of your highway trip. Seems to me this is a scenario the average user experiences on a regular basis making the whole proposition rather more than just theoretical.

  9. @Chris O
    Just to be "accurate," You say "after 20 minutes of highway driving the Volt's battery will be depleted...."
    Maybe at around 100 mph, I might see my Volt's battery depleted after "20 minutes of highway driving..." but in my experience, I am still getting 38-42 miles of total EV range at freeway speed, so that is more like 40 minutes or DOUBLE your assertion. Further I am pretty confident that I have at least NOT YET seen any parallel drive train action in my own 3100 miles of Volt driving.
    Have you any real examples to support your assertions above? It would seem that maybe your "sources" are not too well-informed? Did you miss my observation that common usage is to label something based on it's regular level of function?

  10. George: the 20 minutes are based on the EPA rating for the Volt which state an EV range of 35 miles, which I assume includes a fair share of urban driving so it seems reasonable to assume that highway only miles are in the 25-30 miles range which will mean that at 80MPH you should run out of juice in 18-22 minutes provided you live right next to a highway so you didn't spend any juice getting there. I don't know what you mean by not seeing the parallel action, but I reckon GM didn't go to all the trouble (and scandal...) of fitting it if it was something that has little use in real world driving.

  11. George,
    Thanks for the feedback.
    John C. Briggs

  12. @Chris,
    I am not sure where in this country you find a freeway that "permits" 80 mph? I personally run 62-65 setting on my cruise control, and i do get the 38-42 miles of total EV range noted. Relative to the possibility of the ICE kicking in while the EV range was still functional...maybe at speeds above 70-75 mph or on some kind of bigger hill than I have yet found.
    Clearly for strong passing surges from 65 mph on up, I suspect the ICE might kick in while still in EV range and perhaps in a long steep hill anytime the ICE might also engage, but so far in my driving I have not seen that effect. I have not gone faster than maybe 71-72 mph and then only VERY briefly and we have yet to take the car to Reno or Tahoe (from Sacramento)....

  13. @George,
    Well, Texas allows 80 MPH and has voted to take it to 85 MPH. However, just because you are permitted to go that fast, doesn't mean you should go that fast. Since the wind force increases with the velocity squared, it takes 4 times as much energy to travel at 80 MPH as 40 MPH. 55 MPH seems like a reasonable compromise.
    John C. Briggs

  14. George: from what I can find the "dreaded" parallel hybrid thing only happens in charge sustaining mode at speeds over 70MPH, so Volt owners who rarely hit the highway or don't feel the need for speed when they do will rarely "experience" it. It's clear though that GM chose a series-parallel set up for the Volt and I doubt they did it for no serious real world purpose at all. I think that people who choose to spent the extra $8K on a Volt rather than buy a Nissan Leaf fully intend to take it to the highway regularly, because it's long range highway capability is it's edge over the Leaf. So maybe the parallel hybrid feature is more important for most Volt owners than it is for you from what I get from your driving habits?.

  15. while I find the series / parallel hybrid & planetary gear technical details interesting, I generally take a practical position. when is it on electricity, when is it using gas... what is the electric range, what is the gas mileage. these are the things that matter to me; the underlying tech is not as important as the results.
    a few other thoughts:
    1) I question the application of the word "imminent" to the Karma. I've seen no evidence of its imminent arrival. I hope it is imminent but I'm not holding my breath.
    2) you called the Synergy Drive "technically a parallel hybrid". this is not accurate. the Toyota system is a power-split system, technically a series-parallel hybrid. it is 'commonly' referred to as parallel, but not 'technically'. any of the Honda hybrid line (Insight, Civic & Accord hybrid) are better examples of parallel.
    3) my biggest problem is this story is this "The big problem with a pure electric car is the size, weight, and cost ... for realistic extended driving of 200 to 300 miles in a day." let's break this down. Size: the Volt only seats 4. there are bigger BEVs. Cost: really? you are going to hold up the Volt and the Karma as examples of affordable vehicles? Fail. Range: 200-300 miles are not required for most people on most days. if you really need 200+ miles per day, a PHV (either type) is a great idea, but most people suffer from "range exaggeration". they assume they drive more than they really do.

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