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Toyota VP Continues Discussion: Is Parallel or Series PHEV Better?
Sep 14, 2007 (From the CalCars-News archive)
This posting originally appeared at CalCars-News, our newsletter of breaking CalCars and plug-in hybrid news. View the original posting here.
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Toyota VP Irv Miller's postings at Toyota's great new "Open Road" blog have previously engaged in high-level discussions about conversions­2007/­07/­readers-of-this.html and vehicle-to-grid's potential­2007/­07/­irvs-sheet-the-.html . He now brings out for public discussion the response from last week by other Toyota executives, who say that when Toyota builds a parallel PHEV, it will be better designed than the GM series PHEVs (the Chevy Volt and the Opel Flextreme.

Last week, our introduction to Toyota's comments at­calcars-news/­838.html basically said that much of this won't get sorted out until we have actual vehicles on the road, but that the fact that this two-way discussion is happening is very encouraging. All this contributes to improving the chances the by the time we have mass-production PHEVs, we'll have great cars.

But we've asked CalCars' Technogy Lead, Ron Gremban. to chime in. In what follows, after Miller's blog entry, we include Ron's comments, which are also on the Open Road website among the diverse responses to Miller's posting at­2007/­09/­hybrid-tech-par.html

September 08, 2007 IRV'S SHEET: Hybrid Tech: Parallel vs. Series Contributed by Irv Miller, Group Vice President, Corporate Communications

The term "Sustainable Mobility" is something we'll be telling you more about in coming months. It summarizes Toyota's ongoing multifaceted efforts to develop vehicles that rely not just on conventional sources of energy, but also on alternative sources of energy.

Note my use of the plural, sources. Sustainable Mobility involves research on much more than vehicles powered by our Hybrid Synergy Drive. It involves vehicles powered by fuel cells, by diesel, and much more.

But naturally it also involves our continuing development of Hybrid Synergy Drive.Hsd

And that's what Toyota Executive Vice President Kazuo Okamoto was talking about during a presentation on the Challenges for Sustainable Mobility in Tokyo earlier this week.

We believe that the approach we are taking with our experimental plug-in parallel-hybrid Prius is superior to that being promoted by those who favor an alternate system called the series hybrid. Okamoto's expression of our belief in the superiority of parallel hybrids apparently raised some eyebrows. Those raised eyebrows cause us to wonder what it is about parallel hybrids that some people don't get.

Basically, what Okamoto said was that when we consider the amount of space batteries require, the time it takes to charge them and the distance over which they will propel the car, a parallel hybrid using nickel-metal-hydride batteries makes the most sense.

Remember, now, that a parallel hybrid has two power sources that can propel the vehicle while either working together, in parallel, or working independently. Those power sources are an electric motor and a gasoline engine. It's an either, or, or both kind of thing.

Using a gasoline engine and an electric motor that both drive through the vehicle's transaxle, Hybrid Synergy Drive makes the gas engine available for short bursts of acceleration and for high-speed cruising, a state in which an internal-combustion engine is very efficient. For start-stop traffic, and as an assist during acceleration, the electric motor is used. This blended approach of Hybrid Synergy Drive just makes the most sense to us - even in a plug-in application.

A series hybrid, on the other hand, runs solely by electricity from batteries. These are charged by a generator that is powered by a small, on-board internal combustion engine. There is no mechanical connection between the engine and the drive system. So - and we love this part - a series hybrid hauls around a gas engine that isn't available to directly propel the car.

We're actually quite surprised at the apparent enthusiasm for series hybrids. That's because at this point in time, there are no automotive series hybrids in mass production that actually work. They simply don't exist.

Yes, there was a high-profile showing of a series-hybrid prototype called the Chevrolet Volt at the Detroit auto show last January. But keep in mind that the advanced lithium-ion batteries that the Volt would use, batteries suitable for the long-term rigors of every-day automotive use, don't exist. Indeed, several firms are working hard on perfecting them for automotive use. But for now, the 40 miles between charges that the Volt's engineers talk about, and that have so many people fired up, are purely theoretical. In fact, when the first Volt prototype hits the road next year, its engineers say that it will be capable of no more than 10 miles between charges.

It's also worth noting that when battery technology advances to the point where a series approach finally is viable, thanks to larger and better performing batteries, it would also mean that the Hybrid Synergy Drive system could be improved with those same larger and better performing batteries. It will work even better than it already does. Conversely, however, if battery advances are not rapid, and if performance and cost targets are not met, Toyota's Hybrid Synergy Drive system is still viable and available, just as it has been for the past decade. Meanwhile, a market-ready series hybrid still will not exist - in spite of the considerable hyperbole that's been expended on the concept. And those who are waiting for a series hybrid could have been driving a parallel hybrid all this time, reaping the benefits of that technology.

Series hybrids have of course been around for a long time in non-automotive applications. In fact, for at least the last 50 years, railroadLocomotive_2 locomotives have been series hybrids, with huge diesel engines driving generators that provide the electrical power that turns the locomotives' wheels. As in the Volt, there is no connection between the engine and the drive wheels.

Our parallel approach provides excellent performance because of its range and flexibility, but is not designed to run purely on electricity, though as noted, we're experimenting with plug-in technology that would increase the electric-only range of Hybrid Synergy Drive. Series hybrids can provide a pure all-electric plug-in function, a function in which the vehicle reaches its destination with the gas engine never having been started. But so far, as I've noted, the series hybrid's electric-only range remains very limited.

So the choice seems clear. If you want a product that's easy on the environment, gets great fuel economy and has good performance, the only reasonable choice is Hybrid Synergy Drive. You can buy a vehicle powered by the system right now, today. But if you want a series hybrid, well - you can cross your fingers and wait for a few years until some difficult engineering and production problems are solved. Or, you can look into buying a locomotive.

Toyota's arguments summarized:

  • Series hybrids require Li-ion batteries that aren't yet ready for the road, so no series hybrid vehicles exist yet. The Volt depends on such batteries for its 40-mile electric range and high power output (peaks handled by Hybrid Synergy Drive (HSD)'s gasoline engine), and is therefore still theoretical.
  • Toyota's HSD exists now and can be adapted to pure electric drive, so it is superior.
  • With Toyota's HSD, the internal combustion engine (ICE) handles peak power requirements, and is quite efficient doing so.

Here are other points beyond Toyota's:

  • Existing NiMH batteries similar to those used by Toyota (but designed for deep discharge) could drive the Volt 20 miles electrically without any change in the Volt's weight or other parameters. 20 miles is not as exciting as 40 miles, but can be very useful and is double the 10 miles EV range that Toyota claims Volt prototypes will have.
  • Though there are no series hybrids on the market now, that has more to do with there being no PHEVs of any type on the market yet -- due to auto manufacturer reluctance -- than with an assumed superiority of parallel or HSD hybrids. Series hybrids are naturally PHEVs.
  • Though an ICE can be efficient in handling peak power requirements: 1. A battery can, too, and a battery large enough for 20-40 miles EV range can easily be designed for such power peaks. In fact, as Tesla Motors is demonstrating, an electric propulsion system's low power efficiency can actually be improved (as well as regenerative braking recovery improved) by designing it for high peak output -- whereas, as we know, higher-output ICE's have proportionally higher drag and lower low-power efficiency (look at the Lexus RX600's mileage vs. the hybrid Camry's). 2. An ICE designed to run at a single power and speed (e.g. for a series hybrid) can be even more efficient than any designed to drive the wheels. 3. Series hybrid pollution control is much simpler, can be better, and can avoid most fuel efficiency tradeoffs required of an HSD hybrid's ICE. 4. Though a series hybrid's ICE propulsion has the inefficiency of a generator, motor, battery, and electronics in the loop (not mentioned by Mr. Miller), the ICE efficiency of an HSD hybrid has the electronic torque converter's losses to contend with as well as lower average ICE efficiency. HSD's torque converter losses are quite high; GM claims its two-mode losses are lower, while a continuously variable transmission (CVT)'s losses may be quite low. Which type of hybrid ends up most efficient as an ICE is likely to depend on the quality of each design team's engineering and components more than inherent differences. 5. A series hybrid is more efficient than a parallel hybrid when driven electrically, as there is no ICE transmission or torque converter involved. This difference, too, may be subtle, and depends mainly on the parallel system's mechanical losses. 6. Therefore, while a parallel or series/parallel hybrid (like the HSD) may or may not have a slight efficiency edge over a series hybrid during ICE propulsion, the series hybrid will be best during EV driving, which is good to maximize in a PHEV since most driving miles are local.

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