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Subaru Introduces 40MPG Turbo Parallel Hybrid Concept

B5-TPH concept.

Subaru’s new B5-TPH hybrid concept made its North American debut at the 2006 North American International Auto Show. The B5-TPH concept vehicle applies the company’s Turbo Parallel Hybrid (TPH) powertrain system (earlier post) and lithium-ion battery technology in a two-seat grand touring car that offers fuel economy of 40 mpg.

Subaru’s parent—Fuji Heavy Industries (FHI)—has been developing the TPH powertrain for future mass production and will test-launch TPH-powered Legacys in the Japanese market next year.


Subaru also showcased a test version of its R1e urban electric vehicle (earlier post) equipped with next-generation long-life lithium-ion type batteries from NEC Lamilion Energy.

NEC and FHI jointly established NEC Lamilion in 2002 for the development of secondary batteries. Designed to meet the needs of city mobility, the Subaru R1e is projected to achieve an 80% recharge in about fifteen minutes. The resulting charge can supply enough power to serve most daily commuting needs in congested urban areas. (More on the R1e and the Li-ion technology in a subsequent post.)

Subaru has always been, and will continue to be, committed to safeguarding the natural environment that so many of our customers avidly enjoy. We will continue to make these technologies a priority in our product development, manufacturing and business processes.

—Kunio Ishigami, chairman, president and CEO, Subaru of America

The Turbo Hybrid Powertrain. The Subaru TPH powertrain in the B5-TPH places a thin, 10-kW motor generator between a newly-developed 2.0-liter Miller cycle engine and the automatic transmission.

The engine is based on the standard Legacy 2.0-liter Boxer (horizontally opposed) engine, with modifications to the cams and engine management software to implement the Miller cycle.

A Miller-cycle engine (similar to the Atkinson cycle) leaves the intake valve open during part of the compression stroke. The late closing of the intake valve eliminates the substantial amount of energy normally required to overcome friction (as well as pumping losses) in the process of completing a normal compression stroke. The result is increased engine efficiency, at around 10%–15%, although with a loss of power.

To compensate for the power loss, the prototypic Miller engine uses a supercharger. Subaru instead applies a turbocharger to make up for the power loss in the mid-range. The additional assistance provided by the electric motor compensates for low-end power loss.

The combination of the motor-generator and the turbocharged Subaru Boxer engine creates a system that not only provides power in the mid-speed ranges when the turbocharger is active—as with conventional turbo models—but it also delivers excellent acceleration and fuel economy.

In the Subaru B5-TPH, the Miller Cycle turbo Boxer engine operates up to 30% more efficiently than a conventional gasoline engine. Overall, the engine delivers 256 hp, with 343 Nm of torque.

Tphbreakway Tphgraph
Breakaway view of the TPH powertrain with the motor exposed. Engine performance graph showing the affect of the different components.

Subaru plans to equip the system with high-performance manganese lithium-ion batteries. FHI is also developing Li-ion capacitors, and is currently conducting performance tests on prototype cells. The Li-ion capacitor dramatically enhances energy density, while retaining the inherently superior capability of instantaneous charge/discharge and the high durability of regular capacitors.

The Li-ion capacitor’s negative electrode uses newly developed Li-ion occlusive carbon material, while its electrolyte is also made of Li-ion. Pre-doping enables occlusion of a large amount of Li-ion on the negative electrode, boosting its capacity and increasing the electrical potential difference. This in turn enables high voltage without deterioration in positive electrode performance.

FHI believes that the application of certain new materials to the positive electrode, combined with the pre-doping technique of the Li-ion capacitor, will theoretically double the estimated accumulation capacity of capacitors available in today’s market.

(More on FHI’s work with Li-ion batteries and capacitors subsequently.)

Quick Comparison: 2007 Toyota Camry Hybrid vs. Subaru B5-TPH Concept
  2007 Camry Hybrid B5-TPH Concept
Gasoline engine 2.4-liter Atkinson Cycle 2.0-liter Turbocharged Miller Cycle
Cylinders 4 4
Engine Power 147 hp (110 kW) 256 hp (191 kW)
Engine Torque 187 Nm 343 Nm
Hybrid Type Full (with all-electric mode) Mild (power assist)
Motor Output 105 kW 10 kW
Motor Torque 270 Nm 150 Nm
Battery type NiMH Li-ion
Fuel Economy (combined) 40 mpg US 40 mpg US
Fuel consumption (combined) 5.9 l/100km 5.9 l/100km



A 40mpg screamin' machine, apparently!


Interesting. Not on the chart of course are cost and emissions - both higher for the Subaru I suspect...


No word yet on either.


I'll buy one.


After all, this is a concept. I'll believe it when I see it.

Tripp Bisop

What is the advantage of have the capacitors? Do they have a higher energy throughput? I know that they can't store the energy long term so you need a battery too.



B5-TPH is a smaller two doors concept car while Toyota Camry Hybrid is a 4 doors mid-size production model.

Notice B5-TPH still uses traditional transmission while Hybrid Synergy Drive in TCH is much more simplier (mechanically) with just one Planetary Gearset.

Subaru did not specify if that 40 mpg is combined or just the highway. It is most likely highway fuel economy because there is only 30% more fuel efficient.


Tripp: I believe one advantage of capacitors is that they can accept higher charge rates than can batteries, thus the regenerative braking can be more efficient at converting momentum into stored energy. They probably are also more efficient in discharge mode, although I'm not an engineer.

On another topic, I note that the current production Legacy 2.5GT Sedan (turbocharged 4-cyl) is rated at 250HP and 20/26mpg with manual transmission. Bumping this up to ~40mpg--especially the city number--would be quite impressive, IMO.

Tripp Bisop

Thanks Nick. I suspected that it was something along those lines. It's good to see yet another Auto Manufacturer getting their feet wet in the Hybrid world.

Lance Funston

If this is an AWD car, then I am doubly impressed. As usual EPA 40mpg= about 33mpg in the real world, but still impressive. Finally Subrau's fuel economy starts to match up with the environmental values and outdoor lifestyle of its customers.


This description is fairly thin, but the idea of motor sandwiched between engine and trans sounds much like Honda's IMA system. Given that, I wonder why the release says the electric motor is between the engine and "the automatic transmission". No possibility of a manual? This has been one of the biggest disappointments to me personally of the whole hybrid trend. Manual transmissions don't seem to get a whole lot of attention.

tom deplume

It is possible the electric motor replaces the torque converter which is the major source of AT inefficency.

hampden wireless

Ultracapacitors are not only faster but more efficient. They loose far less energy to heat.


Probably just because this paticular concept has an automatic transmission. As for manuals in the hybrid world, the most fuel efficient production car in the world has a manual transmission... 5spd Insight.

Earlier info from Subaru with a simular setup were with a manual trans. I'd wager either could be used.

Mike GR

The comparison chart between the Subaru and Toyota could have used size and weight columns, as both cars seem quite different.


Toyota's HSD doesn't have the concept of gears so a manual doesn't make any sense. I'd bet that if you put the HSD in a car the size of the insight, along with the 1.0L 3 cyl engine it would get better mileage than the manual insight.


If they took this powertrain and put it into all Outback, Forester, Legacy, and Impreza, I would be interested. Not interested in a 2 place hybrid. My next car is a Toyota Prius for these reasons. If I wanted a 2 seater, I'd get a Honda Insight. The market for a 2 seat hybrid is thin. You need volume to make money on any technology.


This vehicle seems to be more complex than the Mitsubishi MIEV concept, which has In-wheel motors (which is a magnetic-electric ac motor with regen)
And also Li-ion capacitors (as I know).
But using this Miller/Attkinson system after 2000 is a loosy way (in my humble opinion)
Nicola Tesla ideas would be a better direction.
Making a car that using null-point-energy or if its locked inside the oil lobby department, use some opesource pattents of Him.
Tesla made several experimental AC motors, why dont the factories use 'em ???
I believe Pure EV is the Future.
God Bless the Technology :)


I also see pure EV's becoming much bigger in the future. The supercapacitors will be used to take strain off the batteries during stop'n'go and regen and offer better acceleration by increasing the effective c-rate of the batteries. Hydraulic regen systems being developed also show promise for producing non electric 40mpg range hybrids, as well as great acceleration. I love the MIEV concept, the motor in wheel idea is a great idea for many reasons, Ones that comes most to mind is tunable digital traction control schemes, and four motors to fail before the car stops rolling. I want one of those cute R1e's, except with a mini nuclear generator, that would be more fun. I think that eventually supercapacitors will advance to the point that instant charging is possible and can be used as the main power cells like those mini remote control cars. It would seem that the main reason some of Tesla's inventions havent been developed is that few people can comprehend his inventions. I mean, the guy killed a fish in midair with a thrown rock as a young boy

Rene B

Blah Blah Blah!!!

Where do I find and buy the new supercapacitor modules that Subaru made. Give me some specs please anyone that knows whats going on please.

Are they better than Maxwell's. I build REAL Plug In EV's and I need the best Maxwell has 48.6V 165F Cap module (190Kjoules). Less than 15 Kilo's and about $2G.
Whatcha Got?

PS: if any of you are currently working on or designing or thinking of designing. Even if you have used the TERM hybrid in the last 12 months you are a DONE you have no concept of how far behind you are and you will be left there!!!!

4 HR CHARGE thats right I said 4 hours !!!

Thank you Rene-b


Hmm, Subaru is supposed to release a Hybrid to the USA market for the 2013 year, so far no word on what it will be, though it will likely be an Impreza.
Will it just be a regular boxer engine with a small AC electric motor to provide regenerative braking and acceleration? I hope not. Subaru should have the boxer engine be an Atkinson cycle engine, or Miller cycle (Atkinson with supercharger), to get more MPG out of their engines.
Making a mild hybrid by using their regular engines will be a disappointment. But I am glad that they are (Hopefully!) getting into the hybrid game, finally.


Ultimately I believe HCCI engines will provide even more efficiency and will come with time. HCCI engines with a hybrid or plug-in hybrid powertrain would be perfect.

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