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Fuji Heavy’s Turbo Parallel Hybrid for Subaru

The FHI Turbo Parallel Hybrid Powertrain

Fuji Heavy Industries’ (FHI) new Subaru hybrid (earlier post) will use a thin 10-kW motor in combination with the 2.0-liter 16-valve twin-scroll turbo engine currently used in the Legacy to create the Turbo Parallel Hybrid (TPH) powertrain.

The combination of the motor generator and Subaru’s Boxer turbo engine, which is adopting the Miller cycle in this application, creates a motor-assist hybrid (similar in concept to the first generation of Honda IMA) that FHI hopes will provide the performance of the current 2.0 Legacy GT (turbo) while delivering a 20% improvement in fuel consumption.

Compared to the SSHEV (Sequential Series Hybrid) system that FHI had previously developed and shown in 2003, the TPH uses a much smaller, compact motor and a smaller battery—also reducing the system cost.

A Boxer engine is horizontally opposed engine—i.e., the pistons lie in a horizontal plane, with pairs of cylinders on the left and the right, as opposed to engines where all pistons are inline, or positioned in a V.

The Boxer design ideally provides perfect balance because each piston’s movement is exactly counterbalanced by the corresponding piston movement of the opposite side. Subaru uses Boxer engines in all its cars.

The basis for the TPH engine is the turbo EJ20 currently being used in the Legacy. This engine comes equipped with Active Valve Control System (AVCS), but the TPH engine will specially modify the valve timing to implement the Miller cycle.

The Miller cycle, developed in the 1940s (by Ralph Miller), is a modification of the basic Otto four-stroke combustion cycle. Later adapted for use in cars—notably by Mazda in the Millenia—the Miller cycle is similar in approach to the Atkinson cycle engine used by Ford in the hybrid Escape and Mariner, with one major difference.

In both Atkinson and Miller cycles, the engine leaves the intake valves open during the beginning of the compression stroke. This pushes part of the charge back out the normally closed valve.

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.

Put another way, the benefit of this is that the compression stroke effectively becomes shorter than the expansion stroke. The compression work starts when the valve is closed, so the piston gets all the compression for a percentage of the normal work. The result is increased engine efficiency, at around 10%–15%, although with a loss of power.

In the prototypic Miller configuration, however, a supercharger over-feeds the cylinder to compensate for the loss or blowout of charge.

Subaru, however, is not using a belt-driven supercharger with its TPH-Miller cycle engine, but rather a turbocharger, with which it has years of experience in its car lines. Because the turbine’s output decreases as a result of the Miller cycle, the TPH turbine has been designed to provide higher flow than normal.

As shown in Volkswagen’s design work on its dual-charged TSI engines (earlier post), turbochargers are good at higher speeds, but lack in boost at low-engine speed ranges. Without the supercharger to provide low-speed boost, the Miller engine would deliver relatively poor low-end torque.

To provide that low-speed boost, Subaru is turning instead to the electric motor in its hybrid powertrain.

The 10-kW electric motor provides 150 Nm of torque. (Subaru is proud of its higher torque spec. Compare that to the 14-kW motor providing 135 Nm of torque in the Honda Accord hybrid with a larger 3.0-liter engine. The about-to-be-released 2006 Civic hybrid, with a smaller 1.3-liter engine, uses a new 15-kW motor delivering 103 Nm of torque.)

Combining the Miller cycle and TPH systems will result in performance that matches the current 2.0 GT-turbo specs in the Legacy, while providing an approximate 20% improvement in fuel economy, according to FHI.

The current non-hybrid 2.0 turbo Legacy (sold in Japan, the US only offers the 2.5-liter version) delivers 191 kW (256 hp) and 343 Nm of torque. On a combined Japanese cycle, it consumes 7.69 liters/100km, or about 30.6 mpg US.

Applying the 20% hybrid factor to those numbers would yield comparable performance, but at 6.15 liters/100km—or 38.3 mpg US.

Rough Comparison of Hybrid Powertrains
2006 Civic Hybrid2005 Accord Hybrid2007 Subaru Legacy Hybrid1
1 Based on preliminary targets
2 Honda estimates of new 2006 system
3 Japanese 10*15 cycle
Engine displacement 1.3 liters 3.0 liters 2.0 liters
Cylinders 4 6 4
Type Inline V Boxer
Cycle Otto Otto Miller
Elec. Motor Output 15 kW 14 kW 10 kW
Motor Torque 103 Nm 135 Nm 150 Nm
Hybrid Powertrain Power 82 kW (110 hp) 190 kW (255 hp) 191 kW (256 hp)
Hybrid Powertrain Torque 166 Nm 314 Nm 343 Nm
Fuel consumption 4.7 l/100km2 7.4 l/100km 6.15 l/100km3
Fuel economy 50 mpg US2 32 mpg US 38.3 mpg US3

If—and of course, that’s a big if—Subaru is able to deliver on that goal of maintaining current 2.0 GT performance while delivering a 20% improvement in fuel economy, the Legacy hybrid will outperform the Accord hybrid, while using a smaller engine and consuming less fuel.



This is not really good. What we need is more something like 50%-75% at least.


Its what they can handle at the moment and 20% is better then nothing.


38 mpg will make it the best of the larger hybrids and if it speced out at 40+ it would qualify for some tax credits in my state (CT).

My 1997 Subaru Outback is nearing its end-of-life (200,000 miles) and this would go nicely with my 2004 Prius.


Double the motor size and cut the engine size down to 1.5 liters or less and you'd get 40 MPG.  Doubling the motor size would also give you 300 N-m of torque... from stall.

Orville  L.

This makes me very unhappy because of the poor use of engineering talent. The target is 200% gain, not 20% that makes no appreciable difference. Plug-in hybrids are a far better solution. I vote for economy over performance.


I agree that economy trumps performance (especially with $3+ gas), but plug-in hybrids reqire significantly more battery to be effective and the Prius NiMh battery currently costs $3500 (7ah/200v).


Economy over Performance doesn't win sales.
20% Gain in economy without a decrease in the already excellent performance is nothing to scoff at as you all are doing.

If you want hybrids and economy technologies to proliferate then you should embrace this solution, hybrids will never be #1 if they remain #2 in performance.

If everyone in the world could cut thier fuel usage by 20%, it WOULD be a significant difference.


Unfortunately, 20% improvement over the typical vehicle life of 10 years is only 2% per year; depletion looks to be going faster than that.  A 50% improvement would keep pace, and a 70% improvement (some GO-HEV configurations could do this) would yield 7% per year and outpace depletion by a comfortable margin.


"This is not really good. What we need is more something like 50%-75% at least."

Efficiency gains of that magnitude are rare in the engineering world. Besides, it's so easy for all of us to chide Fuji for what "they should have done." Our hindsight is 20-20, eh? Especially when we're spending someone else's money...

Let's not forget that it takes several years and up to several billion dollars to develop a radically different powertrain for production, and that it can be done only if the vehicle manufacturer's decision-makers believe that the project will make money. Car companies can't turn on a dime. Those who tsk-tsk Honda for their conservative design forget this, and also forget that Toyota has said that they originally gave the Prius a 5% chance of success. Given that estimate, I'm surprised the project was approved in the first place.

To beat an overused cliche into the ground, we've got what everyone so cleverly refers to as a "perfect storm" of market pressures driving up gas prices. What would have happened if the perfect storm had gone in the other direction, and gas prices had remained stable from 2000 through 2006, spiking in 2007? The Prius and Insight might have quietly gone away by 2004 or so, and we would have had to revisit hybrids and other alternatives all over again in 2010 or so. Not good.

Let's all hope for the implementation of GO-HEVs as soon as possible, but meanwhile, we might want to consider any new hybrid as another step forward in America's acceptance of hybrid technology. And that will be good for GO-HEVs.


I read once that the modern economy generally improves it's overall efficiency of wealth production by 2% every year..on average. So 2% improvement in their powertrain is at least consistent. We would need, however, a 3% improement overall in order to fight global warming.


As I'm fond of saying, I love to drive, but I enjoy breathing even more. If vehicle manufacturers - who are obviously not benevolent transportation providers - can create things that many/most people want (key) to buy, and make these things safe, pleasant, efficient, low-emissions, reliable, crashworthy, and actually fun to drive, I tip my hat to them. I am physically able to ride a bicycle, and can do so for hours at a time, but I have (unwittingly) designed my life around the need for a car/vehicle (children, distance to necessities, time, etc.). Also, I simply love the exhilaration of driving a great-handling car to its (and its tires' - don't forget the tires!) performance limits. Of course, I feel like a total schmuck for getting enjoyment out of basically burning gasoline (and rubber). Anyway, my point seems to be: support manufacturers, suppliers, and fuel producers who seem to make an effort to be 'better' environmentally. It's a hell of a job. If I was the one who had to mine the ore, process it, drill the wells, refine the crude, plant and maintain the rubber trees, organize countless scores of people to convert all of this into vehicles that people want, build and maintain roads and bridges, coordinate traffic signals, issue licences, and insure drivers, I'd just walk through the forest instead. Some of the best complaining is done by simply withholding dollars/pounds/francs/etc.


(make the dollars/Euros/etc., I presume)


So Subaru already has (in Japan) a non-hybrid Legacy that makes 256hp and gets a combined 30.6 mpg? If that's real world mileage, it's already better than the Accord hybrid (see the mean Accord hybrid mileage in the real world is 29.9).


According to the data sheet, 13 km/liter, which = 7.69 l/100km, which = 30.6 mpg.

But, that’s not a real world number—that’s a Japanese drive cycle number. Their version of the EPA numbers.


Could also be Imperial gallons (4.546092 litres) rather than US gallons (3.785412 litres) - so 30.6 mpg would be equivalent to 9.23 L/100km (we consistently average 9.8 L/100km combined city and rural highway driving with our 05 Legacy 2.5i wagon - about 162 hp). Those darned lab numbers. Consumer Reports' latest issue has an interesting article all about lab vs. real-world mileage ratings. They report that in their testing, some government ratings 'inflate' mileage by up to 50% (especially hybrids, which tend to do very well on the lab dynos vs. real driving), and, surprisingly, a few vehicles actually do better on real roads than on the lab cycle.

Al Santa

You may want to take some more math classes...20% improvement every year, not divided by the life of the car.


Good for Subaru. Got to say, 20% increase in economy at similar output is pretty damn good. But I still want a nice catalyzed diesel that I can run on algae-feedstock B100; alas for our Tier 2 wackiness.

Joseph Willemssen

Good overview of the technology, though I think the fuel economy comparisons with the Accord Hybrid are suspect for several reasons, some of which you mentioned.

The Accord Hybrid isn't sold in Japan, so you're comparing Japan 10/15 with the EPA ratings, which aren't equivalent. For example, to use a couple of vehicles sold in both markets, the Insight's fuel economy rating in Japan is 1.34 times what it is in the US. For the Harrier/RX400h, it's 1.44 times greater in Japan.

As you also mentioned, Subaru uses a 2.5L in their Legacy in the US, whereas they use a 2.0L in Japan. The US version Legacy GT gets a combined 21 MPG - or about 50% less than the Japan 10/15 rating of the 2.0L turbo. Also, some of the Legacy GTs in Japan get 12 km/L, not the 13 km/L figure you're using.

So, in reality, using the higher mileage number you're using (30.6 mpg) and the lower ratio I mentioned (1.34:1), that Japan 10/15 number is roughly equivalent to 22.8 mpg under EPA ratings. Boost that 20%, and you're looking at 27.3 mpg.

Pretty good, considering the performance of the vehicle, but still substantially lower than the 32 MPG rating of the '05 Accord Hybrid. At 15.5K miles per year, one could expect to save 83 gallons in the Accord relative to the Legacy (again, in its 2.0L trim).

Joseph Willemssen

" or about 50% less than the Japan 10/15 rating of the 2.0L turbo"

correction: about 1/3 less


We need to rely on a better "carbon cycle" for power lets move over to bio-diesel or better. Remember the the VW Rabbit of the 70's "50mpg" and now we accept 32 to 50!

over 30 years of technologies later, i'm sure we can do better


Anyone: does B100 have low-temperature gelling problems compared to petroleum diesel, or are there safely-compustible additives that resist gelling?

Glenn Arlt

Hi, Joseph, nice to see you blogging here, too. Of course, you know that I'm going to remind everyone that the new Toyota Prius and presumably, also the new 2006 Honda Civic IMA, improve on mileage a great deal - but I see your point, and as usual you have good arguments, which you generally back up well. I'd like to see every vehicle built a hybrid, and not just improve mileage by 20% as the Subie would but more like 100% as the Prius does (compared to my functionally similar Hyundai Sonata V6). As for biofuels, those of you who have not seen by blogs elsewhere, get yourself a big surprise and check out


Just read an article on Gasoline Direct Injection (GDI), seems to have all the fuel/emmissions savings of a hybrid with no electric motor or batteries, or have a GDI hybrid and really increase fuel mileage/reduce emmissions.

"Typical fuel economy gains of 15% to 20% are possible by using GDI as compared to conventional port injection, but stratified charge GDI can make these savings even higher."

"Stratified charge engines can also reduce emissions by up to 10% because of the lean fuel mixtures used."


In all of the excellent comments there is no mention of Rotary-Engine technology. We reverse the motion of the pistons and accelerate from zero many thousands of times per minute - where is Mazda on that ?



The VW rabbit from the 1970's weighed around 1700 lbs; If you were to reduce the weight of any modern, fuel injected car that much you would achieve similar results. The problem? All that federally regulated safety equipment. Lots more metal for crumple zones, airbags, thousands of sensors, convoluted intake and exhaust systems that are much bigger than necessary, standard equipment air conditioning, etc...
Consumers and/or the government demand these features. That's why current cars weigh so much more (<2800 lbs for a Ford Focus as an example) and mileage doesn't improve.

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