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Report: Toyota To Develop Hybrid Systems With Fuji Heavy for Subarus

Japan’s Nihon Keizai Shimbun reports that Toyota Motor has agreed to develop hybrid systems jointly with Fuji Heavy Industries (FHI), the parent of Subaru, with the aim of using the technology in Subaru Legacy models in two to three years. Executives of both companies will announce details on Monday of a broader Toyota-FHI alliance, including a US manufacturing agreement.

Toyota acquired an 8.7% stake in FHI after GM sold off its stake in the company last year (earlier post).

Toyota originally intended to supply hybrid systems to Fuji Heavy (earlier post), but the companies have decided to work jointly on gasoline-electric hybrid technologies because existing Toyota systems cannot readily be used with the Subaru’s horizontally-opposed engines, according to the report.

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

The TPH 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 a 10 kW electric motor compensates for low-end power loss.

The result is a high-powered mild-hybrid system that is quite different in design that the Toyota Hybrid Synergy Drive technology.


Rafael Seidl

Subaru and Porsche are the only companies that are successfully using opposed-cylinder gasoline engines in the cars these days. Porsche has announced a variable geometry turbocharger for the 2006 Carrera, incorporating very expensive aerospace alloys. The cooler exhaust of a Miller cycle should give Subaru the option of applying more conventional VTG technology at a fraction of the cost.

Cam phasers will presumably be used to nullify the Miller detuning at low RPM, further reducing turbo lag. Any that remains will be masked by the electric motor. More and more companies are suggesting that such mild hybrids may be a better overall compromise (cost/weight/fuel economy/emissions) than full hybrids.

Subaru is an AWD company, and with this hybrid architecture they will have to pay special attention to drivetrain losses. We'll have to wait and see if they stick with their all-mechanical solution. A German company might opt for a dual-clutch transmission and a torsen differential in the transaxle.

However, Subaru could also opt for front wheel drive with electric assist motors in the back as Lexus has done to reduce drivetrain bulk and weight. In this case, it would in addition permit the opposed-piston engine to be mounted lower in the chassis, lowering the overall center of gravity and freeing up room under the hood for a larger intercooler. Another advantage is that the Lexus concept permits recuperative braking on both axles.

It's not clear if Subaru intends to use run-flat tires and put the traction battery in the spare wheel well (as Opel has done in its Zafira hybrid study).


Both Miller and Atkinson types of SI engines call for Continuously Variable Transmission to minimize partial throttle inefficiency of these engines. So most likely it will be Toyota Synergy Drive pared to Subaru flat four with instant start/stop capability. That makes it full hybrid. I surely hope for additional two motors at the back with stability control capability. And when they manage to pack it into Impresa, it will be my dream car.

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