When it comes to building cars, Subaru has always followed its own drummer. Aside from Porsche, it’s the only carmaker using horizontally opposed “boxer” engines—which lower the car’s center of gravity, neatly complementing their signature “symmetrical” all-wheel-drive system. From capacious if agricultural Outback station wagons to balls-to-the-wall screaming rally cars like the WRX, Subaru are often cult-like and deeply loyal to this offbeat manufacturer.
It stands to reason, then, that a Subaru hybrid would be slightly different. The Subaru B5 TPH (for Turbo Parallel Hybrid) concept, a sporty two-seat, all-wheel-drive grand tourer, blends elements of coupe, sporty hatchback, and Outback sport-utility. Its turbocharged 2.0-liter boxer four develops 191 kW (256 hp) and 253 foot-pounds of torque. The company’s engineers added a very thin—just 58 mm—electric motor-generator between engine and transmission. Though it produces just 10 kW (13 hp) at peak power, it pulls like a steam engine, generating 110 foot-pounds of torque from start-up.
This neatly offsets the characteristic “turbo lag” —in which turbochargers provide little power until they have spooled up to full operating speed. The hybrid also compensates for the characteristics of the Miller cycle engine (defined as the Atkinson cycle used in all hybrids plus forced induction from a turbo- or super-charger). What’s the Atkinson cycle? It’s a variation on the standard Otto cycle gasoline engine, in which valve timing simulates a cycle in which the piston moves through strokes of different lengths. This extracts more energy from the fuel because on the power stroke, the combusting air-fuel mixture can expand to a greater volume than it originally occupied on the intake stroke…but low-speed power is terrible, so the hybrid’s low-speed torque fits nicely.
Unlike full hybrids that can run on batteries alone, both power sources—engine and electric motor—operate full time. This let Subaru fit a much smaller and more compact motor, adding just 200 lbs (100 kg) of weight including the battery pack.
A crucial component of the B5-TPH is its manganese lithium ion batteries, co-developed by Fuji Heavy Industries (Subaru’s parent) and NEC Corp. With 50 percent greater power density than the Nickel-Metal-Hydride batteries in hybrids today, Li-ion batteries also offer much faster recharge. The weak spots of Li-ion batteries have traditionally been their life cycle, which varied with use cycles rather than time, and their heat generation. Subaru and NEC claim to have solved these problems, though the company has released very few details. Experimental numbers of the TPH powertrain will appear first during 2007 in versions of the Legacy sedan, in Japan only.