Subaru has built the world's first flat four-cylinder diesel to be sold in a production vehicle. Europeans have been driving it in the Legacy and Outback since January 2008, and it was recently added to the engine lineup in the new third-gen Forester. Americans...well, we'll have to wait, but we could get the boxer-diesel Forester sometime in 2010. If, that is, Subaru gets the message (wink-wink, nudge-nudge) that American buyers are seriously interested.

I recently drove the new Forester 2.0D through the idyllic rural roads of Austria and neighboring Slovenia, and it's an appealing piece indeed. The boxer diesel is essentially an all-new engine, sharing its bore centers with Subaru's 3.0-liter gas six (to help reduce machining costs) but otherwise made mostly of unique parts. The twin-cam, four-cylinder mill displaces 2.0 liters, feeds off a common-rail fuel-delivery system, and breathes via 16 valves and a variable-nozzle turbo. Output is 145 horsepower at 3600 rpm and 258 pound-feet of torque at just 1800 rpm (compared with 226 pound-feet for the Forester's 2.0-liter turbo gas engine).

Subaru notes several advantages to the boxer's design. Most important, the horizontally opposed configuration is naturally balanced, requiring no weight- and friction-increasing counter-rotating shafts for smooth spinning. The engine is extremely compact, thanks to a bore pitch reduced 6 mm over the 2.0-liter gas engine. An aluminum block minimizes weight, while a low-mounted turbocharger helps drop the car's center of gravity. The engine also wears a diesel particulate filter (DPF), a closed silicon-fiber honeycomb that traps particulates passing through the exhaust.

Mated to the new engine is a new six-speed manual transmission designed especially for the boxer diesel. Subaru currently has no automatic capable of handling the diesel's lofty torque output, and given the relatively low volumes anticipated for the diesel, the company isn't inclined to spend big bucks on creating a new one. Expect the manual to be the one and only transmission offering when (if?) the Forester diesel eventually makes its stateside debut.