2010 BMW 335d
A decidedly different approach to efficiency is embodied by the 2010 BMW 335d. The technically sophisticated and more costly approach relies on a sequentially turbocharged, compression-ignition engine.
The 3.0-liter in-line six-cylinder BMW diesel produces 265 horsepower at 4200 rpm. While the horsepower is nothing to write home about, its torque bests the Corvette at 425 lb-ft, and this peak comes at just 1750 rpm. Driving the rear wheels through a conventional six-speed automatic, 0-60 mph performance (about 6 seconds flat) is similar to that of a V-8 sedan, but its fuel economy -- 23 mpg city, 36 mpg highway -- shames some four cylinder economy cars. Top speed for 335d models with the Sport Package is 149 mph.
Here's how the BMW achieves its numbers: Diesel engines are hearty by nature. Their heavy-duty construction is needed to handle static compression ratios that would turn gas-fueled mills into grenades. The BMW 3.0-liter utilizes a 16.5:1 static compression ratio and injects fuel directly into each cylinder at 26,000 psi.
This precisely metered and fully atomized fuel charge combines with a pressurized air charge that's the product of sequential turbochargers. The twin turbos are not a matched pair as they are on the 2010 Ford Taurus SHO. One is small and the other large. The light mass of the small turbo enables it to spin up quickly, and it alone produces boost from approximately 700-1500 rpm.
As engine speed increases, there is enough exhaust energy to spin up the larger of the two turbos. From 1500-3200 rpm, both blowers add boost. Above 3200 rpm and to the engine's red line, the efficiency of the small turbo falls off and the boost duties fall to the larger unit. Maximum boost from the turbos is 23.9 psi above atmospheric.
This approach comes with a cost that BMW pegs at about $2500 for comparably equipped vehicles (335i with an automatic transmission compared to a 335d). This premium is directly related to hardware costs. The high-pressure fuel-injection system and turbos account for roughly 75-percent of the upcharge, while the multi-stage exhaust system consumes the rest.
To make the diesel clean enough for North America, the 335d's exhaust requires an oxidation catalyst, a diesel particulate filter, a urea dosing module, and a selective catalytic reduction catalyst. These components aren't free, nor is the urea refills required to keep the 335d on the road (it will not start if the urea tank is empty).
While costly, the approach works, and if CARB ever gets its head on straight, diesel-powered vehicles like the BMW 335d could be a hit in America.