Most of the updates BMW made to the 2009 3 Series fall under the "mild" category as the car is just three years old. These include items like subtle changes to the interior and exterior styling, a new-and easier to use-generation of iDrive, an expansion of the ConnectedDrive telematics functions, and the addition of revised or new options. But there is one change that falls into the "major" category, the addition of a twin-turbocharged diesel to the list of available engines.
As with its turbocharged gasoline engines, the small turbo spools up quickly and provides boost at low engine speeds, works in concert with the larger unit in the mid-range, and stands down at higher engine speeds. Piezo-electric injectors operating at 1,800 bar pressure provide up to five injections per revolution to keep noise and emission levels under control. As a result, the 3.0-liter inline six-cylinder has a 16.5:1 compression ratio and produces 265 hp @ 4,200 rpm and 425 lb-ft @ 1,700-2,250 rpm. This is enough to accelerate the 335d sedan from 0-60 mph in 6.0 seconds, and provides more torque than every current BMW engine except the V12. Consequently, the diesel is mated to a six-speed automatic as there are no manual transmissions in BMW's arsenal capable of handling this amount of torque. The engine features piston wrist pins that are offset toward the cylinder wall to reduce the tendency for the piston to rock, and a crankcase ventilation system that is heated to improve cold scavenging. In addition, the intake swirl valves are electrically operated, the EGR systems (both low- and high-pressure) are water-cooled, the urea tank is heated to prevent freezing at temperatures below 12° F, the fuel filter is heated to prevent wax crystal formation, and ceramic booster elements are added to the heater box to make up for the diesel's dearth of excess engine heat for passenger compartment heating.
Emissions are handled by a two-way (HC and CO) oxidation catalyst that feeds into a ceramic particulate filter. The exhaust is pushed through a finned mixer plate that contains the urea injection module and adds turbulence to the stream before it is sent through the selective catalytic reduction (SCR) unit. The 335d carries five gallons of a 32% urea/68% water mixture in a separate heated tank that should last between scheduled service visits. When 0.66 gallons (2.5 liters) of fluid remain in the tank, a yellow warning light in the instrument cluster illuminates to indicate there are approximately 1,000 miles before he must replenish the tank. At 200 miles before empty, a red light illuminates. If the tank runs dry while the car is in motion, the engine's power output is gradually cut, and it will not restart after being shut off until the urea mixture is replenished.