The new 335d Sedan: power, torque and remarkable fuel efficiency. All with refinement and sound worthy of a BMW

In Europe, where fuel prices have historically been – and remain – much higher than in the U.S., vehicle manufacturers have achieved dramatic gains in the performance, smoothness, quietness and emission control of diesel engines…while maintaining the diesel’s historic superiority in fuel efficiency. Now, with the new 335d Sedan, BMW brings this progress to the U.S., with brand-new technology that meets the U.S.’ strictest emission standards.

Like all ’09 BMW gasoline engines, the 335d’s new diesel is constructed with a weight-saving aluminum cylinder block – something that’s not taken for granted given the diesel’s much higher compression pressures. (All BMW engines have aluminum cylinder heads.) It is an inline 6-cylinder engine maintaining a defining BMW tradition that means exceptional smoothness and unique sound.

In addition, several diesel-specific technologies of this new engine contribute to its achievement of true high performance, diesel fuel economy and clean exhaust:

Common-rail direct fuel injection. One of the most important innovations of recent diesel development, this has replaced the individual-cylinder or distributor-type injection systems of earlier diesel engines.

Common Rail (CR) means that a single, very high-pressure fuel-delivery pump supplies all cylinders, the fuel traveling along a common “rail” or distributor line. From there, fuel is injected at extremely high pressure (1800 bar/2645 lb./sq in.) directly into the cylinder.

The basic novelty of common-rail injection was to divorce the production of fuel pressure from the actual injection process; this was necessary to give the diesel engine what gasoline engines have long had, namely completely electronically controlled fuel injection. CR also facilitates multiple injections per combustion cycle. In all, this was a breakthrough that made possible dramatic strides in diesel power, efficiency and emission control.

Piezo injectors. A further breakthrough, already applied by BMW in the 335i (and other BMWs) models’ twin-turbo gasoline engines. With direct injection, the injectors – that critical component injecting fuel into each cylinder’s combustion chamber – are subject to especially high temperatures and pressures. Instead of most injectors’ conventional electrical valves, a “stack” of piezo crystals reacts lighting-fast to impulses from the engine electronics governing the injector needle’s opening stroke and duration for ultra-precise control. This, too, facilitates major advances in fuel economy and emission control, in gasoline, as well as diesel engines.

Variable Twin Turbo technology. In contrast to the two small, equal-size turbochargers of BMW’s twin-turbo 6-cylinder and V-8 engines, the diesel employs two turbos of different sizes.

At low engine speeds, intake air does pass through the large turbo, but it’s the smaller, lower-inertia one that does the air compressing (turbocharging). Thanks to its optimum efficiency within this rpm range, it provides effective boost for driving from a standstill up to modest acceleration rates and driving speeds. So it is that this engine develops a good 390 lb-ft. of torque at as low as 1500 rpm – a remarkable achievement that will amaze first-time 335d drivers and continue to thrill those who drive this BMW regularly.

With increasing engine speed, the larger turbocharger begins to take over: first as a pre-compressor for the smaller one, then progressively until it becomes the primary turbo. The engine reaches its maximum of 425 lb-ft. by 1750 rpm, and then maintains this immense torque level until 2250 rpm going on to reach its peak power of 265 hp at 4200 rpm.

Diesel combustion, power and torque characteristics. In its most basic distinction from gasoline engines, a diesel engine achieves its combustion not with a sparkplug, but by much “harder” compression of the fuel-air mixture; the 335d engine, for example, has a compression ratio of 16:5:1, vs. 12.0:1 in today’s ultra-high-performance BMW M gasoline engines and the 10.2:1 of the 335i twin-turbo gasoline unit. For one thing, this requires a significantly stronger engine structure; indeed, the 335d engine weighs more than its 335i counterpart.

The diesel combustion process also produces very different torque and power characteristics. Whereas the 335i engine reaches a maximum torque of 300 lb-ft. @ 1400-5000 rpm and a maximum power of 300 hp @ 5800 rpm, the 335d attains its maximum torque of 425 lb-ft. @ 1750-2250 rpm and its maximum power of 265 hp @ just 4200 rpm. In general, diesels have this low-speed-torque, lower-rpm character and in their vehicular application, they are geared differently. The 335d, for example, comes only with automatic transmission – an automatic is the ideal partner for a diesel engine – and its final drive ratio is 2.81:1, vs. 3.46:1 for the 335i with automatic transmission.

Diesel engines also employ a different fuel, the product of a different refining process from that of gasoline. The two fuels are not interchangeable; gasoline cannot be used in a diesel engine, and vice versa. Their prices typically diverge; at any time or in any location, demand may be different for the two, and government taxation policies for gasoline and diesel fuel also diverge.

Stellar pulling moxie, full-bore acceleration – and fuel efficiency. The huge torque output of 425 lb-ft. at relatively low engine speeds speaks for itself; 335d drivers will marvel at this engine’s robust response at low to medium speeds. The peak power output of 265 hp also speaks for itself, as does the resulting 0-60-mph time of 6.0 sec. While official EPA mileage estimates are not yet available, preliminary tests indicate 23 mpg city and 33 highway.