Special Coverage
Story Behind 10 Best Engines
You can be forgiven if you think the word “diesel” just doesn’t fit with BMW AG’s image of performance and luxury.
Americans, and even many otherwise astute car fanatics, still think diesels are noisy, dirty powerplants that deliver glacial acceleration.
That makes diesels a tough sale against virtually any gasoline engine in the U.S. market, and an almost insurmountable challenge when the in-house competition comes from BMW’s lineup of flawless, and much-lauded, gasoline engines.
But in Europe, where diesels have been preferred over gasoline engines for decades, BMW has taken
the idea of creating fun-to-drive oil burners very seriously. When the auto maker first offered a 6-cyl. diesel in the 524td in 1983, the car quickly became known as the fastest diesel car of its time.
Now, about two-thirds of all BMWs sold in Europe have diesel engines. And it isn’t just because fuel costs Europeans three times as much as it does Americans or that diesels get 30% better fuel economy.
Long ago, car-savvy Europeans fell in love with the diesel’s unmatched low-end torque, which is especially well-suited for driving twisty mountain roads with a minimum of gear changes as well as hurtling down the Autobahn at triple-digit speeds.
BMW’s 3.0L DOHC I-6 Turbodiesel is the culmination of all the Bavarian auto maker’s research and development over the years, and Ward’s editors immediately were smitten when BMW finally decided to offer its oil-burning dreamboat of an engine in the U.S.
Tom Baloga, BMW’s U.S. vice president of engineering, confirms the auto maker made some specific technical changes to U.S.-spec versions of the engine and the vehicles in which it would be offered to address U.S. consumer sensitivities regarding diesel noise and vibration issues.
The wrist pin location in pistons was modified slightly to smooth out engine vibration and minimize a condition known as “piston slap.” A special crankcase reinforcement also was added to U.S. versions to reduce vibration, and additional noise-reduction materials and coverings were added to further quiet noise emanating from the engine compartment.
Because diesel engines are more efficient and don’t run as hot as their gasoline counterparts, an auxiliary electric heater was added so customers would not complain about their cars not heating up fast enough in colder temperatures.
Europeans long have grown accustomed to these differences, Baloga says, but there were concerns U.S. consumers might not accept them.
While the 3.0L is quite a fine engine in itself, what truly makes it a Ward’s 10 Best Engines winner is the innovative and compact twin-turbo technology that provides class-leading thrust and horsepower. In a unique arrangement, the system uses one small and one larger turbine to deliver progressive boost.
The small turbocharger spools up quickly to deliver solid, low-end response with virtually no lag and then allows the larger turbo to take over and deliver maximum boost when necessary.
“The configuration really is a sequential turbocharging system, rather than a twin-turbo system,” says Dean Tomazic, vice president-engine performance and emissions div. at FEV Inc., an independent engine and powertrain systems research-and-development company.
Why hasn’t such an innovative concept been implemented sooner? In the past, packaging, cost and controls of such systems were the prohibiting factors, Tomazic says.
However, he says moving to lower-cost waste-gated turbines and a compressor bypass reduces cost. Furthermore, smart packaging and development of advanced controls allowing for a seamless transition from the small to the larger turbocharger operation, and vice versa, enable BMW to cost-effectively put the system into production.
Tomazic says BMW already is using a sequential turbocharging system on a 2.0L I-4 (outside the U.S.) that produces 200 hp and 295 lb.-ft. (400 Nm) of torque and says the auto maker likely will expand use to other engines.
BMW’s Baloga will not comment specifically about future products, but he says it is conceivable BMW will look at using two and maybe even three sequentially arranged turbochargers in the future to lower costs and enhance performance.
It’s reasonable to assume future improvements to the 3.0L I-6 Turbodiesel will include a simplified, more invisible emissions control system and higher fuel-injection pressures as BMW strives to gain even more control over the combustion process, Baloga says.
If there is one imperfection in this otherwise superlative engine, it would be the alarmingly complex emissions-control system that enables the engine to eliminate oxides of nitrogen in the exhaust.
That system injects urea into the exhaust from an “active” tank containing about 1.6 gallons (6 L) of the fluid by means of a dosage pump. The active tank and dosage pipes are heated because the urea solution freezes at 12° F (-11° C).
The active tank is connected to a second passive tank that holds 4.5 gallons (17 L). Overall, engineers say a full load of urea adds about 50 lbs. (23 kg) to the vehicle's weight. Having this much urea on board means the system only has to be replenished during regular oil change intervals, generally about every 13,000 miles (21,000 km).
Under normal circumstances, the customer never will have to bother with additional service, and the cost of refilling the tanks is covered under BMW's no-charge maintenance program for the first four years or 50,000 miles (80,000 km).
The 3.0L produces 265 hp and has a specific output of 88 hp/L. But what really hooked us was the pavement-wrinkling 425 lb.-ft. (576 Nm) of torque at 1,750 rpm – the bulk of which is available from just 1,500 rpm.
With one goose of the throttle, and puff of pristine exhaust, this engine single-handedly destroys all the old stereotypes about diesels.
In our 335d test car, it delivers head-snapping acceleration, hitting 60 mph (97 km/h) in six seconds. During highway cruising, it is quieter than any of BMW’s gasoline engines we have tested.
The engine also dispels the myth that diesels are bulky and heavy. Its lightweight aluminum construction ensures BMWs won’t be front-heavy and allows the 335d to maintain an almost perfect 51/49 weight balance from front to back.
As far as the exhaust goes, it’s cleaner than most comparable gasoline engines, thanks to a highly sophisticated exhaust aftertreatment system that employs urea injection and a maintenance-free particulate filter it meets even California’s super-tough emissions standards.
Only one stereotype proved to be true: terrific efficiency. U.S. Environmental Protection Agency numbers are 23/36 mpg (10-6.5 L/100 km) city/highway for the 335d.
Even though Ward’s editors are infatuated with the performance of the 335d, Baloga says so far the engine is finding the most favor in the U.S. as an option in BMW’s X5 xDrive cross/utility vehicle.
A quick look at the numbers reveals why: In price, the diesel sits perfectly between the auto maker’s 6-cyl. and V-8 engine options, but it excels over both in fuel economy and performance.
The 6-cyl. gasoline X5 tips the scales at 4,982 lbs. (2,258 kg), and the gasoline V-8 version weighs in at a bloated 5,330 lbs. (2,418 kg).
Despite its heft, the V-8 X5 is plenty fast, hitting 60 mph (97 km/h) in just 6.4 seconds, thanks to the engine’s 350 hp and 350 lb.-ft (474 Nm) of torque.
But fuel economy is a dismal 14/19 mpg (17-12 L/100 km) city/highway. The gasoline 6-cyl. version is not much better, offering 15/21 mpg (16-11 L/100 km). The 260-hp 3.0L I-6 also feels overmatched most of the time, because the engine makes only 225 lb.-ft. (305 Nm) of torque at a higher 2,750 rpm.
Compare that with the diesel, pumping out 75 lb.-ft. (102 Nm) more torque than the V-8 and at 1,000 rpm less than the gasoline I-6 serves up its meager 225 lb.-ft. The xDrive35d also does 0-60 mph one second faster and offers 25% better fuel economy than the gas-powered I-6.
On paper, and during test drives, the diesel turns into a slam-dunk for truly discriminating consumers.
The key to selling a diesel to U.S. car buyers is getting them in the vehicle and allowing them to feel the power and torque first hand, Baloga emphasizes.
