2008 BMW 635d
2008 BMW 635d. Click image to enlarge

Story and photos by Peter Bleakney

Photo Gallery: 2008 BMW diesels

Munich, Germany – Okay folks. The time has come for us North Americans to finally retire that tired old song about diesel-powered cars being noisy, smelly and gutless. Sure, the late-seventies Oldsmobile diesel was a piece of crap, and the similar vintage diesel Rabbit sounded like a blender full of marbles and smoked like a Russian sailor. But those days are gone.

It’s time to wake up and smell — well, nothing.

BMW invited a few journalists to Munich to sample a selection of cars powered by their powerful, squeaky-clean and efficient 3.0-litre variable twin-turbo diesel straight-six. Does an engine that generates 286 horsepower, 427 lb.-ft. of torque and meets Bin 5 emission regulations while sipping 25% less fuel (and thus emitting 25% less CO2) than a comparable gas-powered engine sound interesting?
You bet.

A few vehicles with this engine, mated to a six-speed manumatic, will be coming to North America in the latter half of 2008, but at this point, BMW won’t tell us which ones.

Diesel-powered passenger cars are a huge deal in Europe, where fuel prices are around double of what we pay. Sixty-seven percent of BMW’s European sales are diesels, and the biggest seller of its entire line (gas and diesel) is the four-cylinder diesel 320d.

BMW 335d Touring
BMW 335d Touring. Click image to enlarge

So why is North America so far behind in this regard? Several reasons: gas has historically been cheaper here, the low-sulphur fuel needed for clean diesel engines wasn’t available until recently, and the stringent California Bin 5 emission regulations, which are the world’s toughest, couldn’t be met even by the cleanest diesel cars. As a result, most manufacturers (VW and Mercedes-Benz excepted) never really considered North America when developing compression-engined vehicles.

But a couple of new technologies have emerged that address the two remaining diesel emission bugaboos – particulate matter (soot) and NOx (nitrogen oxide). BMW uses a particulate filter, right at the exhaust manifold that traps the tiny soot particles in a complex honey-comb structure. When deemed necessary by the computing powers that be, the ECU adjusts various parameters of the fuel mixture and valve-timing to increase the exhaust temperature, and the particulates are burned off.

BMW 3.0-litre twin-turbo diesel engine; note urea injection system, just left of engine
BMW 3.0-litre twin-turbo diesel engine; note urea injection system at centre, just left of engine. Click image to enlarge

Now, about that pesky NOx. Just downstream from the particulate filter, liquid urea (a common organic compound made from ammonia and CO2) is injected into the exhaust stream. Here, it reverts back to ammonia and CO2, mixes with the NOx, and then the whole concoction is neutralized in the catalytic converter, emitting only nitrogen and oxygen. A small reservoir of urea is refilled at every service interval.

Mercedes, Audi and VW diesels are using the same technology.
There is also a lot of interesting tech in this light-weight aluminum engine before the exhaust stream. The twin turbos – one larger than the other- work in series, with the smaller spooling up at low rpms, giving instant torque and quick throttle response. As the revs increase, variable exhaust gas routing allows the bigger turbo to gradually take over, huffing more air into the cylinders.

A precisely metered common-rail injection system blasts vapourized diesel fuel into the cylinder at 28,000 psi (and up to five times per compression cycle), ensuring the most efficient combustion possible.

BMW 635d
BMW 635d. Click image to enlarge

As you can see, creating a clean and powerful diesel engine is a hugely complex process, and you may well ask, why bother? The answer came to me on the autobahn behind the wheel of a 335d Touring. At 220 km/h, with tachometer showing 3500 r.p.m., the 3 was stable, serene and, most amazingly, still pulling like a damn freight train. We kissed 235 km/h before traffic slowed our progress.
Sure, lots of cars can reach those velocities, but the smoothness and linear torque delivery of this diesel powerplant is something to behold. When puttering through villages, or driving over the winding roads through rural Bavaria, the 335d was equally impressive.

Inside, the only real giveaway to its diesel-ness is the 5000 r.p.m. redline on the tach. Outside, you can hear the faintest growl coming from under the hood, but that’s about it. There’s no odour.

Following 200 km of mixed, and certainly vigorous motoring, the on-board trip computer said we’d sipped diesel at a parsimonious 8.3 L/100 km. The car is officially rated at 6.7L/100 km on the EU cycle.

2008 BMW 335d coupe
2008 BMW 335d coupe. Click image to enlarge

There were a few diesel 5 Series at the program. Being a family guy, I was particularly taken with the 535d Touring with M Package. I didn’t get to drive it, but my partner and I snagged a 635d. The fact that BMW is putting this 3.0L diesel engine in its flagship coupe says a lot. It suited the car to a tee, offering effortless and refined urge.

Fritz Steinparzer, Director of BMW Diesel Engine Development, is well aware of the challenges BMW faces in selling high-performance diesel cars in the US – a land where the gas/electric hybrid is seen as the eco-knight in shining armour. Pricing will be crucial. These cars carry around a 2000 euro premium in Europe, which may be a bit rich for our market. Driving will be believing for many customers.

Try as we might, we couldn’t get the Bimmer brass to crack on which diesels are coming our way. If I were a betting man, I’d put money on the 3 Series, 5 Series, X3 and maybe the X5.

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