Attack of the Killer V-8s

So you think diesel engines still are the smelly old lumps of the type that served indistinguished duty in the '70s?

Read just your thought process, because the diesel has experienced a radical rebirth in the latter part of this decade - and the transformation is just about complete.

How do we know? The "Big Three" highline German automakers - Audi AG, BMW AG and Mercedes-Benz - are in the opening stages of launching a new generation of V-8 turbodiesels. And they so resolutely believe in the advantages of the new-wave diesels that they're fitting the oil-burners in their flagship models.

What's primarily on offer from the big diesels is genuinely gargantuan torque - just what you want for moving a big car - and meaningful gains in fuel economy that come from the high thermal efficiencies presented by the latest direct-injection (DI), common-rail fueling technology.

BMW is first to production, offering a 3.9L V-8 DI turbodiesel - claimed to be a first of its kind for a production passenger car - in the 740d.

With twin turbochargers and intercoolers, the new BMW turbodiesel engine develops 245 hp at 4,000 rpm, and a massive peak torque of 413 lb.-ft. (560 Nm), the peak being dead flat from 1,750 to 2,500 rpm. For comparison, a Dodge Viper's 8L V-10 produces only 77 lb.-ft. (104 Nm) more torque, from an engine with twice the displacement of BMW's new turbodiesel.

The BMW V-8 turbodiesel is fitted with a dizzying array of advanced ideas. Electric adjustment of the VNTs (variable nozzle turbines) is an innovative feature, and it's said to be 10 times faster and also more accurate than the usual pneumatic control of variable-nozzle turbochargers.

Another advance is the "cracked" main bearing caps for the cylinder block. They are cast integral with the crankcase, then hydraulically broken off along a pre-scored line. Individual fracture profiles created in this way ensure a perfect fit on assembly with the crankshaft, and provide maximum resistance to lateral forces. Hence each cap can be secured with only two vertical bolts instead of the additional splayed pair otherwise needed to withstand a diesel's typically high crankshaft loads, saving engine weight and reducing complexity.

The block is a grey iron casting with vermicular graphite, which is 20% lighter than conventional grey iron. It also has greater strength than aluminum - as generally required for diesel operation - and offers acoustic advantages in noise suppression.

A water-cooled, 2.1-kW alternator is an additional refinement for the engine and the 740d sedan. Noise reduction in the high-frequency range is said to be cut by up to 3 dB by the sound-deadening effect of the water jacket and elimination of the cooling-air fan. Improved efficiency enhanced by heat dissipation is cited as another benefit that contributes to fuel economy.

Despite the substantial curb weight of 4,320 lbs. (1,960 kg), due to the heavy engine, the 740d accelerates to 62 mph (100 km/h) in 8.4 seconds, with a 150-mph (242 km/h) top speed. Those figures are comparable to the V-8 gasoline-engine model.

Consumption according to the Euro 100 combined drive cycle is given as 24 mpg (9.8 L/100 km). This is with the standard 5-speed Steptronic sequential-shift automatic transmission

WAW European correspondents have driven the 740d and swear that it's preferable to the 4.4L V-8 gasoline-engined 7-series. Enough said, eh?

Almost simultaneously, Audi releases details of its newest (and very similar) diesel, a twin-turbocharged, common-rail fed DI 3.3L DOHC V-8. It's expected to be in production for the A6/A8 ranges sometime next year. Mercedes-Benz will complete the troika with the intro of its 4L turbodiesel V-8 later this year.

The Audi turbodiesel V-8 develops an appreciable 225 hp at 4,000 rpm and 354 lb.-ft. (480 Nm) of torque that peaks in a broad 1,800 rpm to 3,000 rpm plateau. These figures surpass Audi's early estimates of the 3.3L V-8's might; when the engine's existence was first announced in late 1997, the company said it would produce just 200 hp.

Audi's diesel engine block is constructed of vermicular graphite cast iron - the same as BMW's - but Audi claims the material offers only a 10% weight savings over cast iron. The cylinder heads are aluminum.

In addition to DI and common-rail fueling, the new 3.3L turbodiesel features a unique main-bearing frame that incorporates the five main bearings, rather than a common design with individual bearing shells. Other meaningful features include an air/water intercooler situated in the 90-degree vee between the cylinder banks - as are a cooler for recirculated exhaust gas and the high-pressure fuel pump. It is difficult to imagine making more efficient use of the "valley" between cylinder banks; Audi says this technique largely is responsible for the engine's short overall length of just 28.2 ins. (71.7 cm). The new 3.3L turbodiesel is 33 ins. (84 cm) wide and 27 ins. (70 cm) tall.

Audi says water-cooling the recirculated exhaust gas provides a reduction in oxides of nitrogen (NOx) emissions and "has a positive effect on the formation of particles."

Dual intake ports in the cylinder head serve swirl and filling functions, and allow for the centrally located, 6-jet injectors to be located "absolutely perpendicular to the cylinder axis." Audi says pre-injection and an optimized piston crown with a wide and shallow combustion-chamber recess allow for a reduced compression ratio of 18.5:1. The company admits the compression ratio is low for a DI diesel, but it produces a good compromise between high performance and reduced emissions, in addition to favorable cold-start characteristics.

Aftertreatment is accomplished with two close-coupled catalytic converters placed just downstream of each cylinder bank's turbocharger, followed by two main underfloor catalysts. The company says its new turbodiesel V-8 meets Euro Stage III exhaust emissions standards.

Okay, but what about U.S. penetration for the new Super Diesels? Big-car customers here might dig on brawny torque and better fuel economy, too.

An Audi spokespersons says, "At this time, it's not on the books. At this point, we do not offer diesels in the U.S. lineup - due in large part to the current economic conditions where relatively inexpensive gasoline is available. Should this situation change, we'll be ready with the advanced diesel technology."

BMW didn't return our call (presumably, it's too busy selling every piece of metal it can wrap around an engine), but we suspect the response would be similar.

We think Audi's product plan is all wet. Flagship buyers aren't that concerned about the difference between 18 mpg and 22 mpg or the cost of fuel - it's power and cruising ability, and the new-generation diesels deliver a surfeit of real-world-useful torque. If Audi and BMW and Mercedes would put a few of these new diesel cars in the hands of long-standing big-car owners, we think the car companies would be astonished at the response.

Don't forget that mantra: "Customers buy horsepower but drive torque."

Just modify that to: Customers buy gasoline but would drive diesel - if they got to know these new techno-marvels.