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Turbo Placement on BMW V-8 May be New Design Trend

Efficient and capable of meeting Euro 5 and U.S. ultra-low emissions (ULEV) standards, experts say the new engine also is capable of producing far more than its current 400 hp without breaking a sweat.

Critics who think the V-8 engine is a bloated dinosaur headed for extinction should take a close look at BMW AG’s new twin-turbo 4.4L in the X6 and upcoming 750Li.

Efficient and capable of meeting Euro 5 and U.S. ultra-low emissions (ULEV) standards, experts say the new engine also is capable of producing far more than its current 400 hp without breaking a sweat.

Environmental Protection Agency fuel-economy numbers are not yet available for the 750Li, which is due in the U.S. in the spring, but it is safe to say it will not threaten the Toyota Prius’ lock on fuel-efficiency leadership. However, it is a plausible replacement for BMW’s much larger, and thirstier, V-10 and V-12 engines.

The new gasoline V-8 has been downsized from 4.8L and replaces the effective-but-cumbersome Valvetronic system with direct fuel injection and twin turbochargers. The new design makes 40 more horsepower and 90 more lb.-ft. (122 Nm) of torque than the previous engine.

Most significantly, the 4.4L is extremely compact and generates a prodigious 450 lb.-ft. (610 Nm) of torque across a broad power band from 1,750 to 4,500 rpm. It is all due to a new design that situates both turbos and catalytic converters in the “V” section of the block, rather than the outboard sides of the cylinder banks.

BMW says the configuration is a world first for an 8-cyl. gasoline engine.

“The design is indeed unique, with the main advantages being higher performance (especially torque for a gasoline engine) and lower fuel consumption,” says analyst Mike Omotoso, senior manager-Global Powertrain at J.D. Power & Associates, noting “600 Nm of torque at only 1,800 rpm is impressive. Those torque specs are what you would expect from a diesel.”

BMW says the torque peak is about the same as delivered by the auto maker’s 6.0L V-12.

For a Detroit comparison, Chrysler LLC’s naturally aspirated 5.7L Hemi produces 376 hp and 410 lb.-ft (556 Nm) of torque.

Of course it was not just a matter of moving turbos to and fro. The change also requires a new configuration of the intake and exhaust ducts, BMW says. This, in turn, demands the exhaust camshafts and valves be placed inboard, so the path for the exhaust gas from cylinders to turbochargers is shorter. The intake camshafts and valves then are positioned outboard.

Dean Tomazic, vice president-Engine Performance and Emissions Div. at FEV Inc., an independent engine and powertrain systems research and development company, says all this rearranging results in three major benefits: better packaging, improved cold-start emissions and the elimination of turbo lag.

Moving the turbos from outside the cylinder banks to the center of the engine shrinks the powerplant’s footprint and creates more room in the engine compartment for the steering column and other mechanisms that cause packaging headaches for vehicle designers, Tomazic says. It also eliminates the issue of installing bulky heat shielding on both sides of the engine to protect other components from the high temperatures the turbos generate.

Locating turbos and catalysts in the center of the engine also improves emissions, he says.

Turbochargers complicate cold-start emissions because they absorb exhaust gas heat and slow the time it takes catalytic converters to “light off” and reach optimum operating temperature. By placing the turbos and converters on top of the engine close to the combustion chamber exhaust ports, they heat up faster, providing cleaner emissions during startup.

The short path from exhaust port to turbine also minimizes heat losses and improves turbocharger efficiency.

The third benefit of a centralized turbo arrangement is what Tomazic calls “improved transient response,” otherwise known as reduced turbo lag.

“A more compact exhaust system means minimizing the volume between the pistons when they push out the exhaust and the (turbocharger) turbine. With this arrangement you can’t get smaller than that, therefore you have optimized transient response,” he says.

Auto makers have experimented with the concept for years, but until recently were fearful. “They were afraid of overheating turbochargers, overheating the entire engine compartment. We know of cases where the paint on the hood (of a test vehicle) peeled off.”

Now, the need to improve transient response and meet more stringent emissions standards is forcing auto makers to deal with the heat issues and move forward.

General Motors Corp.’s new 4.5L light-duty diesel, due soon, features a forced-induction system similar to BMW’s. Tomazic predicts more turbocharged gasoline and diesel V-8s will use the centralized layout in the future, as well as V-6s.

Meanwhile, J.D. Power’s Omotoso predicts the 4.4L will grow stronger. “There is speculation that the next M5 will use this engine instead of the current V-10, and an M5 with less than 500 hp is unlikely,” he says. “I suspect the current engine in the (BMW) 750 and the X6 has been de-tuned to sacrifice horsepower in favor of fuel economy. Fuel economy is not really a consideration in the M5 and M6, so look for 500-550 hp in the next year or two.”

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