Spark Ignition In Peril?

BMW AG unveils in Europe its latest turbodiesel development: advanced 2-stage turbocharging for its 3L I-6 turbodiesel.

The revised diesel's peak output of 272 hp — and specific output exceeding 90 hp/L — surpasses that of many high-performance 6-cyl. gasoline engines of similar displacement.

The new turbocharger system helps BMW deliver a 25% gain — 54 hp in total — over its existing 3L I-6 turbodiesel that uses a single variable-geometry turbo. Torque is increased by 12%, primarily at low revs, with turbo lag “virtually eliminated,” says BMW.

The company's current 3L turbodiesel already is widely considered the premier diesel of its class — so adding another 50-odd horses simply strengthens the case that new-generation diesels have come of age as genuine performance engines.

The operational range of the 3L turbodiesel also is expanded, with crankshaft speed extended to 4,800 rpm, versus the previous 4,000-rpm practical rev limit. The two developments simultaneously boost maximum output and flexibility.

Adapted from high-performance marine-drive systems, the 2-stage turbo is the first application for a production-vehicle engine. It first will power the BMW 535d sedan, which hits the European market this autumn.

The layout has a small turbocharger combined with an exceptionally large one to serve the 3L displacement. Both are in continuous operation and are plumbed to pressurize all six cylinders.

At low engine rpm, a flap-type bypass valve in the two turbochargers' shared exhaust input is closed, and all the exhaust gas is directed to the small turbocharger. With its minimal mass, the impeller spins almost instantly to 180,000 rpm.

Exhaust passing through the small turbine then enters the large turbine, and charge air from that compressor is fed into the small compressor. The intake air, now at compounded pressure from both turbos, then feeds through the intercooler to the diesel's inlet manifold.

The resultant extremely high boost pressure enables 90% of maximum torque to be reached at only 1,250 rpm, barely in excess of idle speed.

As crankshaft speed approaches 3,000 rpm, a pneumatic actuator starts opening the flap valve, and exhaust gas then is progressively split between the two turbines. Both compressors then deliver intake air to the intercooler, supplying the larger volume of intake air required as engine speed increases.

BMW says the 3L turbodiesel's peak of 272 hp comes at 4,400 rpm. Thanks to the large turbine's size and 230,000-rpm shaft speed, 90% of peak horsepower still is on tap at 4,800 rpm — an impressively high rpm for a diesel — giving extended flexibility and drivability at both ends of the rev range.

The twin-turbocharged diesel's peak torque rating of 413 lb.-ft. (560 Nm) comes at 2,000 rpm. With this extraordinary muscle, BMW says the 535d sprints to 62 mph (100 km/h) in 6.6 seconds. The car also is credited with an electronically limited top speed of 150 mph (250 km/h).

BMW says the engine is compliant with new Euro IV emissions standards and includes a standard particulate filter for the 535d. When already diesel-delirious European customers get a taste of this latest enhancement, expect applications of this snorting new diesel in other BMW model lines ASAP.