Power trio: a pair of sixes and a 500 hp V-10 add up to a winning hand for BMW powertrain

Automotive Industries, August, 2004 by Michael Scarlett

BMW has introduced three new engines, all to appear first in the 5-series range. First was the R6, a 3.0-L gasoline inline six that promised to be the first of a new generation; next a highly impressive "variable turbocharger" 3.0L diesel inline six; and finally the power unit for the new M5 sports sedan, a 5.0L V-10.

Given that, in Europe at least, in spite of the countering weight-adding effects of ever mounting passive safety demands, there is constant pressure on manufacturers to reduce fuel consumption in order to cut C[O.sub.2] emissions, so weight reduction is vital. When the carmaker is BMW, where performance is a main plank of their sales appeal, building engines lighter while still improving power and torque is a challenge.

That is the reason for the biggest change in the new gasoline six, which boasts a bedplate, cylinder head cover and crankcase cast largely in magnesium alloy. Such alloys have a density roughly 30 percent less than the aluminum alloy previously used in most BMW engines. On the debit side, no one has yet developed a treatment or alloy of magmesium similar to the various ways of using aluminum alloy cylinder bores without ferrous liners, nor of reducing magnesium's incompatibility with water. That is why the crankcase-cum-cylinder block is a breakthrough technology in casting, being a mix of magnesium alloy and aluminum alloy.

Hyper-eutectic silicon aluminum alloy is used in the form of a cast insert clothed--by casting the one within the other--in magnesium alloy. The insert contains cylinder bores, specially honed to marginally lower the surface aluminum, leaving the hard silicon in contact with piston and rings--a technique used by BMW and others for some years now. The top end of what are effectively aluminum liners carry, the studs for cylinder head clamping. The insert also carries all the internal coolant passages, lost-wax-cast into the insert using polystyrene foam capable of producing relatively small diameter holes the length of the block. Its bottom end forms the upper hall of the main bearing housings, emerging from the magnesium alloy to end at the crankshaft center line. The lower bearing housings are formed in sintered steel, cast into the magnesium alloy bedplate. Further weight is lost in the use of hydroformed camshafts.

On the power and efficiency improvement side, the R6 uses a second generation of BMW's Valvetronic valve operating system, which takes the maximum working speed to 7,000 rpm, while avoiding the need for throttle butterfly valves for improved volumetric efficiency, said to raise fuel efficiency by 10 percent. The water pump claims to be a world first in production car engines, being electrically powered instead of engine driven. Con-ventional pumps consume up to 2kW; maximum demand for the electric pump is 200 watts, and it is not always needed, for example during warm-up from cold. Alternatively, it can be switched on when the engine is idle, to heat the car interior. This means only one ancillary belt drive.

Fruits of all this ate an engine weighing 355 pounds according to BMW's measurement method, with a maximum power of 255 hp at 6,500 rpm, maximum torque of 221 lb.ft. constant between 2,500 and 4,000 rpm, and fuel consumption down by 12 percent.

Those power and torque figures for the gasoline 3.0L compare interestingly with the corresponding ones for the same capacity, ii turbocharged, new inline six diesel engine. This produces 268 hp at 4,400 rpm, and 391 lb.ft, of torque at 1,500 rpm to substantiate BMWs claim that this is the most sporting diesel of all time." They also point out that 268 hp from 183.6 cu.in, corresponds to 89.5 hp/L, the highest specific power output of any production car turbo-diesel.

How does it achieve such figures? Supercharging by means of an exhaust-driven centrifugal turbocharger has long been the essential method for allowing modern direct injection diesels, with, in unblown form, their up to 55 percent lower specific output, to rival gasoline engines.

In Europe, there are V-6 diesels with two turbochargers, one to each cylinder bank. On the new BMW 3.0L inline 6-cylinder diesel, the Steyr-based BM\V Diesel Competence Center in Austria uses two turbochargers, one small, one large, compressing air into the four-valves-per cylinder engine via ah air-to air intercooler. Both turbos are supplied by Garrett, the smaller blower's lower rotor inertia and volume meaning, of course, that it responds more quickly and works at high boost for lower engine speed, while a large turbo delivers higher boost at higher exhaust flows for top-end power and torque.

Therefore, the two turbochargers are mounted on remarkable examples of interwoven aluminum alloy and iron casting, respectively forming the blower part of the inlet manifold and the turbine part of the exhaust.

At low engine speeds, air passes through the large blower and is compressed in the small one, so that 95 percent of maximum The torque is available flora 1,500 rpm. The role of the large turbine increases with increasing speed, so that it pre-compresses to a swelling extent, maximum torque occurring at 2,000 rpm. At high engine rpm, the big blower obviously dominates.