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BMW Improves Powertrains

While all the automotive world seems to consider fuel cell technology as the answer to future powertrain requirements, BMW alone seems to have its doubts.

While all the automotive world seems to consider fuel cell technology as the answer to future powertrain requirements, BMW Valvetronic systemBMW alone seems to have its doubts. Sure, it says, fuel cells do have a part to play in the car, but only as an auxiliary power unit (APU) to power up the vehicle's on-board electronic systems. And putting its money where its mouth is, the Bavarian automaker recently presented just such a system it has been developing with Delphi Automotive Systems over the last couple of years.

Shown in a prototype 750i at BMW's Munich headquarters in mid-February, the basis of the system is the solid oxide fuel cell (SOFC) in which hydrogen is converted into electricity at approximately 800° C via zirconium oxide ceramics. Its advantages over the polymer electrolyte membrane (PEM) fuel cell that BMW has already put into limited series production is that the SOFC is less sensitive to impurities that are produced in a gasoline reforming process since CO can also be converted into electricity and not only hydrogen. In addition, it does not require expensive precious metal electrodes.

 
 

Fitting neatly into the trunk of a car without too much loss of space, BMW claims that the SOFC is twice as efficient as the combination of engine, generaor and battery while also adding greater functionality to the electrical system. It opens up far more possibilities for things like the “mobile office” while requiring less fuel. A normal combustion engine with generator uses up to 1.5 liters of fuel per 100 kilometres simply to supply the car's electric systems with power. With APU, this is reduced to 0.7 liters.

“The customers will have all the advantages of fuel cells, without having to fill up with anything except petrol,” says BMW board member Dr. Burkhard Goschel. “The series APU, which will be ready for series production in 2006, will drive the whole development of fuel cell technology forward. Regarding its use in automotive applications, I am firmly convinced that our APU will come to be seen as one of the most important innovations in automobile development of this decade.”

While it may be the most important innovation this decade, it is not the most revolutionary. Once again one has to nod in BMW's direction for this. Over the years there have been various solutions to replacing the throttle butterfly; the throttling of gasoline engines has always been a frustrating necessity. Both BMW and Renault have shown concepts where valves are opened using electronic solenoids, while Fiat has demonstrated a hydraulic system. But they were all too costly for series production. It looked as though the throttle butterfly was here to stay until someone devised a satisfactory method of reducing fuel intake while maintaining an appropriate mixture.

That someone is BMW with its Valvetronic system. In it, valve lift to control air intake is varied mechanically through an “intermediate arm.” This arm is located between each camshaft lobe and its inlet valve, pivoted at both the top and centrally, close to the cam. An electric motor with worm gear and eccentric drive atop the valvetrain acts upon a shaft close to the upper top pivot point that, in effect, moves the intermediate arm closer to, or further away from, the camshaft, which, in turn, effects the valve opening. The closer the central pivot point is to the cam, the wider the valve opening.

Through this method, valve lift is variable between 0 and 9.7 mm, while adjustment of the worm gear from one extreme to the other takes 300 milliseconds. Combined with BMW's Double Vanos valve timing technology, the camshaft angle relative to the crankshaft can be adjusted by up to 60°.

The intermediate arm is finished to a tolerance of 0.008 mm, and the cams controlling the eccentric shaft are machined to tolerances of a few hundredths of a millimeter. The entire system is pre-assembled and dropped as a module into position in the cylinder head. A 40-MHz 32-bit engine management system CPU reflects the speed at which the system needs to operate with another processor controlling the Valvetronic system.

The benefits of controlling the valve opening in this way, says BMW, is a 10% improvement in fuel consumption due to the elimination of pumping losses found on conventional engines. Other advantages include improved atomisation at low throttle openings due to gas velocity of around 320 per square metre through the part open valves, and cleaner valve seats due to the high velocity. Valve-tronic is also a fundamental building block in the company's aim of meeting the 2008 carbon dioxide fleet requirements of 140 gm/km.

Initially, it will be seen on BMW's new all-alloy 4-cylinder engine that has been the beneficiary of some other tweaks as well. In addition to the new twin balancer shafts and two-stage oil pump, the block is open-decked and pressure diecast with cast-in cylinder liners. There is also a ladder frame assembly between the sump and crankcase.

Another innovative feature is the water flowing across the head rather than along it. Not only does this result in coolant temperatures being reduced by 60%, but the size of the water pump is halved, which in turn reduces power consumption by 60%. The power steering fluid is warmed quickly, reducing the power used by the hydraulic pump. Mounting the water and power pump on the same shaft and a heat exchanger between coolant and engine oil reduces oil temperature by 30%.

Built at BMW's newly opened Hams Hall engine plant in England–the company's “center of excellence” and sole supplier of new-generation 4-cylinder Valvetronic engines between 1.6 and 2.0 liters–it will initially be found on the new all-alloy 1.8-liter, 4-cylinder engine going into the new 316ti Compact in June. It will subsequently be applied to eight and 12-cylinder engines by 2002. However, as the system will not work from around 6,500-7,000 rpm, as stronger valve springs are required which create higher friction losses, it will not be applied to any of the “M” series engines.

While BMW regards Valvetronic as significant a step as that from carburetors to fuel injection a few years ago, there is a cost—the new engine costs around 15% more to produce than the original, despite highly automated manufacturing systems at its new $600-million engine plant. The aim is to produce 60,000 4-cylinder engines in 2001, but BMW expects annual output to increase to around 400,000 units once the new small 1-Series small car, the X3 and the new Z3 arrive in around 2004.