BMW has just announced what it describes as its most fuel-efficient and cleanest 3 Series ever – and the most powerful 7 Series diesel ever.
The confusingly named four-cylinder 316d diesel (it has a 2.0-L engine) achieves CO2 emissions of 118g/km, while combined fuel consumption is 4.5 L/100km. It delivers 260 N•m (192 lb•ft) from 1750 rpm and has an output of 85 kW (114 hp). BMW claims the car can reach 100 km/h in 10.9 seconds.
The 740d with 221 kW (296 hp) and 600 N•m (443 lb•ft) takes the diesel output honors at BMW. Its 3.0-L twin-turbo engine, which employs one small and one larger turbocharger, features an all-aluminum crankcase and third-generation common rail injection system. The car’s performance includes a claimed 0-100 km/h time of 6.3 seconds. Combined fuel consumption is 6.9 L/100km and CO2 emissions of 181 g/km.
The BMW 730d’s engine is also revised to achieve, depending on version, a best CO2 emissions figure of 178 g/km. Injection pressure of both engines is 2000 bar (29,008 psi).
Both also benefit from BMW’s Efficient Dynamics program, which includes brake energy regeneration and ancillary systems, including air-conditioning, that disengage or engage according to demand and conditions.
The power and torque figures now being achieved by relatively modestly sized diesel engines are very impressive, but downsizing and weight savings bring particular challenges. Dr. Frank Doernenburg, Director of Piston Technology at powertrain component supplier Federal-Mogul, said that one of the issues his Germany-based team has been working on is the ability of lightweight aluminum pistons to withstand the increasingly demanding loads produced when engines are heavily boosted.
Doernenburg explained that under high temperatures, with intensive thermal and mechanical cycling, the rim of the bowl could crack.
"The problem is due to the presence of free primary silicon particles distributed throughout the aluminum alloy matrix," he said. "As aluminum expands eight times as much as silicon when heated, stresses are set up within the piston every time the temperature fluctuates. When combined with repeated mechanical loads (each time the cylinder fires), fatigue cracks can be initiated at the corners of the silicon particles."
Federal-Mogul’s solution, called DuraBowl, is to premachine the cast aluminum piston and then remelt the alloy around the rim of the bowl using a TIG (tungsten inert gas) torch, robot-mounted and computer-controlled for repeatable quality. The remelted alloy cools a thousand times faster than it did when originally cast, which leads to much smaller silicon particles – only one-tenth the previous size.
Metallurgists refer to this as refinement of the microstructure, a technique that increases the strength and durability of metal alloys. Says Doernenburg: "The elegance of our solution is that the process is physically simple. The sophistication is in the control of key parameters to ensure consistent quality. This makes the final product very cost-competitive compared to both fiber-reinforced pistons and steel pistons."
Doernenburg concluded that the remelting process substantially increased piston life. "In one example where the standard cast piston failed, none of the remelted pistons had failed when we stopped the test at seven times the duration," he said. "A conservative estimate would be a fourfold improvement in the life of any cast piston which suffers from bowl rim failures."