The new BMW 7 Series is stuffed full of aluminium. That is no surprise because all the makers of high-end cars are frantically trying to get weight and CO2 emissions down and fuel efficiency up.
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But what is a surprise is that the aluminium was not primarily for that purpose. Further, the steelmakers do not expect aluminium to make much impact on car design despite the huge pressure from legislators and environmentalists to build more economical cars.
Firstly, to define the BMW achievement: the new BMW 730d is down 10% on fuel consumption and CO2 emission despite having a power increase of 6%. The weight is down 35kg compared with the last model to 1865kg, though 20kg of additional kit has gone in to the car.
The roof is aluminium. The weight saving is 7kg but the vital feature of the saving is not so much its intrinsic value but the fact that it reduces the centre of gravity of the car and improves its handling.
The doors are now aluminium. That is a weight reduction of 22kg. So too are the bonnet, front side panels and front spring struts. The boot (trunk) by contrast is deliberately high tensile steel to add weight over the rear wheels.
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By GlobalDataGerhard Dillman, the head of integration engineering for the project, says that there was some benefit from the lightness of aluminium but the principal objective was getting the centre of gravity of the car where it was wanted. The fulcrum is around the handbrake, and for the driver, whose bum is alongside the handbrake, that means a 50:50 weight distribution and perfect balance. By weight, the total of aluminium in the car is now 19%. In the last 7 Series it was near zero. It is calculated that a 4% fuel saving arises from the aluminium usage. BMW still prioritises performance over fuel economy because that is where the heartland of its customer demand lies.
BMW engineers talk metaphorically about the refrigerator industry: “When CFCs were banned it was not the end of expensive fridges. It was just the start of a technology shift. This is the same thing.”
They also think that they are going to get a drop on the old enemy, Mercedes, because they reckon Mercedes has been banking on Smart to get the fleet C02 average down while BMW has concentrated on its engine technology. Without too much of a fanfare, BMW cars have become mild-hybrids in that they have regenerative braking.
Tyres have less rolling resistance, the engine has less friction, and a switch from hydraulic to electric actuation is removing a drain of power from the engine. Last week’s Autocar did a comparison between new BMW 730d and the Mercedes S320 and declared BMW the winner because it is more entertaining to drive.
The conservatives will also note that the two cars are pretty level on most comparators, but on CO2 and mpg the balance tips heavily to the newer car. The 730d is 192g/km and 39.2mpg (S320: 220g/km and 34mpg). The only area where the S320 kicks ass is in the boot which is 12% bigger at 560 litres. Note however, the current Mercedes range-topper is in the middle of its six-year life, and will not get the right of reply until the arrival of the new model in 2011.
