Mercedes SL or a Prius: Which Is Greener? – Part Two

MetalMiner welcomes guest commentator Thomas Kase, principal analyst for MetalMiner’s sister site, Spend Matters, and unabashed car aficionado.

This post is continued from Part One.

The new SL is only the 3rd large-scale aluminum-bodied car production effort ever. (While giving credit to Honda for their NSX sports car, Audi and Jaguar are the first two to pull it off otherwise.) The metal used in the Mercedes is largely the AlSi10MgMn alloy – for example, the entire firewall – the single-largest piece in the car – is cast from this material; a design that cuts down the parts count considerably.

MB has increased the new SL’s torsional rigidity by about 20 percent compared to the current version. New and relatively new production techniques are combined in the car – hydroforming is used to create complex curves (the 1997 Corvette was the first mass-made car produced using this technique); extruded profiles with seven sealed chambers are used for the main lengthwise ‘frame’ components; and tailored, welded blanks are used in the massive center section enclosing the drivetrain.

The floor uses a sandwich construction where friction stir welding (FSW) joining processes are used to put the large piece together. FSW is especially beneficial with large pieces that can’t easily be stress-relieved otherwise. As a side benefit, the massive construction with cast firewall and surrounding materials creates two extremely rigid (5-gallon air volume each) corner pockets” in each forward foot well that are eminently suited to serve as bass resonance volumes.

Around 400 feet worth of adhesives are also used to hold the car together. MIG welding is used as well, although I suspect Mercedes wants to minimize the use of more traditional welding since aluminum is inherently inferior to ferrous materials from a welding perspective.

All this effort gets you a body in white (as the naked monocoque, aka unibody, is called in auto-speak) that weighs a scant 560 lb (255 kg), which is less than two-thirds of a steel construction of comparable strength. Over the life of the car, the savings from not constantly having to accelerate and decelerate the extra mass is significant. Expect more aluminum use in the future as CAFE standards are jacked up.

To my mildly incendiary header, I expect aluminum cars to be greener overall than the current batch of hybrids – seriously, anything with redundant drivetrains and that many extra parts can’t be sound engineering or economics.

Compared to the current batch of electrical cars (if the batteries don’t burn up, they’ll ruin you financially when they need replacing every few years), and the losses from the power plant (likely coal, dirtier than exhaust from a car’s catalytical converter) to the grid (transmission losses are between 30 and 50 percent) to the car’s batteries (where storage losses are immense), I think a TCO analysis will give well-designed, traditional combustion engine cars the edge for a good number of years to come. Another important consideration is recycling – aluminum is actually economically viable to recycle, with no federal mandates or incentives needed!

Here in the US, we can buy this marvelous piece of machinery for right around $110,000 or so, whereas the Europeans will have to fork over the equivalent of $150,000 for the same car. You will start to see this car on the streets around my birthday – end of March, that is – but unless Jason feels tremendously generous, I will have to contend with the R107 (1978 450SL) and R113 (1966 230SL) already in the family’s garages.

Happy motoring!

–Thomas Kase

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