Unless the completely wireless car becomes a reality, wiring systems engineers will be working to reduce the weight and complexity of automotive harnesses, while taking steps to increase their reliability as feature content is added. Presenters reviewed the state of the art at an SAE 2013 World Congress session on the subject.
The harnesses used today weigh 15-30 kg (33-66 lb) and despite many steps already taken to lighten them, are seen as having potential for even more weight reduction. The increasing use of data buses, with one or a twisted pair of wires connecting zoned electronic modules, replace thick harnesses. This has helped the auto industry hold the line and even reduce weight despite the large increases in electrical loading. Further, wiring harness makers have been using sophisticated software to optimize wiring cross-sections, explained Dr. Wolfgang Langhoff of wiring harness supplier Leoni AG, in a presentation delivered by U.S. engineer Mark Christian.
He cited Leoni's Tool for Dimensioning of Electrical Distribution Systems (TOODEDIS), an internal Java software program based on the Scilab computational language for numerical optimization. TOODEDIS includes electrical and thermal models and does not require modifications to a vehicle's electrical/electronic architecture, he said.
Accuracy of load data
Although it may get electrical load information from a vehicle manufacturer, Langhoff said, Leoni prefers to use actual vehicles to verify loads, as the typical data from the car maker is "just a snapshot in time." Another presenter, Dr. Ludwig Brabetz of the University of Kassel, noted that the typical 12-V system operates in a range of 9-16 V during an engine start. And he added that multiple voltage supply levels, and the maximum times at each extreme, both increase complexity and affect the reliability of the fused systems to provide the needed electrical power.
Although downsizing wires and use of data busses and modules has reduced harness cross-sections and therefore improved packaging, particularly in doors and under-dash, it may have hit a plateau for weight reduction, which may be a greater priority. The industry has alternative wiring conductors, but those that include copper involve compromises that may increase size to improve electrical performance.
The use of aluminum, therefore, has been re-evaluated, and with corrosion prevention measures now available, is gaining adherents. Aluminum, Langhoff pointed out, is far more available, constituting 82 ppm in the Earth's crust, vs. just 7 ppm for copper. In fact, he noted, in human history copper has often been used as money. As a result, it is typically about 7.5 times as expensive as aluminum and is subject to great price swings. He showed a graph comparing prices for both metals in the 2002-11 period, and said through to 2008 the price of copper was following the stock market.
The lighter weight of aluminum also is significant, about half that of copper for the same electrical conductivity. The conductivity for the same thickness is about 42% lower, which increases cross-sectional thickness required for a match by 61%, and the outer diameter of equivalent aluminum wire is 25% greater. Aluminum does not match copper in ability to flex and endure vibration.
Mitigating the corrosion issue, particularly when an aluminum wire is mated to a copper terminal, requires a well-sealed joint. The solutions vary according to the environment, with the simplest a shrink seal around a joint that is both ultrasonic welded and crimped, and this approach seems to lend itself to automation. Other choices, Langhoff said, are polymeric resin with a UV cure, and total overmolding the joint.
Aluminum is out of the laboratory and into limited production by Leoni for Aston Martin, Jaguar, and Renault. Leoni's Mitchell also said that Toyota is using some aluminum in door harnesses.
Louis Chretien, another Leoni presenter, also focused on aluminum, and he told the meeting that it requires more processing than copper, and because of its weaker mechanical properties, standard crimping force is not enough to verify a connection. As a result, he said, Leoni uses optical monitoring. However, he said, a 15-kg (33-lb) wiring harness could save 2.2 kg (4.9 lb) if aluminum replaced copper.
The wiring core material notwithstanding, a key issue has always been harness reliability, and Leoni's Matthias Groetsch showed preformed harnesses, available in 2- and 3-D design, the former available with a heat protection mat.
A conventional harness is like a totally limp tree with flexible branches, and is moved into a 3-D shape and retained with conduits and tape, he noted. The preformed design gives precise wire routing with terminals that line up without possibly damaging "adjustments." Add in the protection it provides against dirt, humidity, and contamination from oil and other chemicals if used under hood.
Leoni's preformed harnesses use polyurethane conduit, which is thermally stable in a range of -40° to +120°C (-40° to +248°F). The harnesses are foam-filled up to the electrical contacts, which eliminates any internal wiring movements.
The preforming does require a capital investment in manufacturing facilities, but it also enables the harness to be produced in a more automated manner, so can be expected to become cost-effective in high volumes. And the greater ease and accuracy of assembly line installation is apparent.
As engine compartments get smaller and/or more tightly packed, the simplified installation is an even more important advantage if the extra space taken by aluminum wiring is to be accepted. And because of higher under-hood heat, still another potential feature that's available becomes worth consideration: the preformed harness can incorporate thermal protection in the form of reflective outer surfaces. Groetsch described two Leoni systems, a closed tube and a surface mat, both of which he called ready for fabrication to a car maker's order.
The tubular design can be made in a 3-D configuration, but the wiring must be threaded through first, and then the tube can be put into a foam-filling tool. This poses an obvious issue if the wiring already has terminals attached, but where feasible the result is a harness that should be impervious to contaminants as well as excess heat. The mat doesn't require threading the wiring through a tube, but it can be made only in a linear (2-D) shape.