Challenged to a dual

It takes 2.5kW to boil the water in the electric kettle for your morning cup of tea, about the same power as you need to keep a three-bar electric fire burning on a cold day. According to recent car industry predictions, this is the sort of electrical power your family car will be pumping out […]

It takes 2.5kW to boil the water in the electric kettle for your morning cup of tea, about the same power as you need to keep a three-bar electric fire burning on a cold day.

According to recent car industry predictions, this is the sort of electrical power your family car will be pumping out in a few years’ time. Electric windows, power steering, electric water pumps, heated windscreens, heated catalytic exhausts, active suspension, possibly even electronically actuated engine valves they are all pushing up the load on the alternator and battery.

The question facing the car companies and their suppliers is how to generate more electric power under the bonnet while meeting fuel consumption targets for 2005 which will be two-thirds lower than today’s levels. And 2.5kW represents just the minimum demand peak loads could hit 6kW for short spells, equivalent to half the output of a typical domestic central heating boiler.

Industry’s answer, short of an unexpected breakthrough in fuel cell technology, is to generate electricity at much higher voltages, so gaining a step change in electrical efficiency, leading in turn to improved fuel efficiency.

What US and European car makers and suppliers are close to agreeing is a 42V/14V dual voltage system. Power-hungry equipment such as air conditioning would be run at 36V requiring an alternator to generate 42V, while light loads would be run at 12V, requiring a 14V supply.

The principle is that for a given power output, increasing the voltage gives a proportional decrease in current. The loss of energy caused by the resistance of electrical cables and components, however, depends on the square of the current. So increasing the voltage by three reduces the losses by a factor of nine.

Similarly, the relative voltage drop along each length of cable is reduced by a factor of three at the higher voltage. Components such as starter motors now have to handle less current, so they can be made physically smaller, and can also operate more efficiently at higher voltages.

At a meeting in Hanover in Germany last month a consortium of more than 60 European car makers, suppliers and electronics companies met to discuss a draft electrical specification for 42/14 dual voltage equipment.

The lead has been taken by the European car industry, driven primarily by the need for fuel economy. US auto industry companies have been closely involved with the aim, promoting the specification as a global standard over the next 12 months. This will allow manufacturers and suppliers to get on and develop dual voltage equipment as soon as possible.

So far, Japanese car makers appear to be showing little interest in dual voltage schemes. According to Nissan, engineers in Japan have been looking at both 42V and 48V applications, but it remains a low priority.

What you will see under your bonnet in the next millennium depends on how each car maker decides to go about implementing a dual voltage system. In practice there will probably be one alternator pumping out 42V and charging two batteries, one at 36V and the other at 12V, with electronic converters to change from one voltage to the other.

The first cars powered by the new system are expected to be top-of-the-range models from BMW or Mercedes within the next decade. But the luxury tag may not last long. Ford is looking at the development of 42V alternators and starter motors. ‘We want to bring the latest technology down to an accessible level,’ says a Ford spokesman.

Discussions about high voltage systems for vehicles go back a couple of decades, driven mainly by the need to find enough power to meet increasing loads. But it really took off in 1994 with the push in Europe towards the goal of producing a car with a fuel consumption of 3 litres per 100km by the year 2005 the so-called 3-litre car.

According to Malte Keypers at Delphi Automotive Systems’ engineering centre in Germany, a member of the dual voltage consortium, the average fuel consumption of a typical European car today is around 8.7 litres per 100km. Its average power consumption is around 800W.

With each 100W of electrical load equating to around 0.15 litres/100km of fuel consumption, generating this amount of power accounts for 14% of fuel consumption about 1.2 litres.

But if nothing is done to improve the efficiency of the electrical system, then generating 1kW would account for 40% of the fuel consumption of the proposed 3-litre car.

Similarly, in 1994 the US Department of Commerce Council for Automotive Research (USCAR) announced a target fuel consumption of 80mpg at 40mph for US cars.

According to estimates by MIT for the US dual voltage consortium, adding an 800W electrical load would reduce that target immediately to 65mpg, using today’s equipment. In effect, an extra 100W of electrical load has the same effect on fuel consumption as adding another 50kg of weight.

So how does the new system alleviate this?

At 12V, the resistive losses on a power output of 1kW amount to about 64W. Increase the voltage to 36V and the losses drop to 8W. At the not implausible power for a future auto-electrical system of 5kW, losses at 12V would amount to 1.8kW. At 36V, this would drop to 200W a significant saving equating to 2.4 litres/100km of fuel.

A more efficient way of generating electricity is also a prerequisite if the car industry is to introduce more electronic controls.

The typical wiring harness already weighs 40kg, with more than 1,500 separate wires and 300 connectors linked to a couple of dozen sensors and 35 microprocessors. ‘Electrical systems can represent 20% of a vehicle’s cost,’ says Professor John Kassakian of the Massachusetts Institute of Technology.

One of the worries is that development of the electro-mechanical valve train could be held back by the lack of available power. The idea is to get rid of the mechanical valve train reliant on a camshaft. Instead the valve stem would, in effect, become the armature of a solenoid so that its movements could be controlled electronically.

Apart from benefits to engine efficiency, says Kassakian, valves could be left open while the engine is started, reducing the power needed from the starter motor and thus the size and weight of the battery.

Ultimately, a combined alternator-starter could be used, turning the engine via the fan belt. Bosch, the electrical component supplier, is currently working on just such a system.

But these developments will not be possible until a more efficient way of generating and distributing electrical power around a vehicle is developed. Lucas, a member of the European dual voltage consortium, calculates that a starter motor running at 48V would give almost 50% more power than a 12V version of the same size. And, thanks to reduced component costs, there would be significant cost savings for the high voltage version.

Similarly, a 48V alternator could be made 12% smaller and weigh 18% less than a 12V one, says Lucas, although there would only be a modest cost saving.

Higher voltages also mean that wire thickness can be reduced, although this would not benefit the two-thirds of a vehicle’s wiring that carries less than 3A, according to Lucas. Wires would still need to be sufficiently thick to stop them snapping.

The choice of the 42/14 dual voltage system is something of a compromise. There has been a move away from switching vehicle electrical equipment on and off directly with mechanical switches, with car makers now preferring the use of power semiconductors and remote switching. Commercial power semiconductors work better at higher voltages and 48V would have suited them and increased overall electrical efficiency. However, the alternator driving a 48V system would be putting out close to 60V, right on the borderline for electric shock.

Also, electric light bulb filaments need to be made thinner for higher voltages, but this makes them more brittle. If car headlamp suppliers had their way, they would go back to the original 6V system to ensure tough, long-lasting filaments. Under the 42V/14V concept, lighting can remain at 12V.

Battery manufacturers have warned of the high costs of retooling to provide high capacity, high voltage batteries that would match the 12V lead acid for starting on a cold wet day.

Echoing these points, Visteon, the automotive component arm of Ford, says a number of issues have still to be resolved, namely the development of wiring, weight reduction and better energy efficiency. ‘It is too early to say if the 42V/14V concept is the optimal solution,’ it says.

But that may yet all change. According to Chuck Vink, responsible for electrical and electronic architecture development at Delphi, it may be possible to eliminate the 12V lead acid battery. Delphi is already working on the development of lithium polymer batteries which provide three times the power density of lead acid batteries. They are smaller and lighter and can be made into virtually any shape to fit in any space, even between the roof and the roof liner.

Coupled with what Delphi sees as a move to more decentralised electrical power storage using a network of smaller, lighter batteries, the days of jump leads may soon be over.