The Toyota Prius has become synonymous with hybrid technology. But, as well as being incredibly ugly, is it really that green?

The "dust to dust" report, carried out by US market research group CNW says that the car is actually less efficient than a Hummer. It reasons, with some scientific analysis, that although in service emissions are lower, lifecycle emissions - which take into account manufacture and disposal - outweigh any benefit.

Although the document is highly disputed, not least by Toyota, and its methodology is rather clumsy, it is fairly well agreed that electric hybrids require more energy to manufacture. And the issue of recyclability, especially of the batteries, is still very much a burning issue.

Despite innovative and clever engineering solutions, the in-use benefit needs to be proven to be more than the extra 'costs' of manufacture and disposal. One company that says it is time for a rethink is Artemis Intelligent Power based in Loanhead, just south of Edinburgh. Rather than an electrical hybrid system, it uses has developed a hydraulic one.

"The underlying advantage of hydraulics is that the power density is significantly higher than electrical machines," says Dr Win Rampen, a director at Artemis.
"Like an electric hybrid, the principle is the same. When the car brakes, the system captures and stores energy that would normally be lost. This is then used when the car next accelerates."

The Digital Displacement Hybrid Transmission consists of connecting a pump mounted to a standard internal combustion engine, which is hydraulically connected to hydraulic motors and coupled to the wheels. The engine drives the pump, which sends its output power, via hoses, to two motors driving the rear wheels.

"But hydraulic machines typically suffer from poor part load efficiency and indirect controllability," says Rampen. "But what our technology brings is all the good things associated with hydraulics, plus the added advantage of controllability. That removes hysteresis while keeping parasitic losses to a minimum."

When the team recently trailed a modified BMW 5-series at Millbrook Proving Ground, the result was a 50% reduction in fuel use. A computer controller meant the vehicle was able to seamlessly switch between the stored energy in the hydraulic system and the petrol engine. It also ensured the engine operated at its most efficient speed, using a minimum of fuel, at all times.

But despite the impressive statistics, the reception from OEMs varied as senior research and development engineer, Dr. Niall Caldwell explains: "We got a mixture of receptions. From very cold at Damlier Cysler, where we were put in front of electric hybrid guys; they didn't like it.

"But then on the other hand when we presented to BMW we were given a very warm reception. They like to keep in touch with all sorts of alternative drive trains and it just so happened we came up at a time when they were looking to the future."

BMW has placed informal support for the project, and critically, enabled the team to integrate the technology in to a rear wheel drive car. The initial programme used a Ford Focus. "That became more of a packaging exercise because of the already highly integrated frontend," says Rampen. "The space provided by rear wheel drive vehicles means the whole conversion could be done without the need to modify the sub frame, or anything else structurally for that matter."

In terms of a retrofit, Artemis says is currently looking at producing replacement transmissions for commercial vehicles. Although many commercial vehicles may have the same engines over a 20 year period, it is not unusual to replace the transmission after around 10 years. This opens up a window of opportunity for the technology.

"Existing hybrid buses tend to get between 25% and 35% fuel consumption reduction," says Caldwell. "We would aim for the higher end of that, if not 40%.

"But, the main argument is cost. Electric hybrid buses are extraordinarily expensive. When you look at hybrid bus of today's electric technology as a commercial proposition they do not make economic sense because they can cost up to double the cost of the baseline. The Digital Displacement hybrid system can do all of that for much less additional cost.

"We are talking about parts which are machined or cast out of steel or aluminium and are mass produced relatively cheaply to known automotive production methods. And they have much better recyclability."