New tech makes hybrid buses cost-effective

Reuters News
Posted: Oct 27, 2015 12:58 PM

By Jim Drury

Artemis's new Digital Displacement (DD) power system this year won the company a prestigious Royal Academy of Engineering MacRobert Award.

The company, owned by Mitsubishi, hit the headlines earlier this year when a 7MW (megawatt) wind turbine containing a Digital Displacement transmission (DDT) hydraulic system was deployed to operate as a floating wind-turbine in deep water 20 kilometers off Fukushima. It's the largest floating wind-turbine in the world.

Until now hydraulic pumps and motors have been controlled by varying the stroke of pistons with an adjustable mechanism, but have proved inefficient for automotive transmissions and wind turbines.

By contrast, the output of a DD pump or motor is controlled by digitally enabling individual cylinders containing electronically-controlled digital valves. Individual cylinders can be switched from idle to pumping cycle once every shaft revolution, in a pattern determined by an embedded controller. This provides results in a considerably faster and more accurate control response compared to variable-stroke machines.

"We developed digital displacement hydraulics, which is a way of turning off the capacity that you don't need without attracting a parasitic loss, and that was the fundamental starting point to make very efficient hydraulics which was both controllable and scalable to very big scales," explained chairman and founder of Artemis, Dr Win Rampen.

According to managing director, Dr Niall Caldwell, "conventional hydraulics have some weak points, particularly efficiency, controllability, and noise. So digital displacement is a fundamental rethinking of hydraulic power technology, which embeds electronics and software, digital control at the heart of the machine and that revolutionizes hydraulic technology to become much more efficient, much more controllable, and quieter."

Artemis is working on various projects to enable significant reduction in the fuel consumption of commuter trains and buses. A regenerative braking energy storage system using their DD technology showed fuel consumption reductions by 10 percent, and can be retrofitted to existing diesel trains. It also produces less noise and cuts exhaust emissions within stations.

The potential for hybrid buses is even more impressive, with fuel savings of 27 percent demonstrated on tests carried out on vehicles used by Lothian Buses, made by manufacturer Alexander Dennis.

 Caldwell says the technology pays for itself within two or three years, meaning that there is no need for any government subsidy, a particularly important factor in developing countries. In addition, it uses conventional materials, rather than expensive ones often used in mainstream electric hybrid technology.

"What we're trying to do is make a hybrid technology that is very low cost, it's made with conventional materials such as steel, rather than exotic and rare materials such as lithium, which can really be applied globally to make a hybrid system that pays for itself as a business case, without subsidy, and we think that's the key to making all the world's buses hybrids," he said.

Artemis has also adapted DD hybrid transmission for a car, specifically its BMW 530i, which it has been driving visitors to its site around in since 2008. Independent tests showed double the fuel economy in city driving compared to the same car with a six speed manual transmission, along with 30 percent lower carbon dioxide emissions.

As he drove the car around the company's Midlothian factory and demonstrated how it worked, Caldwell told Reuters: "Conventional electric hybrid mixes the power of the engine with that of an electric motor and an electric battery. In contrast our hydraulic hybrid mixes the power of the engine with a hydraulic motor  and a hydraulic battery, which is otherwise known as an accumulator, which stores the energy of braking the vehicle in the form of compressed gas."

According to Rampen, part of the beauty of DD is that its power rating can be increased dramatically by replicating multiple hydraulic components within a single system.

He said: "The power rating can be increased very dramatically, so far up to 7 megawatts, by just making a module and then replicating it around a machine to make a very high-powered machine. But also these modules tend to be rather small within themselves and as a result they're easy to take out and replace, and when you're working with offshore wind, particularly, where access is very difficult and expensive, to be able to dismantle and replace any one component in this very inaccessible place is very important."

Rampen says the MacRobert Award has created huge global interest in Artemis.