Series Hybrid Transmissions
Digital Displacement® technology is ideal for building series or parallel hydraulic hybrid transmissions for automotive applications. Artemis publicly demonstrated its first full Digital Displacement Hybrid Transmission, fitted to a BMW 530i, in May 2008. The project was supported by the Energy Savings Trust. Independent tests confirmed that the prototype car gave double the MPG in city driving compared to the same car with a six speed manual transmission. Overall, including highway driving, the prototype had approximately 30% lower carbon dioxide emissions than it had before Artemis fitted its energy saving transmission.
You can see a video of the Artemis series-hybrid BMW by following this link.
Artemis group photo with hybrid BMW September 2008, photo courtesy of University of Edinburgh.
An enabling technology
Many companies have investigated hydraulic hybrid technology, in both series and parallel configurations. Some of the attractions are:
- Hydraulic drives and energy storage have high power density compared to electric alternatives;
- Hydraulic systems have a long-established reputation for robustness and longevity in countless industrial applications;
- Hydraulic components tend to be relatively easy to manufacture and are well-suited to mass production - and therefore cheap.
The problem has always been that the part load efficiency of hydraulic pumps and motors are too low. Cars spend little of their lives at full power, so high part load efficiency is crucial to the automotive duty cycle. Consequently, until now, hydraulics have generally only been used in specialised applications such as refuse trucks and city buses- often with mixed results.
Digital Displacement® is the enabling technology for hydraulic hybrids because its efficiency remains high throughout the operating range.
Features of the Artemis series-hybrid BMW 530i
Infinitely variable transmission ratio
The engine runs at its optimal RPM at all times, giving 2-litre performance from a 1.6-litre engine and improved fuel efficiency. A DD transmission makes very high overdrive ratios possible but has an almost instant kick-down to allow the engine to achieve higher speed and power during acceleration.
No dissipative clutch or torque convertor
Full wheel torque is possible with the engine at tick-over.
Regenerative braking with accumulator storage
Storing reclaimed energy in a hydraulic accumulator greatly improves urban cycle efficiency.
Engine off operation
The vehicle can start moving using energy stored in the accumulator without any engine power at all, allowing the engine to stop when in traffic yet retaining immediate response to accelerator pedal input.
Four-wheel independent traction control
A DD traction control system can provide completely independent hydraulic supplies for each wheel, each controllable at high bandwidth.
All Wheel Drive and Rear Wheel Drive cars no longer need a driveshaft to transfer power from the front of the vehicle to the rear wheels, increasing the interior space and giving vehicle designers more freedom.
Artemis has also built and successfully demonstrated active suspension systems for vehicles using Digital Displacement® Pump Motors (DDPM) to control multiple single-acting rams. These systems feature high control bandwidth and high efficiency without the use of proportional valves. The ability to go from idle to full-flow in under 30 milli-seconds allows rapid position and stiffness changes even in high performance vehicles. Being directly microprocessor controlled, the DDPM is easily integrated into a drive-by-wire control architecture.
Digital Displacement® active suspension pump.