Hybrid Comparisons

Competitive Advantages of the RDS Technology

Superior Power to Weight Ratio

The RDS technology has an advantage over hybrid electric systems because it provides a superior power to weight ratio. This means that the heavier the vehicle the greater the advantage becomes. Heavy vehicles need high power to move their load and to stop. This requires high power hybrid system motors, controllers and storage systems that hydraulic components can readily provide — and batteries cannot.

The advantage of hydraulic accumulators is that they can cope with energy flow rates well in excess of braking requirements. In contrast, batteries in hybrid electric systems must be large and heavy to absorb energy at the rate demanded by braking. Accordingly, these systems require a battery that will only be partially charged by braking rendering up to 85% of the battery’s capacity redundant, and the spare storage capacity cannot be efficiently utilised. On the other hand, accumulator storage can be matched exactly to any application requirement without redundancy.

Weight Advantage

Hydraulic hybrids are considerably lighter than electric hybrids for urban commercial vehicles, as shown in the graph below. This weight advantage maximises payload, improves fuel economy, providing a considerable market advantage.

The RDS is a parallel hybrid, meaning it taps into the driveline. A series hybrid is the driveline. The graph below showing the weight comparisons for parallel hybrids shows the RDS as 100%. The other examples are shown relative to the RDS.

Electric hybrids use either batteries or super capacitors as their storage whereas the RDS uses accumulators. Typical types of batteries used for vehicles are nickel metal hydride (NiMH) and sodium-nickel-chloride (NaNiCl) which have been used in the graphs below.

For the same type of vehicle an electric hybrid using NaNiCl batteries would weigh double that of a hydraulic hybrid RDS system.  A NiMH system would be around 95% heavier than the RDS and a super capacitor would be more than 40% heavier.

Even when comparing series systems shows that hydraulic hybrids are significantly lighter. If the RDS was presented on the graph below it would be represented at the 100% mark.

Storage Efficiency

Storage efficiency means the efficiency from the time the energy enters RDS from the driveshaft until it is returned to the driveshaft. Storage efficiency means the energy efficiency travelling from the engine to storage out, that is, from point A to point B as described in the illustration below for hydraulic hybrids (RDS) and electric hybrids.

Hydraulic Hybrid (RDS)

The graph below shows that RDS is much more efficient than other types of electric hybrids.

For the same power requirement, a typical hydraulic installation:

  • is considerably lighter (up to 900kg)
  • has better energy storage efficiencies
  • does not introduce 650 volt components and operating systems
  • has a fraction of the heat loss and dissipation into the vehicle maintenance regime
  • integrates with existing vehicle hydraulic accessories (e.g. cranes)

Cost Advantage and Availability

Hybrid hydraulic systems are cheaper to buy and own than other hybrid systems. These components have and reliability. Only minor adaptations for fitting to vehicles are required. Hybrid electric systems however consist of components with a long track record of performance a limited life and require significant development to improve efficiency, power to weight ratio, volume production and cost.

Safety Advantage

The RDS technology is very safe. The pressure in hydraulic accumulators is easily and safely discharged for storage and maintenance. In addition, the RDS technology is easily programmed to automatically discharge when the vehicle is switched off. There is no risk for self-ignition, fire or shock. By contrast, hybrid electric systems use high voltage, high current circuits which pose several significant safety hazards.

Environmental Impact

The RDS is a clean technology. The whole-of-life environmental footprint of hybrid hydraulic components is significantly less than hybrid electric components. The RDS technology components contain long life readily recycled materials and fluids. In contrast, the batteries of an electric system contain noxious materials, ranging from concentrated acid to heavy metals that have a detrimental impact on the environment when manufactured and when they are disposed of at end of life.

Environmental Operating Conditions

Hybrid hydraulic systems are far more tolerant of environmental operating conditions being less affected by extreme heat, cold and moisture than hybrid electric systems.


The hydraulic components of the RDS technology will integrate seamlessly into the existing maintenance practices of fleet owners. Hydraulics systems are in widespread use in the transport industry, so no new skills or tools are needed to service hybrid hydraulic vehicles. Should there be a problem, the system can turn itself off (or be turned off by the operator) until repairs are carried out. This allows the vehicle to continue working productively and with all the original vehicle capabilities.


Hybrid hydraulic systems are suitable for integration with hydraulic ancillaries already on the vehicle, such as cranes and other lifting devices. The RDS technology is easily scalable using available components.

After-Market and Retrofit

The RDS technology will be marketed in kit form to make after-market and retrofit installations very straight­forward. Installation of the system requires minimum change to the vehicle and, only passive interface with the vehicle control systems. This provides RDS with a significant advantage over systems that replace the transmission and/or the engine of the vehicle.