A mechanical hybrid drivetrain is equivalent to the more common hybrid electric system in that it can recover braking energy, provide supplementary power for better acceleration and reduce fuel consumption markedly.

In a mechanical hybrid system, hydraulic or pneumatic accumulators are used to store energy rather than the batteries typical of electrical hybrids. Alternatively, clutches can be used to transfer energy to a flywheel which can store energy in a rotating mass.

Application relevance

The main determinant of suitability within the Australian market appears to be whether sufficient energy can be recovered from braking to offset the disadvantages of carrying the system’s additional weight. The most obvious applications are those that involve frequent stop-start driving, including light commercial vehicles, buses, and medium and heavy trucks operating in urban environments. Residential waste trucks are a good example of a suitable application for mechanical hybrid systems.

Potential benefits

Fuel and emissions reductions from implemented mechanical hybrid systems in light commercial vehicles range from 25–70%; however, real-world use is likely to achieve around 35–50% fuel savings.

Key implementation considerations

The nature of the duty cycle must be considered prior to selection and implementation of any hybrid technology, including mechanical systems. Frequent stop-start conditions are required to generate significant fuel savings. In the wrong conditions, an acceptable fuel saving and payback period may not be achieved.

Capital costs have been cited as being approximately 15% higher than for a conventional vehicle, with a payback period of three to four years. In addition, maintenance and service requirements and the service life of the component systems also need to be taken into consideration when evaluating whole-of-life costs.

Examples of implementation

UPS parcel truck

This brochure details the fuel savings achieved by a courier and delivery company in the USA by using mechanical hybrid technology in an urban application. In 2006, UPS began a trial with the US Environmental Protection Agency using a hydraulic hybrid parcel truck. The trial achieved fuel savings of up to 70%. Based on the success of its early evaluation vehicle, UPS announced (in October 2008) an order for seven additional hydraulic hybrids to trial across its fleet. The trial using UPS mail delivery trucks found a likely payback of two to three years, with the initial cost of the system likely to be around US$7000, or less than 15% of the base vehicle cost. For more information, see US Environmental Protection Agency (2006) Hydraulic Hybrid Vehicles.


This press release details the potential fuel savings that may be realised with the use of hydraulic regenerative braking in light commercial vehicles in a stop-start application (ThomasNet News 2002). In 2002, Ford Motor Company and Eaton Corp tested a system called Hydraulic Launch Assist (HLA) for Ford’s F-series pick-up trucks. The Ford HLA system recorded potential recovery rates of 380 kJ of energy (80% of the vehicle’s initial kinetic energy). This is claimed to be sufficient to accelerate a 4.5 t truck to 40 km/h without requiring the combustion engine. The claimed fuel savings of Ford’s system were 25–35%. The size of the Ford system would likely add US$2000 to the price of an F-series pick-up truck (which is most comparable to a light commercial vehicle in Australia). For more information, see ThomasNet News (2002) Hydraulic Regenerative Braking Boosts Fuel Economy.


The transmission specialist Torotrak has developed a flywheel-based mechanical hybrid Kinetic Energy Recovery System (KERS) for fitment to buses and commercial vehicles. The mechanical hybrid system is expected to offer the commercial vehicle sector an opportunity to deliver fuel efficiency savings of 20%.

For the full report, see Fuel for Thought – Identifying potential energy efficiency opportunities in the Australian road and rail sectors (opens in a new window) PDF 1.5 MB.