Hydraulic Launch Assist

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Hydraulic Launch Assist is a hydraulic hybrid regenerative braking system produced by the Eaton Corporation. It also is referred to as the HLA (reg.) system.[1]


The HLA system recycles energy by converting kinetic energy into potential energy during deceleration via hydraulics, storing the energy at high pressure in an accumulator filled with nitrogen gas. The energy is then returned to the vehicle during subsequent acceleration thereby reducing the amount of work done by the internal combustion engine. This system provides considerable increase in vehicle productivity while reducing fuel consumption in stop-and-go use profiles like refuse vehicles and other heavy duty vehicles.[2]

Parallel vs. Series Hybrids[edit]

The HLA system is a so-called parallel hydraulic hybrid. In parallel systems the original vehicle drive-line remains, allowing the vehicle to operate normally when the HLA system is disengaged. When the HLA is engaged, energy is captured and released during deceleration and acceleration respectively. This is in contrast to so-called series hydraulic hybrid systems which replaces the entire traditional drive-line to provide power transmission in addition to regenerative braking functions.

Hydraulic vs. Electric Hybrids[edit]

Hydraulic hybrids are said to be power dense, while electric hybrids are energy dense. This implies that electric hybrids, while able to deliver large amounts of energy over long periods of time are limited by the rate at which the chemical energy in the batteries is converted to mechanical energy and vice versa. This is largely governed by reaction rates in the battery and current ratings of associated components. Hydraulic hybrids on the other hand are capable of transferring energy at a much higher rate, but are then limited by the amount of energy that is stored. For this reason, hydraulic hybrids lend themselves well to stop-and-go applications or applications with high vehicle weights.


Concept Vehicles[edit]

Ford Motor Company included the HLA system in their 2002 F-350 Tonka truck concept vehicle where it was reported to have reduced fuel consumption by 25%-35% in stop-and-go driving.[2]

Shuttle Bus[edit]

Eaton, Ford, the US Army, and IMACT Engineering built an E-450 shuttle bus as part of the Army’s HAMMER (Hydraulic Hybrid Advanced Materials Multifuel Engine Research) project.[3]


Eaton has been awarded the Texas government’s New Technology Research and Development grant to build 12 refuse vehicles with HLA systems.[4]

Peterbilt Motors has designed a Model 320 chassis that incorporates the HLA system,[5] which was featured on the cover of the December 13, 2007 issue of Machine Design.[6]