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Daimler Buses to offer electrohydraulic power steering to reduce fuel consumption

Daimler Buses will offer electrohydraulic power steering (EHPS) to reduce fuel consumption in coming bus models. The fully integrated system replaces the conventional power steering pumps used currently in all buses with a gross weight of more than 7.5 t.

Conventional hydraulic power steering (HPS) is driven directly by the internal combustion engine via belts or gear trains with a fixed gear ratio which results in a pump speed that is directly proportional to the engine speed. To provide sufficient power for the steering in every situation, the pump must be able to deliver the full volumetric flow rate even when the engine speed is low—meaning that it is over-dimensioned for most operating situations. For this reason, higher engine speeds require a restriction of the volumetric flow rate by means of a bypass, which, however, entails unnecessary losses.

The electrohydraulic power steering pump (EHPS) circumvents this problem specifically for heavy commercial vehicles and buses. The technology retains the steering gear box, however the hydraulic power steering pump is uncoupled from the internal combustion engine and is instead driven by an electric motor with integrated power-control electronics.

This allows for control of the pressure and volumetric flow rate as needed independently of the internal combustion engine speed. In combination with an intelligent operation strategy, this leads to low energy requirements and thus to higher transportation efficiency.

The electrohydraulic power steering (EHPS) consisting of a three-phase current synchronous motor, control unit, high-performance electrical system and hydraulic pump, reduces fuel consumption of buses. Click to enlarge.

As a result of its positioning in the vehicle, the unit—consisting of a three-phase current synchronous motor, control unit, high-performance electrical system and hydraulic pump in a single housing—enables simple visual inspections of the power steering fluid reservoir and its fill level.

Short hydraulic pipes also make sure system-based friction loss is significantly reduced within the hydraulic circuit. The intelligent control system converts vehicle data to a demand-based actuation of the electrically operated 24V pump. Consequently, it provides the right amount of steering assistance depending on the driving situation.

The system’s efficiency gain results from the improved hydraulic efficiency as well as from the operating strategy which allows the EHPS to be controlled according to actual need. The improved efficiency chain and the control that is adapted to actual needs make overall fuel savings of 0.9% - 1.2% possible, according to Daimler’s development partner, the Engineering Center Steyr GmbH & Co. KG.

The 24V EHPS system can be operated directly from the 24 V on-board power network in heavy-duty commercial vehicles with no additional boost converter. Consequently, it can be used in both electric and hybrid vehicles as well as in conventional vehicles.

The system has already been awarded the “Innovation Label” by the specialist panel of the European Coach Week (ECW) at the international Busworld 2015 specialist trade fair in Kortrijk, Belgium.

Daimler’s plan is to offer the system for Mercedes-Benz Citaro city buses as optional equipment from mid-2017.


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