Parallel-Series Hybrid Powertrain Concept for a Wheel Loader

Conventional wheel loaders are characterized by a hydraulic working system and a hydraulic propulsion system to move the machine. These systems share the same engine shaft power. There are several ways to hybridize the powertrain of a wheel loader, depending on the topology and number of electric machines used. For example, a parallel-series hybrid powertrain decouples the working hydraulics from its propulsion system, allowing both subsystems to be controlled independently. This could lead to significantly better system efficiency and simpler operation.

In this context, this paper presents a parallel-series hybrid powertrain concept and an adapted operating strategy for this system. The proposed operating strategy calculates the required minimum speed of the hydraulic pump based on the operator’s input and optimizes engine operating points to reduce fuel consumption while meeting other requirements such as propulsion torque and battery state of charge (SOC). A 7-ton wheel loader demonstrator vehicle was designed and built based on the proposed powertrain concept and operator interface. An adapted hybrid powertrain simulation model and the proposed operating strategy were validated using measured data obtained with the demonstrator vehicle. The simulations and field test results show a fuel consumption advantage of up to 30% compared to the conventional powertrain. The results are analyzed and the impact of the improved system efficiency on key requirements of future engine concepts is discussed.