Abstract
The powertrain efficiency deeply affects the performance of off-road vehicles like wheel loaders in terms of fuel economy, load capability, smooth control, etc. The hydrostatic transmission (HST) systems have been widely adopted in off-road vehicles for providing large power density and continuous variable control, yet using relatively low efficiency hydraulic components. This paper presents a hydrostatic-mechanical power split transmission (PST) solution for a 10-ton wheel loader for improving the fuel economy of a wheel loader. A directly-engine-coupled HST solution for the same wheel loader is also presented for comparison. This work introduced a sizing approach for both PST and HST, which helps to make proper selections of key powertrain components. Furthermore, this work also presented a multi-domain modeling approach for the powertrain of a wheel loader, that integrates the modeling of internal combustion (IC) engine, hydraulic systems, mechanical transmission, vehicle(wheel) dynamics, and relevant control systems. In this modeling, an engine torque evaluation method with a throttle position control system was developed to describe the engine dynamics; a method to express the hydraulic loss of the axial piston hydraulic pump/motor was developed for modeling the hydraulic transmission; and a vehicle velocity control system was developed based on altering the displacement of a hydraulic unit. Two powertrain models were developed, respectively, for the PST and HST systems of a wheel loader using MATLAB/Simulink. The simulation on a predefined wheel loader drive cycle was conducted on both powertrain models to evaluate and compare the performance of wheel loader using different systems, including vehicle velocity, hydraulic displacement control, hydraulic torque, powertrain efficiency, and engine power consumption. The simulation results indicate that the vehicle velocity controller developed functions well for both the PST and HST systems; a wheel loader using the proposed PST solution can overall save about 8% energy consumption compared using an HST solution in one drive cycle. The sizing method and simulation models developed in this work should facilitate the development of the powertrains for wheel loaders and other wheeled heavy vehicles.
Highlights
The energy consumptions of off-road vehicles such as wheel loaders mainly depend on the energy efficiency of their transmission systems, which usually adopts continuous variable transmission (CVT) to improve the comfort
In a power split transmission (PST), the transmitted power is shared by two different transmission paths: a mechanical path with normally high efficiency, and a hydrostatic or hydrodynamic path for CVT
The comparison of simulation results between the PST model and hydrostatic transmission (HST) model shows that the torque or power provided by PST hydraulic system is less than by HST hydraulic system, and the efficiency of the PST is generally higher than that of the HST
Summary
The energy consumptions of off-road vehicles such as wheel loaders mainly depend on the energy efficiency of their transmission systems, which usually adopts continuous variable transmission (CVT) to improve the comfort. The hydrodynamic and hydrostatic solutions have been commonly used in off-road vehicle applications when comprehensively considering efficiency, power density, speed range, cost, etc. In a power split transmission (PST), the transmitted power is shared by two different transmission paths: a mechanical path with normally high efficiency, and a hydrostatic or hydrodynamic path for CVT. For the wheel loader application, Nilsson et al presented a multi-mode hydrostatic-mechanical power split transmission, which achieved at least 15% better fuel saving potential than the present torque converter solution [6]. Liu et al replaced the existing torque converter with a hydrodynamic-mechanical
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