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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.

Hydraulic pumps can be fixed or variable displacement. Fixed displacement pumps are typically smaller, lighter, less expensive, and can be of any design (gear, vane, axial piston, radial piston, ect.)[1]. Variable displacement pumps are often axial piston with an adjustable swash plate. A virtually variable displacement pump (VVDP) is a fixed displacement pump combined with a fast switching control valve that performs the same function as a variable displacement pump. This is done by always pumping full flow, but using the control valve to divert only a certain percentage of flow to the system, and the rest back to tank. A VVDP has several advantages over a traditional variable swash axial piston pump. First, the pump can be of any design, not just axial piston. Second, the flow control bandwidth can be much faster because it is only limited by the bandwidth of the fast switching control valve and system accumulator, not the bandwidth of a swash plate. Third, a VVDP pump can be more efficient because it can operate at its optimum pressure and flow setting. On the downside a VVDP will require a high speed valve. There are also added switching power losses due to constant metering over valves, compressing and decompressing hydraulic oil, and metering during transition between pumping to system and tank. This paper concentrates on modeling these three switching losses.