Evaluating a Pump Controlled Open Circuit Solution

Abstract

Today’s mobile machines most often contain hydraulic valve controlled drives in an open loop-circuit. For the purpose of saving energy the constant pressure pumps have, in the past, often been replaced by load-sensing pumps and load-sensing valves. In mobile applications these load-sensing solutions have significantly reduced the energy consumption. However, in applications with unequal drive pressure levels the load sensing systems still result in energy losses, referred to as metering losses. Throttling losses associated to load lowering have also been paid more attention to in the energy debates. By instead adopting direct pump displacement control, with the capability of energy recuperation in lowering motions, many of these losses can be avoided. In this article the author has studied a hydraulic system configuration where each actuator/supply system comprises an electrically controlled variable displacement pump/motor working in an open-circuit together with four separate electrically controlled valves. The four valves render a solution versatile in control, as the cylinder chambers can be connected to pump and/or tank as well as be closed at any time. Depending on how the valves are controlled different modes of operation are achieved, which in turn yields different total energy efficiency depending on the present working point. In this study the performance, operability and energy consumption is evaluated in a wheel loader that is first looked upon with its original load sensing hydraulic system and then modified with a completely new pump controlled hydraulic system. Measurements published in this article demonstrate the advantages and drawbacks of pump control in an open circuit compared to a load sensing system. Performance is evaluated by looking at productivity, loading capacity and duty cycle time of a typical loading cycle. Operability is evaluated by looking at response times, load oscillations and driver comfort. Theoretical calculations of energy efficiency including metering and throttling losses over the valves as well as pump efficiency, line losses and losses in the recuperation phase are evaluated and validated. The fuel consumption of the pump-controlled wheel loader is measured and put side by side to measurements of the same machine equipped with a load-sensing hydraulic system.