Fuel efficiency optimization of a baseline wheel loader and its hydraulic hybrid variants using dynamic programming

In this paper, fuel consumption of a 5.7-ton municipal tractor in a wheel loader application is studied, and methods for improving the fuel efficiency are compared with each other. Experimental data from the baseline machine with load-sensing hydraulics has been gathered during a y-pattern cycle, and the data is inputted to an optimization function having realistic loss models for a hydraulic pump and diesel engine. Dynamic programming is used to analyze different system configurations in order to determine optimal control sequence for each system. Besides optimization of variable engine rotational speed on the baseline system during the working cycle (considering the point of operation), three hybrid supply systems are studied: 1) a hydraulic flywheel, 2) parallel supply pumps and 3) a throttled accumulator. These systems utilize a hydraulic accumulator as an energy source/sink alongside the diesel engine. The optimal sequence for charging and discharging of the accumulator is examined in order to minimize the fuel consumption of the machine. The idea is to use the lowest acceptable, constant engine rotational speed, to cut down the diesel losses. In addition, the study covers an analysis of adjusting the engine rotational speed for each point of operation also when the hybrid systems are considered. The results show that finding advantageous engine rotational speed for each loading condition can decrease the fuel consumption of the baseline machine around 14%, whereas hybridization of the supply system can further improve the result by a couple of percentage units. Hybrid systems also reduce engine's maximum load by making it more uniform, which allegedly reduces emissions. The possibility of engine downsizing to further improve the fuel efficiency of hybrid systems is not considered, because the maximum engine power is usually determined by the hydrostatic transmission of a municipal tractor. However, the study assumes that actuators are controlled using traditional 4/3 proportional control valves; hence, there are still potential for greater fuel savings. For example, applying independent metering valves on the actuator control can further decrease the system losses.