Fluctuating fuel prices, more stringent emissions regulations and increasing environmental awareness are leading to a growing interest in highly efficient mobile machines, especially in the premium segment. Depending on the duty cycle, today’s machines exhibit total efficiency values of less than 10 %, meaning that only a fraction of the energy contained in the fuel is actually converted into mechanical power. The low efficiency is mainly caused by the inefficient operation of the internal combustion engine and throttling losses across the hydraulic valves. To solve this problem, a number of research institutions and companies have invested a lot of time and money into the development of better systems. The simplest and therefore most popular solution has been to optimise only the hydraulics. Such an approach does lead to some improvement but, unfortunately, neglects the importance of the engine and thereby the efficiency of the whole machine. To develop a truly efficient system, it is necessary to consider the machine as a whole, i.e. the individual subsystems and their interactions with one another and the environment.
Since early 2013 the Institute for Fluid Power Drives and Controls at RWTH Aachen University has been developing and testing the new STEAM mobile hydraulic system for excavators. STEAM represents the first holistic approach to optimise both the energy efficiency and performance of mobile machinery. In contrast to today’s load sensing and flow controlled systems, STEAM does not use the engine and pump to directly supply flow to the actuators, but rather to maintain the pressure level in two separate hydraulic accumulators (High and Medium Pressure). Using this accumulator charging circuit, it is now possible to completely decouple the hydraulic actuators from the engine and pump, allowing these components to operate far more efficiently. This not only enables operation at a considerably lower and quieter engine speed of 1200 rpm but also minimises throttling losses and allows recuperation of both potential and kinetic energy from all the actuators. Consequently, the machine can be referred to as a hydraulic hybrid.
One of the main aims of the project has been to bridge the gap between university research and industrial application. Within the scope of a publically funded project and in cooperation with Volvo Construction Equipment a one of a kind prototype machine with both a standard load sensing hydraulic system and STEAM has been built. Having both systems on the same machine minimises factors that could distort measurement results, thereby allowing an objective comparison to be conducted. A simplified schematic of the prototype machine is shown in figure 1.
Fig1. - Circuit layout of prototype machine with reference load sensing system and STEAM
The final test conducted was an aggressive 90° truck loading cycle. As shown in figure 2, the boom, arm, bucket and swing are all active during the cycle.
Fig2. - 90° truck loading cycle
The movement was repeated 15 times during which approximately 14 t of gravel were displaced and dumped into the back of a truck. The dig and dump cycle is ideally suited to a system with hydraulic accumulators. When raising the boom and swinging towards truck the energy stored in the accumulators is used to assist the engine and pump. During the return stroke considerably less power is required and the pump can be used to charge the accumulators. In addition, potential energy from the boom and kinetic energy from the swing can be recovered and stored in the accumulators.
An analysis of the engine and pump operation shows some interesting aspects. The engine’s specific fuel consumption is plotted as a function of engine speed and torque is plotted in figure 3.
Fig3. - Engine operating points
The resulting engine operating points for both systems (LS and Steam) are also shown. The heights of the individual columns indicate how frequently the engine operates at a certain speed and torque combination. The LS system operates at the higher engine speed of 1800 rpm with an equal distribution of operating points at different torque values. STEAM, on the other hand, operates at 1200 rpm mostly at high torque in the region of optimal fuel consumption when charging the accumulator and at idle for short periods of time when the accumulator is full. Due to the high power demand of the cycle the idling phases are very short. Similarly, the pump operation is depicted in figure 4.
Fig4. - Pump operating points
In the case of LS the points are scattered, emphasizing the coupling between the supply and demand sides. The STEAM system shows two distinct operating points at high displacement settings – one during medium pressure charging, the other during high pressure charging. Despite the lower engine speed, STEAM managed the same cycle time and consumed 27 % less fuel. Measurements show that the improved engine operation is responsible for approximately half the improvement. The rest is a direct consequence of the reduced throttling and energy recovery in the hydraulics. These results underline the importance of the holistic approach.
In contrast to other hybrids, STEAM does not use any electrical storage devices or actuators. The already installed hydraulic system only needs to be modified, which keeps costs low and avoids unnecessary energy transformations. The robust and easy to maintain hydraulic hybrid technology increases profitability for operators, as they can expect a more agile machine with considerably lower fuel consumption. Considering the ever growing concern over depleting fossil fuel resources and more stringent emission guidelines, STEAM is clearly a very relevant and possibly game-changing innovation for the construction equipment industry.