Integration of Engine & Hydraulic Controls for Best Operation

Transcription

1 22.1 Integration of Engine & Hydraulic Controls for Best Operation Gary LaFayette, Stephan Gruettert, Michael Gandrud Sauer-Danfoss (US) Company Boris Laudenbach, Dieter Koenemann Sauer-Danfoss GmbH & Co. OHG ABSTRACT In 212, the Tier 4 interim / stage IIIB emissions regulations will mandate that new Diesel engines emit significantly lower levels of NOx (nitrogen-oxygen compounds) and PM (particulate matter). Diesel engines with power ratings below 56 kw will have less stringent mandates to reach. This paper describes software that, in combination with an electronically controlled engine and an electro-hydraulic system, enables the vehicle designer to mitigate the impact of these requirements by allowing the engine in many machines to be downsized below the 56 kw threshold while maintaining productivity. Even in applications where engine downsizing is not possible, the methods taught in this paper have additional benefits, including reduced fuel consumption. The methods in this paper which are collectively termed Work Function Control (WFC), provide engine anti-stall capability, flow sharing among the machine s hydraulic functions, and intelligent engine speed control for optimal engine efficiency and fuel savings. This software solution enables efficiency and productivity improvement in applications that have a work function focused power demand, such as excavators, aerial lifts, telehandlers, log loaders, forwarders, truck mounted cranes and backhoe loaders. Validation testing of these methods was performed on a backhoe loader. These tests revealed a 19% fuel savings, and a potential to downsize the engine by 18%. INTRODUCTION Upcoming emissions legislation and the risk of rising fuel prices have forced vehicle designers to consider new ways of improving machine efficiency while maintaining or improving productivity. Since the electronic control of Diesel engines is becoming much more common in the off-highway market due to upcoming emissions regulations, vehicle designers should consider new opportunities that the combination of electro-hydraulic systems and electronically controlled engines open up to optimize machine efficiency and productivity. Work Function Control (WFC) is Subsystem Application software for use with Sauer-Danfoss PLUS+1 microcontrollers and associated hydraulic and electronic products. This software provides three advanced functions to improve the productivity and fuel efficiency of the working hydraulics. These three core functions are anti-stall, flow sharing, and intelligent engine speed control. The anti-stall function limits the power consumed by the working hydraulics under heavy engine loads to prevent engine stalling. The flow sharing optimally allocates flow to each function when the flow demand exceeds the flow supply. The intelligent engine speed control sets the engine speed to the most efficient operating point given the flow demand from the hydraulic system. In addition to these advanced functions, WFC also provides the basic functionality expected in an electro-hydraulic control system such as joystick shaping profiles, input smoothing, and fault monitoring and handling. WFC HYDRAULIC SYSTEM REQUIREMENTS An electro-hydraulic work function system is required to enable WFC. An overview of the components that may be included in such a system is given below. PLUS+1 microcontroller Electronic Human Machine Interfaces - (Joysticks, Steering Wheels, Pedals, Displays, etc) Load Independent Electro-Hydraulic Valves Electro-Hydraulic Steering Variable Displacement Load Sensing Pump Electronically Controlled Engine Hydrostatic Propel - Only when intelligent engine speed control is used while propelling the machine SOFTWARE ALGORITHM This section first discusses the structure of the WFC software, then how the three core components of antistall, flow sharing, and intelligent engine speed control are connected. Finally, the functionality and benefits of each of the core components is discussed in detail. GENERAL DESCRIPTION Figure 1 shows a simplified block diagram of a system with WFC. In comparison to a conventional system, a system with WFC is more integrated into the vehicle power train and enables advanced functionality. In the conventional system, the flow from each valve section is proportional to the

2 joystick input and the engine throttle is manually set at a fixed position during the work cycle. With WFC, the software uses the joystick input along with feedback from the engine and a pump displacement sensor to find the optimal valve section flow to provide flow sharing and anti-stall. Additionally, the joystick input and pump displacement are used to dynamically set the optimal engine speed during the work cycle. The valve output block in Figure 1 performs the typical processes of an electro-hydraulic control such as joystick shaping profiles, input smoothing, and fault monitoring and handling. It also uses the output from the flow sharing and anti-stall blocks to provide the additional functionality of WFC. Joystick Inputs Intelligent Speed Control Speed Setpoint CAN Messages Engine Load Feedback Engine Speed Feedback Flow Sharing Anti- Stall Valve Output Speed Flow Engine Pump Valve Torque Pressure Pump Displacement Feedback (Optional) Figure 1: WFC Block Diagram Fault Checking Load Sense Signal To Work Functions The pump displacement feedback as shown in Figure 1 is an optional signal in the control system. The displacement feedback is only required for machines that frequently operate at the pump s pressure compensator limit. For example, machines used for excavation frequently operate at maximum breakout force (pump compensator limit) whereas machines primarily used for material handling do not frequently operate at the pump pressure limit. The flow sharing and intelligent engine speed control portions of the WFC algorithm require knowledge of the total flow output from the hydraulic system to function properly. In applications that do not frequently operate at the pump compensator limit, the precise flow control of Sauer-Danfoss PVG valves allows the total flow to be accurately calculated from the valve input command and spool flow profile. In applications that frequently operate at the pump compensator limit, the valve command cannot be used as an estimate of the total output flow because the pump s pressure compensator is modulating output flow to maintain the maximum pressure setting. In these situations, the pump displacement sensor is used to augment the joystick input and calculate the total flow output from the hydraulic system. It is possible to implement WFC on machines that are not fully electro-hydraulic. For example, on a backhoe loader with a torque converter transmission, it is possible for the intelligent engine speed control to only be active when the transmission is in neutral and the machine is being used as a backhoe. Also, if a limited number of work functions are not electro-hydraulic (steering for example); WFC can assume a minimum reserve flow and speed for that function or compensate for the missing feedback with the pump displacement sensor. When the vehicle is fully electro-hydraulic (including hydrostatic propel), work function control can be integrated into a complete vehicle power management solution that includes both the work hydraulics and the propel hydraulics. ANTI-STALL The anti-stall portion of WFC monitors the difference between the engine speed set point and the actual engine speed as well as the load signal from the engine controller. When the algorithm detects a situation that could lead to engine stall, the command to the flow control valves is reduced to lower the power demand from the hydraulic system. It is common for engines to be sized such that there is sufficient power to allow the hydraulic pump to operate at maximum pressure and maximum flow simultaneously (hydraulic corner power). This is done because equipment operators find it very undesirable for the engine to stall. With the anti-stall provided by WFC, machines do not need engines sized to the hydraulic corner power (a condition that typically represents less than 1% of the duty cycle) because the risk of engine stall can be mitigated with software rather than excess engine capacity. For a given machine, this means it is possible to increase the hydraulic pump size for increased productivity or reduce the engine power for improved efficiency and lower cost. The anti-stall function only reduces hydraulic power consumption when needed, especially useful in applications with multiple power demands from the engine. With hydro-mechanical torque limiting pump controls, the torque limit must be set at a level that will not stall the engine with all power demands active. With anti-stall, hydraulic output is only reduced when the total demand on the engine exceeds its load capacity. If the hydraulic system is the only active power demand the WFC system can provide a greater hydraulic output than the competitive hydro-mechanical torque limiter. The anti-stall function also allows for a priority scheme to be applied to the reduction in hydraulic output. Such an option is not available with the hydro-mechanical torque limiting pump control without the additional expense of priority valves. FLOW SHARING Electronic flow sharing is particularly useful with pre-compensated flow control valves (such as PVG32 valves) since these valves do not provide flow sharing functionality on their own. Even with postcompensated flow sharing valves (such as PVG1 valves), electronic flow sharing offers additional

3 functionality and benefits not provided by the postcompensated valve alone. When the flow demand exceeds the flow supply in a system with pre-compensated valves, the valve section with the lowest load pressure receives flow first. Once that valve section's requirements are met, the remaining flow goes to the valve section with the next highest load pressure; continuing until the available flow is consumed. In some cases the highest pressure function can come to a complete stop (stall) because the available flow is exhausted. This characteristic is undesirable in applications that frequently operate multiple functions simultaneously. The flow sharing portion of WFC monitors the joystick input, engine speed and pump displacement to determine when the flow demand exceeds the flow available from the pump at the current engine speed and maximum displacement. If the demand exceeds the available supply, the commands to the work functions are reduced such that the commanded flow matches the available flow. The commands to the active functions can be reduced proportionally or according to a priority scheme. This prevents the stalling described in the previous paragraph. Since flow sharing is able to divide the available flow to the active work functions, the pump does not need to be sized to provide full flow to each function simultaneously (a rare situation in most applications). This may enable the use of a smaller, lower cost, pump that will operate closer to its maximum displacement on average, resulting in improved pump efficiency. Since WFC implements flow sharing electronically, it is able to provide several features not available with postcompensated flow sharing valves. A list of these benefits is provided below. With WFC, some functions can be configured to receive priority during flow limited situations without the need for additional valves. With the hydraulic flow sharing in conventional post-compensated valves, only a proportional reduction is possible. With WFC and pre-compensated valves, it is possible to have independent load sense pressure relief valves on each work port; an option not available on post-compensated valves due to their design. WFC provides flow sharing using the real-time joystick input rather than reacting to a flow limited situation as with post-compensated valves. Eliminating this lag removes a potential source of instability in the hydraulic system. With WFC, flow sharing can be toggled on or off with an operator selectable switch. This is useful because some experienced operators prefer the feel of individually reducing commands during flow limited situations rather than having it occur automatically. INTELLIGENT ENGINE SPEED CONTROL The intelligent engine speed control in WFC monitors the joystick inputs and pump displacement feedback to calculate the minimum engine speed required to meet the flow demand at maximum pump displacement. The engine speed set point is dynamically adjusted to meet the flow demands while operating the engine most efficiently. This generally means operating the engine at the lowest speed possible to meet the hydraulic flow requirements. This is contrasted to the typical mode of operation where the operator sets the engine to a fixed speed and the variable displacement pump adjusts its output to meet the flow demands. During this typical mode of operation, the operator sets the engine speed to meet his peak flow requirement, however, most of the time the engine speed is significantly higher than necessary. The intelligent engine speed control offers a significant improvement over auto-idle systems that are currently available in the market. These systems reduce the engine speed to low idle after a brief period of time with no operator input. In addition to saving fuel while no work is being done, intelligent engine speed control can also save fuel when the machine is operating at low power levels. During most work cycles, the hydraulic flow requirement fluctuates significantly while working. This is especially true in material handling applications where large inertial loads must be smoothly accelerated and decelerated to maintain vehicle stability. With intelligent engine speed control it is possible to reduce fuel consumption up to 2% during work cycles that have widely variable flow requirements. With the intelligent engine speed control tuned for maximum efficiency, the maximum rate that the hydraulic pump flow can be increased is limited to the maximum acceleration of the engine speed. During work cycles requiring precise commands and smooth increases and decreases in flow, this rate limit is unnoticeable. During work cycles with jerky operator input where rapid increases in flow output are expected, the rate limit due to the engine may become noticeable. The rate limit posed by engine acceleration is partially mitigated with anti-stall. When the system receives a step input in demanded flow, the anti-stall function senses the difference between desired speed and actual speed and limits the rate at which the hydraulic output increases. By limiting how quickly the hydraulic load is applied to the engine, excess torque is available to the engine for acceleration. By balancing these competing demands, the flow response (a function of both pump displacement AND engine speed) can be maximized. Most machines are operated in work cycles that are a mix of smooth precise movements and quick, rapidly changing movements. The intelligent engine speed control can be adjusted dynamically to account for the differences in required flow response. It may be desirable to provide the operator with a toggle switch to select a Normal Mode / Maximum Eco-Mode of

4 operation. To provide added responsiveness in the Normal Mode of operation, the engine speed would be set to a margin of several hundred rpm above the speed which is required to provide the commanded flow. This added speed margin will reduce the fuel savings that are possible, but allows more rapid response to flow commands since the pump is able to increase its displacement more quickly than the engine is able to accelerate. In the Maximum Eco-Mode setting, the engine speed would be set to more closely match the speed that is required for the pump to produce the commanded flow with little or no additional engine speed margin. The change in modes may also be handled automatically with an adaptive control that monitor s joystick inputs and determines how aggressively the machine is being operated. When the operator inputs are smooth with relatively slow changes in pump flow, the system would operate at maximum economy. When operator inputs are jerky and rapidly changing, the system would operate at maximum responsiveness. WFC provides the operator the ability to operate anywhere between maximum responsiveness and maximum efficiency based on the requirements of the work being completed. TEST RESULTS To demonstrate the benefits offered by WFC, a backhoe loader was equipped with an electro-hydraulic system and WFC software. A professional operator used this backhoe loader to excavate a series of trenches across a flat open field. Excavation with the backhoe portion of the machine was the focus of our testing because the machine under test was equipped with a torque converter transmission. This precludes the use of WFC while using the loader and propelling the machine. As mentioned in the introduction, hydrostatic propel is required to enable intelligent engine speed control while propelling the machine. The total length of excavation was 14 m with an average depth of 1.6 m and was dug over 2.7 hours. The bucket was.46 m wide and the soil conditions were primarily compacted sandy loam. The excavated material was piled at the edge of the trench. This test scenario was designed such that the machine would be operated at maximum digging capacity and machine utilization. The machine used in this test weighs 7, kg and has a 4.4 m dig depth. The engine is a 4.4L turbocharged Tier 3 engine that is rated at 68 kw and was equipped with an electro-proportional engine speed control actuator. The hydraulic system consists of a Sauer-Danfoss 75cc S45 open circuit pump with a pressure compensated / load sensing (PC/LS) control and a PVG1 flowsharing load independent valve stack with PVED-CC electro-hydraulic actuators that transmit information and receive commands from the vehicle CAN bus. The valves are controlled with Sauer-Danfoss JS6 joysticks and a PLUS+1 MC5-1 microcontroller. The software in the microcontroller was configured to offer two personalities. The first personality was a baseline personality that resulted in conventional machine operation. The second personality enabled WFC which was tuned to Maximum Eco-Mode. By equipping the machine with a selectable personality, the productivity and fuel efficiency of each personality could be compared. The machine was also extensively instrumented to record hydraulic pressure and flow values, engine speed, cylinder position, joystick and valve commands, and engine fuel consumption flow rate. The cylinder pressure and position measurements allowed the work (energy) exerted by the cylinders to be calculated. The fuel consumption was normalized to the cylinder work to eliminate the effect of variable soil conditions on the test results. ENGINE DOWNSIZING The engine and hydraulic pump on this machine are sized in the conventional way where the corner power of the hydraulic pump matches the maximum engine power. To demonstrate the potential for engine downsizing on a machine with a conventionally sized engine, Figure 2 is a quantile plot that shows the pump power requirements during the trenching test with the baseline personality. The y-axis is obtained by sorting the time history of required pump power and the x-axis is a normalized timescale that represents the percentile of a particular operating power. Pump Power Requirement (kw) Base Engine Size Reduced Engine Size 68 kw Anti-Stall Active 56 kw Normalized Time (%) Figure 2: Pump Power Requirement While Trenching Figure 2 clearly shows that full engine power is rarely utilized, even during a test cycle designed to operate the machine at maximum capacity. If the engine power were downsized 18% to 56 kw, the small triangular area shaded in the figure would no longer be an achievable operating condition. If we compare the shaded area (an indication of productivity) to the total area under the

5 curve, we find that productivity would only be reduced 1.3% with an 18% reduction in engine power. A downsized engine will eliminate the ability to operate in the shaded area, but it will not prevent the operator from attempting to operate the machine in that area. If the operator attempts to operate the machine in this area without anti-stall, there is a potential for engine stall. Since operators view a machine that frequently stalls very negatively, engine stall must be prevented if engine downsizing is to be feasible. If the engine was downsized to 56 kw, Figure 2 shows that the engine would stall during approximately 1% of the work cycle without anti-stall enabled. The anti-stall function offered by WFC makes downsizing feasible by sensing attempted operation in the restricted area and reducing the output of the hydraulic system to stay within a smaller engine s capabilities. PUMP SIZING The post-compensated PVG 1 valves on the tested machine provide flow sharing hydraulically. By examining the operator input and comparing it to the actual pump output; we can see how the flow sharing valves enabled a small pump to operate at a productivity level near that of a larger pump. On this machine, up to four functions may be simultaneously operated while digging with the backhoe. If all four functions were simultaneously given a full flow command, the total flow requirement would be 5 lpm. The maximum pump flow, however, is only 17 lpm at full engine speed and maximum pump displacement. Figure 3 is a quantile plot of the joystick input command while trenching in the baseline machine configuration. The y-axis is the operator requested flow based on the joystick input. This plot shows that simultaneous full flow commands to all functions would not be expected to occur during normal digging activities. 6 Figure 4 is a quantile plot of the actual pump output. The y-axis is the actual pump output flow. This curve has a distinctly different characteristic than Figure 3. The pump output curve is relatively flat near maximum output approximately 5% of the time. Flow (lpm) Available Demand Flow Sharing Active Normalized Time (%) Figure 4: Pump Output Flow While Trenching The flat characteristic in the curve near maximum pump output indicates that the operator is requesting more flow than the pump is able to provide. With the flow sharing valve, however, the impacts to productivity are minimized because the available flow is proportionally distributed to the active functions. EFFICIENT ENGINE OPERATION The test scenario used here contained virtually no idle time. There are, however, many work cycles with a backhoe that do require frequent idle time, including when a work task requires waiting for other machine on the jobsite or when interacting with manual labor to accomplish a task. Figure 5 below demonstrates the significant fuel savings that is possible by reducing engine speed to low idle during these times. Joystick Commanded Flow (lpm) Flow at Maximum Simultaneous Command Maximum Pump Output Normalized Time (%) Fuel Consumption (l/hr) Baseline Personality WFC Personality Hi Idle Lo Idle Working Fuel Consumption No Load Fuel Consumption Figure 3: Joystick Command While Trenching Figure 5: Working and Idling Fuel Consumption

6 Even when the machine is operating near maximum capacity as in our testing, there are times when the engine speed can be reduced with intelligent engine speed control to improve fuel efficiency. Figure 6 shows one dig cycle with the baseline personality. The actual engine speed is plotted along with the minimum calculated engine speed that would be required to provide the commanded flow at maximum pump displacement. With the baseline personality, the operator used a manual engine speed lever to set the engine at maximum speed during the work cycle. During this work cycle, it is common for the flow requirement to be reduced while digging and briefly when swinging into and out of the trench. While digging, the machine is typically at maximum break-out force and the pump pressure compensator (set at 25 bar) is reducing the output flow to maintain the maximum pressure setting. While swinging into and out of the trench, there is a period when only the swing is active because the bucket must be maintained above ground level. This is one of the few times that multiple functions are not operating simultaneously. Engine Speed, rpm / 1 Pump Pressure, bar Return to Dig Dig Actual Speed Min Req d Speed Dump Figure 6: Dig Cycle, Baseline Personality Based on the results in Figure 6, it is possible to estimate how much fuel savings would be provided by dynamically adjusting engine speed during the work cycle. At 7.2 seconds with the baseline personality (noted with an arrow in Figure 6), the minimum required speed is only 131 rpm, however, the actual engine speed is 23 rpm. At this condition the pump requires 4 kw to meet the pressure and flow requirements. Figure 7 demonstrates that reducing the engine speed while maintaining the 4 kw hydraulic output would reduce fuel consumption approximately 16%. At lower power levels the percent savings is even more dramatic. Fuel Consumption (l/hr) % Savings Engine Speed, rpm Figure 7: Fuel Consumption versus Speed 5 kw 4 kw 3 kw 2 kw Figure 7 shows how fuel consumption varies with engine speed at a constant pump power level. When engine speed is reduced at a constant pump power level, the displacement of the pump must increase to maintain the power output. This causes the engine and pump to operate at a point of lower speed and higher torque which is generally more efficient. Figure 8 shows one dig cycle, similar to Figure 6 except with the WFC personality. It is clear from this plot that the intelligent engine speed control matches the actual engine speed to the minimum required engine speed. With the WFC personality, a 19% reduction in fuel consumption was demonstrated during the trenching work cycle. The minimum required engine speed in Figure 6 and Figure 8 is a measure of the total hydraulic flow. When we compare these two figures we see that the minimum required speed (total hydraulic flow) is significantly smoother with the WFC Personality. This means that the rate of change of hydraulic flow with the WFC personality is less than the baseline personality. With the baseline personality, the rate of change of hydraulic flow is limited only by the control response of the hydraulic pump control because the engine is always operated at maximum speed. With the WFC personality,

7 the rate of change of hydraulic flow is also limited by the rate of change in engine speed. Engine Speed, rpm / 1 Pump Pressure, bar Return to Dig Dig Dump Actual Speed Min Req d Speed Figure 8: Dig Cycle, WFC Personality During this testing, the WFC personality was tuned to provide maximum fuel savings. This meant having the actual engine speed follow the minimum require engine speed as closely as possible. The result was a slight reduction in flow response that was noticeable to the operator, but not detrimental to productivity. The advantage of WFC is that it can be adjusted by the operator to provide maximum flow response or maximum efficiency depending on the needs of the work being executed. CONCLUSION This paper proposes new thoughts on how to execute vehicle designs in order to meet the future emissions regulations. The idea of matching the actual work with the hydraulic power is discussed. With the pressure on overall vehicle cost, total cost of ownership, Tier 4 regulations and the availability of processor controlled communication between engine and the hydraulic system; the challenge can be managed with electrohydraulic systems with Work Function Control software. The three functions of the Work Function Control (WFC) software that are presented in this paper allow operator feel, productivity, and fuel efficiency to be maintained or improved, even after a downsized engine is installed in a machine. Due to the key 56 kw engine power threshold within the Tier 4 engine emissions regulations, the engine power rating in many machines will be downsized to this power level. Even for machines which must use an engine with a power rating above this threshold, the anti-stall, flow sharing, and intelligent engine speed control functions of the WFC software offer many benefits. The WFC software and resulting fuel savings of 19% was demonstrated with tests on a backhoe loader machine. ABOUT THE AUTHORS Gary LaFayette is a Senior Engineer in the Advanced Systems Engineering team for Sauer-Danfoss at the Ames, IA location. He has worked at Sauer-Danfoss for 8 years. Gary graduated from Iowa State University with a degree in Mechanical Engineering. He may be contacted at glafayette@sauer-danfoss.com. Boris Laudenbach is an System Application Engineer in the SAE team for Sauer-Danfoss at the Offenbach location in Germany. He has worked at Sauer-Danfoss for 4 years. Boris graduated from the University of Applied Science in Fulda with a degree in Electrical Engineering, Robotics and Controls. He may be contacted at BLaudenbach@sauer-danfoss.com.