IFPE Paper 8.1 Engine Overspeed Protection for Tier 4 Machines with Hydrostatic Transmissions Simon L. Nielsen Danfoss Power Solutions Frank J. Rozycki Danfoss Power Solutions This study evaluates ISL as an engine overspeed protection solution based on the technical criteria that are often of highest importance to an original equipment manufacturer (OEM). This study also presents the recent application of ISL to an agricultural sprayer as a case study for braking performance testing. INTRODUCTION Figure 1 Combine harvester ( Blue Graphics Concept Danfoss) ABSTRACT The progression of diesel engine emissions standards to Tier 4/Stage IV levels has impacted the on- and off-road vehicle industry in numerous ways. One impact that has been observed in some off-road vehicle applications as they become compliant is reduced engine drag torque due to engine downsizing and efficiency improvements. Reduced engine drag torque availability may limit the braking performance of a vehicle unless means are taken to support the drivetrain braking torque required; loading an engine above the drag torque available runs the risk of overspeeding and damaging the engine and any driven components. For applications with hydrostatic transmissions, a number of technical strategies exist to limit the drivetrain torque applied to the engine and prevent overspeed during braking. One strategy is to dissipate some vehicle energy as heat via pressure reduction of the system flow to prevent the pump s kit pressure from exceeding a level that would overspeed the engine. The method known as Integrated Speed Limitation (ISL) consists of a piloted pressure reducing valve and a bypass orifice which are both sized to an application and function automatically during heavy braking. VEHICLE BRAKING Braking occurs when there is a net force acting against a vehicle s direction of motion. This force is typically generated with tires or tracks via dedicated service brakes, the drivetrain, or a combination of these two sources. Engines are often only considered to be a means for converting chemical energy into mechanical energy for vehicle functions; however, they also serve a role in braking. The engine supports the braking torque generated by the drivetrain. The braking capability of an engine is referred to as its drag torque, also referred to as friction torque or braking torque. Engine drag torque is the torque necessary to turn an engine without combustion and is primarily composed of pumping and friction losses 1). An engine s drag torque increases approximately linearly with speed in its range of operation and generally increases with displacement. An engine s drag torque is typically less than one-third of its rated torque. Drag torque acts on the crankshaft against the engine s direction of rotation and must be overcome with combustion whenever the engine is providing a net torque output. If, during braking, the torque input to the engine exceeds the total drag torque of the engine and any driven accessories at some maximum allowable speed, then the engine is considered to be overspeeding. Engines and most driven accessories are typically rated to some maximum speed, above which they are more likely to suffer damage or catastrophic failure. THE IMPACT OF TIER IV Since the introduction of regulations by the Environmental Protection Agency (EPA) 2) and European Commission 3) that mandate a significant reduction of nitrogen oxides, carbon monoxide, hydrocarbons, and particulate matter emissions from off-road diesel engines, original equipment manufacturers (OEMs), engine
manufacturers, and subsystem suppliers have invested significant effort to ensure compliance. In order to meet these new standards, engine manufacturers have introduced engine technologies, such as common rail injection and variable-geometry turbochargers, that enable higher power densities to be achieved 4). As discussed by Bennink 5) as well, increases in engine power density and improvements in system efficiency have enabled some OEMs to downsize engines for greater overall efficiency and/or more appropriate matching of load. Downsized, or right-sized, engines will tend to benefit the overall efficiency of a vehicle and can be expected to have a lower mass and purchase cost, but there will likely be less drag torque available when the engine is motored during braking. Some applications are finding that they now need an engine overspeed protection strategy; others are finding that their existing need for overspeed protection is increased and more challenging to meet than before. HYDROSTATICS In many off-road vehicle applications with hydrostatic transmissions, it is not as common to utilize the equipped service brakes for speed management, or there may not be any service brakes installed at all. Example applications include harvesters (Figure 1) and agricultural sprayers (Figure 2). In these cases, it is necessary for the engine and drivetrain to be able to provide all of the torque necessary to brake the vehicle. Figure 2 - Agricultural sprayer. ( Blue Graphics Concept Danfoss) Furthermore, off-road vehicles are becoming larger, heavier, and faster than ever before. These factors, combined with standards 6) and legislation 7) that impose rigorous requirements for vehicle braking, make the challenge of braking a vehicle even more difficult. RESEARCH OBJECTIVES Given the challenges posed by the braking of off-road vehicles with hydrostatic transmissions and the impact of current emissions regulations, one objective of this study is to outline the technical solutions that exist for engine overspeed protection for these applications. In addition, this study evaluates the technical feasibility of Integrated Speed Limitation (ISL), a technology used in hydrostatic pumps. ENGINE OVERSPEED PROTECTION TECHNOLOGY A review of global patent files reveals numerous inventions for braking heavy vehicles and for preventing engine overspeed, dating back to the early days of the automobile. Solutions typically function by increasing the effective drag torque capability of the engine and/or controlling the torque input to the engine by the transmission. Today s solutions are increasingly integrated and sophisticated. This section outlines the technologies relevant to modern off-road vehicles with hydrostatic transmissions. Engine Solutions Most engine-based solutions for engine overspeed protection function by increasing an engine s effective drag torque when necessary. There are several key engine braking technologies 8). The compression-release brake functions during braking by opening an engine s exhaust valves near the top of their respective piston s compression stroke, releasing energy into the exhaust system instead of back to the piston on the subsequent power stroke. Other engine brakes throttle the engine s exhaust flow to increase drag due to pumping losses. Another method to increase drag due to pumping losses is to control a variable-geometry turbocharger so as to increase the backpressure on the engine. Hydraulic Solutions Most hydraulic solutions for engine overspeed protection can be grouped into one of the following categories: Dissipative Pressure Reduction The principal of engine overspeed protection via dissipative pressure reduction is that kinetic and potential energy of a vehicle is converted into heat during braking. Hydraulic retarding systems 9),10) have been developed in order to increase the effective braking capability of an engine. In these cases, an auxiliary hydraulic pump is attached to the engine and pumps flow through some type of restriction on command in order to dissipate energy when needed during braking. Other technology uses the closed-circuit, hydrostatic system itself for engine overspeed protection. These solutions 11),12),13) limit the torque generated at the pump kit and input to the engine by porting system flow over a restriction and generally enable maximum hydrostatic braking to be achieved at the wheel motors. Pressure Limiting via Ratio Control Ratio control (generally motor displacement control) limits the hydrostatic braking of a vehicle to the capability of the engine. If, during vehicle braking, the braking capability of the engine is exceeded, the displacement of a motor is reduced. This reduces the system flow rate and prevents the system pressure from increasing to a value that causes overspeed. Kinetic Energy Recovery Systems Instead of converting vehicle energy into heat, it can be stored during braking and added back into the drivetrain when needed; see, for example, reference 14). These systems typically utilize an accumulator for hydraulic energy
storage. This concept can enhance the hydrostatic braking capability of a system and prevent engine overspeed while energy is being stored but requires additional means for protection once the energy storage device has reached its capacity. Other Solutions Other solutions for engine overspeed protection generally function by increasing the power consumption of some type of engine-driven accessory during braking. Methods 15) have been developed to activate compressors, fan drives, and other accessory loads when the risk of engine and/or pump overspeed is present. These methods act to increase the effective drag torque of an engine. transmission system. It is a hydraulic solution to provide engine overspeed protection that reduces the hydrostatic system pressure to the braking capability of the engine without reducing vehicle braking capability, as described above in Hydraulic Solutions. This portion of the study explains this technology in more detail and then evaluates it according to the Evaluation Criteria presented above. DESCRIPTION Integrated Speed Limitation consists of a pilot-operated, pressure-reducing valve and a bypass orifice and can be integrated into a closed-circuit hydrostatic pump as shown in Figure 3. (A full-page version of this schematic is provided in the Appendix.) EVALUATION CRITERIA In evaluating the technical feasibility of any engine overspeed protection solution, the following criteria are often of high importance to OEMs: Method of Activation and Transient Performance Is the activation of the overspeed protection solution automatic, or does it require operator activation? Does the solution function proactively, or does it only respond to overspeed once it is detected? Does the solution provide responsive vehicle braking once initiated, or does it cause vehicle control to feel delayed or lagged? Can the solution function over the duration of a braking event? Does the solution function without affecting the desired control response of the vehicle? Ability to Maximize Vehicle Braking Does the solution enable braking performance requirements (e.g., stopping distance and/or deceleration rate) to be achieved? Can the solution utilize the maximum pressure capability of the hydrostatic system at any vehicle speed, or is the performance limited? Utilization of the Available Drag Torque Does the solution utilize the braking capability of the engine or is excessive energy converted into heat? Suitability for Different Drivetrain or Control Architectures Can the solution be applied independently of a vehicle s drivetrain, control, or software implementation, or does it have specific requirements that must be met? Ease of Application Can the solution be configured, installed, tested, and validated clearly and with minimal resource investment, or are repeated iterations of tuning and testing required to meet performance requirements? Packaging/Installation Can the solution be applied with little to no impact on the physical installation for the vehicle, or does it have significant physical requirements in order to be implemented? INTEGRATED SPEED LIMITATION (ISL) Figure 3 - Schematic of a closed-circuit, hydrostatic pump with ISL. The shaded/labeled components make up the ISL assembly. THEORY OF OPERATION During a forward braking, or motoring, event, the B port of the pump will be at a high pressure, and flow will enter the pump s B port from the motor(s). (Figure 4 shows a zoomed-in view of Figure 3 and includes colored labels showing the flow directions and relative pressures in the circuit.) System flow passes through the pressure reducing valve, which is normally open unless the system pressure exceeds the pilot valve pressure setting. Once this pressure setting is exceeded, flow passes through the pilot valve, and the pressure reducing valve closes such that the system flow is automatically throttled to regulate the kit pressure (as measured at the M13 gauge port). At lower system flow rates, the pressure reducing valve becomes completely closed, and all system flow is throttled by the bypass orifice. The system pressure is reduced according to the system flow rate over the orifice. Therefore, the pressure reduction through the bypass orifice decreases as the system flow rate decreases, and the pressure at the pump kit increases as the pump approaches neutral. Integrated Speed Limitation (ISL) 16) is an engine overspeed protection technology utilized in high power closed-circuit pumps that are part of a hydrostatic
the high system flow rates that can occur during braking. Therefore, ISL enables maximum hydrostatic braking to be achieved at the wheels without the risk of overspeeding the engine. Utilization of the Available Drag Torque The performance of ISL is configured with two settings the pressure setting of the pilot valve for the pressure reducing valve and the area of the bypass orifice. The settings are made to maximize the use of the engine s inherent braking capability without causing overspeed. Figure 4 - Pump pressures and flow directions labeled on a zoomed-in view of a pump schematic when ISL is active. Red = high (braking) pressure, green = reduced (kit) pressure, blue = low (charge) pressure. If, during this braking event, the B-side pressure limiter valve setting is exceeded, the pressure limiter valve acts to port flow to the low-pressure side of the servo piston (as measured at the M5 gauge port), causing the swashplate angle to increase toward maximum forward displacement. This allows the pump kit to accept higher levels of flow. In all cases, the purpose of ISL is to limit the torque generated at the pump and input to the engine. The pilot valve pressure setting and bypass orifice sizing are the variables that govern how much braking power is converted to heat and effectively diverted away from the engine. Note that when the pump is in pumping mode and flow is exiting the B port (i.e., the vehicle is being driven in the reverse direction), flow passes through an integrated bypass (check) valve in order for ISL to not be active. EVALUATION The following is a qualitative evaluation of Integrated Speed Limitation according to the Evaluation Criteria presented above. Suitability for Different Drivetrain or Control Architectures ISL has no specific drivetrain, control, or software requirements. Being purely hydromechanical in nature, ISL is equally applicable for systems using mechanically-, hydraulically-, or electrically-controlled pumps and motors. ISL is also equally applicable for systems using fixed- or variable-displacement motors. These features make ISL particularly useful for retrofitting applications faced with lower engine drag torques due to Tier 4 requirements because they do not need to redesign their transmission or control system. Ease of Application ISL is configured based on the total drag torque available at the maximum allowable engine speed during braking. The drag torque of an engine is commonly available from its manufacturer as a specification. After configuring the pilot valve pressure setting and the bypass orifice area, ISL is installed and tested on an application to confirm that it meets performance requirements. The configuration may be adjusted during testing by installing a pilot valve with a different pressure setting and/or installing a bypass orifice with a different area. The suitable ISL configuration can usually be confirmed in one vehicle testing session, freeing up valuable testing and engineering resources and minimizing the impact on the customer in delivering their product to market on time. Packaging/Installation As shown in Figure 5, ISL is integrated into a hydrostatic pump and utilizes little to no additional space compared to a standard pump without ISL installed. No changes in the system line connections are required. Method of Activation and Transient Performance ISL s overspeed protection functionality is active as soon as the braking pressure of the hydrostatic system exceeds the pilot valve pressure setting, which is sized to not exceed the braking capability of the engine. The activation of ISL is automatic and purely hydromechanical in nature. The normal transient performance of the hydrostatic system is not impacted with ISL installed. Ability to Maximize Vehicle Braking The strategy of ISL is to allow for high system pressure during braking but to reduce the pressure at the pump kit in order to not overspeed the engine. ISL is also designed to handle
setting and a bypass orifice with an equivalent diameter of 5.5 mm were selected for the single pump with 165 cc displacement in order to keep the engine/pump speed below 3100 rpm, the maximum speed rating of the pump. The selection was determined by using a performance prediction tool that estimates engine overspeed during a braking event. Validation The configuration was validated with a series of vehicle braking tests that subjected the pump to sustained, high, motoring pressures. The plots in Figure 6 show the results of one high-speed stopping test at loaded mass on a flat surface. All of the braking was performed with the hydrostatic propel system; the service brakes were not used. Figure 5 - Closed-circuit, hydrostatic pump with ISL. The red components are the pressure reducing assembly and the pilot valve for the ISL function. CASE STUDY Integrated Speed Limitation was applied to a large agricultural sprayer (Figure 2) requiring engine overspeed protection. This sprayer was equipped with a hydrostatic transmission composed of one pump and four motors. Table 1 summarizes the key specifications of the machine. Although the sprayer s installed engine was manufactured under the regulatory standards of EPA Tier 1, its drag torque capability is representative of values typically seen in today s sprayer market. Table 1 Key specifications of agricultural sprayer used in case study for application of ISL. Specification Value Units Vehicle mass (loaded) 14500 kg Engine displacement 8.3 L High idle engine speed 2200 rpm Engine drag torque at high idle 180 Nm Maximum engine speed 3750 rpm Pump drive ratio 1:1 [ ] Propel pump displacement 165 cc/rev Maximum pump speed 3100 rpm Maximum system pressure 480 bar Motor displacement (each) 80 cc/rev Final drive ratios 22:1 [ ] Wheel radius 0.85 m Maximum vehicle speed 65 km/h ISL Configuration After considering the combined drag torque of the engine, accessory pumps, and fan drive and other parasitic losses, a 120 bar pilot valve pressure Figure 6 High-speed stopping test of the loaded sprayer on a flat surface. At the time of 0.6 seconds, the vehicle speed was commanded from approximately 58 km/h to zero. As shown in the plots, a system pressure of approximately 480 bar was sustained during the stopping event. However, ISL functioned automatically to limit the pressure at the pump kit and, therefore, limit the torque input to the engine. At higher vehicle speeds, Kit Pressure B was limited to a nearly constant value by the pilot-operated pressure reducing valve; at lower speeds, this pressure was regulated by the bypass orifice and approached Loop Pressure B as the speed decreased. ISL is shown to keep the engine/pump speed below 3100 rpm. The stop was completed in 5.6 seconds, and the stopping distance was approximately 50 meters. These test results validate the ISL configuration selected with the performance prediction tool and show that the engine and pump are protected from overspeed while meeting vehicle performance requirements.
CONCLUSION Engine overspeed protection has been and will likely continue to become a more significant topic as vehicle braking performance requirements increase and as offroad vehicle OEMs design applications with less available engine drag torque. Integrated Speed Limitation (ISL) is an engine overspeed protection technology available for applications with hydrostatic transmissions that meets the technical requirements of primary concern to OEMs as they ensure compliance with EPA Tier 4 Final regulations and look toward future challenges. REFERENCES 1) Stone, R., Introduction to Internal Combustion Engines, Macmillan Publishers Ltd., 1985, pp 232-233. 2) EPA, Nonroad Compression-Ignition Engines Exhaust Emission Standards, Emission Standards Reference Guide, <www.epa.gov/otaq/standards/nonroad/nonroadci.ht m>, accessed 7 Nov, 2013. 3) European Commission, Emissions from non-road mobile machinery, Enterprise and Industry, <ec.europa.eu/enterprise/sectors/mechanical/nonroad-mobile-machinery>, 5 Aug 2013. 4) Korane, K.J., Off-highway diesel engines meet Tier 4 emissions regulations, Machine Design, 25 Aug 2011. 5) Bennink, C., The trend toward higher power densities, OEM Off-Highway, Sep 2013, pp 26-29. 6) ASABE, Braking System Test Procedures and Braking Performance Criteria for Agricultural Field Equipment, ANSI/ASAE S365.9 NOV 2011, American Society of Agricultural and Biological Engineers, Nov 2011. 7) European Commission, Council Directive 76/432/EEC of 6 April 1976 on the approximation of the laws of the Member States relating to the braking devices of wheeled agricultural or forestry tractors, Official Journal of the European Communities, No L 122/2, 8.5.76. 8) Tennant, H., Engines That Slow as Well as Go, Cummins Turbo Technologies, HTi edition 10, 2008, pp 11-12. 9) Crull S.W. and MacIntosh, D.J., Overspeed protection control for an engine, US Patent 3999386 A, 28 Dec 1976. 10) Taylor, L. and Göllner, W., Hydrostatically driven vehicle with retarder valve, US Patent 6202783 B1, 20 Mar 2001. 11) Drin, B., System for controlling a hydraulic vehicle drive, US Patent 6338247 B1, 15 Jan 2002. 12) Widemann, A., System for controlling a hydraulic vehicle drive, US Patent 6360537 B1, 26 Mar 2002. 13) Thoms, R., Valve arrangement in a hydraulic circuit, use of the same and arrangement for controlling a hydraulic vehicle drive, US Patent 7201183 B2, 10 Apr 2007. 14) Shore, D.B. and Dix, P.J., Energy recovery system for a work vehicle including hydraulic drive circuit and method of recovering energy, US Patent 6971463 B2, 6 Dec 2005. 15) Letang, D.M., Babcock, D.J., Weisman, II, S.M., Method for engine control, US Patent 6330873 B1, 18 Dec 2001. 16) Danfoss Power Solutions. ISL-Integrated Speed Limitation. Technical Information. 11053026 Rev AD, Jan 2013. CONTACT Simon L. Nielsen is a Systems Engineer on the Propel Systems Development team in the Hydrostatics division of Danfoss Power Solutions in Ames, Iowa. He has worked for Danfoss for the last three years in this position and previously for a year as an engineering intern in both Ames and Nordborg, Denmark. His areas of focus include system modeling, controls, software, and vehicle testing in the development of propel subsystems for off-road vehicles. Simon earned a BS degree in Mechanical Engineering in 2009 and a MS degree in Agricultural Engineering in 2011, both from Iowa State University. He may be contacted at: SLNielsen@danfoss.com. Frank J. Rozycki is a Systems Engineer on the Propel Systems Development team in the Hydrostatics division of Danfoss Power Solutions in Ames, Iowa. He has worked for Danfoss for seven years. His areas of focus include system modeling, controls, software, and vehicle testing in the development of propel subsystems for offroad vehicles. Frank earned a BS degree in Mechanical Engineering in 2006 from Iowa State University. He may be contacted at: FRozycki@danfoss.com. DEFINITIONS, ACRONYMS, ABBREVIATIONS EPA: Environmental Protection Agency ISL: Integrated Speed Limitation OEM: Original Equipment Manufacturer
APPENDIX Schematic of a closed-circuit, hydrostatic pump with ISL. The shaded/labeled components make up the ISL assembly.