A work machine, such as a construction work machine, an agricultural work machine or a forestry work machine, typically includes a prime mover in the form of an internal combustion (IC) engine. The IC engine may either be in the form of a compression ignition engine (i.e., diesel engine) or a spark ignition engine (i.e., gasoline engine). For most heavy work machines, the prime mover is in the form of a diesel engine having better lugging, poll-down and torque characteristics for associated work operations.
The step load response of an IC engine in transient after a load impact is a feature mostly influenced by the engine displacement, the hardware of the engine (e.g., whether it has a standard turbocharger, a turbocharger with waste gate or variable geometry, etc.), and by the software strategy for driving the air and fuel actuators (e.g., exhaust gas recirculation, turbocharger with variable geometry turbine (VGT), fuel injector configuration, etc.) with respect to the requirements of emissions legislation (e.g., visible smoke, nitrous oxides (NOx), etc.), noise or vibrations. The load impact may be the result of a drivetrain load (e.g., an implement towed behind the work machine) or an external load (e.g., an auxiliary hydraulic load such as a front end loader, backhoe attachment, etc.).
Engine systems as a whole react in a linear manner during the application of a transient load. Initially, the load is applied to the drive shaft of the IC engine. The IC engine speed decreases when the load increases. The engine speed drop is influenced by whether the governor is isochronous or has a speed droop. The airflow is increased to provide additional air to the IC engine by modifying the air actuators. A time delay is necessary to achieve the new air flow set point. The fuel injection quantity, which is nearly immediate, is increased with respect to both the smoke limit and maximum allowable fuel quantity. The engine then recovers to the engine speed set point. The parameters associated with an engine step load response in transient after a load impact are the speed drop and the time to recover to the engine set point.
An IC engine may be coupled with an IVT which provides continuous variable output speed from 0 to maximum in a stepless fashion. An IVT typically includes hydrostatic and mechanical gearing components. The hydrostatic components convert rotating shaft power to hydraulic flow and vice versa. The power flow through an IVT can be through the hydrostatic components only, through the mechanical components only, or through a combination of both depending on the design and output speed.
One example of an IVT for use in a work machine is a hydromechanical transmission which includes a hydraulic module coupled with a planetary gear set. Another example of an IVT for a work machine is a hydrostatic transmission which includes a hydraulic module coupled with a gear set.
A work machine including an IVT may be prone to loss of traction control and wheel slip when the IVT ratio changes to match load conditions. The IVT controller senses engine speed and deepens the IVT ratio as engine speed decreases under load. When at low ground speeds, the amount of power required for the work machine is a low percentage of what the engine can generate, so the engine may not lug down when the output torque from the engine increases. The operator will then not be aware that the torque at the wheels is increasing. In this case, the drive wheels can lose traction and spin out without notice.
In many construction or agriculture machinery applications it is desirable to limit or eliminate wheel spin (tractive effort) while the machine is under load so as not to disturb the surface upon which the machine is working. Current four-wheel-drive (4WD) front end loaders manufactured by the assignee of the present invention, such as the model 644J, 724J, and 824J, contain torque converter-driven powershift transmissions. Torque converter-driven machines limit tractive effort by naturally providing torque input control to the transmission via the speed differential across the torque converter. This speed differential is a function of vehicle ground speed and engine speed. As the ground speed approaches zero, the converter output torque approaches the stall torque for the present converter input speed (engine speed). The stall torque therefore is proportional to engine speed. The operator controls engine speed via a foot throttle pedal, and thus controls torque to the transmission and therefore controls machine tractive effort. Tractive effort control is especially important in a 4WD loader application when the machine is loading the bucket. The operator wants the machine to “push” the pile with a consistent force without spinning the tires in order to fill the bucket completely.
One problem with this configuration has to do with the very features that make it desirable for controlling tractive effort. Consider the 4WD loader bucket loading case: As the machine digs into the pile, the load increases significantly. The torque converter can only supply a finite amount of torque for the given engine and ground speeds, and so the ground speed decreases as the load from the pile overcomes the converter torque capacity. As the ground speed decreases, the speed differential across the torque converter increases. The operator may then wish to increase tractive effort, and thus increases engine speed to increase torque. This cycle can continue until the machine either cannot provide enough tractive effort to dig further into the pile, or the tires spin out. In both cases, there is significant speed differential across the torque converter, and the converter is providing a significant torque to the transmission at very low speeds. Thus, the problem presents as a significant power loss across the torque converter, with the energy being dissipated as heat into the converter fluid. With power loss comes added fuel burn, which translates into reduced fuel efficiency which gets worse with increased converter speed differential and torque.
One attempted solution to address the fuel efficiency problem has been to place a hydrostatically driven transmission into a 4WD loader. The hydrostatic transmission offers the benefits of increased efficiency in the above case by providing near infinite speed ratios which allow the transmission output speed to be controlled to near zero at any engine speed, with no “slipping” across any elements. Thus the fuel efficiency improves. However, as before the problem with this configuration results from its very features. As the transmission input-to-output speed ratio gets very large (near zero output speeds), the transmission output torque rises proportionally. Thus, the operator has no limit controls over the tractive effort. For the bucket loading case above, the machine simply spins the tires during loading, with no way to eliminate it.
What is needed in the art is a work machine configured with an IVT which is not prone to losing traction in low ground speed conditions.