The operation of an internal combustion engine, such as, for example, a diesel, gasoline, or natural gas engine, may cause the generation of undesirable emissions. These emissions, which may include particulates and oxides of nitrogen (NOx), are generated when fuel is combusted in a combustion chamber of the engine. An exhaust stroke of an engine piston forces exhaust gas, which may include these emissions, from the engine. If no emission reduction measures are in place, these undesirable emissions will eventually be exhausted to the environment.
Efforts are being made to decrease the amount of undesirable emissions that are exhausted to the environment during the operation of an engine. It is expected that improved engine design and improved control over engine operation may lead to a reduction in the generation of undesirable emissions. Many different approaches, such as, for example, exhaust gas recirculation, water injection, fuel injection timing, and fuel formulations, have been found to reduce the amount of emissions generated during the operation of an engine. After treatments, such as, for example, traps and catalysts have been found to effectively remove emissions from an exhaust flow. Unfortunately, the implementation of these emission reduction approaches typically results in a decrease in the overall efficiency of the engine.
Additional efforts are being focused on improving engine efficiency to compensate for the efficiency loss due to the emission reduction systems. One such approach to improving engine efficiency involves selectively adjusting the actuation timing of the engine valves. For example, the actuation timing of the intake and exhaust valves may be modified to implement a variation on the typical diesel or Otto cycle known as the Miller cycle. In a “late intake” type Miller cycle, the intake valves of the engine are held open during a portion of the compression stroke of the piston.
An engine may be equipped with a variable valve actuation system that allows the actuation timing of the engine valves to be selectively varied to meet the current operating conditions of the engine. As described in U.S. Pat. No. 6,237,551 to Macor et al., issued on May 29, 2001, a variable valve actuation system may be incorporated with a conventional cam-driven valve actuation system. In a conventional cam-driven valve actuation system, the engine valves are driven by a cam arrangement that is operatively connected to the crankshaft of the engine. The rotation of the crankshaft results in a corresponding rotation of a cam that drives one or more cam followers. The movement of the cam followers results in the actuation of the engine valves. The shape of the cam governs the timing and duration of the valve actuation.
As also described in U.S. Pat. No. 6,237,551 to Macor et al., a variable valve actuator may be disposed between the cam arrangement and the engine valve. In the described system, the variable valve actuator includes a chamber in which fluid may be sealed to establish a hydraulic link between the cam and the engine valve. When the hydraulic link is established, all of the valve motion provided by the shape of the cam is transferred to the engine valve to actuate the engine valve. To vary the actuation timing of the engine valve, a control valve may be opened to allow fluid to flow from the chamber. The release of the fluid breaks the hydraulic link between the cam and the engine valve, and the engine valve is allowed to close, independently of the shape of the cam. In this manner, a variable valve actuator may be used to selectively vary the actuation timing of an engine valve.
To achieve the greatest benefits from selectively implementing variations on valve actuation timing, the variable valve actuation system should precisely control the time at which the engine valves are opened and closed to meet the particular operating conditions of the engine. However, the operation of a variable valve actuation system, such as the system described above, may depend upon the properties of the fluid used to operate the valve actuator. Some fluid properties, such as, for example, the fluid viscosity, may change with the operating conditions of the engine and thereby change the operation of the valve actuator. For example, the valve actuator may have a shorter response time when the operating fluid has a low viscosity and a longer response time when the operating fluid has a higher viscosity. If the variable valve actuation system does not account for these types of changes in fluid properties, the time at which the valve actuator opens or closes the valve may not match the operating conditions of the engine. Accordingly, the operation of the variable valve actuation system may result in marginal gains in engine efficiency.
The system and method of the present disclosure solves one or more of the problems set forth above.