Valve timing systems and assemblies are used in internal combustion engines in order to alter the timing of the opening and closing of the engine's intake and/or exhaust valves. Prior valve timing systems typically include one or more camshafts and one or more cam timing assemblies which selectively advance and/or delay the rotation of the one or more camshafts relative to the crankshaft, thereby controlling the actuation of the intake and exhaust valves.
These prior systems and the valve timing profiles used by these systems suffer from several drawbacks. For example and without limitation, due to their inherent physical limitations, these prior cam-driven systems are ineffective to adjust or compensate for variations in the compression ratios of the various cylinders of a vehicle's engine. Due to manufacturing variability, the compression ratios of the various cylinders of many vehicle engines often vary significantly. The higher the compression ratio of a cylinder, the more susceptible that cylinder is to knock. Hence, many conventional engines have cylinders with varying susceptibility to knock. When one or more cylinders have a greater susceptibility to knock, the calibration of the ignition or "spark" system is typically retarded in order to prevent engine knock and excessive activation of the vehicle's knock control system. Particularly, due to the inability of these prior systems to independently adjust the valve timing of particular cylinders, the spark calibration of engines utilizing these prior cam-driven systems must be undesirably reduced.
Efforts have been made to increase the efficiency of vehicle engines and reduce emissions by eliminating the camshafts and operating the intake and exhaust valves by use of selectively controllable electromagnetic or electrohydraulic actuators. These types of systems eliminate many problems related to the physical characteristics of cam-driven systems and offer additional precision in valve timing. However, these prior systems are typically implemented using conventional valve timing profiles or strategies, and thus, suffer from many of the same previously delineated drawbacks associated with cam-driven systems. For example and without limitation, the valve timing profiles used within these systems and strategies cannot compensate for variation in the compression ratios of the various cylinders of the engine. As a result, the spark calibration of these engines is likewise undesirably reduced to prevent knocking and excessive activation of the engine's knock control system.
There is therefore a need for a new and improved system and method for valve timing for use with a camless engine including several cylinders having varying compression ratios.