Conventional automotive internal combustion engines operate with one or more camshafts controlling the engine valves, i.e., intake and exhaust valves, according to a predetermined lift schedule. With this type of mechanical structure, the lift schedule is fixed. A fixed lift schedule, however, will not allow for optimum engine performance since in general different engine operating conditions require different optimum lift schedules.
The enhancement of engine performance obtainable by varying the timing and lift as well as the acceleration, velocity and travel time of the intake and exhaust valves in an internal combustion engine is generally appreciated in the art. Nonetheless, the technology for providing a straight-forward, relatively inexpensive and highly reliable system has not been forthcoming. Increased use and reliance on microprocessor control systems for automotive vehicles and increased confidence in hydraulic as opposed to mechanical systems is now making substantial progress in engine valvetrain design possible.
There are several reasons why a generally fixed lift schedule is not optimum. Control of gas exchange in a conventional engine with cam driven valves is limited and cannot be optimized for all engine operating conditions. Control of gas exchange, however, in a camless engine is fundamentally different. In an engine with a conventional mechanical valvetrain, with its fixed valve timing, the intake air flow is controlled by air throttling, which results in throttling losses. Further, the amount of residual exhaust gas retained in the cylinder cannot be controlled by a mechanical valvetrain, thereby requiring the addition of recirculated exhaust gas to the intake air by an external exhaust gas recirculation (EGR) system in order to reduce nitrogen oxide emissions.
The latter limitation is also a concern with engines having lost motion engine valve systems connected between camshafts and engine valves, since they are still limited somewhat by the inflexibility of a camshaft. Lost motion control systems can control the amount of lift, but are very limited in controlling the timing of valve opening and closing, thus limiting their ability to control the residual gas content in a cylinder. Further, a camless electrohydraulic system has the advantage of completely eliminating the cost and weight of camshafts while providing increased flexibility in the timing and amount of opening of each engine valve. In general, variation of the timing of engine valve opening and closing is preferred, rather than controlling the lift only, to determine the amount of air that is inducted into a cylinder.
In an engine with an electrohydraulic camless valvetrain, the engine valve events are flexible. The quantities of intake air and residual exhaust gas in each cylinder can be controlled by varying the timing of opening and/or closing for the intake and exhaust valves, which eliminates the need for intake air throttling and an external EGR system. On the other hand, while an electrohydraulic camless valvetrain provides more flexibility to enhance engine performance, there can be drawbacks not encountered with systems employing mechanical camshafts.
For all of the inflexibility and inefficiency associated with a mechanical valvetrain, it has one major advantage: the accuracy with which a camshaft can be ground is such that a reasonably good cylinder-to-cylinder air distribution is inherently assured. In the case of an engine with a camless valvetrain, equal distribution of air and residual gas among cylinders is not inherent. While a lost motion type of system may not have as great of an inherent variation as a camless system, due to the fact that it is still driven by mechanical camshaft, it still has other disadvantages as noted above.
In a camless valvetrain system, instead of an air throttle and an external exhaust gas recirculation system, changes in the timing of control valves can be used to control the amount of air inducted into and the amount of residual gas retained in the combustion chamber. The engine valves can be electrically controlled by these control valves, such as solenoid valves, which respond to electric control signals from an on-board computer. To assure that the actions of the intake and exhaust valves in all cylinders are substantially equal, substantially identical performance of respective control valves in all cylinders must by achieved, which is a challenging task.
A camless valvetrain can accomplish both proper intake air and exhaust gas distribution among cylinders combined with the elimination of exhaust gas recirculation in order to provide a complete engine optimization package. The need, then, arises to ensure that the system can accomplish the optimization continually while operating, as well as correct for any variations that tend to be inherent in this type of system.
The timing and duration of the voltage signals that activate the control valves can be controlled with great accuracy and uniformity. Unfortunately, this does not translate into uniformity of control valve performance. Individual control valves tend to respond differently to identical voltage signals, due to inevitable minor differences in their physical systems. To achieve substantially identical performance by all control valves requires a set of control signals, each individually tailored to the needs of the specific control valve that it controls.
This control is required to assure substantially even distribution of intake air and residual gas from cylinder-to-cylinder due to this inherent control valve-to-control valve variability. In addition, system sensitivity to changing ambient conditions, to gradual deterioration in performance of individual components and in quality of the working fluid can further contribute to deviation from the required performance.
Tightening up manufacturing tolerances and applying post-manufacturing adjustments can reduce, but not totally eliminate, the control-valve-to-control-valve differences. Further, this does not resolve the problem of possible changes in control valve performance and quality of working fluid over time. This inherent variability creates the need for a camless valvetrain system that has an adaptive control system continuously monitoring the results of its performance under various engine operating conditions and which adjusts the system to account for the system tolerances to assure correct and equal distribution of intake air and residual gas among the cylinders at all times.
Thus, a control system is needed that accounts for various engine operating conditions by changing the valve event of each engine valve based on values of required intake air and residual gas quantities placed in a computer memory and which has a feedback loop that monitors the actual air and residual gas quantity independently for each engine cylinder to create a correction memory that corrects for deviation from the required parameters in each cylinder for the various engine operating conditions. This will allow engine optimization for best fuel economy, emissions and torque as well as for best idle quality.