| The regulations for reducing greenhouse gases and pollutant emissions and improving vehicle security have greatly accelerated the development of advanced engine technologies in recent years. With the potentials for improving the engine performance, variable valve technology, variable stroke technology, cylinder deactivation technology, auxiliary brake technology, variable combustion mode technology, variable compression ratio technology, variable EGR technology and the others have been investigated widely, and abundant achievements have been obtained. Depended largely on the flexibility of variable valve events, application effects of those technologies have a big discount for the limited flexibility of variable valve actuation systems. The research tendency is to combine those technologies to improve the engine performance, but it is impractical to apply all the actuation systems of those technologies to an engine. In the view of scientific research and application, it is necessary to develop a variable valve actuation system suitable for those technologies mentioned.Based on the strategy of partition optimization of engine performance across the whole drive and brake operational range, a multi-mode engine was presented with four modes, including two stroke drive mode adopted in low speed high torque drive conditions, four stroke drive mode adopted in the other drive conditions, two stroke brake mode adopted in heavy load brake conditions and four stroke brake mode adopted in light load brake conditions. In each mode, based on the current condition, the fuel injection and valve operational processes are adjusted to form the best fuel-air mixture and to achieve efficient and clean combustion. The variable valve actuation system suitable for the multi-mode engine must have the four modes mentioned, and flexible variable valve events with the independence among individual cylinders could be achieved in each mode. Obviously, existing variable valve actuation systems, mostly developed for four stroke engines or two stroke engines in drive mode, could not meet the requirements mentioned.As the pilot research part and important technical support of the multi-mode engine, a compact multi-functional variable valve actuation system (CMVVA) was proposed, including four types:CMVVA-C4, a continuously variable valve actuation system for four-cylinder engines, CMVVA-F4, a fully variable valve actuation system for four-cylinder engines, CMVVA-C6, a continuously variable valve actuation system for six-cylinder engines, and CMVVA-F6, a fully variable valve actuation system for six-cylinder engines. The CMVVA system can operate in four stroke drive mode, four stroke brake mode, two stroke drive mode and two stroke brake mode. The mode switch is designed to switch between the two stroke mode and the four stroke mode. The camshaft phaser is adopted to adjust the valve opening timing and to switch between the drive mode and the brake mode within the fixed stroke mode. Compared with existing master-slave piston type systems, with the help of a distributor, continuously variable valve events with the independence between the adjustion of the valve opening timing and the adjustion of the valve maximum lift and the valve closing timing were achieved with two oil-supply sets and two drain valves for all intake/exhaust valves in the CMVVA-C system, and fully variable valve events with the independence among the valve opening timing, the valve maximum lift and the valve closing timing were achieved with two oil-supply sets, two solenoid valves and one drain valve for all intake/exhaust valves in the CMVVA-F system. The valve operational processes of the individual cylinders could be adjusted independently in all CMVVA system.Simulation models for the CMVVA-C system and for the CMVVA-F system were bulit with AVL Hydsim. The valve opening delay, the valve maximum lift, the valve opening duration and the valve seating velocity were adopted as system characteristic parameters, the effects of the system structure parameters on the system operational process were analyzed to qualitatively investigate the working mechanisms of the systems. The calculation results revealed that the effects of the system structure parameters on system characteristic parameters were complex and changed with the speed in both the CMVVA-C system and the CMVVA-F system. The system structure parameters, which had approximately linear relationships with the system characteristic parameters, and the corresponding effects were quantitatively investigated through correlation analysis, to in-depth investigate the working mechanisms of the systems and to obtain the evidence for system optimization and engine matching. Numerical studies on the flexibility and controllability of the systems were carried out with the optimized simulation models, with the valve opening timing, the valve maximum lift, the valve opening duration and the valve opening time-area value adopted as valve operational parameters. The control parameters of the CMVVA-C system were the oil-supply starting timing and the oil-draining timing, and those of the CMVVA-F system were the oil-supply starting timing, the oil-supply stopping timing, the oil-returning timing and the oil-draining timing. The calculation results of the CMVVA-C system revealed that the valve maximum lift, the valve opening duration and the valve opening time-area value could be adjusted synchronously by controlling the drain valve in full speed range; the valve opening timing could be adjusted synchronously at low speed, and when the speed was increased, it was unchanged, while the valve opening velocity was decreased. Further correlation analysis revealed that except the valve opening timing had approximately linear relationships with the oil-supply starting timing at low speed, and the valve opening duration had approximately linear relationships with the oil-draining timing at low and moderate speed, all the other valve operational parameters had approximately linear relationships with the corresponding control parameters, and the corresponding correlation coefficients were more than 0.9, indicating good controllability of the CMVVA-C system. The calculation results of the CMVVA-F system revealed that the valve opening timing, the valve maximum lift, the valve opening duration and the valve opening time-area value could be adjusted independently by controlling the solenoid valves and the drain valve. Further correlation analysis results revealed that all the valve operational parameters had approximately linear relationships with the corresponding control parameters, and all the correlation coefficients were more than 0.95, indicating good controllability of the CMWA-F system.An improved CMVVA-C4 system was proposed and tested on the experiment rig. Through the comparison between the simulation and experimental results, the simulation model had been verified, the method of improving the calculation accuracy was pointed out, and the valve actuator with a through-hole type piston sleeve should be adopted. Experimental results revealed that synchronously variable valve events could be achieved with the drain valve and further correlation analysis results revealed all the valve operational parameters had approximately linear relationships with the corresponding control parameters, and all the correlation coefficients were more than 0.9, indicating good controllability of the CMWA-C system. |
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