U Waterloo team develops novel variable valve actuation system; 58% less energy use than cam-driven valvetrain
A team at the University of Waterloo has developed a novel variable valve actuation (VVA) system, optimized to pursue minimum energy consumption and minimum sensitivity of the valve lift to cycle-to-cycle variations of engine pressure. The product of a decade of research, this patented system could improve fuel efficiency by more than 10%, said Amir Khajepour, a professor of mechanical and mechatronics engineering at Waterloo.
The new VVA system, described in a paper in the journal Mechatronics, uses an energy recovery technique to remove the trade-off that exists between the system’s power consumption and sensitivity. The results show that the optimized system has a low variability of about 5% (0.5 mm) to cycle-to-cycle variations of 50% in the in-cylinder gas force. The optimized VVA with the proposed energy recovery system (ERS) also consumes about 58% of the energy used in a conventional cam-driven valvetrain.
In general, VVA systems are either cam-based or camless systems. The former is directly driven by the engine crankshaft. Due to its high reliability, accuracy, and robustness, many of these systems have been already applied to vehicle engines. Among different cam-based VVA systems, cam-phasers and cam-profile–switchers (CPS) are two well-known technologies. A limited degree of flexibility is still a major disadvantage of cam-based valvetrains compared to the existing camless systems.
In contrast to cam-based VVA, camless ones are completely disconnected from the engine crankshaft. High levels of flexibility in valve timing and valve lift are the main advantage of these systems over others. Electro-hydraulic, electro-mechanical, electro-pneumatic, and electro-magnetic valvetrains are all in this class. Although these systems are highly flexible, some issues (e.g. the high cost, high power consumption, low robustness, and low reliability) deter manufacturers from their application.
To address the aforementioned issues, a novel hydraulic VVA was designed, manufactured, and tested in our previous study. The numerical and experimental results have shown its flexibility, reliability, and the repeatability is comparable with current camless valvetrains. To precisely control the engine valve timings, opening duration, and lift, the proposed VVA system has been equipped with linear and non-linear controllers.
… This paper presents a novel variable valve actuation system for internal combustion engines. In addition, a simplified model is built to optimize the energy consumption by GA [genetic algorithm]. Furthermore, a novel ERS [energy recovery system] is designed to save the energy to its maximum. More importantly, the results show that the proposed system can save about 58% energy used in a conventional cam driven valvetrain.—Pournazeri et al.
The system comprises two rotary spool valves, two phase shifters, a single-acting spring-return hydraulic actuator for each engine valve, and a hydraulic power unit. The engine valve is connected to the hydraulic actuator piston. Both the low-pressure and high-pressure rotary spool valves (i.e. LPSV and HPSV) are responsible for transferring oil to or from the hydraulic cylinder. These valves are rotated by the crankshaft and the engine speed is two times larger in four-stroke engines.
|Schematic of the proposed VVA. HPSV = High pressure rotary spool valve. LPSV = low pressure rotary spool valve. Pournazeri et al. Click to enlarge.|
However, two electric phase shifters are utilized to phase shift their shafts independently. These phase shifters are able flexibly to change the angular position of the output shaft with respect to its original position without changing the input/output speed ratio.
The hydraulic system is designed to guarantee fully closing the engine valve at the maximum engine speed such that early valve closure occurs at lower engine speeds. During the closing stage of the engine valve at lower engine speeds, only a part of the spring potential energy is utilized to empty the cylinder by discharging the oil; the rest is wasted.
To conserve the surplus spring potential energy during the valve’s closing stage, the upstream pressure of the main pump is able to be changed by a secondary pump coupled to the shaft of the main pump. The engine valve actuator’s downstream pressure is regulated such that the surplus spring energy is used to maintain the main pump’s upstream pressure during engine valve operation. This reduces the power consumption of the main pump.
|The proposed ERS for VVA system. Pournazeri et al. Click to enlarge.|
This method has the potential to bring the well-established benefits of a fully variable valve system out of the lab and into production engines because cost and complexity aren’t issues.—Amir Khajepour
Mohammad Pournazeri, Amir Khajepour, Yanjun Huang (2018) “Improving energy efficiency and robustness of a novel variable valve actuation system for engines,” Mechatronics, Volume 50, Pages 121-133 doi: 10.1016/j.mechatronics.2018.02.002