Development Of Cam-Less Valve Train And Its Application On A Controlled Auto-Ignition Engine

Controlled auto-ignition (CAI) is an innovative combustion process which has been receiving worldwide attention from major automotive manufacturers and engine research institutions. Characterized by its homogenous charge auto-ignition by compression and low temperature combustion, CAI mode is able to realize high thermal efficiency and low engine-out emissions showing great research potential. Trapping or re-breathing high temperature burnt gas via cam-less valve train proves to be a simple and effective method to obtain CAI combustion.An electro-hydraulic cam-less valve train was developed in this thesis aimed to achieve variable engine valve phase and valve lift. Two-stage hydraulic piston mechanism was featured by fast engine valve opening and low flow ratio requirement. Cushioning system was used to avoid mechanical impact between hydraulic piston and its stop, which reduced the working noise and improved the valve train endurance. A Matlab model of this electro-hydraulic valve train was previously built to assist system design. Items showing effect on the system performance were numerically explored. Thanks to this numerical model, working parameters was finally determined and optimized.Test bench for this cam-less valve train was established and valve train system functionalities were analyzed through the data acquisition system. The results indicated a normal distribution for the intake and exhaust valve opening and closing delay at constant working fluid pressure and temperature. While the valve closure has nothing to do with the working pressure, higher fluid temperature claimed a faster valve closing response. Soft landing was achieved with seating velocity effectively reduced to 0.08 m/s. Valve lift swept from 3.1 mm to 11 mm while changing the working pressure. Obtained valve opening and closing profile showed great repeatability at constant working pressure.Engine test platform was established with cam-less valve train mounted on the engine head and fully controlled with self-developed electrical management system. With open-loop feedforward control strategy by looking up valve phasing map, cam-less valve train ensured engine speed up to 2200 r/min. Basic fuel injection pulse and optimal ignition advanced angle were sampled. After calibration, cam-less engine reached the power of original engine with camshaft, and even higher at some operating points. Fuel economy and emission performances reached the same level with original engine. Stable CAI combustion with second exhaust valve event was firstly achieved on the engine test platform in our country. CAI combustion, emission features and operation region were studied. Mode transition between SI and CAI was also analyzed. In this study, exhaust gas recompression process was observed on the pressure history due to the trapped burnt gas as a result of advanced first exhaust valve closing. Recompression peak pressure was 0.5MPa, lower than that of NVO strategy,1.2MPa. Experiment results revealed that indicated thermal efficiency was 0.3644 at selected operating points,10% higher than that of SI mode.Stable CAI combustion region was obtained as a function of IVO and SEVO, with fixed equivalence F/A ratio of approximately stoichiometric and with fixed exhaust valve event and fixed IVC at engine speed of 1000 r/min. Within this operation region, IVO timing swept from 18°CA ATDC to 60°CA ATDC and SEVO timing swept from 18°CA ATDC to 46°CA ATDC. Left side of this region with early IVO was limited by misfire due to its lower EGR rate; on the other hand, right side of this region was restricted by misfire owing to the extreme lean fresh charge. Even delayed IVO timing implied CAI misfire. EGR rate, adjusted by varying IVO or SEVO timing, ranged from 40% to 57%. Within the entire CAI region, BMEP swept from 0.227MPa to 0.401MPa. With second exhaust valve closing kept constant, engine load could be altered by adjusting the IVO timing. The brake thermal efficiency swept from 0.25 to 0.31, higher than 0.284 in most of this region, with corresponding EGR rate from 39.6% to 43% and NOx emission lower than 140ppm. Experiment tests indicated that temperatures, includes cooling water temperature, intake charge temperature and especially, exhaust gas temperature, had significant influence on the CAI combustion. Misfire occurred for the operating points with exhaust gas temperature lower than 633K.Mode transition from spark ignition (SI) to CAI could be realized within two cycles and stable CAI combustion was in turn obtained. Due to the high temperature of burnt gas from SI mode, first CAI combustion was advanced and the in-cylinder peak pressure appeared near the TDC.