Based on the self-developed variable-mode valve actuation system，a combined simulation of the valve actuation system and the engine used was carried out to study their coupling operation characteristics，which provides a theoretical proof for vehicle application. The results show that there is a strong coupling relationship between the in-cylinder pressure and the valve operation，especially in the exhaust valve operation stage near top dead center(TDC) and below the engine speed of 2300r/min. The higher the engine speed is，the greater the valve dynamic lift loses，and the higher the maximum cylinder pressure is. While their change degree is gradually reduced at engine speed over 2300r/min. Adding a transitional lift on the exhaust brake cam after TDC can eliminate the exhaust valve recoil，and further improve contradiction relationship between the braking power and the maximum in-cylinder pressure. Compared with the ideal four-stroke brake，braking power of the two-stroke brake is obviously increased，and the maximum in-cylinder pressure is obviously reduced. The actual two-stroke braking power is increased by 35.94%，45.61% and 27.54%，and the maximum in-cylinder pressure decreased by 45.42%，27.20% and 7.35% at 1600，1900 and 2400r/min，respectively.

An electric compressor matching to a free-piston engine generator was optimized using a genetic algorithm and the internal flow characteristics were investigated. The flow losses due to various irreversible factors were quantified by entropy generation analysis. Moreover，the entropy generation and vorticity distribution of the compressor before and after optimization were compared and analyzed to further clarify the optimization strategy. The results show that the ratio of turbulent dissipation，heat transfer dissipation and viscous dissipation in the impeller and diffuser is nearly 6∶3∶1，respectively. Irreversible losses are mainly concentrated in the shroud gap and blade pressure surface. The source of loss is the tip leakage flow with part of the main stream mixed with the reverse vortex. Turbulent dissipation in the volute accounts for more than 85% of the dissipation，specifically in the form of local losses and friction head loss. The isentropic efficiency of the optimized compressor has been increased by 6% and the range and magnitude of the vortex clusters in the flow passages are substantially reduced. The irreversible flow losses are also reduced. The above work reveals the influence of irreversible factors of different dissipation mechanisms on the energy loss and provides guidance for the aerodynamic optimization design of the compressor.

The influence of different inlet conditions on the oil film cavitation phenomenon of connecting rod bearings of a heavy-duty diesel engine was explored through the dynamic boundary calculation. A method of slotting on the bearing bush was proposed to suppress the cavitation phenomenon during the bearing movement. The effects of the number，depth and form of grooves on the suppression of oil film cavitation were further analyzed. It is found that the increase in the number and depth of grooves can improve the effect of oil grooves to inhibit cavitation，but the degree of improvement of the inhibition effect decreases accordingly. If cavitation occurs and collapses frequently，it will be more likely to destroy the bearing bush surface and cause obvious cavitation erosion compared with the stable cavitation state. Compared with the full-groove model，the half-groove model has a higher suppression effect，which can reduce the maximum gas phase volume fraction of the oil film from 0.94(ungrooved)to 0.39. Combined with the reliability requirements of the overall structure of the bearing bush，selecting a half-ring single groove with an appropriate groove depth can effectively suppress the cavitation of the oil film during the bearing movement.