To meet the stringent emission regulations and fuel economy demands of the spark ignition (SI) engine, more and more new technologies such as turbocharging, variable valve actuation (VVA), and exhaust gas recirculation (EGR) are being developed. For the turbocharged SI engine, the high boost pressure can lead to higher laminar combustion velocity with higher maximum burned gas temperature, which induces more emissions; it also carries a risk of serious knocking, which can not only deteriorate the brake-specific fuel consumption (BSFC) but also destroy the engine. As is well known, the dilution mixture gas methods, which include VVA and EGR, are effective techniques to advance the combustion phase and suppress knocking in the SI turbocharged engines. The effects of VVA and EGR rates on the BSFC and combustion characteristics of an SI engine were analyzed through 100 groups of engine experiments, and the quantitative analysis of the influence saliency of VVA and EGR rates has been introduced. Then, the optimal level of each factor was obtained by a comprehensive balance method. The results indicated that the EGR rate has the most significant influence on the BSFC and CA50. At the same time, the BSFC was only 211.7 g/kWh, which has been improved significantly, and CA50 was 12.55° CA ATDC, which effectively enhances the knock resistance when applying the optimization parameters.

A variable valvetrain system is the key part of the variable stroke engine (VSE), which could achieve higher power performance and low-speed torque. An innovative axial shift valvetrain system (ASVS) was put forward to meet the air-charging requirements of a 2/4-stroke engine and complete a changeover within one working cycle. Two sets of intake and exhaust cam profiles for both intake and exhaust sides in the 2/4-stoke mode were designed for 2/4-stoke modes. Furthermore, a simulation model based on ADAMS was established to evaluate the dynamic valve motion and the contact force at different engine speeds. The dynamic simulation results show that the valve motion characteristics meet the challenges at the target engine speed of 3000 r/min. In two-stroke mode, the maximum intake valve lift could achieve 7.3 mm within 78°CaA, and the maximum exhaust valve lift could achieve 7.5 within 82°CaA on the exhaust side. In four-stroke mode, the maximum intake valve lift can achieve 8.8 mm within 140°CaA, and the maximum exhaust valve lift can achieve 8.4 mm within 140°CaA. The valve seating speeds are less than 0.3 m/s in both modes, and the fullness coefficients are more than 0.5 and 0.6 in the 2-stroke and 4-stroke mode, respectively. At the engine speed of 3000 r/min, the contact force on each component is acceptable, and the stress between cam and roller can meet the material requirement.

Traditional heavy-duty vehicle service brakes use the friction method to realize vehicle deceleration, which means that longer-term use of the service brake will lead to overheating. It will make the braking capacity significantly reduced, at the same time, the service brake system also will wear much faster. Engine brake has been developed continuously in recent decades since it has the advantages of small install space and weight, no attenuation of braking power, rapid response, and endurance braking. However, the complex structure and large valve load of the valve train are the main obstacles to the widespread use of two-stroke compression release braking. Two-stroke compression release braking power is the effective indicator of the braking capacity and the maximum cylinder pressure (Pmax) can reflect the load of the valve train, therefore, the braking power and Pmax need to be optimized at the same time. In this paper, the multi-objective non-dominated sorting genetic algorithm II (NSGA II) was introduced to optimize the two-stroke compression release braking performance and it was compared with the orthogonal design method. The results indicated that the braking power and Pmax of optimal solution 1 by NSGA II achieved −395.64 kW and 59.37 bar, which were 1.17% and 2.78% improved than that of orthogonal analysis, respectively. In addition, the calculation process shows that NSGA II provides a more comprehensive and reliable method to optimize the valve parameters of the two-stroke compression release brake.

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