TCD燃烧系统对柴油机燃烧和排放性能改善效果的试验研究

Diesel engines find wide use in the mechanic industry depending particularly on high power density and reliability. Energy saving and emission reduction of diesel engines can remarkably contribute to national security and environmental protection. However, the introduction of spray injection pressure has restricted the performance of diesel engines in recent years. The reason is that the spray impingement can concurrently increase soot emission subjected to a prolonged combustion period. Therefore, a wall-flow-guided combustion system was developed to greatly apply for many products using the fuel momentum produced by the high injection pressure. Deutz's TCD2015 (T for turbocharger, C for charge air cooling, and D for diesel particle filter) type engine is one of the typical diesel engines with a wall-flow-guided combustion system. The TCD combustion system is characterized by a shallow basin to form the circular ridge, especially for high performance and emission reduction. Current similar combustion system also appears in many types of engine with a prominent ability. This study aims to explore the effect of the TCD combustion system on the combustion process and emission in a diesel engine. A single-cylinder diesel engine experiment was performed at different speeds, loads, and excess air coefficients (φ). Test results showed that the brake specific fuel consumption (BSFC) and the soot emission of the TCD combustion system were lower than those of the conventional ω combustion system at different operation conditions. The BSFC drop tended to be the maximum of 7.01% at 1 800 r/min, φ=1.2 operation. The maximum drop of 86.67% on soot emission appeared at 1 800 r/min, and the brake power of 54 kW. Furthermore, the TCD combustion system outperformed at a low excess air coefficients between 1.2 to 1.6. A diesel engine simulation model was also built in AVL Fire to reveal the improvement mechanism of fuel-air-mixing quality in the TCD combustion system. The simulation model configured k-ζ-f turbulence, wave spray breakup, Dukowicz spray evaporation, and ECFM-3Z combustion model. A verification for mesh size independence confirmed the unit size around 1mm. The spray total penetration (including vapor phase), in-cylinder pressure, and heat release rate were evaluated to ensure the accuracy of simulation in spray liquid penetration. The operations at 1 800 r/min with different loads and excess air coefficients were chosen to evaluate the TCD combustion system performance. Simulation results for equivalent ratio distributions showed that the cyclic ridge formed the fuel stream towards the inner and outer chambers, and then guided the fuel diffusion towards the combustion chamber center and the cylinder head, where the shallow basin wall became the fuel guider to avoid fuel gathering in the clearance at 28℃A. The velocity distributions demonstrated that a smaller region of low-velocity presented in the TCD combustion system, compared with the ω combustion system, indicating smaller collision energy loss and better fuel-air-mixing quality. A better performance was achieved at the brake power of 72 kW and φ=1.2 operations, due to the faster spray motion. The fuel mass ratios in four equivalence ratio intervals were smaller than 1, 1-2, 2-4, and larger than 4, further to evaluate the fuel-air-mixing quality in the cylinder. Results showed that the fuel mass ratios of TCD combustion system in the equivalence ratio 2-4 and larger than 4 were smaller than those of ω combustion system in 0-30℃A with a maximum of 9.75% at φ=1.2 operation, while in 30-60℃A the fuel mass ratios in the equivalence ratio smaller than 1 and 1-2 were larger with 7.45% the maximum drop at the brake power of 72 kW. Hence, the great performance of the TCD combustion system relied mainly on the sufficient fuel-air-mixing facilitated by the chamber structure. The finding can provide a sound technical reference for the development and optimization of the combustion system in diesel engines.