Combined Effects of Late IVC and EGR on Low-load Diesel Combustion

Artikel i vetenskaplig tidskrift, 2014

The increasing demand for improved efficiency of diesel engines requires more advanced combustion solutions. These solutions include the use of variable valve timings in combination with more traditional methods such as EGR, turbocharging and advanced injection systems. By modifying the characteristics of the charge air, further hardware optimization becomes possible. In the current investigation, the effect of late intake valve closing (LIVC) was investigated together with the effect of (external) exhaust gas recirculation (EGR) in a single cylinder heavy duty diesel engine. Different injection timings and injection pressures were investigated. The mass flow of oxygen was kept constant in order to show how the density and temperature of the reactant mixture affect the combustion and emission characteristics. The combustion results showed that if the oxygen mass flow is kept constant, an EGR approach is more efficient than LIVC in lowering fuel consumption due to the effects of increased cylinder gas flow which improves fuel conversion efficiency. It was shown that the ignition delay for a fixed combustion phasing was independent of EGR but could be increased by LIVC. The peak pressure was more strongly affected by EGR due to the larger gas flow but this response can be reduced by means of LIVC. After compensating for combustion timing effects, the reduced peak pressure was mainly attributed to reduced effective compression ratio resulting from the LIVC. The results show how variable valve timing can be used as one important tool to obtain better combustion characteristics and thus enable more efficient powertrains.