Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD diesel engine

An experimental investigation has been performed on the modification of in-cylinder gas thermodynamic conditions by advancing the intake valve closing angle in a HD diesel engine. The consequences on the diffusion-controlled combustion process have been analysed in detail, including the evolution of exhaust emissions and engine efficiency. This research has been carried out at full load (100%) and low engine speed (1200 rpm) with the aim of generating a long and stable diffusion-controlled combustion process. The intake oxygen mass concentration was kept at 17.4% to obtain low NOx levels in all cases. The required flexibility on intake valve motion has been attained by means of an electro-hydraulic variable valve actuation system. The results obtained from advancing the intake valve closing angle (IVC) have shown an important reduction on in-cylinder gas pressure and density, whereas the gas temperature showed less sensitivity. Consequently, the diffusion-controlled combustion process is slowed down mainly due to the lower in-cylinder gas density and oxygen availability. Important effects of advancing IVC have also been observed on pollutant emissions and engine efficiency. Where NOx production decreases, soot emissions increase. Finally, the results of pollutant emissions and engine efficiency have been compared with those obtained retarding the start of injection.