Ehleskog, M., Gjirja, S., and Denbratt, I., "Effects of Variable Inlet Valve Timing and Swirl Ratio on Combustion and Emissions in a Heavy Duty Diesel Engine," SAE Technical Paper 2012-01-1719, 2012, doi:10.4271/2012-01-1719.
In order to avoid the high CO and HC emissions associated with low temperature when using high levels of EGR, partially premixed combustion is an interesting possibility. One way to achieve this combustion mode is to increase the ignition delay by adjusting the inlet valve closing timing, and thus the effective compression ratio.The purpose of this study was to investigate experimentally the possibilities of using late and early inlet valve closure to reduce NOx emissions without increasing emissions of soot or unburned hydrocarbons, or fuel consumption. The effect of increasing the swirl number (from 0.2 to 2.5) was also investigated. The combustion timing (CA50) was kept constant by adjusting the start of injection and the possibilities of optimizing combustion using EGR and high injection pressures were investigated. Furthermore, the airflow was kept constant for a given EGR level.The engine used in the experiments was a single-cylinder heavy-duty direct injection diesel engine equipped with an electronically controlled unit injector with variable needle opening pressure. Its geometry is based on the Volvo D12C production engine with the compression ratio lowered from 18.5 to 17. The engine is also equipped with a fully flexible pneumatic inlet valve system, enabling both early and late inlet valve closing times and varied swirl levels.The results show that late or early IVC can be used to achieve partially premixed combustion (and thus low soot and NOx emissions) without a fuel consumption penalty.