Study Finds That Late Intake Valve Closing and Increased Intake Pressure Can Reduce NOx, Soot, HC, and CO Emissions Simultaneously in Diesel PCCI
|Summaries of the different effects of late IVC on engine emissions and performance from recent studies. (LIVC: late IVC; ←: advance; ↓decrease; ↑increase; – remain constant). Credit: ACS, Peng and Jia (2009). Click to enlarge.|
A study by a pair of researchers from the University of Sussex (UK) and Dalian University of Technology, China found that using late intake valve closing (IVC) in diesel PCCI (premixed charge compression ignition) engines, can, with the assistance of increasing intake pressure, reduce NOx, PM, HC, and CO emissions simultaneously.
A certain EGR (exhaust gas recirculation) rate and optimized SOI (start of injection) timing were always necessary to maintain satisfactory NOx and soot emissions for diesel PCCI combustion. Their paper was published online 14 October in the ACS journal Energy & Fuels.
PCCI in diesel engines offers the potential for substantially reducing NOx and particulate matter simultaneously while maintaining engine efficiency of conventional diesels. Late IVC as a mechanism to optimize PCCI combustion has a complex influence on engine performance and emissions, and those influences can be significantly combined with other operating parameters such as EGR rate, intake pressure, injection timing, and fueling rate.
The low combustion temperature induced by late IVC could help the reduction of NOx emissions and engine noise. However, it also leads to the increase of HC emissions. Late IVC prolongs ignition delay and improves charge homogeneity inside the cylinder,which reduces NOx and soot emissions. On the other hand, since late IVC allows some intake gases to be expelled out the cylinder during the compression stroke, the amount of oxygen available for the combustion is reduced, and this could cause the increase of soot emissions. Higher boost pressure can help to maintain the in-cylinder gas density when late IVC is used, but this increases the compression work and thus possibly leads to higher ISFC. CO emissions have a more complicated behavior for late IVC, and it is sensitive to both combustion temperature and local air-fuel ratio.
—Peng and Jia (2009)
In their study, Zhijun Peng and Ming Jia performed a full engine cycle simulation using a three-dimensional computational fluid dynamics (CFD) model coupled with detailed chemical kinetics to investigate the effect of late intake valve closing (IVC) on combustion and emission characteristics in a PCCI diesel engine.
Specifically, they focused on the potential of late IVC combined with variable intake pressure, EGR rate,and start of injection (SOI) for emissions reduction.
Based on the results, they concluded:
Ignition timing could be efficiently controlled by IVC timing because it could change the effective compression ratio.
Late IVC avoided high NOx and soot concentrations by keeping lower combustion temperature and providing better premixing. But it probably led to increases of HC and CO emissions due to the lower combustion temperature and insufficient oxygen amount.
Increasing intake pressure helped reductions of soot, HC, and CO emissions due to the improved homogeneity of the fuel-air mixture and increased oxygen amount. It is extremely important to increase the intake pressure for soot reduction when late IVC was adopted, they noted. By properly retarding IVC timing and increasing intake pressure simultaneously, all four regulated emission components could be reduced.
For diesel PCCI combustion, a certain EGR rate was necessary for maintaining low NOx emission when IVC timing was retarded, although higher EGR rate increased soot emissions almost for all investigated IVC timings.
SOI timing should be carefully optimized when variable IVC timing was employed for gaining both low NOx and soot emissions for diesel PCCI combustion.
Zhijun Peng and Ming Jia (2009) Full Engine Cycle CFD Investigation of Effects of Variable Intake Valve Closing on Diesel PCCI Combustion and Emissions. Energy Fuels doi: 10.1021/ef900688v