Reducing pressure fluctuations at high loads by means of charge stratification in HCCI combustion with negative valve overlap
Artikel i vetenskaplig tidskrift, 2009
Future demands for improvements in the fuel economy of gasoline passenger car engines will require the development and implementation of advanced combustion strategies, to replace, or combine with the conventional spark ignition strategy. One possible strategy is homogeneous charge compression ignition (HCCI) achieved using negative valve overlap (NVO). However, several issues need to be addressed before this combustion strategy can be fully implemented in a production vehicle, one being to increase the upper load limit. One constraint at high loads is the combustion becoming too rapid, leading to excessive pressure-rise rates and large pressure fluctuations (ringing), causing noise. In this work, efforts were made to reduce these pressure fluctuations by using a late injection during the later part of the compression. A more appropriate acronym than HCCI for such combustion is SCCI (Stratified Charge Compression Ignition). The approach was evaluated in tests with a single-cylinder metal research engine and a single-cylinder optical engine. The latter was used to characterize the combustion in laser-based analyses including laser-induced florescence (LIF) determinations of fuel tracer, OH and CH 2O (formaldehyde) distributions. A high speed camera was also used for direct imaging of chemiluminescence. The effects of two main parameters were studied: the proportion of fuel injected late to create a stratified charge and the timing of the late injection. In addition, two fuels were used: a certification gasoline fuel and a blend of n-heptane, iso-octane and 3-pentanone. Both fuels were used in the metal engine for comparison. Use of a stratified charge allowed the maximum pressure-rise rates and ringing intensity to be reduced at the expense of increases in NOx and CO emissions, regardless of fuel type. Optical results indicated that both the fuel distribution and combustion were not homogenous.