The Effect of EGR Dilution on the Heat Release Rates of Boosted Spark-Assisted Compression Ignition (SACI) Engines 2020-01-1134
This paper presents an experimental investigation of the impact of EGR dilution on the tradeoff between flame and end-gas autoignition heat release in a Spark-Assisted Compression Ignition (SACI) combustion engine. The mixture was maintained stoichiometric and fuel-to-charge equivalence ratio (ϕ') was controlled by varying the EGR dilution level at constant engine speed. Under all conditions investigated, end-gas autoignition timing was maintained constant by modulating the mixture temperature and spark timing.
Experiments at constant intake pressure and spark timing showed that as ϕ' is increased, lower mixture temperatures are needed to match end-gas autoignition timing. Higher ϕ' mixtures exhibited faster initial flame burn rates, which were attributed to the higher estimated laminar flame speeds immediately after spark timing. At constant intake pressure and mass fraction burned at the onset of autoignition, end-gas autoignition rates increased significantly at higher ϕ' conditions.
The increasing trends in peak autoignition rate and end-gas energy at autoignition onset were found to be consistent for all intake pressures ranging from 80 kPa to 150 kPa. For a constant spark timing, the mass fraction burned at the onset of autoignition correlated well with ϕ', regardless of intake pressure.
Increasing the mass fraction burned at the onset of autoignition was effective at reducing the peak heat release rates. This behavior was attributed to the reduced energy content of the end-gas but also to the reduced end-gas burn rates. Possible explanations for the latter observation were the greater buffer gas effect due to the larger burned gas mass, and the higher reactivity stratification of the end-gas being closer to the walls.
Vassilis Triantopoulos, Stanislav Bohac, Jason Martz, George Lavoie, Andre Boehman, Jeff Sterniak, Dennis Assanis
University of Michigan, Robert Bosch LLC, University of Delaware