SI Engine Operation with Retarded Ignition: Part 1 - Cyclic Variations

Engine operation with spark ignition retarded from MBT timing is used at cold start to reduce HC emissions and increase exhaust gas temperature; however it also results in increased cyclic variations. Steady-state cold fluids testing was performed to better understand the causes of the cycle-to-cycle variations. Detailed analysis of individual cycles was performed to help gain an understanding of the causes of cyclic variations. The important results were:

• The primary cause of cyclic variations in IMEP is variations in the combustion phasing (location of 50% mass fraction burned). The expansion ratio decreases rapidly during combustion for retarded spark timing and therefore the phasing determines individual cycle thermal efficiency and IMEP. Variations in the late burn have little impact on the IMEP as this combustion occurs close to EVO and does little expansion work.
• A combustion diagram analysis indicates that flame quench and misfire at ignition do not contribute to the increase in cyclic IMEP fluctuations as is the case at the lean limit with standard spark timing. Prior cycle effects which occur at the lean limit with standard spark timing also were not found at cold, retarded spark conditions.
• Improved in-cylinder motion (swirl and tumble) decreased cyclic variations which enabled additional spark retard (relative to MBT timing) and produced lower HC emissions, presumably due to improved mixing.
• Dual spark plugs further improved stability. This was thought to be due to more repeatable combustion phasing as inhomogeneities in the chamber were “averaged out” by the two spark locations.