Impact of retarded spark timing on engine combustion, hydrocarbon emissions, and fast catalyst light-off

摘要

An experimental study was performed to determine the effects of substantial spark retard on engine combustion, hydrocarbon (HC) emissions, feed gas enthalpy, and catalyst light-off. Engine experiments were conducted at cold engine conditions for various ignition timings and air/ fuel ratios. Chemical and thermal energy of the exhaust gas was tracked from cylinder-exit to the catalytic converter inlet using a variety of experimental techniques. Time-resolved exhaust port and exhaust runner hydrocarbon concentrations were converted to an exhaust HC mass flow rate and compared to time-averaged downstream HC levels. Quenching experiments quantified cylinder-exit HC emissions by rapidly cooling exhaust gas at the valve seats, effectively freezing HC oxidation reactions. Combustion stability was observed to decrease as the phasing of the 50% mass fraction burned location occurred later in the expansion stroke. A thermodynamic burn rate analysis indicated combustion was complete by exhaust valve opening with spark timings as late as 200 after top-dead-center (ATDC). Engine operation with a relative air/fuel ratio 10% lean of stoichiometric resulted in the lowest observed tailpipe-out HC emissions. Retarded spark timings increased exhaust system oxidation, with port HC oxidation ranging from 15% to 37% with additional HC reductions (40-50%) in the runner for ATDC spark timings. Catalyst light-off times were reduced by 5 seconds and cumulative catalytic converter-in HC emissions were reduced by 44% prior to light-off. A phenomenological model of exhaust system oxidation was developed to provide insight into HC burn-up with late combustion phasing.