On flame propagation of primary reference fuels and reformer gas: Implications for improving cold-start performance of a SI engine

A major reason for the increased hydrocarbon (HC) emissions during the cold-start phase of the Federal Test Procedure is that the engine must be operated fuel rich in order to obtain suitable idle quality and drivability. Significant reductions in HC emissions would be obtained if the engine could be operated at the stoichiometric air/fuel ratio, or preferably lean of stoichiometric. In this work we investigate the possibility of starting an engine on reformer gas/air mixtures during the cold-start and then adding increasing amounts of hydrocarbon fuel to transition into fully warmed-up operation on gasoline.; First the laminar flame speeds and lean flammability limits of reformer gas/air mixtures with different amounts of n-butane addition are investigated numerically. In the second stage, the experimental study is carried out in a hierarchical manner by measuring the laminar flame speeds of various fuel mixtures. First, the combustion of pure primary reference fuel (PRF) and PRF blends, as well as reformer gas is characterized. Next, the effects of small amount of hydrocarbon addition on reformer gas combustion and the effects of small amount of reformer gas addition on hydrocarbon combustion are systematically investigated. Through this experimental effort, we aim to enhance the understanding of combustion of the primary reference fuels and characterize the interactions of reformer gas and hydrocarbon fuel. The results indicate that the flame speeds are dramatically reduced with even as little as 5% hydrocarbon fuel additions. Although the flame temperature is increased with hydrocarbon fuel addition, the fact that the H and O radicals react preferentially with hydrocarbon fuel leads to a reduction in the radical pool and a consequent flame speed reduction. The present results indicate that in order to obtain the full benefit of reformer gas operation during cold-start, the amount of hydrocarbon fuel in the fuel mixture must initially be kept to a minimum. Immediately following catalyst light-off the level of liquid fuel being fed to the engine can then be increased.; A digressional study on the flammability limits of ultra-dilute n-butane/air mixtures are calculated over ranges of temperature, pressure, and dilution levels relevant to HCCI operation. The results indicate that with the elevated temperatures required to obtain HCCI combustion the in-cylinder charge is capable of supporting a propagating flame.