Effects of n-butane addition on reformer gas combustion: Implications for the potential of using reformer gas for an engine cold start

Abstract

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 driveability. 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 HC fuel to transition into fully warmed-up operation on gasoline. The laminar flame speeds and lean flammability limits of reformer gas/air mixtures with different amounts of n -butane addition are investigated numerically. The results indicate, that the flame speeds are dramatically reduced with even as little as 10% n -butane addition. Although the flame temperature is increased with n -butane addition, the fact that the H and O radicals react preferentially with n -butane leads to a reduction in the radical pool and a consequent flame speed reduction. While the lean flammability limit of reformer gas/air mixtures is leaner than n -butane/air mixtures at 1 atm, at 10 atm the trend reverses and the flammability limit of pure reformer gas/air mixtures is richer than that of n -butane/air mixtures. 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 miniumum. Immediately following catalyst light-off, the level of liquid fuel being fed to the engine can then be increased.