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Many different chemical measurement methodologies of air to fuel (A/F) ratio have been documented in technical publications [1, 2, 3, 4, 5, 6, 7 and 8]. Each of these methods is derived from the same physical principles but they vary in simplifying assumptions and physical constants. All are well proven over time with test data, producing excellent results near stoichiometry. Few technical publications, however, include data at lean A/F ratios and none include data at the very lean A/F ratios at which new high-technology engines such as gasoline direct injection spark ignition engines may operate with stratified combustion. This paper presents a comparison of three A/F ratio measurement methods based on exhaust gas composition. The methods produce similar results when applied to feed-gas emissions, but results vary when applied to post-catalyst emissions measurements. Some theories that may explain this behavior are discussed.

Low-Pressure Exhaust Gas Recirculation (LP-EGR) has been shown to be an effective means of improving fuel economy and suppressing knock in downsized, boosted, spark ignition engines. LP-EGR is particularly beneficial at low-speed, high-load conditions, but can lead to combustion instability at lower loads. The transport delays inherent in LP-EGR systems slow the reduction of intake manifold EGR concentrations during tip-out events, which may lead to excessive EGR concentrations at low load. This paper explores leveraging Variable Valve Timing (VVT) as a means of improving the rate of reduction of intake manifold EGR concentration prior to tip-out. At higher boost levels, high valve overlap may result in intake manifold gas passing directly to the exhaust manifold. This short-circuiting behaviour could potentially improve EGR evacuation rates.