| Summary: | Partially Stratified Charge (PSC) technology is one of many ideas that can be used to extend the lean limit of operation. It is accomplished by introducing a small quantity of pure natural gas, usually on the order of 5% of weight or less of the overall fuel quantity, in the local area of the spark plug electrode, whereas the rest of the mixture in the combustion cylinder is homogenous and very lean. This creates opportunity for more stable ignition, which in turn increases the likelihood of flame propagation throughout the combustion cylinder. The PSC insert technology herein requires neither engine nor spark plug modification, as the insert itself provides the passageway to move natural gas from the injector to the spark plug electrode.
2 different PSC inserts and corresponding spark plug sizes were tested, one accepting an 8mm spark plug with different injection configurations possible, and one accepting a 14mm spark plug with only injections aimed directly at the spark plug gap possible. It was demonstrated that PSC insert technology can successfully extend the lean limit of operation with both spark plug sizes. It was also demonstrated that different injection patterns and electrode offsets can be more beneficial than others. Using the herein defined homogenous lean limit cut off range of 5% coefficient of variation, an 8mm spark plug using an injection pattern creating more "swirl" in the local area and offset slightly from the electrode is able to decrease the created nitrous oxide emissions by up to 71% at 2500rpm, and an extended lean limit of lambda = 1.67 at 2000rpm versus the homogenous limit of 1.47. This same PSC geometry also increased brake mean effective pressures by up to 9.7%.
The PSC insert using a 14mm spark plug achieved an extended lean limit of lambda = 1.71 at 2000rpm versus the homogenous limit of 1.56, brake specific fuel consumption improvements of 4%, and brake mean effective pressure improvements of up to 4%. At the extended relative air-fuel ratio, nitrous oxide values were higher than, but still very close to, the homogenous lean limit values.
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