Quantitative evaluation of n-butane concentration on knock severity of a natural gas heavy-duty SI engine

Knocking combustion of a natural gas spark ignition engine with the addition of n-butane was investigated. Specially, the effects of n-butane concentration on knock severity were quantitatively evaluated with engine full-load at various engine speeds. The results show that with an increase in n-butane concentration, the peaks of in-cylinder pressure and heat release rate increase, combustion duration is shortened, and CA50 is advanced. At high speeds, n-butane concentration has a more pronounced influence on combustion and knock. With increased n-butane concentration, cyclic pressure traces oscillate more and more violently, the maximum pressure rise rate (MPRR) and the maximum amplitude of pressure oscillation (MAPO) in some cycles increase significantly. Here the MPRR threshold of light knock cycle is defined at 1.0 MPa/CAD, and the one of heavy knock cycle is 4.0 MPa/CAD. Meanwhile the MAPO threshold of light knock cycle is set at 0.1 MPa, and the one of heavy knock cycle is 0.3 MPa. Based on these thresholds and the probability of knock severity, light and heavy knock conditions are identified. Moreover, the combustion close to light knock may increase thermal efficiency, and heavy knock has caused the erosion of aluminum piston crown and top land in the experiment. •Effects of n-butane concentration and engine speed on knock severity evaluated quantitatively.•Natural gas combustion is more sensitive to butane concentration at high speeds.•Knock concentration limit of n-butane decreases at high speeds.•Light and heavy knock conditions are identified.