Spark Assist for CA50 Control and Improved Robustness in a Premixed LTGC Engine – Effects of Equivalence Ratio and Intake Boost 2018-01-1252

Low-temperature gasoline combustion (LTGC) engines can deliver high efficiencies, with ultra-low emissions of nitrogen oxides (NO_x) and particulate matter (PM). However, controlling the combustion timing and maintaining robust operation remains a challenge for LTGC engines. One promising technique to overcoming these challenges is spark assist (SA). In this work, well-controlled, fully premixed experiments are performed in a single-cylinder LTGC research engine at 1200 rpm using a cylinder head modified to accommodate a spark plug. Compression ratios (CR) of 16:1 and 14:1 were used during the experiments. Two different fuels were also tested, with properties representative of premium- and regular-grade market gasolines. SA was found to work well for both CRs and fuels. The equivalence ratio (ϕ) limits and the effect of intake-pressure boost on the ability of SA to compensate for a reduced T_in were studied. For the conditions studied, ϕ=0.42 was found to be most effective for SA. At lower equivalence ratios the flame propagation was too weak, whereas ϕ=0.45 was closer to the CI knock/stability limit, which resulted in a smaller range of CA50 control and T_in compensation. At ϕ=0.42, SA worked well from P_in = 1.0 to 1.6 bar, but the range of effective T_in compensation dropped progressively with boost from 21 °C at P_in = 1.0 bar to the equivalent of 12 °C at P_in = 1.6 bar. The amount of control authority using SA was demonstrated by varying the spark timing, advancing CA50 to the onset of strong knocking and then retarding CA50 to near misfire. SA provided good control, however the CA50 control range decreased from 7.2° CA at P_in = 1.0 bar to 4.2° CA at P_in = 1.6 bar. For all intake pressures at these well-mixed conditions, NO_x emissions for SA were less than for compression ignition only, and all were below the US-2010 Heavy Duty limit.