Development of Fast Idle Catalyst Light-Off Strategy for Gasoline Compression Ignition Engine - Part 2 2020-01-0314
The present investigation expands on our previous work on development of fast idle catalyst light-off strategy for a light duty gasoline compression ignition (GCI) engine. In part 1, the steady state experimental investigation in a single cylinder GCI engine indicate an optimum strategy for effective catalyst light off during cold start fast idle operation. According to this strategy, the strategy includes (1) dispersing a first fuel injection during the intake stroke, (2) dispersing a second fuel injection during the expansion stroke, and (3) igniting a spark during the expansion stroke. This strategy increases the exhaust temperature during cold starts thereby assisting in lighting the oxidation catalyst, and reduce emissions and provide greater combustion stability as compared to other injection and spark strategies. In this study, based on the reported optimum strategy, a strategic sensitivity study was performed to investigate the impact of lambda, load, speed, spark plug design and injector spray pattern on effective cold start catalyst light off. The strategy, hereby specifically termed as tailed injection strategy, is as follows: 1) start of first injection (SOI1st) = -350 CAD aTDC 2) start of second injection (SOI2nd) = 15 CAD aTDC and 3) spark timing = 15 CAD aTDC. The results of the sensitivity analysis indicate that higher load is useful to obtain better combustion stability and higher exhaust temperature at expense of increased fuel consumption and emission. At higher speeds, increased exhaust temperature is attained but with the penalty of increased NOX emission and higher COV. Lean operation (Lambda = 1.05) has a better compromise in terms of COV and emissions. In order to ascertain the feasibility of the proposed strategy during fast idle period, dynamic testing with fast (transient) HC measurement was performed in part 2 of the study besides performing the steady state sensitivity study. Dynamic operation at the optimum strategy resulted in few initial misfired cycles due to poor combustion stability (for less than 4 secs after). This was identified by the initial HC spike in the time resolved fast HC measurement profile. Therefore, either the engine was run richer or the spark timing was advanced during the first part of fast idle catalyst warm up period to reduce the misfiring and minimize hydrocarbon and CO breakthrough. Amongst these two operating strategies, spark timing advancement during the first phase of idle period is effective during dynamic operation as running richer cost for increase HC cumulative emissions in the tail pipe. Based on the dynamic testing results, we split the initially proposed control strategy in two parts - firstly, transient combustion control strategy during first part of fast idle phase (less than 4 secs) for rapid engine warm up and effective catalyst light off (light off phase 1) secondly, split fuel injection and extremely retarded spark timing strategy is proposed during the second part of catalyst warm up period to generate high enthalpy flow in the exhaust with reduced hydrocarbon and other emissions, improved combustion stability, and fuel economy (light off phase 2).