Benefits and drawbacks of compression ratio reduction in PCCI combustion application in an advanced LD Diesel engine

The present paper describes an experimental study on the effect of the compression ratio on the performance of a LD diesel engine operating with a PCCI calibration, near the estimated EURO 6/Tier2 Bin5 NOx emission limits. The research activity is the result of a collaborative project between Istituto Motori and Centro Ricerche Fiat aimed to carry out an exhaustive analysis of the compression ratio (CR) influence on the performance of a LD diesel engine. Starting from a reference engine configuration the CR was reduced in two steps sequentially. Each CR value was characterized under PCCI operation mode and, under conventional diesel operating mode, at maximum torque. The exploration of the PCCI application in the NEDC operating area was performed prefixing limits on maximum fuel consumption, maximum pressure rise and maximum tolerable smoke. The main result was that no significant increment in PCCI application area reducing the CR was possible without overcoming the limits. But thanks to a drastic drop in smoke emission for the low CR values, a further enlargement of near EURO 6 PCCI application area was possible by means of engine recalibration. The results allowed the definition of the optimum range values of the CR able to minimize the NOx-particulate trade-off coupled with acceptable increment in unburned compound emissions (HCs+CO) and FC, helping to the definition of the line-guides for the future EURO 6/Tier2 Bin5 engine design.