Combustion System Optimization of a Low Compression-Ratio PCCI Diesel Engine for Light-Duty Application

A new combustion system with a low compression ratio (CR), specifically oriented towards the exploitment of partially Premixed Charge Compression Ignition (PCCI) diesel engines, has been developed and tested. The work is part of a cooperative research program between Politecnico di Torino (PT) and GM Powertrain Europe (GMPT-E) in the frame of Low Temperature Combustion (LTC) diesel combustion-system design and control. The baseline engine is derived from the GM 2.0L 4-cylinder in-line, 4-valve-per-cylinder EU5 engine. It features a CR of 16.5, a single stage VGT turbocharger and a second generation Common Rail (1600 bar). A newly designed combustion bowl was applied. It features a central dome and a large inlet diameter, in order to maximize the air utilization factor at high load and to tolerate advanced injection timings at partial load. Two different piston prototypes were manufactured by changing the internal volume of the new bowl so as to reach CR targets of 15.5 and 15. The baseline and the new in-piston bowls were experimentally analyzed in conjunction with different injector nozzles to assess their best match for PCCI application. To such an end, the ratio of premixed fuel-air was enhanced by advancing injection and increasing fuel-air contact surface. Hence, in addition to baseline injectors with 7 holes and 148 deg spray cone angle, two nozzle configurations featuring 9-hole and reduced spray cone angles (135 deg and 120 deg) were considered in order to assess the benefits of increased fuel-air contact surface and advanced injection timings, respectively, in charge premixing. The experimental tests were carried out on the highly dynamic test bed of the IC Engines Advanced Laboratory (ICEAL) at Politecnico di Torino. Combustion analysis was also performed by means of an innovative premixed-diffusive multizone diagnostic tool, which has recently been developed at ICEAL. For the different combustion system configurations, engine performance and pollutant emissions were investigated at characteristic engine working points, for both partial and full loads, where wall impingement and air utilization are critical, respectively. In addition, the low CR engine prototypes were tested for NEDC cycle by reproducing the engine operation on a D-class passenger car at the dynamic test bed. Finally, the combustion system which presented the best compromise between partial- and full-load performance was selected.