SAE International The Influence of Injection Strategy and Glow Plug Temperature on Cycle by Cycle Stability Under Cold Idling Conditions for a Low Compression Ratio, HPCR Diesel Engine 2012-01-1071

Experimental studies have been undertaken on a single-cylinder HPCR diesel engine with a compression ratio of 15.5:1 to explore the effect of fuel injection strategy on cycle by cycle stability. The influence of the number, separation and quantity of pilot injections on the coefficient of variation of IMEP has been investigated at -20°C, 1000 rev/min, post-start idling conditions. Injection strategy and glow plug temperature trade-off has also been investigated at a range of soak temperatures. Up to four pilot injections have been used. For timing of the main injection near to the optimum, CoV IMEP values of 10% or better can be achieved. Closer spacing of injections improved stability and extended the range of timings to meet target stability. The best combinations of pilot number and pilot quantity varied with total fuel delivered. For a 20 mg/cycle total, twin-pilot injections delivering a total pilot fuel of 4 mg/cycle, or three or four delivering a total pilot fuel of 4 mg/cycle or 6 mg/cycle, gave best stability, but at lower total fuelling (16 mg/cycle) stability was not so good and achieved by fewer combinations. Target stability was not achieved through hotter glow plug temperatures alone; a multiple pilot strategy or an increase in total fuelling was necessary. Increasing the number of pilot injections and glow plug temperature produced a higher premix contribution and a following strong development of main combustion, which is generally consistent with better cycle by cycle stability. At high glow plug temperatures and using three and four pilot injections, stability improved with warmer soak temperatures. At -5°C, stability was particularly poor when single- or twin-pilot injections were used irrespective of glow plug temperature.