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, CoVIMEP 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.
