Injection pressure and hydraulic nozzle flow rate are significant parameters that influence mixture formation and combustion in the direct injection (DI) diesel combustion process.

An injection pressure increase enhances mixture formation as well as combustion. Decreasing the hydraulic flow rate, by means of reducing spray hole orifice diameter, affects mixture formation primarily through improved gas entrainment processes. Both injection pressure increase and hydraulic flow rate decrease therefore benefit mixture formation and combustion, which cause less soot emissions. Thus, the application of higher exhaust gas recirculation rates to decrease nitrogen oxide emissions is possible. The objective of this paper is to determine the effects of hydraulic flow rate variation in combination with very high injection pressures.

—Rollbusch 2011

Testing was on a single-cylinder research engine, based on a light-duty direct injection diesel engine, with a displacement of 755 cm³ per cylinder. The engine featured a prototype common rail fuel injection system that is capable of delivering system pressures larger than 250 MPa (2,500 bar). Four different hydraulic nozzle layouts were tested.

For each engine operating point, exhaust gas recirculation variations with different injection pressure levels and boost conditions were conducted. Engine test results were performed at a constant soot/nitrogen oxide emission ratio of 1:10.

Rollbusch observed that the lower hydraulic flow rate at part-load causes a higher mean relative air/fuel ratio within the fuel spray and thus induces less soot formation. Moreover, the premixed fraction of combustion is enhanced through both injection pressure and hydraulic flow rate increases.

In full load operation, parameters such as injection duration, combustion duration and spray momentum during spray/wall interaction are identified as more important.

Boost pressure variation had only a minor effect on part-load, whereas full-load results are greatly influenced. In part load and full load operation, a limitation of the potential to reduce emissions was not observed within the investigated injection pressure range.

Resources

• Carsten Rollbusch (2011) Effects of hydraulic nozzle flow rate and high injection pressure on mixture formation, combustion and emissions on a single-cylinder DI light-duty diesel engine. International Journal of Engine Research doi: 10.1177/1468087411430304