The Effects of Leaner Charge and Swirl on Diesel Combustion 2002-01-1633
Substantial reduction of NOX and particulate emissions from diesel engines will be required by the emission legislation in the future. In a diesel engine, the combustion and emissions formation are governed by the spray formation and mixing processes. Parameters of importance are droplet size, droplet distribution, injection velocity, in-cylinder flow (convection and turbulence) and cylinder charge temperature/pressure. The mixing is controlled by convective and turbulent mixing due to in-cylinder charge motion, momentum transfer and turbulence induced by the injection process. The most important processes are known to be the turbulent macro- and micromixing.
Smaller nozzle orifices are believed to increase mixing rate, due to smaller droplet size leading to faster evaporation. Dimensional analysis suggests that the turbulent mixing time, τmix, scales with orifice diameter, d. Smaller orifices do, however, reduce the spray penetration length (proportional to the square root of d), which may lead to problems with air utilization.
In order to investigate the effects of turbulent mixing, two different nozzles with different orifice diameters were tested, 6 × ∅0.227 mm and 15 × ∅0.130 mm, at different air-to-fuel ratios. The experiments were performed on a single-cylinder heavy-duty direct-injection diesel engine using two different compression ratios, rC=12.0 and rC=18.5.
Small orifices decrease soot at light and medium load, but increase soot at high loads due to worse air utilization. The results show that with increased boost pressure (leaner global air-to-fuel ratio) smoke is also reduced at high loads with small orifices. The leaner conditions with increased boost pressure also influence the trade-off NOX versus soot.
Reduced compression ratio increases soot at high load when using big orifices, while small orifices are insensitive to compression ratio. The decrease in soot emissions with leaner global air-to-fuel ratios is greater for the smaller orifices than for the bigger orifices regardless of compression ratio.
Swirl and a leaner air-to-fuel ratio both reduce soot for a big orifice diameter, while for a small orifice a leaner air-to-fuel ratio is more effective to reduce soot than the use of swirl. For a lean condition with a small orifice diameter, the use of swirl does not give any further reduction in soot emission.