Non-Sooting, Low Flame Temperature Mixing-Controlled DI Diesel Combustion 2004-01-1399
Methods of producing non-sooting, low flame temperature diesel combustion were investigated in an optically-accessible, quiescent constant-volume combustion vessel. Combustion and soot formation processes of single, isolated fuel jets were studied after autoignition and transient premixed combustion and while the injector needle was fully open (i.e., during the quasi-steady mixing-controlled phase of heat-release for diesel combustion).The investigation showed that fuel jets that do not undergo soot formation in any region of the reacting jet and that also have a low flame temperature could be produced in at least three different ways during mixing-controlled combustion:
First, using a #2 diesel fuel and an injector tip with a 50 micron orifice, a fuel jet was non-sooting in ambient oxygen concentrations as low as 10% (simulating the use of EGR) for typical diesel ambient temperatures (1000 K) and densities. Non-sooting combustion was achieved by rapid air entrainment prior the lift-off length that produced a reacting fuel-air mixture was too lean for soot formation. Second, using the same fuel and injector tip at an ambient gas temperature of 850 K and 21% oxygen, it was shown that non-sooting, mixing-controlled combustion occurred under conditions for which the cross-sectional average equivalence ratio at the lift-off length was approximately 0.6. This suggests that the combustion was fuel-lean and the formation of a high-temperature diffusion flame at a near-stoichiometric mixture was avoided. Quasi-steady fuel-lean combustion conditions were also realized using an oxygenated fuel (21 wt% oxygen) with a 100 micron orifice. Third, using extensive EGR (8% ambient oxygen), a more conventional 180 micron orifice, and an oxygenated fuel, non-sooting combustion was also achieved at typical diesel ambient temperatures and densities. For the third set of conditions, the lack of soot formation was most likely caused by jet mixture temperatures in fuel-rich regions of the jet that were too cool for soot inception.
The adiabatic flame temperature reached when employing the above techniques was approximately 2000 K or lower. The lack of soot formation and low flame temperature realized suggests that these mixing-controlled combustion methods offer the potential for a simultaneous soot and NOx reduction in an engine. However, the above results are for single, isolated jets, unlike practical engines, and further research is needed to determine whether these methods can be implemented in real engines.