Mechanism of Soot and NOx Emission Reduction Using Multiple-injection in a Diesel Engine

Engine experiments have shown that with highpressure multiple injections (two or more injection pulses per power cycle), the soot-NOx trade-off curves of a diesel engine can be shifted closer to the origin than those with the conventional single-pulse injections, reducing both soot and NOx emissions significantly. In order to understand the mechanism of emissions reduction, multidimensional computations were carried out for a heavy-duty diesel engine with multiple injections. Different injection schemes were considered, and the predicted cylinder pressure, heat release rate and soot and NOx emissions were compared with measured data. Excellent agreements between predictions and measurements were achieved after improvements in the models were made. The improvements include using a RNG k-e turbulence model, adopting a new wall heat transfer model and introducing the nozzle discharge coefficient to account for the contraction of fuel jet at the nozzle exit. The present computations confirm that split injection allows significant soot reduction with out a NOx penalty. Based on the computations, it is found that multiple injections have a similar NOx reduction mechanism as single injections with retarded injection timings. Regarding soot reduction, it is shown that reduced soot formation is due to the fact that the soot producing rich regions at the spray tip are not replenished when the injection is terminated and then restarted. With split injections, the subsequently injected fuel burns rapidly and does not contribute significantly to soot production. The present work also demonstrates the usefulness of multidimensional modeling of diesel combustion to reveal combustion mechanisms and to provide design insights for low emission engines. EXTENSIVE RESEARCH is in progress to reduce both nitrogen oxides (NOx) and particulate (soot) emissions from diesel engines due to environmental concerns. One of the emission-control strategies is in-cylinder reduction of pollutant production. It is well known that it is very difficult to reduce both NOx and soot production simultaneously during the combustion process. Many emission-reduction technologies developed so far tend to increase soot emission while reducing NOx emission, and vice versa. For example, retarding fuel injection timing can be effective to reduce NO formation. However, this usually results in an increase of soot production. On the other hand, although increasing fuel injection pressure can decrease soot emissions, it can also cause higher NOx emissions at the same time [1]*. Recently, it has been shown experimentally that with high-pressure multiple injections, the soot-NOx trade-off curves of a diesel engine can be shifted closer to the origin than those with single-pulse injections, reducing both soot and NOx emissions significantly [2-4]. Nehmer and Reitz experimentally investigated the effect of double-pulse split injection on soot and NOx emissions using a single-cylinder Caterpillar heavy-duty diesel engine [2]. They varied the amount of fuel injected in the first injection pulse from 10 percent to 75 percent of the total amount of fuel and found that split injection affected the sootNOx trade-off. In general, their split-injection schemes reduced NOx with only a minimal increase in soot emissions and did not extend the combustion duration. Tow et al. [3] continued the study of Nehmer and Reitz [2] using the same engine, and included different dwells between injection pulses and triple injection schemes in their investigation. They found that at high engine load (75%), particulate could be reduced by a factor of three with no increase in NOx and only a 2.5% increase in BSFC compared to a single injection, using a double injection with a relatively long dwell between injections. They also found that triple injection could reduce NOx and soot emissions at both light and high loads. Another important conclusion of Tow et al. [3] is that the dwell between injection pulses is very important to control soot production and there exits an optimum dwell at a particular engine operating condition. The optimum dwell of a double-injection was found to be about 10 degree crank angles at 75% load and 1600 rev/min for their engine conditions. * Numbers in brackets designate References at the end of the paper. Pierpont et al. [4] confirmed that the amount of fuel injected in the first pulse affects the particulate (smoke) level in experiments where the NOx emission level was held constant. However, the best double injections were found to also depend on the spray nozzle included angle. For a production injector with a 125 included angle, which results in significant wall impingement on the piston bowl, the best double injections were found to be those with 50% to 60% of the fuel injected in the first pulse. They also found that with a combination of EGR and multiple injections, particulate and NOx were simultaneously reduced to as low as 0.07 and 2.2 g/bhp-hr, respectively, at 75% load and 1600 rev/min. Other multiple injection studies can be also found in the open literature [5, 6]. The published experimental works indicate that multiple-injection is an effective mean to control NO and particulate production during the diesel combustion process. In general, multiple injections allow the injection timing to be retarded to reduce NOx emission while holding the particulate at low levels. Both the amount of fuel injected in the first pulse and the dwell between pulses are important for an optimum injection scheme. With the application of multiple injection technology, the goal of improved injection scheme design and better control of engine combustion is made difficult by the fact that design variables are added with flexible injectors. It is thus helpful to simulate the engine processes with the use of computational models, which can provide detailed temporal and spatial information of precisely parameter-controlled injection and combustion processes. Patterson et al. [7] performed multidimensional computations of multiple injections using an improved KIVA code. They tried to reproduce the experimental results of Nehmer and Reitz [2] and achieved a fair success. However, the accuracy of their model prediction deteriorated for double-pulse injections as the amount of fuel injected in the second pulse increased. Kong, Han and Reitz [8] modified the code by including a modified RNG k-e turbulence model and turbulence boundary conditions [9]. Predictions of combustion and emissions of single-injections were shown to be improved significantly [8]. These successes motivated the application of the code to multiple injections in the present study. It is clear that a good model is necessary in order to predict engine combustion and emissions accurately. Accordingly, the submodels used by Kong, Han and Reitz [8] were implemented together with improved heat transfer and injection models. The models were first applied to the experimental results of the double injections of Nehmer and Reitz [2]. For better understanding of the formation of NO and soot during multiple-injection combustion processes, a set of designed singleand double-injection schemes were computed. Based on the computational results, a mechanism of emission reduction using multiple-injection is suggested.