Direct injection of both fuels in RCCI may offer a way to decrease HC and CO emissions without sacrificing efficiency and NOx/PM benefits
|Diagram of direct injector placement used in the study; future work will use a more parallel alignment of the injectors. Wissink et al. Click to enlarge.|
By using a new strategy entailing the direct, separate injection of both low- and high-reactivity fuels, researchers at the University of Wisconsin have found another method for combustion phasing control of RCCI (reactivity controlled compression ignition) that offers the potential of decreasing the emissions of unburned hydrocarbons (UHC) and CO without sacrificing the benefits of RCCI: high efficiency and near zero levels of NOx and soot. (Earlier post.) Martin Wissink from the team presented a paper on the work at the ASME 2012 Internal Combustion Engine Division Fall Technical Conference in Vancouver, BC, this week.
RCCI is a promising dual-fuel, compression-ignition engine low-temperature combustion (LTC) strategy that uses in-cylinder fuel blending with at least two fuels of different reactivity and multiple injections to control in-cylinder fuel reactivity to optimize combustion phasing, duration and magnitude.
By using two different fuels, both the reactivity and equivalence ratio are stratified throughout the cylinder. This adds an additional fast-response control parameter in the form of fuel chemistry, enabling the global fuel properties to change across the operating map.
Researchers have found gross indicated thermal efficiencies in excess of 56% and in-cylinder levels of PM and NOx below the EPA 2010 Heavy Duty standards at specific points in the operating map.
However, similar to other LTC strategies, RCCI suffers from levels of UHC and CO that are considerably higher than in conventional diesel combustion.
Prior work on RCCI has used port injection of the low-reactivity fuel and direct injection of the high. In an effort to decrease the emissions of UHC and CO while retaining the benefits of RCCI, the team, led by Professor Rolf Reitz, investigated the direct injection of both fuels, using separate injectors.
When using a combination of port- and direct-injection of low- and high- reactivity fuels, respectively, the reactivity gradient in the cylinder is directly proportional to the equivalence ratio gradient because every location which contains direct-injected fuel will increase in both equivalence ratio and reactivity. By independently injecting both fuels directly into the cylinder, these gradients become decoupled and independent stratification of reactivity and equivalence ratio becomes possible. By direct-injecting both fuels, a more controlled distribution is possible through spray targeting, potentially reducing the amount of unburned fuel in crevice regions.—Wissink et al.
The researchers evaluated two basic cases using diesel and gasoline:
A baseline case (PFI Gasoline) with port-injection of gasoline and two direct-injections of diesel with a common rail injector (they tested both a 6-hole and 8-hole injector) in which all parameters were held constant except for the ratio of fuel in the first and second diesel injections.
Earlier work had shown that increasing the relative amount of fuel in the second diesel injection increases stratification in certain zones, thereby increasing local equivalence ratio and reactivity. These zones ignite earlier, thereby advancing the combustion event and increasing cylinder temperatures, causing an increase in NOx and PM and a decrease in CO. By sweeping the ratio of fuel inthe two diesel injections, the researchers established a range for the combustion phasing, efficiency, and emissions profile at a particular set of operating conditions.
CR Gasoline, in which gasoline was injected through the higher-pressure common rail injector, allowing for much later injections of large quantities of gasoline. This allows for stratification of the gasoline charge before any diesel is injected. Because the space between the valves was not large enough to accommodate two CR injectors, the team injected diesel through a gasoline direct injection (GDI) injector.
CR Gasoline used two gasoline injections, with the timing of the first gasoline injection held constant at 360° BTDC, while the second injection timing and the relative quantity were swept simultaneously in order to examine the effect of changing the amount of premixed gasoline. During the sweep, the diesel injections were fixed in order to isolate the effect of gasoline stratification.
The operating conditions for PFI and CR Gasoline were nearly identical, with the intake temperature decreased slightly in the CR Gasoline case to match the charge density and combustion phasing range of the PFI Gasoline case. The engine was operated at 1300 rev/min and a nominal load of 9 bar IMEP for all tests.
One caveat is that the CR Gasoline case was not optimized. Optimization requires consideration of the timing and quantity of the different injections for a given set of operating conditions. With the injection controller allowing for up to five, and considering other factors such as intake temperature and pressure, EGR, turbocharger efficiency, etc., the optimization space for this strategy becomes quite large, the authors noted.
At a high level, they found that the results of the CR Gasoline case were largely similar to the ranges established in the PFI Gasoline case, indicating that different methods of fuel stratification can result in similar performance.
The experimental results showed that direct injection of both fuels provides performance at least as good as the standard port-fuel strategy, but with significantly less NOx and similar HC and CO. The direct injection of diesel through the GDI injector rather than the CR system resulted in a significant PM reduction. This suggests that the injector hardware requirements for RCCI may be substantially different from those of CDC, and that further investigation into the optimum diesel injector design for RCCI will be worthwhile.
The computational results suggest that crevice flows arethe primary source of UHC, and that proper spray targeting can reduce UHC and CO by reducing the amount of fuel entering the crevice and increasing the temperature in the near-crevice region, while simultaneously reducing NOx emissions by reducing the peak temperature in the bowl.
Due to the scale of the variable space, further optimization of this strategy will require a continued collaboration ofexperimental and computational study. Important variables will include the nozzle included angle, the quantity, timing, and number of the gasoline and diesel injections, the ratio of gasoline to diesel, the temperature and pressure boundary conditions, and EGR levels.—Wissink et al.
Martin L. Wissink, Jae H. Lim, Derek A. Splitter, Reed M. Hanson, and Rolf D. Reitz (2012) Investigation Of Injection Strategies To Improve High Efficiency RCCI Combustion With Diesel And Gasoline Direct Injection. (ICEF2012-92107)