|Local fuel/air equivalence ratio versus temperature. Supercritical dieseline combustion “navigates between the Scylla and Charybdis” of soot and NOx. Source: George Anitescu. Click to enlarge.|
A team from Syracuse University and the National Institute of Standards and Technology (NIST) is exploring the use of supercritical (SC) dieseline (a diesel-gasoline blend) as an optimized fuel for advanced diesel engine designs.
Diesel-gasoline blends are of interest for Low Temperature Combustion (LTC) modes as a means of avoiding high EGR rates often used to prepare a fully premixed charge before the start of combustion (SOC). High EGR reduces in-cylinder oxygen content, causing deterioration of combustion efficiency, leading to increased hydrocarbon (HC) and carbon monoxide (CO) emissions. One way to avoid the disadvantages of high EGR rates is to use a lower-cetane, higher-volatility fuel. (Earlier post.)
The resistance to autoignition of a low-cetane fuel can provide sufficient ignition delay for air-fuel mixing, while a faster vaporization by high volatility can increase mixing rate, notes Dr. George Anitescu of Syracuse University, who is leading the SC dieseline work. (Dr. Anitescu had earlier been investigating supercritical diesel, earlier post.) Blends of diesel fuel and gasoline (dieseline) have therefore been suggested for LTC, in particular at light loads. Unfortunately, the mentioned advantages can be achieved only across a relatively narrow load window, Anitescu says.
Alternative methods to the conventional combustion are being recently focused on injecting heated fuels as well as new fuel blends. A solution to simultaneously increase fuel efficiency and mitigate the engine emissions is to inject diesel fuels as supercritical (SC) fluids. However, at high temperatures needed for SC injection, these fuels thermally decompose (coke) and can plug the injector nozzle [Eaton research, 1983]. In order to avoid this problem, an anticoking agent such as CO2, H2O, EGR, natural gas, and gasoline can be added to diesel fuels before injection.
Given its specific properties, gasoline appears as a good choice for a blending fuel and an anticoking agent. With dieseline blends, the (pseudo)critical temperature of diesel fuel (~450 °C) can be significantly lowered since that of the gasoline is around 300 °C. Thus, under SC conditions, it can be possible to expand the gasoline proportion in blends with diesel fuels. The ultimate goal will be to use a single fraction from the crude oil distillation which will include both gasoline and diesel fuel ranges.—Anitescu et al., “Dieseline for Advanced Supercritical Combustion in Diesel Engines: Volatility, Phase Transitions, and Spray/Jet Structure”, unpublished
At supercritical conditions, Anitescu says, the fuel is ready for combustion since no droplets to vaporize exist, combustion takes place in a larger volume than around the envelope of a spray, and combustion also takes place at lower temperature (avoiding NOx formation). The team’s work, using KIVA-RIF CFD modeling shows about a 10% increase in thermal brake efficiency and about 80% reduction in both PM and NOx. They developed an experimental SC injector but are still working on a commercial prototype.
Anitescu and colleagues are currently studying the volatility of automotive gasoline - diesel fuel blends using the advanced distillation curve method. Volatility is a key property for an optimized combustion of near- or super-critical fuel blends. In a paper being prepared for submission, they constructed distillation curves for blends of 10, 30, 50, 70, and 90% vol/vol and compared them to those of neat automotive gasoline with octane number 97 and diesel fuel No. 2.
Among their results, they found that dieseline volatility leans toward that of gasoline at the first fraction distilled and toward that of diesel fuel at the end of distillation; the more labile components of dieseline samples thermally decomposed toward the end of the distillation process. Parallel experiments on thermal stability showed no significant thermal decomposition of dieseline during heating time up to one hour at 400 °C. Additional experiments showed that this fuel behavior substantially improved blend transition from liquid to supercritical states with the more chemically-stable gasoline acting as an anticoking agent for heated diesel fuel.
Further, they found that the mixing of heated dieseline with the air upon injection was substantially improved when compared to unheated fuel sprayed as droplets. The results could inform efficient fuel system and combustion chamber designs to optimize supercritical fuel utilization in diesel engines, decrease fuel consumption and practically eliminate harmful emissions without any aftertreatment, they suggest.
In an earlier poster presented at the US Department of Energy (DOE)’s 2011 Directions in Engine-Efficiency and Emissions Research (DEER) Conference Anitescu had presented work on the preparation, injection and combustion of SC diesel-gasoline blends (90% diesel fuel-10% gasoline, vol/vol) to study the thermodynamic and transport properties of the fuel-diluent mixtures. A separate poster described work on supercritical biodiesel fuels.