The soot mitigation potential of ducted fuel injection of natural gas and other light hydrocarbon fuels in compression ignition engines
Keywords: Engineering.
Mechanical Engineering.

Despite their load and range benefits, Diesel engines suffer from high levels of soot emissions. There is a desire to reduce soot emissions in situ to reach acceptable levels without the need for costly aftertreatment. Further, when unimpeded by soot, operating Diesel engines fuel-rich would enable the use of mixed electrochemical-combustion engine cycles for added gains in efficiency. Natural gas and mixtures of alcohol fuels have already proven to result in less soot than traditional Diesel fuel. Nonetheless, when operating at rich equivalence ratios, in situ soot mitigation is likely to be required. Ducted fuel injection (DFI) is a combustion strategy developed by Charles Mueller's group at Sandia National Laboratories aimed at reducing soot emissions in situ. DFI is accomplished by injecting fuel through small tubes located slightly downstream of the fuel injector nozzle rather than freely into the engine cylinder. By impeding entrainment of air over the length of the duct, the development of the outer sheath flame is delayed, and as result, the lift-off length is extended. While the technique has proven effective in mitigating soot for traditional Diesel fuel in optical access engine tests, this dissertation seeks to determine whether the success of DFI can be extended to the Diesel-style combustion of difficult-to-autoignite fuels like natural gas and alcohols. Experiments were performed in a single-cylinder, low-heat rejection engine. The higher in cylinder temperatures afforded by the low-heat rejection engine made the ignition of low-cetane fuels possible. A DFI assembly was designed for use in this engine in a process informed by Schlieren imaging of fuel injection, literature exploring various duct geometries, and engine constraints. Three representative natural gas mixtures, starting with methane and followed by additions of ethane and propane, were used. Overall, the experiments showed DFI to be ineffective in reducing soot emissions for all fuel mixtures over a range of equivalence ratios. Apparent heat-release rate profiles showed that methane combusted predominantly in the premixed combustion phase where a diffusion flame has not yet formed and, consequently, there is no defined lift-off length. Adding ethane and propane decreased the magnitude of premixed burning, allowing more fuel to combust in the mixing-controlled phase. However, DFI again had negligible effect, hypothesized to be as result of long lift-off lengths. A phenomenological N-Stage Lagrangian model was used to assess the effect of the duct on the overall amount of air entrained in the case that the lift-off length of the outer diffusion flame was already long. The model showed negligible difference and, as such, the development of the diffusion flame would not be held off substantially to compensate for entrainment lost over the length of the duct before autoigniting. An easier-to-autoignite propanol ethanol mixture was used to explore the intermediate region between natural gas and \#2 Diesel fuel. Once again DFI did not result in meaningful soot mitigation. However, as equivalence ratio increased, DFI led to significant reductions in the magnitude of premixed burning as compared to conventional Diesel-style combustion of the same mixture. This indicated that entrainment was significantly impeded by the ducts. Despite this, as the lift-off length was higher than that of \#2 Diesel, the diffusion flame was likely not held off enough to realize the full benefit of the strategy. Results indicate that DFI is not a fuel agnostic approach to soot mitigation. The technique is not suitable for Diesel-style combustion that results in long or non-existent lift-off lengths where fuel is consumed mainly through premixed combustion. As such, the in-cylinder temperature and autoignition quality of the fuel dictate the utility of DFI.