Exploration of combustion strategies for high-efficiency, extreme-compression engines [electronic resource]
Keywords: Mechanical Engineering

Increasing the compression ratio of an internal combustion engine to 100:1 or greater could potentially enable efficiencies greater than 60%. Understanding and managing the combustion process is a critical component to achieving this in practice. This thesis explores strategies for combustion at extreme compression ratios. First, the setup of a free-piston device capable of operating at 100:1 compression ratio is described. Initial performance results are reported for air-only experiments. Diesel-style combustion was the first approach taken, as it provides facile ignition phasing. Results are reported from initial lean Diesel combustion experiments at compression ratios ranging from 30 to 100:1. Indicated efficiency peaked at 60% for these experiments. To further understand Diesel-style combustion at extreme compression ratios, a study of Diesel sprays in the extreme compression apparatus was performed. The setup of a combined schlieren and direct luminosity imaging system with full-bore optical access is described. Spray penetration, dispersion, liquid length, and ignition delay are reported for combusting and non-combusting sprays. Compression ratios for these experiments ranged from 30 to 100:1. Spray behavior followed expected trends as a function of primary variables such as gas density. However, rapidly varying gas density from the free-piston profile impacts the spray penetration. Furthermore, at the highest compression ratios in-cylinder fluid motion dramatically affects the spray behavior, enabled by the low ratio of fuel to gas density. Systems added to the extreme compression apparatus to measure gaseous and particulate emissions are described. Emissions measurements from Diesel-style combustion of isooctane at 35:1 compression ratio are reported, to provide a reference case at conditions similar to conventional engines. Emissions were similar to those from production Diesel engines, with the exception that soot, HC, and CO increased more rapidly with equivalence ratio in the present study. Results from experiments with Diesel combustion up to 100:1 compression ratio are also reported. The combustion efficiency was 99% up to 100:1 compression ratio, and HC, CO and soot emissions were low. Emissions of NOx were 5 times higher at 100:1 than at 35:1, and would require aftertreatment. Stoichiometric, premixed-charge combustion enables the use of a three-way catalyst and produces low soot levels. Using this approach at extreme compression ratios requires delaying autoignition until the minimum volume is reached. Options for control of autoignition are discussed, and gas cooling is identified as the most effective. Pre-refrigeration, intercooling, and evaporation of a liquid are modeled and shown to effectively achieve the desired ignition timing at 100:1 compression ratio, without impacting the overall engine efficiency. Experimental results are reported for premixed methane-air combustion with intercooling control of autoignition, for 0.96 to 1.04 equivalence ratio and 35 to 90:1 effective compression ratio. The gas cooling requirement for autoignition control was higher than predicted by the models, but still within practical reach. The indicated efficiency peaked at 57%. Emissions levels from these experiments were similar to stoichiometric spark-ignited natural gas engines reported in the literature, and indicate that a three-way catalyst could be successfully used even at extreme compression ratios. Results are also reported for water injection control of autoignition. Autoignition was successfully controlled up to 60:1 effective compression ratio, but the mass of water required was an order of magnitude higher than predicted. This is shown to result from practical limitations of the current water injector setup.