Operational Characteristics of Oxygenate-Water Fuel Blends Studied in an Optical DI Diesel Engine with Simulated Exhaust Gas Recirculation 2007-01-2017

Engine combustion strategies that preserve high cycle efficiency while minimizing engine-out pollutant emissions are the focus of major research efforts around the world. Such high efficiency clean combustion (HECC) strategies typically employ compression ignition of a charge that exhibits an elevated degree of fuel/air premixing and/or dilution with combustion products. Prior studies have shown that a highly dilute, mixing-controlled combustion strategy using a high-cetane, oxygenated fuel can achieve HECC while avoiding the control, high-load knock, and light-load incomplete combustion difficulties that are often experienced with approaches that use a high degree of charge premixing. On the other hand, employing high dilution levels (e.g., by using large amounts of cooled exhaust gas recirculation, EGR) can place excessive burdens on engine heat exchangers and air-handling systems.

The current study explores the concept of using the water in an oxygenate-water fuel blend as a diluent to help offset the need for excessive levels of cooled EGR, thereby enabling a mixing-controlled HECC strategy with highly favorable attributes. In the first part of the study, a class of oxygenate-water blends with acceptable ignition, corrosion, toxicity, and mixture-stability characteristics is identified. Next, heat-release rate, in-cylinder natural luminosity, and emissions data are quantified in initial screening experiments using blends with four different water concentrations over a range of simulated EGR levels. Results show that such blends can be highly effective at simultaneously suppressing NO_x and smoke emissions in the absence of EGR, though combustion efficiency degrades rapidly if the water concentration is increased beyond a threshold level. Interestingly, when EGR is used in conjunction with an oxygenate-water blend, NO_x emissions do not decrease as quickly as would be expected based on results from experiments with fuels that do not contain water.