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Prospects for High-Temperature Combustion, Neat Alcohol-Fueled Diesel Engines
Abstract:
The use of neat alcohols, namely methanol and ethanol, in direct-injection, compression-ignited engines is difficult, most notably due to their poor ignitability. By employing a high-temperature combustion strategy, this challenge may be overcome, thus creating the opportunity for using these oxygenated and inherently low-sooting fuels for heavy-load applications.
Experimental data are provided from a single-cylinder research engine that shows particulate matter (PM) emissions for Diesel-style combustion of both methanol and ethanol that are below the current US Government regulation limit. The level of particulates remained low up to stoichiometric ratios of fuel and air. A complete emissions analysis indicates a high combustion efficiency of ∼ 96% at stoichiometric conditions.
In order to achieve reliable combustion, some form of intake-air preheating was required. The issue of ignitability is addressed with modeling which indicates that highly turbocharged, non-intercooled air into a cylinder with low heat rejection (LHR) surfaces can achieve conditions that satisfy acceptable ignition delay requirements. With increased exhaust enthalpy, opportunities exist to use thermal or mechanical exhaust regeneration strategies. All of these features contribute to a clean, high-efficiency Diesel engine with heavy-load capability.
To explore the nature of soot formation within alcohol spray jets, images are provided from another single-cylinder device with optical access. The images show single-plume combustion for both methanol and ethanol into an air environment similar to that of an engine. Broadband luminosity is observable for both fuels within the interior of each jet. This indicates that a balance exists between soot formation and oxidation, the difference of which is responsible for engine-out emissions.