With an intention to improve the performance of reciprocating engines used for distributed generation US-Dept. of Energy has launched ARES program. Under this program, the performance targets for these natural gas-fuelled stationary engines are ≻ 50% efficiency and NOx emissions ≺ 0.1 g/bhp-hr by 2013. This paper presents two technologies developed under this program.
Lean-burn operation is very popular with engine manufacturers as it offers simultaneous low-NOx emissions and high engine efficiencies, while not requiring the use of any aftertreatment devices. Though engines operating on lean-burn operation are capable of better performance, they are currently limited by the inability to sustain reliable ignition under lean conditions. Addressing such an issue, research has evaluated the use of laser ignition as an alternative to the conventional Capacitance Discharge Ignition (CDI). Initial tests in a static chamber and a rapid compression machine have shown laser ignition to be capable of ignition at very higher pressures and leaner conditions than those ignitable using CDI. Subsequent tests in a lean-burn single-cylinder engine show NOx reductions up to 70% for a given efficiency, or efficiency improvements up to 3% for a given NOx emission. Further developments in fiber optics and laser technology are required to reduce laser ignition to practice.
NOx emission reduction through charge dilution by using Exhaust Gas Recirculation (EGR) is a common practice. However, EGR has negative performance connotations as it impedes component durability and affects lubrication oil quality. The alternative to EGR is the use of Nitrogen Enriched Air (NEA) that can be facilitated by polymeric membranes: Selective diffusion of oxygen over nitrogen in such membranes enables creation of nitrogen-enriched air. Tests were conducted in a natural gas-fuelled single-cylinder engine showed up to 70% NOx reductions for modest efficiency penalties. Subsequent tests comparing NEA and EGR showed that both techniques had comparable performance. While this technology is suited for stoichiometric (rich-burn) operation as well as lean-burn operation, regulatory compliance is likely to dictate the method of operation.