Improving the Efficiency of the Advanced Injection Low Pilot Ignited Natural Gas Engine Using Organic Rankine Cycles

Intense energy security debates amidst the ever increasing demand for energy in the country have provided sufficient impetus to investigate alternative and sustainable energy sources to the current fossil fuel driven economy. This paper presents the Advanced injection Low Pilot Ignition Natural Gas (ALPING) engine as a viable, efficient and low emissions alternative to conventional diesel engines, and discusses further efficiency improvements to the base ALPING engine using Organic Rankine Cycles (ORC) as bottoming cycles. The ALPING engine uses very small diesel pilots, injected early in the compression stroke to compression-ignite a premixed natural gas–air mixture. It is believed that the advanced injection of the higher cetane diesel fuel leads to longer incylinder residence times for the diesel droplets, thereby resulting in distributed ignition at multiple spatial locations, followed by lean combustion of the higher octane natural gas fuel via localized flame propagation. The multiple ignition centers result in faster combustion rates and higher fuel conversion efficiencies. The lean combustion of natural gas leads to reduction in local temperatures and oxides of nitrogen (NOx) emissions, since NOx emissions scale with local temperatures. In addition, the lean combustion of natural gas is expected to produce very little particulate matter (PM) emissions (not measured). Representative baseline ALPING (60° BTDC pilot injection timing) (without the ORC) half load (1700 rev/min, 21 kW) operation efficiencies reported in this study are about 35 percent while the corresponding NOx emissions is about 0.02 g/kWh, which is much lower than EPA 2007 tier 4 heavy duty diesel engine statutes of 0.2 g/kWh. Furthermore, the possibility of improving fuel conversion efficiency at half load operation with Organic Rankine Cycles using “dry fluids” are discussed. Dry organic fluids, due to their lower critical points, make excellent choices for bottoming Rankine cycles. Moreover, previous studies indicate that “dry fluids” are more preferable compared to wet fluids because the need to superheat the fluid to extract work from the turbine is eliminated. It is estimated that ORC–turbocompounding results in fuel conversion efficiency improvements of the order of 10 percent while maintaining the essential low NOx characteristics of ALPING combustion.Copyright © 2007 by ASME