MULTI-DIMENSIONAL MODELING OF A 1 kWe FREE PISTON LINEAR ALTERNATOR
Free Piston Linear Alternators (FPLA) have the potential to provide solutions for high efficiency distributed power generation by operating at low-temperature combustion modes and by reducing frictional losses compared to conventional crank-slider engines. The current study is focused on modeling, simulation, and analysis of a natural gas FPLA for small-scale Combined Heat and Power (CHP) applications. The objective of this study is to investigate combustion processes and quantify efficiency losses for operation at 1 kWe power output with low NOx emissions. The proposed FPLA operates in Homogeneous Charge Compression Ignition (HCCI) mode with dilute mixture (φ' = 0.25-0.5) at high compression ratio (~23:1). Mixture dilution is accomplished by residual gas trapping. Modeling results of closed cycle simulations indicated that gross indicated efficiency of 43.1% can be realized, while NOx emissions are kept below regulated standards. Effective equivalence ratio (φ') operating range was identified between φ' = 0.25-0.4 to achieve low pressure rise rates, high efficiency, and NOx emissions below regulated standards. Varying the operating frequency was found to introduce a tradeoff between heat losses and combustion efficiency, which led to maximum gross indicated efficiency occurring at 50 Hz. Reducing stroke to bore ratio (S/B) and increasing surface to volume ratio (S/V) at top dead center (TDC) led to increased heat transfer losses to the cylinder walls and lower combustion efficiency.