Sandia National Laboratories has been investigating a new, integrated approach to generating electricity with ultra low emissions and very high efficiency for low power (30 kW) applications such as hybrid vehicles and portable generators.Our approach utilizes a free piston in a double-ended cylinder. Combustion occurs alternately at each cylinder end, with intake/exhaust processes accomplished through a two stroke cycle. A linear alternator is mounted in the center section of the cylinder, serving to both generate useful electrical power and to control the compression ratio by varying the rate of electrical generation. Thus, a mechanically simple geometry results in an electronically controlled variable compression ratio configuration.The capability of the homogeneous charge compression ignition combustion process employed in this engine with regards to reduced emissions and improved thermal efficiency has been investigated using a rapid compression expansion machine. Eight different fuels, including propane, natural gas, hydrogen, methanol, n-pentane, hexane, n-heptane, and isooctane have been used at low equivalence ratio (ϕ ∼ 0.35) and initial temperatures of 25°C, 50°C and 70°C.The results indicate that the cycle thermal efficiency can be significantly improved (56% measured) relative to current combustion systems, while low NOx emissions are possible (<10 PPM). HC and CO emissions must be controlled through some aftertreatment technology. The primary cause of this high conversion efficiency is nearly constant volume combustion at high compression ratio (∼ 30:1).