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Dimethyl Ether (DME) is a promising future compression ignition fuel, particularly when derived from renewable, CO2-neutral feedstocks. While it is generally well-known that DME produces very little soot when burned, few studies have explored its low-temperature combustion behavior, where the potential for ultra-low engine-out emissions of both NOx and soot may exist. The present work shows the results of a single-cylinder engine operating with DME below the level of the US 2010 Heavy Duty Onroad Standard for NOx, without NOx aftertreatment. A high-pressure oil-over-fuel intensified injection system was used to maintain proper air utilization and high combustion efficiency, in combination with intake oxygen control using relatively high levels of EGR for low NOx. Fuel-related material issues notwithstanding, the engine results point toward a potentially cost-effective and efficient means of utilizing bio-derived fuels.

A hydraulic free-piston engine (FPE), which converts combustion energy directly to hydraulic energy, is being developed by the U. S. EPA due to its potential as a lower-cost and higher-efficiency prime mover for hydraulic series hybrid vehicles. Two prototype engines were designed, fabricated and tested: a two-cylinder engine operating primarily with a two-stroke compression-ignition, direct-injection (CIDI) cycle and a six-cylinder engine operating with a four-stroke CIDI cycle. These engines successfully achieved up to 39% peak hydraulic efficiency under continuously fired operation, while demonstrating exceptional repeatability and control of the cylinder compression ratio. A basic description of the engine design, along with the initial test results from these two prototypes, is presented below.