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Conventional crank-based engines are limited by mechanical, thermal, and combustion inefficiencies. The free piston of a linear engine generator reduces frictional losses by avoiding the rotational motion and crankshaft linkages. Instead, electrical power is generated by the oscillation of a translator through a linear stator. Because the free piston is not geometrically constrained, dead center positions are not specifically known. This results in a struggle against adverse events like misfire, stall, over-fueling, or rapid load changes. It is the belief that incorporating springs will have the dual benefit of increasing frequency and providing a restoring force to aid in greater cycle to cycle stability. For dual free piston linear engines the addition of springs has not been fully explored, despite growing interest and literature.

The Stiller-Smith mechanism is a new mechanism for the translation of linear motion into rotary motion, and has been considered as an alternative to the conventional slider-crank mechanism in the design of internal combustion engines and piston compressors. Piston motion differs between the two mechanisms, being perfectly sinusoidal for the Stiller-Smith case. Plots of dimensionless volume and volume rate-change are presented for one engine cycle. It is argued that the different motion is important when considering rate-based processes such as heat transfer to a cylinder wall and chemical kinetics during combustion. This paper also addresses the fact that a Stiller-Smith engine will be easier to configure for adiabatic operation, with many attendant benefits.

The EPA implemented the Urban Bus Retrofit/Rebuild (UBRR) Program for transit buses built before 1994 in an effort to lower the amount of PM emissions in densely populated urban areas. The objective of the program is to provide certified emission control technologies that reduce PM emissions from older buses by 25% or to below 0.1 g/bhp-hr. This paper reviews the development of a retrofit kit that has been certified under the UBRR program to meet the 0.1 g/bhp-hr PM emission requirements on DDC 6V92TA engines with both mechanical (MUI) and electronic (DDEC) fuel injection controls. The kit is a combination of specific and modified engine parts and a catalytic exhaust after-treatment device. The kit replaces existing parts with a new camshaft, a uniquely configured cylinder kit and specified turbocharger, blower and injector. For the MUI engines the cam timing, injector height and fuel modulator are set at specific values to achieve the lowest possible PM level.

Linear engine alternator (LEA) design optimization traditionally has been difficult because each independent variable alters the motion with respect to time, and therefore alters the engine and alternator response to other governing variables. An analogy is drawn to a conventional engine with a very light flywheel, where the rotational speed effectively is not constant. However, when springs are used in conjunction with an LEA, the motion becomes more consistent and more sinusoidal with increasing spring stiffness. This avoids some attractive features, such as variable compression ratio HCCI operation, but aids in reducing cycle-to-cycle variation for conventional combustion modes. To understand the cycle-to-cycle variations, we have developed a comprehensive model of an LEA with a 1kW target power in MATLAB®/Simulink, and an LEA corresponding to that model has been operated in the laboratory.