The company has been developing its MICE (Miniature Internal Combustion Engine) generator technology to address the need for high energy density portable electric power. The MICE generator offers energy densities that are five to ten times higher than current rechargeable batteries, and therefore is an enabling technology for powered prostheses and other high power portable devices such as power tools. As discussed in the MICE generator description below, the company has demonstrated the ability to achieve very low levels of acoustic emissions and vibration from a packaged system.
This innovative motor-generator consists of a miniature linear engine coupled with a linear alternator. It takes advantage of the high energy content of hydrocarbon fuels while eliminating most of the parts found in a standard internal combustion engine-generator set. The basic MICE generator design, shown in the adjacent figure, consists of a two-stroke engine, a spring, and an alternator in a linearly oscillating configuration. MICE is inherently an electric power generator, since there is no mechanical linkage with which to extract power. Pure linear motion is ensured by the use of a unique double helix or multiple helix spring. The pure linear oscillation provides sliding motion with no side forces. The piston does not use rings to seal the combustion chamber, using instead a close fit between the piston and cylinder to hold the leakage to a small enough level that essentially no cycle penalty is incurred. The small amount of leakage, in fact, provides a hydrostatic bearing force to center the piston in the cylinder. Thus, the MICE generator has low frictional losses since there are no bearing surfaces having a direct load. The low friction characteristics and absence of stress generated by direct loads allow the MICE generator to operate at very high cycle speeds, leading to high energy and power density, particularly at smaller size scales. The pure linear motion, in addition to having low frictional losses, allows operation with a solid film lubricant alone – in other words, no oil – with a pressure-balanced cylinder design.
Major differences exist, between the MICE generator and conventional engines. One key difference is that MICE is a free piston design that uses a spring for energy storage. A second major difference is that the MICE two stroke engine is designed to operate with a scavenging ratio less than unity, using HCCI (Homogeneous Charge Compression Ignition) combustion with glow plug assist as the mode of combustion. This gives the MICE generator its low emissions and high efficiency characteristics. Also, the MICE generator is a high Q system, operating at the resonant frequency of the spring-mass system with very low frictional losses.
The company has developed the MICE generator in three power ranges. The adjacent figure shows the 5 – 10 W MICE generator next to the 300 – 500 W MICE generator which is designed to use both light (propane, butane) and heavy (JP-8, diesel) fuels. An AA battery and ruler provide a dimensional reference. Extensive testing has been performed on the 300-500 W MICE generator using both propane and JP-8 fuels. The third size range, a 100 W MICE generator, has recently developed specifically for the powered prosthesis applications. The 100 W MICE generator, which was designed to operate with a butane or propane fuel cartridge, is shown in the lower figure with a military BB-390 battery (5 inch height) for relative size comparison. The long and narrow aspect ratio of the MICE generator will allow easy integration with lower limb prostheses, though a shorter form factor is possible for other applications. Recharging the MICE generator consists in simply replacing the fuel cartridge.